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CHAPTER 5MISSILE CONTROL SYSTEMS

A. Introduction

5A1. General

This chapter will introduce some of thenumerous devices that may be used tocontrol the flight of a guided missile. Wewill discuss four types of control systems:pneumatic, pneumatic-electric, hydraulic -electric, and electric. Throughout thechapter we will deal with general principles,rather than the actual design of any specificmissile.

5A2. Definitions

A missile GUIDANCE system keeps themissile on the proper flight path fromlauncher to target, in accordance withsignals received from control points, fromthe target, or from other sources ofinformation. The missile CONTROLsystem keeps the missile in the proper flightattitude. For example, the missile axis mustlie along the desired trajectory, rather thanat an angle. The missile must be rollstabilized; that is, a fixed plane through themissile axis must remain parallel to a fixed

reference plane outside the missile. Flightattitude stabilization is absolutely necessaryif the missile is to respond properly toguidance signals. For example, assume thatthe missile has rolled 90 degrees clockwisefrom the proper attitude. Now, if it receivesa "right turn" command from the guidancesystem, operation of the control surfaceswill actually turn the missile downward,rather than to the right. But if the controlsystem keeps the missile in the properattitude, guidance signals will be correctly

system. We can therefore say that the outputsignals from the guidance system are put intoeffect by a part of the control system.

To summarize: the missile control system,discussed in this chapter, is responsible formissile attitude control. The guidance system,discussed in chapter 6, is responsible formissile flight path control.

5A3. Purpose and function: basicrequirements

The control system is made up of severalsections that are designed to perform, insofaras possible, the functions of a human pilot.To accomplish this purpose, the controlsurfaces must function at the proper time andin the correct sequence. For example, indriving your car, you remember that youmust make a turn at a certain distance fromthe starting point. You therefore anticipate theturn. In a missile control system, theremembering is done by INTEGRATINGDEVICES and the anticipation is done byRATE DEVICES. These devices will be

described later.

The first requirement of a control system is ameans of a sensing when control operationsare needed. The system must then determinewhat controls must be operated, and in whatway. For example, the system may sense thatthe missile nose is pointing to left of thedesired course. Obviously, right rudder isrequired. (Other missiles may make use ofdifferent control surfaces.) The length of timerudder control is needed depends on the size

attitude, guidance signals will be correctlyinterpreted, and will produce the desiredcorrection in the missile flight path.

When the control system determines that achange in missile attitude is necessary, itmakes use of certain controllers andactuators to move the missile controlsurfaces. The guidance system, when itdetermines that a change in missile courseis necessary, uses these same devices tomove the control surface. Thus theguidance and control systems overlap. Forconvenience, we will assume that thecontrollers and actuators are a part of thecontrol system, rather than the guidance

rudder control is needed depends on the sizeof the error. Should the attitude deviation beto one side and also either up or down,simultaneous action by rudder and elevatorcontrols would be needed.

5A4. Factors controlled

Missile course stability is made possible bydevices which control the movement of themissile about its three axes. The three flightcontrol axes are shown in figure 5A1. Theseare the pitch, yaw and roll axes.

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Figure 5A1.-Three control axes of a missile.PITCH. In certain missiles, pitch control isobtained by the use of elevators similar tothose used on light airplanes. Othermethods will be described in the next

rudder is also used in yaw stabilization of theplane or missile.

The center view of figure 5A2 shows how the

methods will be described in the nextsection of this chapter. For the present, it issufficient to say that pitch control meanscontrol of the up-and-down movements ofthe missile, as shown in the illustration.

YAW. Missile movement about the yawaxis is controlled by the rudder. Othermethods for controlling yaw will be coveredin the following section of this chapter.

ROLL. Roll deviations are controlled bydifferential movements of rudders, elevons,or other flight control surfaces.

5A5. Methods of control

CONTROL SURFACES. The primarycontrol surfaces of aircraft, and of somemissiles, are rudder, aileron, and elevator.The functions of these surfaces are shownin figure 5A2. The top view shows how therudder controls the direction of travel. Therudder is attached to a section of the tailstructure called the vertical stabilizer. Inaddition to course control, the

ailerons can control roll. The ailerons areattached to the trailing edges of the mainlifting surfaces. When one aileron is lowered,the opposite aileron is raised. Usually, theailerons are coupled to other surfaces in sucha manner that good roll control is obtained.

The elevators are attached to a section of thetail assembly called the horizontal stabilizer.The elevators give pitch control; bothelevators go up and down simultaneously.

A study of the drawings will show thatcontrol action is obtained by the controlsurfaces when they present opposition to airflow in such a manner that a force isproduced. This force, pushing against thecontrol surface, causes the wing or tail towhich the surface is attached to move in adirection opposite to the control surface

movement.

But this type of control is not suitable for useat high altitudes, because the air is so thin thatit produces very little force against the controlsurfaces. High speeds introduce otherproblems so that the basic control surfaces

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Figure 5A2.-Functions of primary control surfaces.

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just described are seldom used with guidedmissiles. They are presented here becausethey illustrate the basic principles of control

If the four jets can not provide enough thrustto propel the missile at the desired speed, afifth jet, fixed in position, can be centered in

they illustrate the basic principles of controlfunctions.

JET VANES. As explained in chapter 2, jetvanes maybe used to control the path of amissile. Figure 5A3 shows how a movablevane is installed directly in the jet exhaustpath. When the position of the vane ischanged, it deflects the exhaust and causesthe engine thrust to be directed at an angleto the missile axis. Because of thetremendous heat built up by the burningfuel, the life of a control vane is short.

MOVABLE JETS. Figure 5A4 shows agimbaled engine mounting. The engine ismounted so that its exhaust end is free tomove and thus direct the exhaust gases in adesired direction. There are two seriousobjections to this method of control. Allfuel lines must be flexible, and the controlsystem that moves the engine must furnishconsiderable power.

The gimbaled engine mounting does notgive full control about all three axes. Itcannot control roll. To get control on allaxes, two gimbal-mounted jets can bepositioned as shown in figure 5A5. Bothjets must be free to move in any direction,and each jet must respond to signals fromany of the three control channels (pitch,roll, and yaw).

A control system using four jets is shown infigure 5A6. Each jet turns in only oneplane. Two of the jets, Nos. 1 and 3,control yaw. Jets 2 and 4 control pitch, andall four jets are used together to control roll.

fifth jet, fixed in position, can be centered inthe space between the movable jets.

Positions of the jets are controlled byhydraulic cylinders linked to the enginehousing. One cylinder and linkage is requiredfor each engine. The direction in whichhydraulic pressure is applied is determined byan electric actuator.

FIXED STEERING JETS. Figure 5A7 showsa fixed jet steering system. The jets areplaced around the missile so as to givedirectional control by exerting a force in onedirection or another. A missile using thiscontrol

Figure 5A3.-Jet vane control

Figure 5A4.-Jet control of direction.

Figure 5A5.-Control by two jets.

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Figure 5A6.-Four movable jet control.

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Figure 5A7.-Fixed jet steering.

system has a smooth outside surface, sincecontrol surfaces are eliminated.

5A6. Types of control action

The basic control signals may come frominside the missile, from an outside source, orboth. To coordinate the signals, computersare used to mix, integrate, and rate the signalimpulses. We will first briefly discussindividual computer section functions so thatyou may see the part the computer plays in acomplete control system. A more detaileddiscussion of computers appears later in thischapter.

MIXERS. The mixer combines guidance andcontrol signals in the correct proportion,sense, and amplitude. In other words, acorrection signal must have the correctproportion to the error, must sense thedirection of error, then apply corrections inthe proper amplitude.

PROPORTIONAL. The proportional controloperates the load by producing an error signalproportional to the amount of deviation fromthe control signal produced by a sensor.

RATE. Rate control operates the load by

pneumatic system. It is necessary to showthem to explain how the system isactuated.)

Keep in mind that the rate signal isproportional to the speed at which a missiledeviation is changing in magnitude. Thischange is different from the displacementsignal, which is proportional to the missileangular deviation at any instant. At theazimuth rate gyro, the rate signal appears asan unbalanced air pressure between twoholes in an airblock pickoff.

Now, let us see what happens when themissile yaws and its nose veers to the right.A displacement gyro signal develops at thepickoff (azimuth control air jet). The jetthen pivots to increase air pressure in theleft hole of the pickoff. The air pressure isfed through the lower of the two air tubes tothe diaphragm of the air relay. This forcesthe diaphragm to the left which admits high-pressure air that, in turn, forces the azimuthservo motor to the right. This motion is thentransferred through a mechanical linkage tothe rudder, which moves to the left andcorrects the nose-right deviation.

Another stabilizing action takes place as themissile nose veers off course to the right. Asignal is produced by the azimuth rate gyroas the nose moves. The azimuth rate gyroexerts a force on the right restraining spring,because the force on the gimbal precessesthe gyro. As it precesses, more air isreceived by the left hole of the azimuth ratepick off. This increases the pressure in the

RATE. Rate control operates the load byproducing an error signal proportional to thespeed at which the deviation is changing.This output is usually combined with aproportional signal to produce the desiredchange in missile attitude or direction.5A7. Types of control systems

We have previously discussed fouraerodynamic control methods. These werecontrol surfaces, jet vanes, movable jets, andfixed steering jets. There are four basicmethods of moving these devices.

PNEUMATIC. A pneumatic control systemis shown in figure 5A8. (Some of theoperating controls shown are not directly apart of the

pick off. This increases the pressure in thesame tube that contains the high pressuresignal from the displacement gyro. The rategyro is supporting the correction beingexerted by the displacement gyro.

As the missile path deviates from its desiredheading, the rate signal increases thecorrective action of the displacement gyro.Therefore, if the deviation from properheading were increasing at a rapid rate, thecorrective signal from the rate gyro wouldbe large and would quickly reduce thedeviation of the missile.

At the instant when the missile has veeredas far to the right as it is going to go, theerror signal from the displacement gyro isgreatest because the error is greatest.However, there is no signal from the rategyro because the missile is not changing itsheading at that instant. The rate gyro hasbeen returned to its mid-position by therestraining springs.

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Figure 5A8.-Pnuematic control system.

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An instant later the heading error isdecreasing as the missile begins to correctits heading. The error from thedisplacement gyro still shows greatestpressure from the left hole of the azimuthpickoff because the heading error is still tothe right. However, the signal is decreasingand the error signal from the rate gyro hasreversed direction. The missile nose is nowmoving to the left. This puts a force on therate gyro gimbal in a direction opposite tothe former force. The precessing gyrocreates a pressure in the right hole of therate pickoff which is partially counteractedby the displacement gyro signal. Figure 5A9shows the effects of combining the rate anddisplacement signals.

A study of this drawing will show theadvantages of having a counteractionbetween the rate signal and thedisplacement signal. If the two signals werein the same direction, the rudder would befarther away from the center axis of themissile and the missile would be headingback to the desired course at a faster rate.However, the rate of return would be sorapid that the missile would swing past the

is called oscillation, or hunting, and is veryundesirable. The rate circuit action helpsprevent hunting.

The same kind of action is used in thedisplacement and rate controls in the pitchchannel. The fundamental rate gyro or ratecircuit output is the same for all missile flightsurface control systems.

PNEUMATIC-ELECTRIC. The pneumaticcontrol system we have just described can becombined with other systems to refine thecontrol action. Electric signal pickoff areaccurate and dependable. They can provide asignal voltage that is proportional todisplacement. They have a decided advantageover pneumatic systems in transportinginformation over wires, instead of throughtubing.

It is difficult to design a small electric motorwith sufficient speed and power to actuatemissile flight control surfaces. But we cancombine the best features of electric andpneumatic systems as in figure 5A10.Electrical equipment is used in the front end,and operates pneumatic servos at the

rapid that the missile would swing past thecorrect heading in the opposite direction.The missile would then be veering offcourse to the left so that the control systemwould need correction signals for thatdirection.

The cross-course variations wouldcontinue, with the missile wobbling backand forth on both sides of the desiredheading. This action

and operates pneumatic servos at theactuating end. A system like that in figure5A10 is suitable for controlling a small,subsonic, short-range missile.

Pneumatic controls are slow because air iscompressible, and time is required to build upenough pressure in a cylinder to move thepiston. Since the piston is linked mechanicallyto the flight control surfaces, there is a time

Figure 5A9.-Effect of combining rate and displacement signals.

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Figure 5A10.-Pneumatic-electric control systems.lag between the control signal and themovement of the control surface. But theslow response can be speeded up by addinga booster cylinder, as shown in figure5A10.

The increase in response speed is obtained

The servo amplifier receives a followup signalfrom the control surface, in addition to thegyro error signal. The voltage signal voltage isfed back with a polarity that opposes theinput voltage. The feedback voltage cancelsthe control input voltage when the controlsurfaces have deflected a certain amount.

The increase in response speed is obtainedby allowing air to escape, through ports,into a relief valve after the servo valve hasmoved a certain distance from midposition.The relief valve lets high-pressure air intothe boost cylinder, which then acts inparallel with the actuator cylinder to movethe flight control surfaces. The additionalforce provided by the boost cylinder makesit possible to obtain large control surfacedeflections in either direction.

The sensors for a pneumatic-electricsystem are electric pickoffs that detect gyrodisplacement and produce a voltageproportional to the heading deviation angle.This voltage is small, and must be amplifiedbefore it can operate a solenoid and airservo valve.

The change from electric to pneumaticoperation takes place at the air servo valve.The air servo motor rotates the torquetubes which are connected to the controlsurfaces and extend into the center sectionof the missile. The system shown in figure5A10 may be used for either pitch orazimuth control.

surfaces have deflected a certain amount.The deflection of the control surfaces istherefore proportional to the input signal.

HYDRAULIC-ELECTRIC. Thiscombination is similar to the pneumatic-electric, except that the actuators are movedby hydraulic fluid pressure instead of airpressure. This removes some of thedisadvantages of a pneumatic system, sincethe fluid is not compressible.

In a hydraulic-electric system, a continuouslyoperated pump maintains hydraulic pressureduring the flight. The hydraulic fluid iscirculated in a closed system, so that it can beused over and over. Thus the operating timeof the hydraulic components is unlimited, andthe system is suitable for long range missiles.

Variations in pitch, roll, and yaw are sensedby gyro reference units with electric pickoffs.The pickoff voltages are fed to amplifiers andcomputers and are then used to operate acontroller. The controller is usually ahydraulic

515354 O-59-6

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transfer valve, which regulates the amountand direction of fluid flow to the actuator.

Figure 5A11 shows a hydraulic-electricsystem for roll control. (In some missilesthe ailerons are replaced by control devicescalled "rollerons".) The system usesproportional control only, which means thatthe controls react to information that showsthe deviation of the missile axis from thedesired flight path. The displacement signalis proportional to the deviation.

