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Tactical Missile Guidance and ControlNotes

Contents

1 Introduction 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Autopilot Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Subsystem Interrelationships . . . . . . . . . . . . . . . . . . . . . . . . . Aerodynamic Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Autopilot Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . Integrated Guidance and Control . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 1 2 2 3 4 4 5 6 6 6 6 7 8 10 11 11

2 Missile Instruments 2.1 2.2 2.3 2.4 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gyroscopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Types of Gyroscopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanical Gyroscopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.1 2.4.2 2.5 2.6 Free or Position gyros . . . . . . . . . . . . . . . . . . . . . . . . Rate or Constrained Gyros . . . . . . . . . . . . . . . . . . . . . .

Accelerometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Resolvers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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2.7 2.8 2.9

Altimeters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Current Trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

13 13 13 14 14 14 15 16 17 17 19 19 21 21

3 Missile Servos or Actuators 3.1 3.2 3.3 Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Types of actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pneumatic Actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 3.3.2 3.4 3.5 3.6 3.7 3.8 Stored Cold Gas Servos . . . . . . . . . . . . . . . . . . . . . . .

Hot Gas Servos . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Ram Air Servos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydraulic Actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electro-Mechanical Servos . . . . . . . . . . . . . . . . . . . . . . . . . . Recent Developments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Chapter 1 Introduction1.1 Overview

Aerodynamics is part of the missiles airframe subsystem, the other major parts being propulsion and structure. In a broader sense, it is closely related to the autopilot and controls that, in turn, form a part of the overall guidance loop.Thus in discussing aerodynamic considerations for autopilot design we follow the steps given below: (a) First examine the interrelationships between the subsystems closely connected with both aerodynamics and the autopilot. (b) Then delineate the design requirements for autopilots as they are aected by aerodynamic input. These requirements are, in turn, aected by the choice of steering policy chosen for the particular missile and its mission. The steering policy may be dictated by the type of airframe conguration and propulsion system chosen for the mission. (c) Having developed an appreciation of the functions of aerodynamics and autopilot in the overall system, then proceed to describe how the autopilot designer and aerodynamicist work together in developing their subsystems to meet the design goals of the missile system. Both start with simplied equations of motion and generally add additional terms until the nal design is checked out with the standard Euler equations of motion.

1

Recently, ballistic missiles, in particular, Intercontinental ballistic missile (ICBM), with their high speed and high maneuverability, have posed great challenges for traditional intercepting techniques and methods. Despite the promising prospect of a new method, head-pursuit (HP) guidance method, there still exist problems that need to be solved.

1.2

Autopilot Design

A typical approach to autopilot design is