Should roll develop, the gyro (fig. 5A11)will detect it and cause the synchro toproduce an error signal. The correctionsignal to the servo amplifier is thedifference between the followup signal andthe gyro signal, as indicated by the minussign in the circle between the synchro blockand the servo amplifier triangle.

The difference signal is amplified and usedto operate the controller, which is ahydraulic transfer valve that regulates the

ELECTRIC. In an electric control system, allcomponents are powered by electricity.Except for the controller and flight controlsurface actuator, the components are similarto those that have been described for othersystems.

Variations in roll, pitch, and yaw are sensedby electrical components such as synchros orreluctance pickoffs. The signals from thesecomponents are fed to a computer whichdetermines the amount of error, if any. Ratesignals are obtained from the electricallydriven rate gyro, or from an electric ratecircuit operated by a displacement gyrosignal. The control voltages are amplified andfed to controllers, which vary the powersupplied to the motors that operate the flightcontrol surfaces. A small motor running athigh speed has the same power capability as alarge motor running at low speed. Therefore asmall, high-speed motor can be connected tothe control surfaces through a reduction gearto secure the necessary torque.

hydraulic transfer valve that regulates theflow of fluid to the cylinder. The piston inthis cylinder operates the ailerons

(rollerons; controllable jets; etc.) throughmechanical linkages.

The equipment represented by the blocklabeled "jitter" provides an a-c voltage witha frequency of about 25 cycles per second.This is applied to the transfer valve andother equipment, to keep them in constantvibration and prevent the friction that maydevelop when the parts are not moving.

A constant-speed motor, operating through aclutch, is best suited for rapid controloperation, because the gear train inertia tendsto cause an undesirable lag in control surfaceresponse. The lag may be great enough tomake the missile oscillate about the desiredtrajectory. The use of a clutch helps toovercome this effect.

Figure 5A11.-Hydraulic-electric system for roll control.

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Figure 5A12.-Electrical system for pitch control.An electrical system for pitch control isshown in figure 5Al2. The controllersection converts the power of the controllerdrive motor to power for the three-channelvariable-speed motors. These motorsoperate when a signal is received from theamplifiers. Let us assume that the pitchamplifier furnishes a signal. This signal is

a pneumatic pump may be mechanicallyconnected to a turbo-jet or gas turbine engine.If a pump is used, it will provide the pressureneeded for a hydraulic or pneumatic system.A power source of this type is practical eventhough some of the power developed by theengine is used to drive accessory equipment.There is ample power left for thrust.

amplifier furnishes a signal. This signal isapplied in such a way that the magneticfield of the pitch generator is increased.This causes the generator to develop anoutput voltage, which is fed to the variable-speed motor. The shaft of the variablespeed motor then begins to turn, putting anadditional load on the controller drivemotor.

The speed of the controller drive motormust remain reasonably constant,regardless of loading. Otherwise, if thepitch output decreases, the speed of themotor would decrease; this would result indecreased output from the roll and yawgenerators at the same time. As a result,there would be undesirable cross-couplingbetween control channels so that the pitchsignal would affect other channels, and viceversa.

5A8. Energy sources

The energy required to operate the controlsurfaces may be taken from any of thefollowing sources.

MISSILE ENGINE. The propellant of themissile may be used as a source of energyto operate the control system. Figure 5A13shows how a generator, a hydraulic pump,or

There is ample power left for thrust.

Figure 5A13.-How energy is obtained fromthe missile engine.

Figure 5A14.-Auxiliary engine power.

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AUXILIARY ENGINE. A similar system,shown in figure 5A14, uses fuel from themain missile supply to drive a small engine,which in turn drives a generator for electricpower or a pump for a pneumatic orhydraulic system.

This system does not take power from themain engine and would be suitable for usewhen the main propulsion unit was cut off.The auxiliary engine will furnish power solong as fuel is available. If necessary, theauxiliary engine could be used to drive agenerator and pump simultaneously toobtain power for hydraulic-electric orpenumatic-electric systems.

BATTERIES. Storage batteries can beused either by themselves or to drivemotor-generator sets. The generator can

electric power to operate a completely electriccontrol system, or to drive pumps forcombined control systems. Of course,batteries can supply large amounts of powerfor only short periods of time.

COMPRESSED GASES. Air and other gasescan be stored in tanks under pressure for usein operating control systems. As explained inthe section on pneumatic controls, thecompressed gas is exhausted during thecontrol operation and cannot be used again.

Turbine generators can be driven by the gasesgiven off by BURNING FUELCARTRIDGES if power is needed for a shorttime only. Such a power system is suitablefor air-to-air missiles.

motor-generator sets. The generator cansupply

B. Requirements of a Missile-Control Servo System

5B1. General

Missile control is similar to any automaticcontrol function. The system corrects somecontrollable quantity, and then checks theresults as a basis for further corrections.

There are four requirements of anyautomatic control system. Obviously, thefirst is something to control. The second isa means of determining when anycontrollable item has departed from adesired condition. The third is a means ofconverting an error signal into a form thatcan be used to regulate the controllingdevice. The last is the device that performsthe actual control operation.

5B2. Controllable factors

Factors that must be controlled by the missilecontrol system are pitch, roll, and yaw. Thesystem must provide a means of determiningwhen the missile has departed from thedesired attitude. Deviations are sensed bygyros. Electrical, mechanical, and electroniccomponents are interconnected to form acomplete control system.

The control system shown in figure 5B1 canset up fixed reference lines in space, fromwhich deviations in attitude can be measured.It provides a mechanical and electrical meansfor operating the missile flight control

Figure 5B1.-Block diagram of missile control system.

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surfaces, a means for measuring themagnitude and direction of errors, and ameans for translating the error signals intocontrol surface movement.

5B3. Error-sensing devices

Deviations in missile pitch, roll, and yaware detected by gyros. A minimum of twogyros is necessary for missile flightstabilization. Each gyro sets up a fixedreference line from which deviations aremeasured. One such reference is the spinaxis of a vertical gyroscope; from this axisdeviations about the pitch and roll axes canbe measured, as shown in figure 5B2.

A second reference line is the spin axis of a

Figure 5B3.-Horizontal reference line.

stabilizing (control) purposes and gyros usedfor both guidance and stabilization. If turns orother maneuvers are necessary, a third gyro is

A second reference line is the spin axis of ahorizontal gyro, set up parallel to thehorizontal axis of the missile as shown infigure 5B3.

Gyros used for missile control applicationsare divided into two classes: gyros used for

Figure 5B2.-Vertical reference line.

other maneuvers are necessary, a third gyro isrequired so that there will be one gyro foreach sensing axis.

In addition to the control signals from thevertical and horizontal gyros, which areproportional to the deviation of the missilefrom the desired trajectory, a signal that isproportional to the rate of deviation isrequired for accurate control and smoothoperation. A RATE GYRO furnishes the rateof deviation signal.A gyro that is being used for rate deviationindications has a restricted gimbal that is freeto rotate about one axis only. The spin axis ofa yaw rate gyro is mounted parallel to themissile line of flight. The roll rate gyro spinaxis is parallel to the missile pitch axis, and atright angles to the line of flight. The pitch rategyro spin axis is parallel to the yaw axis of themissile and at right angles to the line of flight.

Figure 5B4 shows, in block form, a systemused to sense motion of a missile with respectto one control axis. This system uses both arate gyro and a free gyro. The gyro outputsignals are fed to an amplifier, which addsthem and gives an output voltage proportionalto their sum. This voltage is applied to aservo motor, which positions the flight controlsurface so as to drive the error amplitude andthe rate of change toward zero.

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Figure 5B4.-A control channel using rate and free gyros.TRANSDUCERS. A transducer is a devicewhich is operated by power from onesource and supplies power to anotherdevice in the same or a different form. Afamiliar example is the phonograph pick-upwhich converts the lateral motion of theneedle in a record groove to electricalimpulses which are then amplified. In most

Although the sensor output represents anerror to be corrected, it is seldom used tooperate control surfaces directly. It must bechanged to include additional information,and then amplified in order to operate thecontrols. These operations are represented bythe block labeled "computer" (fig. 5B1). Thecomputer section is normally composed of

missile applications, a transducer is used ina similar manner-to change mechanicalmotion to an electrical voltage. Moreinformation on transducers will be givenlater in this chapter.

5B4. References

In order to accurately determine errors, thecomplete control system must havereference values built in. The system isthen capable of sensing a change,comparing the change to a reference,determining the difference, then starting aprocess that will reduce the difference, tozero.

The reference units in a missile controlsystem are of three kinds-voltagereferences, time references, and physicalreferences. A more detailed discussion ofreferences will appear in the next section ofthis chapter.

5B5. Correction-computing devices

We have shown that sensor units detecterrors in pitch, roll, and yaw, and that areference unit furnishes a signal forcomparison with the sensor output.

mixers, integrators, and rate components.

5B6. Power output devices

The amplification of error signals isperformed by a conventional vacuum-tubeamplifier or a magnetic amplifier. Regardlessof the method used, the prime purpose of anamplifier is to build up a small sensor signal toa value great enough to operate the controls.

5B7. Feedback loops

For smooth operation of the controls, it isnecessary to feed back some of the controlpower so that it counteracts some of theoriginal force. There are two feedback paths.The major path represents information on theangular movement of the missile. Thisinformation is fed back to the sensor. Thesecond feedback path, called the minor path,returns information on the reaction of acontrol surface, rather than the reaction of themissile. The use of feedback prevents controlsurfaces from swinging to the limit of motion,and thus avoids overshooting as the missilereturns to the proper attitude.

C. Reference Devices

5C1. Purpose and function

The reference device provides a signal forcomparison with a sensor signal, so that

equipment in the missile will "know" whenthe missile has deviated from the desiredattitude. Figure 5C1 shows how the referencesection is connected to the computer section.If the

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reference section were omitted from thecontrol section, the computer would be

unable to set up correction signals.

5C2. Types of reference

In the following discussion, the three typesof reference signals will be describedseparately to show how each type functionsin the complete control system.

VOLTAGE. In some control systems, theERROR SIGNALS are in the form of an a-c voltage which contains the two

The controller unit (fig. 5C1) usually requiresa d-c signal, which must include the

information contained in the original errorsignal. The amplitude of the d-c signal showsthe amount of deviation. The direction ofdeviation is indicated by the polarity of the d-c signal. To keep the d-c signal frombecoming so large that it would causeovercontrol, a LIMITER CIRCUIT is used.Limiters require a d-c reference voltage, andfunction as a part of the reference unit.

TIME. The use of time as a reference isfamiliar to everyone. One common

c voltage which contains the twocharacteristics necessary to make propercorrections in the flight path. These are theamount of deviation, and the direction ofsense of the deviation.

The amount of deviation maybe indicatedby the amplitude of the error signal so that,as the deviation increases, the amplitudeincreases; and if the deviation decreases,the amplitude decreases. Therefore, whenthe missile attitude has been corrected andthere is no longer a deviation, the errorsignal amplitude drops to zero.

The direction of deviation may be carriedby the a-c signal as a phase difference withrespect to the phase of a reference signal.Only two phases are required to showdirection of deviation about any one controlaxis. When a phase-sensitive circuit, suchas a discriminator, is used to compare theerror signal with the a-c reference signal,the direction of error is established and theoutput containing this information is fed toother control sections.

In most cases, the a-c reference voltage isthe a-c power supply for the controlsystem. It also furnishes the excitationvoltage for the sensor unit that originatesthe error signal.

familiar to everyone. One commonapplication is in the automatic home washer.A clock-type motor drives a shaft, whichturns discs that operate electric contacts.These contacts close control circuits thatoperate hot- and cold-water valves, start andstop the water pump, change the washerspeed, spin the clothes dry, and finally shutoff the power. Each operation runs for aspecified time interval. This kind of timer canbe used for certain missile control operations.

Timer control units vary considerably inphysical characteristics and operation. All ofthem require an initial, or triggering, pulse.Since all timers in a complete system are nottriggered at the same time, each must have itsown trigger. This is usually an electricalsignal. It may be fed to a solenoid whichmechanically triggers the timing device.

Another triggering method involves theapplication of an electrical signal to a heatercoil which heats a bimetal strip and causes itto bend, thus opening or closing electricalcontacts. This method maybe more familiarwhen you contemplate the operation of atypical thermostat like the one found in thehome. Still another triggering method is toapply an

Figure 5C1.-Basic missile control system.

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electrical signal to a motor. The motor,which is apart of a timing device, thenstarts the control sequence in much thesame manner in a home washer.

Mechanical timers are used in some missilecontrol systems. In operation, these timersare similar to mechanical alarm clocks. Theenergy is stored in a main spring. If amechanical timer is used in a missile, the

an electrical signal, and the time intervalbegins when the trigger voltage is applied. Asimple motor timer is shown in figure 5C2.The speed of the motor shaft is reduced bythe gear reduction box so that the output shaftrevolves at the speed needed to time theoperation.

An arm connected to the output shaft servesas part of a switch contact system. The length

mechanical timer is used in a missile, theclock mechanism is not started until themissile is in flight, and therefore some formof triggering linkage is necessary. Thisusually consists of a catch that can bereleased by a solenoid. Since the springcannot be rewound after the missile hasbeen launched, a mechanical timer can beused only once during a flight.

Electrical timers in missile control systemsare divided into motor types and thermaltypes. The triggering of either type is doneby

as part of a switch contact system. The lengthof time required for the arm to travel fromthe starting position to the point wherecontact is made is the delay time of the unit.Normally, this mechanism is used only onceduring a missile flight. If recycling isnecessary, a more complex unit is required.

Thermal delay tubes and relays may also beused to control time delay actions. Thermaldelay devices have the advantage over clocktimers in that they can be made to recycle

Figure 5C2.-Simple motor timer.

Figure 5C3.-Thermal delay tube.

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without additional circuitry or mechanisms.However, they do not have the accuracy ofclock timers.

One type of thermal delay tube is shown infigure 5C3. Its components are the twobimetal strips, the contacts, a heating coil,and the strip supports. When a triggeringvoltage is applied, the heating coil heatsONE of the bimetallic strips. As thetemperature rises, the strip deforms and itscontact moves toward the other contact.When the bimetal strip has heatedsufficiently, the contacts touch and the

because of outside temperature effects, theother strip deforms the same amount in thesame direction. This maintains a more or lessconstant spacing between the contacts.

PNEUMATIC. Pneumatic timers may beused in certain missile applications. Timedelay action is obtained by compressing air ina cylinder and then allowing the air to escapethrough a small orifice.

There are two general types of pneumatictimers-piston and diaphragm. The piston typeis shown in figure 5C4. The felt washer acts

sufficiently, the contacts touch and theoutput circuit is completed.

The amount of time between application ofthe triggering voltage and closing of thecontacts is determined by the contactspacing, the temperature characteristics ofthe metals in the strips, and thecharacteristics of the heater coil. The delaytime is preset by the manufacturer; theassembly is then placed in a tube-typeenclosure, and the air is pumped out of thetube. This type of construction preventsany adjustment of the time delay.

The effect of ambient temperaturevariations can be avoided by making bothstrips of the same metals. Then, when onestrip deforms

as an air seal. As the plunger is pulled up, thespring is compressed and the contacts areopened. The spring is held in compression bythe inertia block.

The inertia block forms the trigger for thetimer. The block is of metal and will bethrown backward when it is subjected tosufficient accelerations. Thus, when a missileis launched, the block will fly back andrelease the catch that holds the spring undercompression. Spring pressure will then pushthe piston downward, forcing air out throughthe orifice. The orifice opening may bechanged by adjusting the needle valve. Thesmaller the orifice,

Figure 5C4.-Piston-type pneumatic timer.

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Figure 5C5.-Diaphram-type pneumatic timer.the longer it will take the piston to come downfar enough to close the contacts.

The diaphram-type pneumatic timer, shown infigure 5C5, operates on essentially the sameprinciple as the piston-type timer that has beendescribed.

Most missile control systems use some form oftimer. Remember that an individual timer maybe used to start a variety of control functions.Sometimes a timer is used strictly as a safetydevice.

PHYSICAL REFERENCES. There are anumber of references for missile controlsystems other than the voltage and timeclassifications we have discussed. Theremaining types have been grouped under theheading of physical references. They includegyros, pendulums, magnetic devices, and themissile airframes.

GYROS. We have already explained how agyroscope establishes a reference line in space.A gyro pickoff system can sense any change inmissile attitude with respect to that reference.

PENDULUM. The mass of the earth hasa strong gravitational attraction forobjects near its surface. If a weight ishung on a string and suspended from abeam or other support, the string andweight form a pendulum. The weightmay swing around when it is firstsuspended, but it will eventually come torest. The string will be on a line betweenthe point of support and the earth's centerof gravity. The pendulum can thereforebe used to establish a vertical referenceline.

Some gyros are precessed to a verticalposition by a pendulum device called a"pendulous pick-off and erectionsystem." The complete gyro system iscalled a vertical gyro; it maybe used tomeasure the pitch and roll of a missile.

MAGNETIC DEVICES. Magneticcompasses have been used for centuriesto navigate the seas. The compassenables a navigator to use the lines of fluxof the earth's magnetic field as areference. A similar device, known as a"flux valve" is used in some missilecontrol systems. Its primary purpose is tokeep a

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directional gyro aligned with a given magneticheading. The directional gyro can then be usedto control the yaw of a missile.

movement of flight surfaces arereferenced to the missile airframe.

Synchro indicators can be used to

MISSILE AIRFRAME. The airframe of themissile must be used for certain references. Forexample, the movement of flight controlsurfaces cannot be referenced to the vertical, orto a given heading, because such referenceschange as the missile axes change. Therefore,

Synchro indicators can be used toindicate the angular position of controlsurface with respect to the missileairframe. A potentiometer can be used inthe same way by mounting in on themissile airframe so that its shaft will bedriven by the flight control surfacemovements.

D. Sensor Units

5D1. General

The sensor unit in a guided missile controlsystem is a device used to detect deviation fromthe desired attitude. In this section, we willdiscuss the use of gyroscopes, altimeters, andtransducers as sensing units. Gyroscopes aregenerally considered to be the basic sensor unitin any missile control system. Other types ofsensors, such as altimeters and transducers, areclassed as secondary units.

5D2. Gyros

A gyroscope contains an accurately balancedrotor that spins on a central axis. Figure 5D1shows a FREE GYRO that is mounted so it cantilt, or turn, in any direction about its center ofgravity.

GYROSCOPIC INERTIA. The characteristicof a gyroscope that resists any force whichtends to displace the rotor from its plane ofrotation is called "gyroscopic inertia." Threefactors determine the amount of inertia. Theseare: the weight of the rotor, the distribution ofthis weight, and the speed at which the rotorspins.

A gyro with a heavy rotor has more rigidity thanone with a light rotor, if the speed of rotation isthe same for both. Distributing the weight of thegyro to the outer rim of the rotor will giveincreased rigidity even though there is noincrease in the weight of the rotor. An increasein gyro rigidity can also be obtained byincreasing the speed of rotation.

PRECESSION, REAL AND APPARENT. Thecharacteristic of a gyro that causes the rotor tobe displaced in a direction 90 degrees from thatof the applied force is called precession. Thereare two types of gyro precession: REAL andAPPARENT. Real

Figure 5D1.-Free gyroscope.

precession is sometimes calledINDUCED PRECESSION.

The direction in which a gyro will

precess, when an external force isapplied, is shown in figure 5D2. A forceapplied to a gyro at its center of gravitydoes not tend to tilt the spin axis from itsestablished position, and therefore doesnot cause precession. A spinning gyro canbe moved in any direction withoutprecession, if its axis can remain parallelto its original position in space.Therefore, the gyro can measure onlythose movements of the missile that tendto tilt or turn the gyro axis. Two gyrosare needed for vertical and horizontalstabilization of missile flight. The spinaxes of these gyros would be at rightangles to each other.

APPARENT PRECESSION. The axis ofa spinning gyro points in a fixed direction

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Figure 5D2.-Gyro precession.

because inertia fixes it in space. Over a periodof time a gyro axis will appear to tilt. This iscalled apparent precession, and is due to therotation of the earth.

Figure 5D3 represents a gyro at the equator,with the spin axis horizontal and pointed east-west. The earth turns in the direction shown bythe arrow. If you could observe the gyro spinaxis from a point out in space, it would appearto always point east. To an observer standing onearth, the spin axis appears to gradually tilt ordrift, so that after three hours, the spin axis hastilted 45

Figure 5D3.-Gyro position in space about theearth.

degrees. Notice the apparent precessionshown in figure 5D4. After 12 hours thespin axis is again horizontal, but ispointing west instead of east. At the endof 24 hours, the spin axis is pointing eastagain.

This action gives the impression that thegyro has turned end for end, and that acomplete revolution is made every 24hours. But this is not true. Actually, thegyro axis has maintained its fixeddirection in space; only the earth hasmoved.

The apparent precession of a gyro makes

it unfit for use as a reference over anextended time unless some kind ofcompensation is used to keep the gyro ina fixed relation to the earth's surface.

GYRO DRIFT. Gyro error caused byrandom inaccuracies in the system iscalled drift. It has three principal causes-unbalance, bearing friction, and gimbalinertia.

Dynamic unbalance may occur becauseof operation at some speed ortemperature other than for which thegyro was designed. Some unbalanceexists in any gyro because ofmanufacturing tolerances.

An even amount of bearing friction allaround a shaft does not cause drift. Itwill, however, cause the speed of rotationto change.

Figure 5D4.-Apparent precession.

85Friction in the gimbal bearings causes a loss ofenergy and incorrect gimbal positions. Drift willbe caused by friction in the spin axis bearingsonly if the friction is not symmetrical.

Energy is lost whenever a gimbal rotates,because of inertia. The larger the mass of thegimbal, the greater the drift from this source.

MOUNTING SYSTEMS. The main cause ofrandom drift in gyros is friction in gimbalbearings. Figure 5D5 shows one type ofmounting that has been developed to reduce thisfriction. It is called FLOATED GYRO UNIT.

The floated gyro unit is a viscous-damped,single-degree-of-freedom gyro with a microsyntorque generator and a microsyn signalgenerator mounted on its output shaft. Themicrosyn torque generator places a torque onthe gyro gimbal.

The term "single-degree-of-freedom" means thegimbal is free to rotate with respect to the gyrocase about a single axis. This axis is called theoutput axis; it is perpendicular to the gyro spin(reference) axis. If an angular velocity acts onthe gyro case with a

component about the input axis, aprecessional torque develops about theoutput axis.

In figure 5D5 the gyro wheel is containedwithin the damper housing. The microsynsignal generator units are mounted on thegyro shaft as shown in the drawing. Thespace between the damper housing andthe gyro case is filled with a viscousdamping fluid. Because of the highspecific gravity of the fluid, it serves tofloat the gyro damper housing and gyrogimbal shaft, and thus reduces the gimbalbearing friction and drift.Thermostatically controlled heatersaround the damping fluid space keep thefluid viscosity constant. If the gyroshown in figure 5D5 were mounted withits input axis parallel to the pitch of yawaxis of the missile, the torque applied tothe output shaft would be proportional tothe difference between the desiredangular velocity of the missile and itsangular velocity about the axis.

Another form of gyro support, shown infigure 5D6, is known as an air bearing.This type of support reduces friction tosuch a low value that for all practicalpurposes it can be

Figure 5D5.-Floated gyro unit.

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Figure 5D6.-Air bearing gyro.considered zero. Its operating principle maybe explained by using a fairly commonadvertizing display for comparison. Nodoubt you have seen vacuum cleanerdisplays where the air stream from thecleaner was used to keep a number of pingpong balls or a large rubber ball virtuallysuspended in the air.

ERECTING SYSTEMS. Figure 5D7 is ablock diagram that shows how signals froma precession sensor are used to maintaingyro stability. If the gyro spin axis in thissystem is vertical, the gyro output signalwill be zero. If the spin axis moves awayfrom the vertical, the gyro will send avoltage to the precession sensor. Theamplitude of this voltage will depend on theamount of precession, and its phase of thedirection of precession. The precessionsensor output is amplified to operate thetorque motor, which returns the gyro to thevertical position.

unit is suspended on a universal jointenclosed in a bowl filled with fluid, to preventexcessive swinging in flight.

The gyro's spin axis, kept tangent to theearth's surface and slaved to the earth'smagnetic field, provides a basic reference formissile heading. When the missile turns ineither direction, the flux valve turns with it.When the flux valve turns, the angle betweenits coils and the earth's magnetic field ischanged, and an error signal is generated. Theerror signal is amplified and used to operatecontrols that correct the missile heading.

RATE GYROS. The control signals furnishedby the vertical and horizontal (free) gyros areproportional to the deviation of the missile.Another signal, proportional to the RATE ofdeviation, is required for smooth control. Thisis the RATE-OF-DEVIATION signal; it issupplied by a RATE gyro. A rate gyro has arestricted gimbal that is free to rotate about

vertical position.

Horizontal gyros use a leveling system tokeep the spin axis horizontal, and a slavingsystem to stabilize its direction. The slavingsystem uses a fluxvalve to sense thedirection of the earth's magnetic field. Theflux valve

restricted gimbal that is free to rotate aboutonly one axis. Its construction is shown infigure 5D8.

A YAW RATE GYRO is mounted with itsspin axis parallel to the missile line of flight.

Figure 5D7.-Vertical gyro erection system.

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Figure 5D8.-Basic rate gyro.A ROLL RATE GYRO is mounted so thatits spin axis is parallel to the pitch axis, atright angles to the line of flight. A PITCHRATE GYRO is mounted with its spin axisparallel to the yaw axis of the missile and atright angles to the line of flight.

Displacement signals alone would give themissile a tendency to over-correct its errors,and yaw or pitch about its desired course.The displacement and rate of changesignals minimize over-correction, andensure stability.

greatest at sea level and decreases steadily asthe altitude increases. Since the atmosphericpressure at a given altitude is predictable, it ispossible to calibrate a pressure-sensitiveinstrument in terms of altitude.

A pressure altimeter is a form of aneroidbarometer. Its mechanism includes a bellows-like chamber from which most of the airhas been removed. The pressure of theatmosphere tends to collapse the bellows.The surface of the bellows is connected to ascale pointer through a mechanical linkage,

ensure stability.

PICKOFF SYSTEMS. A "pickoff" is adevice that produces a useful signal fromthe intelligence developed by a sensor. Thesensing devices for missile control generallyindicate angular or linear displacement,measured with respect to some fixedquantity. The pickoff must be able tomeasure the amplitude and direction of thesensor displacement, and produce a signalthat represents both quantities. Electricalpickoffs use phase relation or polaritydifference to indicate direction. The idealpickoff should have a linear output and

minimum friction loss.

5D3. Altimeters

An altimeter measures altitude. There aretwo main types: PRESSURE andABSOLUTE.

The PRESSURE type operates on theprinciple that air (atmospheric) pressure is

scale pointer through a mechanical linkage,which magnifies the bellows surfacemovement.

As the pressure does not remain constant atany one level, this type of altimeter may havean error due to variable atmosphericconditions.

The ABSOLUTE altimeter is sometimescalled a radio altimeter. It indicates altitudeabove the ground, rather than above sealevel. It is actually a form of radar, since itmeasures the time required for a radio pulseto reach the ground, be reflected, and return.

The transmitter antenna sends an FM signalstraight down. The reflected energy is pickedup by a separate antenna. A detectorcombines the reflected signal with a sample ofthe transmitted signal and generates adifference frequency. This frequency isdetermined by the height above the ground.The detector output is amplified and fed to anindicator such

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as a cathode-ray tube, a meter, or adiscriminator that operates a control circuit.

This system accurately indicates heightabove the ground, but it can not indicateheight above sea level.

5D4. Air-speed transducers

A guided missile may use a transducer tomeasure ram air pressure, and provide anoutput voltage that indicates missile airspeed.

One type of airspeed transducer uses bellowscoupled to the shaft of a potentiometer. Asthe bellows is actuated by ram air pressure, itturns the potentiometer shaft and thuschanges the circuit resistance.

Another type of airspeed transducer uses asynchro generator. The rotor of the synchrois so connected that expansion or contractionof the bellows causes the rotor to turn. Thistype of sensor will be described in thefollowing section.

E. Pickoffs

5E1. Function

The pickoff device is important to themissile control system because it producesa signal from the intelligence developed bya sensor unit.

5E2. Requirements

The signal produced by the pickoff must besuitable for use in the control system it isserving. The pickoff must have an output

shaft can be turned by a mechanicalconnection to the sensor unit, or by a motor.Regardless of the method used to turn thegenerator shaft, the synchro motor shaft willmove the same amount. The synchro motordoes not develop enough power to operatemissile flight control surfaces. Therefore, it isused to operate other parts of the systemwhich in turn operate the control surfaces.

To get better action as the null point isapproached, a differential synchro system is

serving. The pickoff must have an outputsense. That is, it must be able to determinethe direction of displacement and thenproduce a signal that indicates the direction.

In electrical systems the indication may bea phase or polarity difference.

The ideal pickoff should have aconsiderable change in output for a smallmovement of the pickoff. It should alsohave minimum torque or friction loss sincethese losses would be reflected to thesensor element and affect its operation.Small physical dimensions and light weightare additional requirements for pickoffsused in missiles. The null point (no output)should be sharply defined.

5E3. Type

Electrical pickoffs in common use fall intofour categories. Each has somecharacteristic that makes it suitable forcertain applications.

SYNCHRO PICKOFFS. A synchro pickoffdevice is normally composed of a pair ofsynchro units wired as a generator andsynchro motor. When an exciter voltage isconnected to the pair, movement of thegenerator rotor will produce acorresponding movement of the synchromotor rotor. The generator rotor

approached, a differential synchro system issometimes used. In this system, two inputs-one electrical and the other mechanical-arefed to a synchro differential generator unit,which then furnishes a voltage equal to thesum or difference between the two inputs.

Synchro pickoffs are sometimes calledselsyns, autosyns, or microsyns.

POTENTIOMETERS. A potentiometer is avariable resistance that is normally used as avoltage divider. The resistance element isformed into a circular shape and a movingarm makes contact with the element. Byconnecting leads to the ends of the strip froma voltage supply and then connecting a loadto the moving arm and one end of the strip,the source voltage may be divided by varyingthe position of the arm on the strip.

The resistance used for many electronicapplications is composed of a thin film ofcarbon deposited on an insulating material.This type of resistance element is not suitablefor servo applications because the resistancechanges with temperature, humidity andwear. These disadvantages are overcome byusing a wire-wound resistance strip, as shownin figure 5E1.

Figure 5E2 shows how a potentiometerdivides voltage. The source voltage is applied

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difference between adjacent turns of wire. Inorder to remove this objection, the resistanceelement is sometimes wound in the form of ahelix. Units using this construction are usuallyidentified by the name HELIPOT instead ofpotentiometer.

Potentiometers are often used in bridgecircuits. The potentiometer forms two armsof the bridge, and fixed resistors form theother two. It is also possible to use twopotentiometers to comprise all four arms ofthe bridge.

RELUCTANCE PICKOFF. A variablereluctance pickoff and gyro rotor are shownin figure 5E3. (The gyro rotor is thecylindrical object in the drawing.) The rotor ismade of ferrous material, which has beenslotted lengthwise and the slots filled with

Figure 5E1.-Wire-wound potentiometer.

to points A and B. One side of the load isalso connected to B. The section of theresistance strip between the moving armand A acts as a resistance in series with theload, and there is less voltage at the loadthat is being furnished by the source. If themoving arm is all the way down to B, theload will get no voltage.

Thus, the position of the moving armdetermines the amount of voltage. It is alsopossible to use the variation in resistance asa control medium. Since the resistancebetween A and the moving arm andbetween B and the moving arm vary as thearm is moved, a null can be indicated whenthe two resistances are equal.

If the shaft of the potentiometer ismechanically coupled to the sensor, theoutput voltage will vary according to themoving arm displacement. However, thevoltage does not change smoothly with thistype construction. The jumpy output is dueto the voltage

Figure 5E2.-How a potentiometer dividesvoltage.

slotted lengthwise and the slots filled withbrass.

The pickoff element is an E-shaped metalmass, of which the center arm is a permanentAlnico (special alloy of aluminum, nickle, andcobalt) magnet. The outer arms, and the longside of the E, are made of soft iron. Coils arewound on each leg and connected in seriesopposition. As the gyro rotates, it causesregular variations in the magnetic flux pathsas the brass and ferrous strips pass the endpieces. This establishes regular variations influx density and induces an a-c voltage in thecoils. However, because of the opposingconnection of the coils, the induced voltagescancel.

The gyro spin and precession axes are shownby dashed lines. As the rotor precesses, theair gaps at each end vary oppositely (oneincreases and the other decreases) inproportion to the precessing force, and causedifferent induced voltages in the pickoff coils.Since the two voltages are different they nolonger cancel, and the output voltage is thatproduced by acceleration. The output voltageis rectified by the crystal diode and filtered bythe condenser. The resulting d-c voltage canbe used as an acceleration signal. The polarityof the signal voltage depends upon which coilhas the greatest induced voltage, andtherefore indicates the direction of theacceleration.

Figure 5E4 shows another type of reluctance

pickoff. The stator has four coils divided intotwo pairs. One pair is supplied with aconstant-amplitude a-c voltage from areference oscillator.

Voltage from one pair of coils is induced inthe second pair through an armature. The

515354 O-59-7

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Figure 5E3.-Gyro rotor and reluctance pickoff.

Figure 5E4.-Externally operated reluctance pickoff.armature is fastened to a shaft that ismechanically coupled to the gyroscopegimbal. As the gimbal moves, it causes thearmature position to change, and alters thecoupling between the two sets of coils. Theamplitude of the induced voltage changes inproportion to the

gimbal movement. This change produces aphase shift that depends on the direction ofshift in missile position.

CAPACITANCE PICKOFF. As shown infigure 5E5, a capacitance pickoff is composedof two outer plates that are fixed in position.

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Figure 5E5.-Capacitance pickoff.A movable plate is centered between thetwo fixed plates and connected to thesensor. The capacity between the centerplate and the two outside plates is equalwhen there is no output from the sensor. If,however, a signal from the sensor causesthe center plate to move toward the bottomplate, the capacity between these two plateswill increase and the capacity

between the top plate and the center plate willdecrease.

This change in capacity can be used to varythe tuning of an oscillator. The change inoscillator frequency is then used for sensecontrol. This type of pickoff is the mostsensitive of all, since a very slight change inplate spacing will cause a large change in

frequency.

F. Computing Devices

5F1. General

Computers appear in missile systems in avariety of forms. The computer maybe asimple mixing circuit in a missile, or it maybe a large console type unit suitable for useat ground installations only.

5F2. Function and requirements

One important function of a computer isthe coding and decoding of informationrelating to the missile trajectory. It isnecessary to code and decode controlinformation in order to offset enemycountermeasures and to permit control ofmore than one missile at the same time.

Another function of the computer is themixing of signals from sensor and referenceunits to produce error signals. Figure 5C1shows, in block form, how the computer islinked with other sections of the completesystem. The signals from the sensor andreference units may be mixed in a presetratio, or they may be mixed according toprogrammed instructions.

The error signals produced by mixing areamplified and passed to the controlactuating system and the followup section.The output of the followup section is thenfed back to the computer for reprocessing.The purpose of

feedback is to reduce over control that wouldcause the missile to oscillate about the desiredattitude.

The computer section may also compare twoor more voltages to produce error signals. Forthis purpose, voltage or phase comparatorcircuits are added. The synchro unitsdiscussed in the previous section are used incomputers to convert signal voltages intoforms that are better suited for processing.

Airborne computers are generally classifiedaccording to the phase of missile flight inwhich they are used. The computers may beseparate units or they may be combinationsof prelaunch computer, launch computer,azimuth computer, elevation computer,program computer, and dive-angle computer.

5F3. Types of computers

In a missile control system, computerelements are of general types-mixers,integrators, and rate components.

MIXERS. As you will recall from the firstpart of this chapter, a mixer is basically acircuit or device that combines informationfrom two or more sources. In order tofunction correctly, the mixer must combinethe signals that are fed to it in the properPROPORTION, SENSE, andAMPLITUDE.

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The type of mixer used will depend mostlyon the type of control system. Most

of the other two shafts, because the verticallever arms from shaft 3 are not of the same

systems use electronic mixers. However,mixers may also use mechanical,pneumatic, or hydraulic principles.

Electronic mixers may use a vacuum tubeas a mixing device. Probably the mostcommon type of tube mixer is the one usedin conventional superheterodyne radio sets.Here a tube mixes an incoming RF signalwith the signal of a local oscillator toproduce a difference frequency. It is alsopossible to use a network composed ofinductors, capacitors, and resistors formixing. Regardless of the type of mixer, thesignals to be combined are represented bythe amplitude and phase of the inputvoltages. Voltages from such sources aspickoffs, rate components, integrators,followup generators, and guidance sourcesmay be combined by the mixer section toform control signals.

Mechanical mixers consisting of shafts,levers, and gears can also be used tocombine information. Figure 5F1 showshow lateral signals from two sources can becombined by using plain levers. To see howthis works, assume that shafts 1 and 2operate independently, and that theirpositions represent information that must becombined. The three connections pivotfreely. The position of shaft 3 represents aweighted average

length. The direction of shaft movement givessense information. The output of shaft 3 maybe used to operate an electrical pick-off, suchas a potentiometer.

Another mechanical mixer uses gears tocombine position or angular velocityinformation. The gear arrangement is similarto that of an automobile rear axle differential.If the input shafts contain positioninformation, they will move slowly andmaintain approximately the same averageposition. The position of the output shaftconstantly indicates the difference betweenthe two shaft positions. If the information isrepresented by the speed of the shaft rotation,the angular velocity of the output shaftrepresents the difference between the twoinput shaft speeds.

It is possible to arrange the input shafts sothat the output represents the sum of theinputs rather than the difference. Weightingfactors can be controlled by changing the gearratios in the differential.

Sometimes information is transferred throughair or hydraulic tubes. The signals are createdby varying the pressure inside the tube. Twosignals can be combined by joining two tubesinto one.

INTEGRATORS. An integrator performs amathematical operation on an input signal.The

Figure 5F1.-Mechanical mixer.

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integral of a constant signal is proportionalto the amplitude multiplied by the time thesignal is present. Assume that the integratoroutput is four volts when the duration ofthe constant input signal is one minute.Then if the same input signal had lasted forone-half minute, the output would havebeen two volts.

But, an actual missile error signal is notconstant, as we assumed in the aboveexample. The amplitude and sense of theerror change continuously. Even so, theintegrator output is proportional to theproduct of the operating time and theaverage error during that time. Should thesense of the error change during theintegration period, a signal of oppositesense would cause the final output of theintegrator to decrease. The integrator canbe considered as a continuous computer,since it is always producing a voltage that isproportional to the product of the average

input voltage and time. Therefore, theintegration of an error with respect to timerepresents an accumulation of intervals oftime and errors over a specified period.

Any integrator has a lag effect. To see whythis is true, let us visualize a situation likethat shown graphically in figure 5F2. Thesolid lines forming the rectangles representon-off signals plotted with respect to time.The polarity is represented by the positionof the rectangle above or below the timereference line. The heavy white linesrepresent the integrated output signal.

Although the input signal goes from zero tomaximum with zero time lag, there is nooutput at that instant. The graph shows thelag effect; note that time is required beforethe output reaches an appreciableamplitude. Approximately the same lengthof time is required for the output amplitudeto drop to zero after the input pulse ends.

proportional control alone is not enough toovercome a strong, steady force that iscausing the missile to deviate from the correctpath. In a case of this kind, the proportionalerror signal will have a steady component thataffects the integrator. The error signal senseremains constant, so that the integrator outputincreases with time as shown at the right offigure 5F2. This output increase reinforcesthe proportional signal until correction of theflight path takes place.

Integration may be performed by a motor, thespeed of which is proportional to theamplitude of the input signal. The motordrives a pickoff, and the distance the pickoffmoves is proportional to the integral of theinput signal.

The direction of motor rotation will dependon the polarity or phase of the input signal.The amplitude of the error signal variesirregularly; the sense of the signal may

reverse, causing reversal of the motorrotation.

Other types of integrators use ball-and-diskmechanical arrangement, resistance-capacity(R-C) circuits, resistance-inductance (R-L)circuits, and thermal devices.

RATE SYSTEMS. The rate section in amissile control system should produce anoutput signal proportional to the RATE OFCHANGE of the input signal amplitude.

The preceding section showed that a time lagis present in integrator circuits. It is this timelag that makes rate circuits necessary. Missiledeviation cannot be corrected instantly,because the control system must first detectan error before it can begin to operate.

The ideal control system would have zerotime lag, thus permitting zero deviation duringthe missile flight. All design efforts are towarda control system with this degree of

to drop to zero after the input pulse ends.

The figure also shows the additive effect oftwo successive negative pulses. This actionis made possible by the time lag, and isused to give more precise control action.

The output signal from the integrator isused to support the proportional errorsignal, to make sure that enough correctionwill always be made by the control system.

Keep in mind that the degree of controlexerted by a pure proportional(unamplified) signal is limited. Over-control, or undercontrol, cause excessivemovement of the missile about the desiredtrajectory. There are times when

a control system with this degree ofperfection. Control surfaces are designed tocorrect missile flight deviations rapidly. Thecontrol surfaces are moved rapidly byactuators, which are operated by amplifiederror signals. But it is possible to have a signalso large that the missile is driven beyond thedesired attitude, and an error occurs in theopposite direction. This error drives themissile back in the first direction. The endresult is a series of swings back and forthacross the desired trajectory.

These unwanted swings are known asoscillation (or hunting) and the addition of arate signal has the effect of damping(retarding)

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Figure 5F2.-Integrator time lag and sense.

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the oscillation. The amount of dampingmay be classed as CRITICAL,UNDERDAMPING, or OVERDAMPING.

The end effect of a rate signal is areduction in the time between the initialcontrol pulse and the output action. Toreduce this time, the rate signal is combinedwith the proportional signal to produce aresultant signal that leads the originalproportional signal.

However, there is output from the ratedevice only when the missile deviation ischanging. The amount of output isdependent on the rate of change. Bycombining the rate signal and the errorsignal, the system can be made to respondto a constant error. It is also

possible to combine an attitude rate signalwith a guidance signal.

Perhaps the most common method ofproducing a rate signal is by using a separatesensor unit, such as a rate gyro. As explainedpreviously, the rate gyro construction is suchthat it can precess only a few degrees and inonly one plane. Precession is restrained by aspring that tends to return the gyro to themidpoint. Any precession in this plane iscaused by a force acting on the gyro gimbals.Such a force would be developed by anyangular movement of the missileframe. Themagnitude of the force would be proportionalto the rate of movement. The gyrodisplacement is detected by a pickoff, and theoutput of the pickoff is the rate signal.

G. Amplifiers

5G1. Purpose

Amplifiers are divided into two groups-POWER and VOLTAGE. Both are used inmissile control systems to build up a weaksignal from a sensor so that it can be usedto operate other sections of the controlsystem. These sections normally requireconsiderably more power or voltage than isavailable from the sensor. Most amplifiersuse electronic tubes, but in this section wewill discuss some of the less conventionalamplifiers.

5G2. Operating principles

Some functions in missile control systemsrequire a series of flat-topped pulses, calledsquare waves, at a definite frequency. It ispossible to convert other wave shapes tosquare waves with vacuum tube amplifiersand clippers. It is also possible to

accomplish the same result with amechanical device known as a chopper.

The coil is excited by an a-c voltage thatcauses the vibrating arm to move at thefrequency of the exciting voltage. Normally,the reed would vibrate at twice the a-cfrequency-once each half-cycle. This can beprevented by incorporating a permanentmagnet in the structure. Then, on one half ofthe a-c cycle the a-c field about the coil isreinforced by the permanent magnet field,and on the other part of the a-c cycle the fieldabout the coil is opposed by the permanentmagnet field. As a further aid to operation onthe desired frequency, the vibrating reed istuned for that frequency by weighting.

The contact arrangement is shown near thebottom of the drawing. Leads are brought outseparately from each of the two fixedcontacts and the vibrating reed to pins on thebase. These pins are arranged so that thechopper can be plugged into a conventionalradio tube socket. In order to reduceoperating noise, the entire mechanism isenclosed in a sponge rubber cushion before itis placed in the metal can. By using the

The chopper is a mechanical switchdesigned to operate a fixed number of timesper second. A cutaway view of amechanical chopper is shown in figure 5G1.This unit has the contacts arranged forsingle-pole double-throw switching, centerOFF position.

is placed in the metal can. By using thechopper in connection with a conventionaltransformer, amplification can be obtained atthe pulse frequency.

Vacuum tubes can be used as electronicchoppers. Other amplifiers, known assaturable reactors, are used for a-c motorcontrol. This type of amplifier maysometimes be used in combination withvacuum tubes.

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Figure 5G1.-Cutaway view of mechanical chopper.

H. Controller Units

5H1. Function

A controller unit in a missile control systemresponds to an error signal from a sensor.In certain systems an amplifier which isfurnishing power to a motor serves as acontroller. In this section we will discusscontroller units other than amplifiers.

5H2. Types

There are several types of controllers and

SOLENOIDS. A solenoid consists of a coil ofwire wound around a nonmagnetic hollowtube; a movable soft-iron core is placed in thetube. When a magnetic field is created aroundthe coil by current flow through the winding,the core will center itself in the coil. Thismakes the solenoid useful in remote controlapplications, since the core can bemechanically connected to valve mechanisms,switch arms, and other regulating devices.Two solenoids can be arranged to give double

There are several types of controllers andeach type has some feature that makes itbetter suited for use in a particular missilesystem than other types.

Two solenoids can be arranged to give doubleaction in certain applications.

TRANSFER VALVES. Figure 5H1 shows anapplication in which two solenoids are used tooperate a hydraulic transfer valve. When

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Figure 5H1.-a. Transfer valve (closed). b. Hydraulic transfer valve and actuator.neither coil is energized, the valve is closed(fig. 5H1a). If S2 receives more energy, thecenter part of the valve section is pulled tothe right, and the actuator is caused tomove. The converse is true if more currentflows thru S1. The actuator can be used tophysically position a control surface.

RELAYS. Relays are used for remotecontrol of heavy-current circuits. The relaycoil may be designed to operate on very

small

signal values, such as the output of a sensor.The relay contacts can be designed to carryheavy currents.

Figure 5H2 shows a relay designed forcontrolling heavy load currents. When the coilis energized, the armature is pulled downagainst the core. This action pulls the movingcontact against the stationary contact, andcloses the high current circuit. The relaycontacts will stay closed as long as the

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Figure 5H2.-Low current relay.

magnetic pull of the coil is strong enough toovercome the pull of the spring.

The relay just described has a fixed core.However, some relays resemble a solenoid inthat part of the core is a movable plunger.The moving contacts are attached to theplunger, but are electrically insulated from it.

Figure 5H3 shows a form of relay that can beused in a penumatic control system. Two airpressure lines are connected to the air inputports. The relay operates when its arm isdisplaced by air pressure. A modified designof this type relay might be used in ahydraulic-electric system in which case thediaphragm would be moved by hydraulic fluidpressure.

AMPLIDYNE. An amplidyne can be used asa combined amplifier and controller, since asmall amount of power applied to its inputterminals controls many times that amount ofpower at the output. Figure 5H4 shows anamplidyne.

The generator is driven continuously, at aconstant speed, by the amplidyne drivemotor. The generator has two control fieldwindings that may be separately excited froman external source. When neither fieldwinding is excited, there is no output from thegenerator, even though it is running. It

Figure 5H3.-Air-actuated relay.generator, even though it is running. Itfollows that no voltage is then applied to thearmature of the load driving motor. (The fieldwinding of the motor is constantly excited bya d-c voltage.)

The control field windings of the generator

are arranged so that the polarity of theexcitation voltage from the sensor willdetermine the polarity of the generator outputvoltage. The generator output is connected tothe load driving motor armature through thelatter's commutator. Since the field of themotor is constantly excited by a fixedpolarity, the polarity of the voltage applied tothe armature will determine the direction ofarmature rotation.

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Figure 5H4.-Amplidyne controller.

I. Actuator Units

5I1. Function

In a missile control system, any errordetected by a sensor must be convertedinto mechanical motion to operate theappropriate control device. The device that

accomplishes this energy transformation isthe actuator unit.

The actuator for a specific control systemmust be selected according to thecharacteristics of the system. The actuator

pneumatic, or electrical. Each of these hascertain advantages, as well as certain designproblems. We will discuss each systembriefly.

5I3. Hydraulic actuators

Pascal's Law states that whenever a pressureis applied to a confined liquid, that pressure istransferred undiminished in all directionsthroughout the liquid, regardless of the shapeof the confining system.

characteristics of the system. The actuatormust have a rapid response characteristic,with a minimum time lag between detectionof the error and movement of the flightcontrol surfaces. At the same time, theactuator must produce an outputproportional to the error signal, andpowerful enough to handle the load.

5I2. Principal types

Actuating units use one or more of threeenergy transfer methods: hydraulic,

of the confining system.

This principle has been used for years in suchfamiliar applications as hydraulic door stops,hydraulic lifts at automobile service stations,hydraulic brakes, and automatictransmissions.

Generally, hydraulic transfer units are quitesimple in design and construction. Oneadvantage of a hydraulic system is that iteliminates complex gear, lever, and pulleyarrangements. Also, the reaction time of a

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Figure 5I1.-Hydraulic system with equal piston displacement.hydraulic system is relatively short, becausethere is little slack or lost motion. Ahydraulic system does, however, have aslight efficiency loss due to friction.

Figure 511 shows that equal input pistondisplacement will produce the same outputpiston movement. Actually, this statementis not wholly true because of slight frictionand compressibility losses. But for allpractical purposes, the motion can beconsidered to be the same for equal pistondisplacements.

A different condition is shown in figure512, where the output piston force hasbeen increased. Because we can't getsomething for nothing, increased forceresults in a decrease in piston travel. Keepin mind also, that the output piston could beused as the input piston, and vice versa.Hypothetically then, a small

gyro displacement acting on a piston can bemade to produce a large control surfacemotion if the gyro is connected to the largepiston and the control surface connected to amuch smaller piston.

In the practical application of hydraulics,something must be done to keep fluid in theproper lines. To accomplish this a circulatingsystem is used. This requires pressure, whichis furnished by a pump.

PUMPS. The pump used in a hydraulicsystem must be driven by some powersource, usually an electric motor, within themissile. Pumps used in missile systemsgenerally fall into two categories-gear andpiston.

A gear type pump is shown in figure 513. Itconsists of two tightly meshed gears enclosedin a housing. The clearance between the gearteeth and the housing is very small.

Figure 5I2.-Proportional hydraulic piston displacement.

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Figure 5I3.-Gear type pump.One gear is driven by an external motor.The other has an idler-type mount, andturns because its teeth are meshed withthose of the driven gear. In operation, theintake port (top of figure) is connected to ahydraulic fluid reservoir, and the outputport is connected to the high-pressuredelivery line.

As the gears turn past the intake port, fluidis trapped between the gear teeth and thehousing. This trapped fluid is carriedaround the housing to the output port.Because the fluid then has no place else togo, it is forced into the high-pressuredelivery line.

A double-action piston pump is shown infigure 514. This arrangement is calleddouble action because fluid is pumped fromthe reservoir as the piston moves in eitherdirection. To see how this happens, assumethat the piston is at the extreme right of thecylinder, and that it has started to move tothe left. A slight vacuum will be created asthe piston moves, and this will reduce thepressure on valve No. 1. At the same time,system pressure will force valve No.4 shut.

delivery tube and additional fluid is pulledfrom the reservoir through valve No. 1.

When the piston reaches the left side of thecylinder, it reverses direction. This createspressure against valves Nos. 1 and 4 so thatvalve 1 closes and valve 4 opens. At the sametime, valve 3 closes and valve 2 opens.

RESERVOIR. The reservoir shown in figure514 is a storage compartment for hydraulicfluid. Fluid is removed from the reservoir bythe pump, and forced through the hydraulicsystem under pump pressure. After the fluidhas done its work, it is returned to thereservoir to be used again. The reservoir isactually an open tank because of theatmospheric pressure inlets.

VALVES. The valves in the illustrated pistonpump are of the flap type, which operate withvery small changes in pressure. Another typeof valve used in hydraulic systems is thepressure relief valve. As its name implies, it isused to prevent damage to the system by highpressures. Some combination systems usehydraulic pressure regulating switches insteadof pressure relief valves.

system pressure will force valve No.4 shut.Atmospheric pressure, which is admitted tothe reservoir through regular inlets, willthen act on the fluid which opens valve No.1. At the same time, fluid to the left of thepiston is being compressed. This forcesvalve No. 2 to close and block the path tothe reservoir. The pump pressure forcesvalve No. 3 to open and, as a result, thefluid on the left side of the piston is forcedinto the

of pressure relief valves.

A typical hydraulic relief valve is shown infigure 515. It consists of a metal housing withtwo ports. One port is connected to thehydraulic pressure line and the other to thereservoir return line. The valve consists of ametal ball seated in a restricted section of

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Figure 5I4.-Double-action piston pump.

Figure 5I5.-Pressure relief valve.the pressure line. The ball is held in place bya spring, the tension of which is adjusted tothe desired lifting pressure. This pressure ischosen so that it will be within the safeoperating limits of the system.

Should the system pressure become greaterthan the spring pressure, the ball will beforced away from the opening, and fluid will

flow into the port that leads to the reservoirreturn line. Thus the pressure can neverexceed a safe limit; and, since the fluid isreturned to the reservoir, no fluid is lost.

ACCUMULATOR. We have shown thathydraulic fluid is stored in a reservoir underopen tank conditions. When it becomesnecessary to store hydraulic fluid underpressure,

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Figure 5I6.-Floating-piston hydraulicaccumulator.

a storage space called a hydraulicaccumulator is used. The accumulator alsoserves to smooth out the pressure surgesfrom a double-action pump, which wouldotherwise cause unsteady operation ofcontrol devices to which the actuators areconnected.

Figure 5I6 shows a floating-piston hydraulicaccumulator. It consists of a closed metalcylinder separated into two compartments by

a floating piston. One compartment is an airchamber and the other a fluid chamber.

In operation, the air chamber is charged,through the bottom fitting, with compressedair until the chamber pressure equals theline pressure desired in the hydraulicsystem. This pressure forces the piston uptoward the top of the cylinder. The cylinderis connected to the hydraulic pressure linethrough the fitting at the top. If the linepressure becomes greater than the airpressure, fluid is forced into theaccumulator, and forces the piston downagainst the air pressure. Should line pressuredrop, the air pressure forces the piston upand puts fluid back in the line. This actionsmooths out variations in pressure duringperiods of heavy loading, or when thepressure pump lags.

Another type of hydraulic accumulator, thediaphragm type, is shown in figure 5I7. It isbuilt in the form of two hemisphericalchambers, which are separated by a flexiblediaphragm. Air pressure forces thediaphragm upward. If the system pressurebecomes greater than the air pressure, fluidis forced into the fluid chamber. When thesystem pressure drops, air pressure in theaccumulator forces the working fluid backinto the system. The diaphragm-typeaccumulator serves the same purpose in ahydraulic control system as the piston type.

ACTUATOR. The purpose of a hydraulicactuator in a missile control system is toconvert fluid pressure into mechanical forcegreat enough to move a control device.

A basic actuator consists of a cylinder withfluid intake and exhaust ports, and a pistonwhich is mechanically connected to the

Figure 5I7.-Diaphragm-type accumulator.

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load. It is possible to have a double-actingpiston-type actuator in which hydraulicfluid under pressure can be applied to eitherside of the piston. A double-action actuatoris shown in figure 5H1b.

5I4. Pneumatic

The principal difference between ahydraulic system and a pneumatic system isthe use of air rather than hydraulic fluid, asthe working medium.

In a pneumatic system, air from a pressuretank passes through delivery tubes, valves,and pressure regulators to operatemechanical units. After the air has done itswork, it is exhausted to the atmosphere. Itcannot be returned to the tank for reuse.Consequently, air must be stored at a muchhigher pressure than is necessary foroperating the loads in order to have enoughpressure to operate the controls as the airsupply in the tank diminishes.

A pneumatic control system is shown infigure 5A8, and a brief explanation of theoperating sequence is given in theaccompanying text.

5I5. Electrical

Generally, motors are used as actuators inelectrical control system. The size of the load,and the speed with which it must be moved,determine the type of motor to be used. D-cmotors are most often used for driving theheavy loads encountered in missile systems,because d-c motors develop a higher stalltorque than do a-c motors. In addition, it ismuch easier to vary the speed of a d-c motor.

Electrical systems were described in sectionA, and an electrical pitch control system isshown in figure 5A12.

5I6. Mechanical linkage

We have discussed the various controlsystems, but have not discussed in detail themechanical means of linking the flight controlsurfaces to the actuator. In addition toproviding a coupling means, the linkage mayalso be used to amplify either the forceapplied or the speed of movement.

A mechanical linkage between an actuatorand a load is shown in figure 5I8. Thedistance,

Figure 5I8.-Actuator and load linked by lever arm.

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d, on the drawing represents the distancefrom the control surface shaft to the pointwhere the force is applied. The controlsurface moves because force exerted by thepiston is applied at a distance from the axisof rotation, and thus produces a torque.Other mechanical linkages may consist ofan arrangement of gears, levers, or cables.

5I7. Combination systems

Often a number of mechanical systems willbe grouped together to form a combinationsystem, as shown in figure 519. Thissystem uses levers, cables, pulleys, and ahydraulic actuator. However, a systemusing this kind of control is not suited forhigh speed missiles.

5I8. Followup units

The followup unit in a missile controlsystem plays an important part in obtaininga smooth trajectory with minimumoscillation. It does this by providingcontinuous information on flight controlsurface position in relation to the missileaxes. The followup signal indicates how theoutput section of the control system isfollowing the correction data initiated by thesensor unit.

Without a signal of this type, the controlsurface would swing to their limit stops anytime the actuator got a signal from the sensor.The followup signal makes possible a controlsurface deflection proportional to themagnitude of the error.

In operation, the followup signal combineswith the error signal so as to oppose it. But,the error signal is the larger of the two, and isstrong enough to produce the necessarycontrol surface deflection. The error signalamplitude decreases as the missile approachesthe correct flight path. When the amplitudesof the error and followup signals are equal,there is no further deflection of the flightcontrol surfaces, because the sum of the twosignals is zero.An electrical followup system is shown infigure 5110. In this system, the error signal issupplied to an electronic mixer where it iscombined with the smaller signal from thefollowup generator. The difference, orresultant, of these signals is fed through anamplifier and controller to the actuator sectionthat operates the control surface. A portion ofthis signal is also fed to the followupgenerator, so that the followup signal isproportional to the flight surface deviationfrom the axis line.

Figure 5I9.-Combination mechanical linkage.515354 O-59-8

515354 O-59-8

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Figure 5I10.-Followup loop of missile control system.

Figure 5I11.-Air relay with mechanical followup.

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It is also possible to use a mechanical followup.When this method is used, the follow-up

spring is compressed and tends to turn thefollowup arm in a counter-clockwise

When this method is used, the follow-upmechanism maybe a part of an air relay asshown in figure 5I11. The control surfaceposition in relation to the missile axis isindicated by a force which is reflected to thecontroller by a spring.

To see how this system operates, assume thatthe signal from a pneumatic pickoff moves theair relay diaphragm up. The follow-up arm willthen move clockwise. This movement causesthe valve spool of the air valve to moveupward. The valve action admits high-pressureair to the relay, and the pressure forces thepiston of the pneumatic actuator to the left.When this happens, the followup

followup arm in a counter-clockwisedirection. Since the followup force is inopposition to the original motion of thefollowup arm, we have the desired inversefeedback.

A large signal will create a larger flight controlsurface deflection before the feedback forcebecomes great enough to return the followuparm to zero. The spring will then push thefollowup arm and the air valve in the oppositedirection, to move the flight control surfaceback. Therefore, the spring acts to limit flightsurface deflection to a value determined bythe error signal, and to return the flightcontrol surface to a position parallel with themissile axis.

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CHAPTER 6PRINCIPLES OF MISSILE GUIDANCE

A. Introduction

6A1. General

From an offensive viewpoint, a missile can beconsidered as a long-range artillery projectile. Inits basic form, a missile is a self-propelled, high-explosive weapon. Like a projectile, it must becarefully aimed in order to follow a trajectorythat will take it to the target.

Missiles like projectiles, are influenced bynatural forces such as cross winds. But,because a missile usually makes a much longerflight than a projectile, it is subjected to thesenatural influences for a longer time.

A missile is a very expensive piece of ordnance.If the conventional artillery practice of firingseveral ranging shots were followed, the costwould be prohibitive. And unguided missiles

would be of little value against moving targets,which can take evasive action.

All of these factors add up to this: an accuratemeans of guiding a missile to its target is anabsolute necessity.

Modern guidance systems are far advanced.But progress in electronic and allied equipmentsis rapid, and our present guidance systems maybe out of date within a few years. The ultimate

The word GUIDED means that the missileflight surfaces are operated by a controlsystem within the missile, in much the sameway as if a human pilot were aboard themissile guiding it to the desired target.

6A3. Purpose and function

The purpose of a guidance system is tocontrol the path of the missile while it is inflight. This makes it possible for personnel atground or mobile launching sites to hit adesired target, regardless of whether thattarget is fixed or moving, and regardless ofwhether or not it takes deliberate evasiveaction. The guidance function may be basedon information provided by sources inside themissile, or on information sent from fixed ormobile control points, or both.

6A4. Basic principles

The guidance system in a missile can becompared to the human pilot of an airplane.One guidance system uses an optical devicethat guides the missile to the target in muchthe same way as a pilot, using landmarks forbearings, guides a plane to a landing field.

If landmarks are obscured, the pilot must useanother system of guidance. He could, for

be out of date within a few years. The ultimatesystem may be a composite formed from thesystems you will read about in this and thefollowing chapters, or it may be entirelydifferent from any of them.

6A2. Definitions

A GUIDED MISSILE may be defined as anunmanned projectile that carries its own flightcontrol equipment. In addition, the missilecarries a payload-either of explosives or ofscientific equipment. High-altitude missiles areused to obtain data on conditions existing farabove the earth. Missiles equipped withguidance control and scientific instruments havebeen shot into the "mushroom cloud" that formsafter an atomic blast, to obtain data onradioactivity, temperatures, and other effects.Since missiles have a number of peacetimeapplications, they should not be consideredexclusively as weapons.

another system of guidance. He could, forexample, use radio beams. One missileguidance system uses radio or radar beamsfor guidance; another uses radio to sendinformation to the missile, just as a groundcontrol station might send instructions to apilot.

We have mentioned radio and radar asprimary guidance controls, but these are notthe only methods by which a missiletrajectory can be controlled. Heat, light,television, the earth's magnetic field, andloran have all been found suitable for specificguidance purposes. Information on all thesesystems will be given in later chapters.

When an electromagnetic source, such asradio or radar, is used to guide the missile, anantenna and receiver are installed in themissile to form what is known as a SENSOR.The sensor section picks up, or senses, theguidance instructions. Missiles that are guided

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by other means use different sensor elements.But the missile control sections, which follow

the sensor section, are basically similar for alltypes of guidance.

B. Phases of Guidance

6B1. General

For purposes of explanation, missile guidancemay be divided into three separate phases. Thefirst is known as the LAUNCHING, orINITIAL phase. The second is called theMIDCOURSE phase, and the last is called theTERMINAL phase. These names refer todifferent parts of the missile flight path.

6B2. Initial phase

Missiles may be launched from a point at somedistance from the guidance equipment. Becausethe missile does not have aerodynamic stabilitywhen it is first launched, the flight controls arelocked in the neutral position, and remainlocked for a short time after launching. As soonas the controls are unlocked, and the guidancesystem assumes control, the initial phase ofguidance is completed.

6B3. Midcourse phase

The second, or midcourse phase of guidance is

time. During this part of the flight, changesmay be required to bring the missile onto thedesired course, and to make certain that itstays on that course. During this guidancephase, information can be supplied to themissile by any of several means. In mostcases, the midcourse guidance system is usedto place the missile near the target, where thesystem to be used in the final phase ofguidance can take over. But, in some cases,the midcourse guidance system is used forboth the second and third guidance phases.

6B4. Terminal phase

The terminal phase is of great importancebecause it can mean a hit or a miss. The lastphase of surface-to-air missile guidance musthave high accuracy as well as fast response toguidance signals.

Near the end of the flight, the missile maylack the power necessary to make the sharpturns that are required to overtake and score ahit on a fast-moving target. In order to

The second, or midcourse phase of guidance isoften the longest in both distance and

hit on a fast-moving target. In order todecrease the possibility of misses, specialsystems are used. These systems will bedescribed in the following chapters.

C. Components of Guidance Systems

6C1. General requirements

A missile guidance system involves a means ofdetermining the position of the missile inrelation to known points. The system mayobtain the required information from the missileitself; it may use information transmitted fromthe launching station or other control point; or itmay obtain information from the target itself.The guidance system must be stable, accurate,and reliable.

In order to achieve these basic requirements,the guidance system must contain componentsthat will pick up guidance information fromsome source, convert the information intousable form, and activate a control sequencethat will move the flight control surfaces on themissile.

Because it is difficult to separate the controland guidance operations, we will go throughthe entire guidance system. However, theflight control section is concerned with flightstability. Missile accuracy is primarily afunction of the guidance section. Missilereliability depends on both sections. We willlist the components and briefly describe thebasic function of each before going into theindividual types of guidance systems.

6C2. Sensor

In some respects, the sensor unit is the mostimportant section of the guidance systembecause it detects the form of energy beingused to guide the missile. If the sensor unitfails, there can be no guidance.

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The kind of sensor that is used will bedetermined by such factors as maximumoperating range, operating conditions, the kindof information needed, the accuracy required,viewing angle and weight and size of the sensor,and the type of target and its speed.

Acoustic sensors called hydrophones are oftenused in torpedo guidance systems. Essentially, ahydrophone is a microphone that worksunderwater. It picks up the vibrations of ships'propellers, and the torpedo can "home" on thisnoise. Acoustic sensors are not well suited forairborne missiles.

Heat, or infrared sensors use an active elementcalled a THERMOCOUPLE, or an elementknown as a BOLOMETER. Either sensor maybe used with a lens and reflector system.

Sensors that respond to light use an activeelement called a PHOTOELECTRIC cell.Another light-sensing system uses a televisioncamera to pick up information and send it backto the launching site by means of a TVtransmitter in the missile.

gravitational force. Some gyros are arrangedfor vertical reference by a pendulous pick-offand erection system. Gyros used in thismanner are called vertical gyros; they may beused to control the pitch and roll of themissile.

An instrument called a FLUX VALVE, hasthe ability to sense the earth's magnetic field,and can be used for guidance. The primarypurpose of this device is to keep a directionalgyro on a given magnetic heading. A gyrooperated in this manner may be used togovern the yaw controls of a missile.

Barometric pressure can be used to determinealtitude. A guided missile that is set to travelat a predetermined altitude may use analtimeter to sense barometric pressure. Shouldthe missile deviate from the desired altitude,an error signal will be generated and fed tothe control section.

Another pressure-type sensor is used todetermine airspeed. It compares staticbarometric air pressure with ram air pressure.The difference between these two pressures

Electromagnetic sensors use radio or radarantennas as active elements. The phase ofguidance determines the location of the antennain the missile structure. For initial andmidcourse guidance the antenna is normallystreamlined into the tail of the missile. For finalphase guidance the sensor may be located in thenose of the missile.

All of these sensors have advantages anddisadvantages. Some are well suited for finalphase applications and totally unsuited for initialand midcourse guidance. The advantages anddisadvantages of each will be covered in laterchapters.

6C3. Reference units

The signals picked up by the sensor must becompared with known physical references suchas voltage, time, space, gravity, the earth'smagnetic field, barometric pressure, and theposition of the missile frame. The sensor signaland the reference signal are compared by acomputer, which will generate an error signal ifa course correction is necessary. The errorsignal then operates the missile control system.

Gyroscopes are used for space reference. Areference plan is established in space, and thegyro senses any change from that reference.

The earth's gravity can be used as a reference;a pendulum can sense the direction of the

The difference between these two pressuresprovides an air speed indication.

The axis of the missile frame is used as areference to measure the displacement of themissile control surfaces. (The movement ofthe control surfaces cannot be referenced tothe vertical, or to a given heading, becausethe reference would change when the missileposition changes.)

Selsyns may be used to indicate the angularposition of the flight control surfaces withrespect to the missile axis. It is also possibleto use potentiometers (variable resistors) forthis purpose. When this method is used, thepotentiometer is fastened to the missile frameand the potentiometer wiper-arm shaft ismoved by the control surface.

6C4. Amplifiers

Each of the sensor units we have discussedproduces an output; in most cases this outputis a voltage. A computer is used to comparethe sensor output voltage with the referencevoltage. If the missile is off course, the twovoltages will not be the same. The computerwill then generate an error signal, which willbe used to operate the missile control surfacesand bring the missile back on course.

But the computer output power is usually toosmall to do the actual work of moving thecontrol surfaces. And the output of a sensor

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unit is often too small for accurate comparisonwith a reference voltage. In such cases themissile uses an AMPLIFIER to increase eitherthe signal voltage or power, or both, to a usefulvalue. A transformer is a simple, familiar devicethat can increase an a-c voltage. The voltageacross the secondary winding may be manytimes as high as the voltage applied to theprimary. But the current available in thesecondary circuit is proportionately smaller thanthat applied to the primary, so that there is noincrease in power. There is no device that willgive out more power than you put into it; inother words, you can't get something fornothing.

In an amplifier, the small power available in the

electrically heated, is connected to thenegative side of the power supply. The plate,which is not heated, is connected to thepositive side. Under these conditions, anelectric current will flow from the cathode tothe plate, through the vacuum in the tube.The control grid is usually a spiral of wiresurrounding the cathode, and much closer tothe cathode than it is to the plate. The voltageapplied to the grid can be used to control theflow of current through the tube.

Let's consider a very simple application, inwhich a vacuum tube is used as a switch.Let's say that we want a relay to operatewhen light falls on a photoelectric cell (fig.6C1). If we apply a voltage across a

In an amplifier, the small power available in theinput signal is used to CONTROL the amountof power that is supplied from another source.Thus, in a sense, a relay or a switch is anamplifying device. A small amount of powerapplied to the relay primary, or to the switchhandle, will close the contacts and thus apply amuch larger amount of power to the load. But aswitch or relay is an all-or-nothing device; thecontacts are either open or closed. Many missileapplications require an amplifier whose output isnot only greater than the input, but alsoproportional to the input. For example, let's saythat a sensor provides a one-volt input to anamplifier, and the amplifier output is 15 volts.Then, if the sensor supplies a three-volt input,the amplifier output must be 45 volts.

Electronic amplifier circuits can be designed togive proportional amplification for a limitedrange of input voltages. Electronic amplifiershave become familiar devices. An electronicamplifier is used to amplify the output of arecord-player pickup to the power level requiredto drive a loudspeaker cone. Electronicamplifiers are used to amplify the signals pickedup by a radio antenna. (But note that, in bothcases, the actual power of the input signal is notincreased. The input power is used simply tocontrol the power supplied from anothersource-either batteries or the a-c line.)

At present, most electronic amplifiers are basedon vacuum tubes. As you probably know, avacuum tube is a sealed envelope of glass ormetal, from which most of the air has beenexhausted. A vacuum tube used in an amplifiercircuit contains a cathode, a plate, and one ormore grids. The cathode, which is

6C1). If we apply a voltage across aphotoelectric cell, and the cell is dark, thecurrent flowing through it will be close tozero. But the current will increase slightlywhen light falls on the cell. Because thecurrent in the photo tube circuit is only a fewmillionths of an ampere, it is not sufficient tooperate a relay. But if we connect a resistancein series with the photo cell, this current willdevelop a voltage across the resistance, inaccordance with Ohm's law. By using a verylarge resistance in series with the photo tubewe can develop several volts across it, eventhough the power is very small. This voltagecan be applied to the grid of a vacuum tube.Then, when light falls through the photo cell,current will flow through the vacuum tube;when the cell is dark, no current will flow. Inthis way, the flow of current CONTROLLEDby the photo cell may be several thousandtimes as much as the current that actuallyflows through it. Finally, we can connect thecoil of a suitable relay in series with thevacuum tube, between

Figure 6C1.-Photoelectric cell with vacuumtube amplifier.

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its plate and the positive terminal of the powersupply. Then, when light falls on the photo cell,the relay contacts will close; when the light isremoved from the cell, the relay contacts willopen again.

A vacuum tube is more often used to amplifyan a-c input signal (fig. 6C2). For this purpose,a plate load resistor is connected in series withthe tube, between the plate and the positiveterminal of the power supply. A changingcurrent through the tube will then produce acorresponding change across the load resistor,in accordance with Ohm's law. In most circuits

than vacuum tubes; they are considerablysmaller, and they require no cathode heatingpower. On the other hand they are unreliableat high temperatures, and are not available onthe market in as wide a variety of types.Finally, they are ineffective at the extremelyhigh frequencies present in many vacuumtube circuits. In guided missile circuitry,transistors have begun to displace vacuumtubes in some applications.

Another amplifying device is the MAGNETICAMPLIFIER. In its simplest form, this deviceis basically a transformer with a saturable

in accordance with Ohm's law. In most circuitsa steady, negative d-c voltage is applied to thegrid. When no input signal is present, thisnegative "bias" voltage maintains the tubecurrent at about half its maximum value. Whenan a-c input signal is applied to the grid, it willalternately add to and subtract from the fixedbias. The plate current, and consequently theplate voltage, will follow the input signal, but ata much increased amplitude. The output signalis taken

Figure 6C2.-Vacuum tube used as a-c amplifier.

from the plate, usually through a capacitor. Thisoutput can, of course, be used as the input for asecond stage of amplification, so that extremelysmall signals can be built up to useful levels.

For many applications, transistors can be used

in place of vacuum tubes. In a transistor, acurrent flows through a metallic semiconductor,rather than through a vacuum. The flow ofcurrent can be regulated by an input signalapplied to a third terminal, so that the transistorcan be used as an amplifying device.Transistors are less sensitive to vibration

is basically a transformer with a saturablecore, and a third winding in addition to theusual primary and secondary (fig. 6C3). Withno current in the third winding, the deviceacts like a transformer, and an a-c voltage isdeveloped in the secondary circuit. Bypassing d-c through the third winding, thecore can be saturated; there is then notransformer action, and the secondarydevelops no voltage. A varying input signal onthe third winding produces a correspondingvariation, of greater amplitude, across theoutput. Most of the magnetic amplifiers nowin use are more sophisticated than the onedescribed here; they have additional windings,and serve a variety of purposes.

A magnetic amplifier is a reliable devicebecause it is rugged, and resistant to shock,vibration, and temperature changes. It is usedin a number of missile applications.

Figure 6C3.-Simplified diagram of magneticamplifier.

6C5. Computer

A computer is necessary in missile guidancesystems in order to calculate coursecorrections rapidly. In one type of missile thecomputer is simply a mixing circuit. On the

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other hand, the computers used at launchingsites may be large consoles with many stages.

An important function of a computer is thecoding and decoding of information relating tothe missile trajectory. In order to make theproper selections, the computer usesdiscriminator circuits to select pulses of theproper width, amplitude, frequency, phase, ortime difference, and reject all other pulses. Byusing a number of different discriminators, awide variety of pulse characteristics can behandled. The discriminator is relatively

slides, linkages, and other mechanical devices.Computers using these principles have beenused to solve navigation and fire controlproblems for a number of years.

In an electronic analog computer, the inputand output variables are represented byvoltages, and the computations are performedby electronic circuits. A type of electronicanalog computer that has proved useful inmissile design is the DIFFERENTIALANALYZER. This device is sometimes calleda SIMULATOR, because it can be given

handled. The discriminator is relativelyinsensitive to manmade electrical noises andatmospheric static.

A second important computer function is tocompare the signals from the sensor andreference units, to compute the missile'sposition with respect to the desired referenceplanes, and to generate error-signal voltages ofthe polarity and amplitude required to bring themissile back on course.

Computers may be divided, according to theway they operate, into two classes: ANALOGand DIGITAL. An analog computer deals withquantities that are continuously variable. Atarget bearing angle, for example, is such aquantity; in an analog computer, such a quantitycan be represented with considerable accuracy

by a voltage, or by the angle of rotation of ashaft. Digital computers, on the other hand, dealonly with quantities that vary by distinct steps.For example, an angle might be represented aseither 67° or 68°, but not as anything betweenthose values. (A more complex computer mightrepresent the same angle as either 67.43° or67.44°, but would be incapable of dealing withany value between those two.) An ordinaryslide rule is a simple analog computer, in whichthe position of the slide is "analogous" to thequantity represented. A desk calculatingmachine, or an abacus, is a simple digitalcomputer.

Both analog and digital computers haveapplications in guided missiles and theirassociated ground equipment, although analogcomputers are more often used. Computers ofeither class may operate either electrically ormechanically, or by a combination of these twomeans. In a mechanical analog computer, theinput and output variables are usuallyrepresented by angles of shaft rotation. Thecomputer performs its various calculationsthrough the movement of shafts, gear trains,

a SIMULATOR, because it can be givenelectrical inputs that simulate both thecharacteristics of a proposed missile and theconditions under which it will operate. Theaction of the computer will then show howthe proposed missile will perform under thespecified conditions. It is thus possible to testnew missile designs without building actualprototype missiles, and this procedure resultsin a considerable saving in both time andmoney.

6C6. Controllers and actuators

If a missile wanders off its proper course, thisfact will be detected by the sensingmechanism previously described. Thecomputer within the guidance system willevaluate the information provided by the

sensing mechanisms, determine the directionand magnitude of the error in missile courseor position, and produce a suitable error signaloutput.

At this point, the functions of the guidanceand control systems overlap. The primarypurpose of the control system is to correcterrors in the attitude of the missile. Theprimary purpose of the guidance system is tocorrect errors in the missile flight path. Bothtypes of error are corrected in the same way:by moving the missile flight-control surfaces.Movements of these surfaces are governed bythe same controllers and actuators, regardlessof whether the error signal is developed bythe guidance or the control system.

Controllers include such devices as relays,solenoids, magnetic transfer valves, andamplidynes. The actuators that apply powerto the control surfaces may be electric,hydraulic, pneumatic, or a combination ofthese. Both controllers and actuators weredescribed in chapter 5.

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6C7. Feedback systems

The final section of a guidance system is knownas a "feedback" or "follow-up" unit. This unitmeasures the position of the flight controlsurfaces in relation to the reference axis of themissile, and compares this value with the errorsignal generated by the computer.

becomes strong enough to exactly equal theerror signal.

As the missile approaches the desired course,the error signal becomes less than thefeedback signal, and the resultant voltagedifference reverses polarity. The reversal inpolarity moves the flight control surfaces in

signal generated by the computer.

Without the follow-up signal, there would benothing but the varying air pressure to preventthe flight control surfaces from swinging to theirmaximum limits any time the sensor caused anerror signal to be generated. By using feedback,the deflection of the flight control surface canbe made proportional to the size of the error.The feedback loop thus gradually returns theflight control surfaces to neutral as the error iscorrected.

To accomplish these results, the feedback signalis used to oppose the error signal. When thefeedback signal becomes as large as the errorsignal, no further deflection of the flight controlsurface takes place because the two signals areequal and opposite, and their sum is zero.

If the error signal voltage is large, a largedeflection of the flight control surface can takeplace before the feedback signal voltage

polarity moves the flight control surfaces inthe opposite direction until they are in neutral.This action is smooth and rapid, and cannotbe duplicated by systems that use ON-OFFswitching.

Figure 6C4 shows the relationship betweenthe follow-up and error signals, and showshow the flight path is smoothed by combiningthese signals. Note how much smoother thelower path is than the upper path, which usesthe error signal alone.

The discussion of feedback loops completesthe basic discussion of the individual stages ofa missile guidance system which is shown inblock form in figure 6C 5.

You should remember that block diagramsmay vary as to form of presentation, but thefinal results will be the same. Block diagramsare related to outside factors as well as thoseinside the missile. By keeping these facts inmind, you will later be able to see how thereference unit may refer to a ground base unitthat is setting up reference points. Thecomputer sections may or may not be in themissile.

D. Types of Guidance Systems

6D1. Preset guidance

The term PRESET completely describes oneguidance method. When preset guidance isused, all of the control equipment is inside themissile. This means that before the missile islaunched, all information relative to targetlocation and the trajectory the missile mustfollow to strike the target must be calculated.After this is done, the missile guidance systemmust be set to follow the course to the target, tohold the missile at the desired altitude, tomeasure its air speed, and at the correct time,cause the missile to start the terminal phase ofits flight and dive on the target.

A major advantage of preset guidance is thatonly limited countermeasures can be usedagainst it. One disadvantage is that after themissile is launched, its trajectory cannot be

changed from that which has been preset at thelaunch point.

6D2. Command guidance

The term COMMAND is used to describe aguidance method in which all guidanceinstructions, or commands, come fromsources outside the missile. To receive thecommands, the missile contains a receiverthat is capable of receiving instructions fromground stations or from another aircraft. Themissile receiver then converts thesecommands to guidance information, which isfed to the sections following the sensor unit.

We will list several command guidancesystems and briefly describe them. Otherchapters in this book will describe thesesystems in more detail.

HYPERBOLIC SYSTEM. Missiletrajectories follow many types of curves.

Most of the curves that are followed aredetermined by the position of the missile inrelation to the target position. However, onetype of

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Figure 6C4.-Relationship of follow-up to error signal.exactly predicted path is a hyperboliccourse laid out by a loran-type system.

LORAN PRINCIPLE. A loran system is amodern electronic aid to navigation. Thename "loran" was derived from the words"LOng RAnge Navigation." The effective

range of loran is as much as 1400 miles atnight and about 750 miles during the day.The accuracy is comparable to that whichcan normally be expected from goodcelestial observations.

Figure 6D1 shows how hyperbolic lines-ofposition are generated by synchronizedtransmitters separated by several miles.

A navigator can think of loran as a fairly newmethod of determining lines-of-position.These loran lines can be compared with otherloran lines, with sun lines, star lines,soundings, radar range circles, or bearings, toprovide navigational fixes.

Loran lines are fixed with respect to theearth's surface, and their determination is notdependent on a compass, chronometer, orother mechanical device. The signals are onthe air and available 24 hours per day, andcover the major part of the seas and oceansof the world.

Loran operates on the following principles:

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Figure 6C5.-Block diagram of missile guidance system.1. Radio signals consisting of short pulsesare broadcast from a pair of shore-basedtransmitting stations.

2. These signals are received aboard ship,plane or missile on a special loran indicator.

3. The difference in times of arrival of thesignals from the two radio stations ismeasured on a special loran indicator.

4. This measured time-difference is used todirectly determine, from charts, a line-ofposition on the earth's surface.5. Two lines-of-position, determined fromtwo pairs of transmitting stations, arecrossed to obtain a loran fix.

Since radio signals travel at a constantspeed, a direct relationship exists betweenthe time of travel and the distance coveredduring that time. Therefore, a measure oftime, is, in essence, a measure of distance.With these features it is easy to see howloran could be used as the basis for amissile guidance system.

The transmitters are fixed components of aloran system. Because loran is concernedwith the measurement of radio signals fromtwo different sources, the transmittersoperate in pairs. The function of eachstation in a loran pair is somewhat differentfrom that of

its companion station, and the names"master" and "slave," given to each describesthe part it plays in the operation.

The master starts the transmission cycle bysending out a pulse of radio energy which isradiated in all directions. After traveling thedistance between the two transmittingstations, which is known as the "baseline,"the pulse arrives at the slave. The signal isreceived at the slave by a loran receiver andthe time of the signal arrival is used by theslave as a reference for the transmission of its

own signal.

After the slave has sent its pulse, the wholeprocess is repeated again and again.

If loran is to be used as a form of commandguidance, the sensor in the missile must besuitable for loran use. A suitable receivingsystem is described in chapter 7.

RADIO COMMAND SYSTEMS. Radio hasbeen used as a guidance link for suchpurposes as model airplane flying, steeringmodel boats and cars, controlling targetdrones, and even for maneuvering oldbattleships during bombing tests. Therefore,when the question of command guidance formissiles came up, radio was among the firstmethods used. But once a radio commandsystem was developed,

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Figure 6D1.-Hyperbolic lines-of-position generated by two transmitters.

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a new problem arose-that of keeping track ofthe missile when it was beyond the range of

normal vision.

Since radar can locate objects not visible byordinary means, it can be used for missiletracking. The radar set transmits a highlydirectional pulse, which is reflected by objectswithin the lobe of energy sent out from theradar antenna. The pulse is returned to theradar set as a reflection. The time between thesending of the pulse and the arrival of thereflection can be accurately measured. Fromthis time measurement, the distance to theobject can be determined. The course of amissile can be followed by radar, and coursecorrection signals sent by radio.

However, even with radar tracking and radioguidance, it is difficult to keep the missile oncourse. Consequently, other items have beenadded to the guidance system.

In order to keep track of the missile, a plottingboard can be used. The missile position isdetermined at intervals, and its successivelocations marked (plotted) on a chart. Bydrawing a line through the successive plots,the missile course can be determined. Once apart of the course has been plotted, theinformation can be fed into a computer whichwill determine the desired course for theremainder of the flight.

Since missiles travel at high speeds,mechanical plotters and computers are used inmodern systems. Commands to correct themissile course can be transmitted as soon as adeviation from the required course is detected.

The use of radio for command guidance ofhigh-speed missiles makes it necessary to usea transmitter that can do more than sendsimple ON-OFF pulses. Otherwise, a separatetransmitter would be required for each controlfunction. This would require several radio

channels for each missile.

In order to get simultaneous operation ofseveral functions, modulated transmissions areused. To overcome other difficulties,frequency modulation using pulse techniques,and radar with pulsed modulation, have beenfound suitable. The various modulation

6D3. Navigation guidance systems

When targets are located at great distancesfrom the launching site, some form ofnavigational guidance must be used. Anexample, which shows the need for thisguidance method, is the Polaris, which canbe launched from a submarine againstinland targets.

Accuracy at long distances is achieved onlyafter exacting and comprehensivecalculations of the flight path have beenmade. The mathematical equation for anavigation problem of this type maycontain factors designed to control themovement of the missile about the threeaxes-pitch, roll, and yaw. In addition, theequation may contain factors that take intoaccount acceleration due to outside forces(tail winds, for example) and the inertia ofthe missile itself.

In this section, we will describe threesystems that may be used for long-rangemissile guidance.

INERTIAL GUIDANCE. The simplestprinciple for guidance is the law of inertia.In aiming a basketball at a goal, youattempt to give the ball a trajectory thatwill terminate in the basket. In otherwords, you give an impetus to the ball thatcauses it to travel the proper path to thebasket. However, once you have let theball go, you have no further control over it.If you have not aimed correctly, or if theball is touched by another person, it willmiss the basket. However, it is possible forthe ball to be incorrectly aimed and thenhave another person touch it to change itscourse so it will hit the basket. In this case,the second player has provided a form ofguidance. The inertial guidance systemsupplies the intermediate push to get themissile back on the proper trajectory.A simple inertial guidance system is shownin figure 6D2. This system is designed todetect errors in the trajectory by measuringthe lateral and longitudinal accelerationsduring the missile flight. To do this, twomain channels are used-one for directionand the other for distance.

found suitable. The various modulationsystems are described in other chapters of thisbook, along with the advantages anddisadvantages of each method.

As shown by figure 6D2, there is somesimilarity between the two channels. Eachcontains an accelerometer, which is used todetect missile velocity changes without theneed for an external reference signal. Theacceleration signals are fed to a computer,which continuously produces an indicationof

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Figure 6D2.-Inertial guidance system.

lateral or longitudinal distance traveled as aresult of acceleration. This is accomplished byintegrating missile acceleration signals toobtain a missile velocity signal. When thevelocity signal is integrated, the result ismissile distance traveled. This method ofdouble integration is built into each channel.

An accelerometer, as its name implies, is adevice for measuring the force of an

acceleration. In their basic principles, suchdevices are simple. For example a pendulum,free to swing on a transverse axis, could beused to measure acceleration along the fore-and-aft axis of the missile. When the missile isgiven a forward acceleration, the pendulumwill tend to lag aft; the actual displacement ofthe pendulum from its original position will bea function of the magnitude of the acceleratingforce. Another simple device might consist ofa weight supported between two springs.When an accelerating force is applied, theweight will move from its original position in adirection opposite to that of the applied force.The moving element of the accelerometer canbe connected to a potentiometer, or to avariable inductor core, or to some otherdevice capable of producing a voltageproportional to the displacement of the

However, the acceleration may changeconsiderably over a period of time. Underthese conditions, integration is necessary todetermine the speed. This operation ineffect makes it possible to multiply a fixedquantity (such as elapsed time) by avarying quantity (such as acceleration).This can be done by dividing the elapsedtime into a large number of "increments" oftime. If we multiply acceleration by timefor each such increment, the result willshow the increment of speed during thattime. If we add up all the increments ofspeed, the sum will be a very closeapproximation to the speed of the missile.

The actual electronic circuits used forintegration are rather complex, but hereagain the basic principle is simple. In onetype of integrator the input consists of aseries of evenly spaced electrical pulsesrepresenting increments of time. Theamplitude of each pulse is controlled by theaccelerometer, so as to represent theinstantaneous value of acceleration. Thusthe quantity of electricity in each pulserepresents an increment of speed. Thepulses are passed through a rectifier (sothat no current can flow in the oppositedirection) and stored in a capacitor. Thecapacitor will, in effect, add up all of theinput pulses. Thus the voltage across thecapacitor will, at any given instant, providean indication of missile speed at thatinstant.

If the missile speed were constant, wecould calculate the distance covered simplyby multiplying speed by time. But becausethe acceleration varies, the speed alsovaries. For that reason, a secondintegration is necessary.

An acceleration may, of course, be applied

proportional to the displacement of theelement.

If the acceleration along the fore-and-aft axiswere constant, we could determine the speedof the missile at any instant simply bymultiplying the acceleration by the elapsedtime, in accordance with the formula

v = at

An acceleration may, of course, be appliedto the missile in any direction. Thus, if themissile is to determine its own position atany given instant, two accelerometerchannels are necessary. For any givenacceleration, one of these measures thecomponent of force along the fore-and-aftmissile axis; the other measures thecomponent across that axis.

The distance and direction channels areidentical in operation. The output voltageof the first integrator indicates the missilevelocity. The output voltage of the secondintegrator is proportional to the distance themissile has traveled.

In order to determine when the missile hasreached the target, the distance traveled bythe missile must be compared to theknown

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Figure 6D3.-Specified velocity reference.distance from the launch point to the target.This comparison may be made by settingup a comparison voltage, representing theentire distance the missile is to travel, as aninitial condition at the time of launching.This voltage is added, with oppositepolarity, to the output of the secondintegrator. As the missile approaches thetarget, the output voltage of the distancechannel decreases. At zero volts output, thedestination has been reached.

This method of determining when themissile reaches the target has thedisadvantage of requiring large integratorand comparison voltages if long distancesare involved. However, this disadvantagemay be overcome by specifying shortdistance comparison points throughout theflight. The specification of comparisonpoints can be done by a recorder carried inthe missile.

error may be as much as half a mile for aflight that lasts 45 minutes. For longer flights,the error would naturally increase.

One method that may be used to overcomethe random drift error involves the use of starsights. The checking is done in much thesame manner as a human navigator wouldcheck his position by observing an object,such as a star, having a known position.

To make the check, an automatic sextant ismounted on a platform in the missile so that itcan be turned on elevation and azimuth axes.An automatic sextant is shown in figure 6D4.

The position data is recorded on a tape,which is placed in the missile prior tolaunching. The tape is pulled through a"reader" head, which contains a series ofcontacts that are actuated by holes in thetape. The motor that pulls the tape receivesits voltage from a timing circuit. Thus the rate

the missile.

Another method that may be used to holddown required voltages for long trips is tospecify the velocity and use that as areference condition on the first integratoroutput. This method is shown in blockdiagram form in figure 6D3.

When this system is used, any error aboveor below the specified velocity is fed to thesecond integrator. The resultant output isproportional to the missile error in distanceat a given time.Other information on inertial guidancesystems will be given in a later chapter inthis book.

CELESTIAL REFERENCE. Navigation byfixed stars and the sun has been practicedfor years, and is very dependable.

A missile guidance system which usescelestial reference most likely would bemade up of an inertial system supervised bya series of fixes on the sun or stars. Thegyro that controls the position of theaccelerometers is subject to random drift,and the result is an error that tends toincrease with time. The

its voltage from a timing circuit. Thus the rateat which the tape is pulled through the"reader" is carefully controlled.

Figure 6D4.-An automatic sextant.

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Figure 6D5.-Sextant positioning system.Figure 6D5 shows a sextant positioningsystem in block form. Note that theelevation servo generator and the azimuthservo generator both receive signals from

The phototube output is amplified, rectified,and fed to a resolver that compares therectified photocell voltage to a fixed referencevoltage. The resolver output consists of two

servo generator both receive signals fromthe tape reader. The generators areconnected to servo motors. The shafts ofthese motors are mechanically connected tothe sextant positioning gears, so that thesextant position is actually controlled by theinformation on the tape.

The position of the sextant is checked by asection called the STELLAR ERRORDETECTION CIRCUIT, whichdetermines whether or not the star iscentered in the telescope field. If the star isnot centered in the field, an error signal isgenerated and processed to show theamount of sextant error. The errordetection circuit is shown in block form infigure 6D6.

The outputs of this circuit are voltages thatare proportional to the missile error in pitch,roll, and yaw. The output voltages areobtained by feeding the starlight picked upby the sextant to a mechanical scanner unit.This unit contains a chopper whichmodulates the light beam at a given rate.The modulated light beam falls on aphototube, causing the phototube output tovary in proportion to the light intensity.

voltage. The resolver output consists of twoerror signals, one in azimuth and the other inelevation. The azimuth error is fed to the yawcomparator along with the azimuthcommands from the tape. The resultantvoltage, obtained by comparing the error andcommand signals, is the yaw error signal.

The elevation error signal from the directionresolver is fed to the pitch and roll resolverwhere it is compared to the tape signal. Theoutput of the resolver is divided into twovoltages-the pitch error signal and the rollerror signal. It is not possible to obtainproportional control with this system becauseof the delay in signals getting through thecircuits, and damping by the rate function.However, the system does tend to return themissile to the correct course as soon aspossible without over-control oscillations.

TERRESTRIAL REFERENCE. Threecharacteristics of the earth's magnetic fieldthat are useful for missile guidance are (1)lines of equal magnetic deviation, (2) lines ofequal magnetic inclination, and (3) lines ofequal magnetic intensity. Refinements of the

515359 O-59-9

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Figure 6D6.-Stellar error-detection circuit.magnetic compass, such as the flux valve,can be used in missile guidance. A completemagnetic guidance system is shown inblock diagram form in figure 6D7.

The flux valve consists of primary andsecondary windings on an iron core. Theprimary is supplied with a-c of a fixedvoltage and frequency. When no externalmagnetic field is present, the device acts asa simple transformer; the frequency of thesecondary voltage is the same as that of theinput voltage. But when an external field ispresent, it will alternately add to andsubtract from the field generated by theprimary current, during successive half-cycles. As a result a second harmonic, attwice the input frequency, is superimposedon the output voltage. If the flux valve isproperly aligned with the external magneticfield, the amplitude of the second harmonicvoltage will be proportional to the strengthof that field.

The magnetic guidance system represented

in figure 6D7 uses three flux valves; in thediagram they are called the axial orient coil,the transverse orient coil, and the detector

coil. The three flux valves are rigidly securedtogether, with axes mutually perpendicular.The outputs of the axial and transverse fluxvalves are used to operate servo systems thatcontrol the position of the three-valveassembly. The effect of these servos is toorient the assembly so that its detector coil isconstantly aligned with the earth's magneticfield.

The output of the detector coil consists of the400-cycle input frequency, combined with the800-cycle harmonic that results from theaction of the earth's magnetic field. Thiscombined output is fed to a filter amplifierthat amplifies the 800-cycle voltage, buteliminates the 400-cycle signal. The amplifieroutput is then fed to a frequency divider,which changes the 800-cycle signal back to a400-cycle signal suitable for operating themissile control system. The amplitude of thissignal will be proportional to the strength ofthe earth's magnetic field.

A 400-cycle reference voltage can be preset

before launching, at a value equal to thatresulting from any given strength of theearth's

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Figure 6D7. Magnetic system.

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magnetic field. The flux valve outputvoltage can be compared with the referencevoltage by a computer. In this way it ispossible to guide the missile along anydesired line of constant magnetic intensity.

6D4. Beam-rider guidance

We have described some basicelectromagnetic (radio and radar) guidancemethods and have shown that it is possibleto send commands to a missile in order tocorrect its trajectory. Since the commandsare sent on a radio beam, why not use thebeam itself for guidance?

PRINCIPLES. The answer to the questionis the present beam rider guidance systems.Basically, the system uses the beam patternof a highly directional radar antenna as atrack between the missile launching pointand the target. Electronic equipment on theground modulates the beam in such a waythat electronic equipment in the missile canderive guidance instructions from it.

FIXED COMPONENTS. The fixedcomponents of a beam rider system areusually a target-tracking radar, a computer,and a guidance radar. It is possible tocombine the guidance and tracking functioninto one beam. However, this method is notas satisfactory as a two-beam system. Boththe one-beam and two-beam systems aredescribed in chapter 8 of this book.

MISSILE COMPONENTS. The missilemust contain receiving equipment suitablefor picking up and deciphering guidanceinformation. This information must then beconverted to appropriate motion of the

is to obtain guidance information from thetarget itself, rather than from some otheroutside source.

Homing guidance may be accomplished by

one of several types of sensors. Radar sets inthe missile may send out pulses, so that themissile may home on the echoes returningfrom the target. Heat sensors may be used topick up infrared radiations that can be usedfor homing. Light sensors may be used topick up light given off by the target, and themissile can use the signals generated by thelight cell for guidance. Television is anotherhoming guidance medium which, whileotherwise suitable for this purpose, haslimited value because it can be used only indaylight, and only when visibility is good.

6D6. Composite systems

We have shown how several guidancesystems may be used. Some of these systemsare better suited for one purpose than others.No one system is best suited for all phases ofguidance. It is logical then to combine asystem that has excellent midcourse guidancecharacteristics with a system that hasexcellent terminal guidance characteristics inorder to increase the number of hits.Combined systems are known as compositeguidance systems.

CONTROL MATRIX. When compositesystems are used, components of eachsystem must be carried in the missile.Obviously, the sections must be separated sothere is no interaction between them and yetbe located close to the circuits they are to

converted to appropriate motion of theflight control surfaces. The Navy's surface-to-air missiles are beam riders. However,the system is also used in otherapplications. Missile components aredetailed in chapter 8 of this text.

6D5. Homing guidance

Missile trajectories are divided into three

phases (1) launch, (2) midcourse, and (3)terminal. Homing guidance is especiallysuitable for use during the terminal phase.

At a predetermined distance from thetarget, the homing guidance system takesover control of the missile flight surfacesfrom the system used during theintermediate phase of guidance. Thepurpose of the homing system

be located close to the circuits they are tocontrol. In addition, some provision must bemade to switch from one guidance system tothe other.

Control of the missile guidance system maycome from more than one source. A signal isset up to designate when one phase ofguidance is over and the next phase begins.This signal may come from a tape, anelectronic timing device, or from a radio

command.

The device that switches control systems iscalled a control matrix. It automaticallytransfers the correct signal to the controlsystem regardless of conditions. If themidcourse guidance system should fail, thematrix switches in an auxiliary guidancesystem to hold the missile on course. Shouldthe original midcourse guidance systembecome active again, the matrix will switchcontrol from the auxiliary back to the primarysystem.

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The matrix can be considered as anautomatic guidance switchboard, orguidance sequence computer.

VELOCITY-DAMPING DOPPLERRADAR. One homing guidance systemmakes use of doppler principles. No doubtyou have listened to a train whistle whilethe train was approaching rapidly. As thetrain moved past you, its whistle appearedto change in pitch (frequency). This samedoppler effect is present in radar waveswhen there is relative motion between thetarget and the antenna of the radar.

Doppler homing equipment can be dividedinto two groups-FM-CW doppler systems,and pulse doppler systems. There are majordifferences in the circuitry of the twosystems. In the FM-CW system, thefrequency of an echo signal has arelationship to the speed of the target withrespect to the receiving antenna. This echosignal can be converted into an indication oftarget velocity with respect to the missile.

The difference between the frequency of

the transmitted signal and the frequency ofthe echo is due to the doppler effect. When

automatic frequency control (AFC), in themissile receiver, maintains the receiver on theselected target. At the closest approach pointto the target, the doppler shift becomes zerobecause there is then no relative motionbetween the missile and the target. The zeroshift can be used to detonate a missile anddestroy a target that would otherwise bemissed. This system does not provide ameans of range measurement. If this featureis desired, additional circuits are required.

A pulse doppler system performs the samefunctions as an FM-CW system and, inaddition, can select a target by its range. Likeother pulsed radar systems, it has greateroperating range for a given average poweroutput than a CW system.

The guidance control signals are sent as aseries of timed pulses. The receiving systemin the missile must contain circuits that willmatch the transmitted pulses in both pulsetiming and r-f cycles. The matching isaccomplished in electronic circuitry known asthe coherent pulse doppler system. In thissystem the transmissions are short pulses at arepetition frequency that can be continuouslyvaried. Low-intensity power, which is used to

the echo is due to the doppler effect. Whenthe two signals are mixed in an electroniccircuit, the circuit will develop a "beat"frequency equal to the difference betweenthe two signal frequencies. The beat notedeveloped in this manner will have a pitchthat is proportional to the relative velocitybetween the target and the radar antenna.To eliminate the possibility of homing onobjects other than the target, a band-passfilter (which will pass only a narrow bandof frequencies) is inserted in the controlcircuit to eliminate interfering signals.

A receiver which is automatically tunedover the frequency range passed by thefilter is used to choose and lock on a target.An

varied. Low-intensity power, which is used toobtain phase coherence between successivepulses, is generated by the stabilized localoscillator. A duplexer provides low-impedance paths to keep the oscillator energyin the desired circuits.

The stabilized oscillator also provides asuitable local oscillator signal which is mixedwith the receiver signals to generate a receiverintermediate frequency. The doppler receivercontains a type of filter, called a velocity gate,which filters out all undesired dopplerfrequencies.

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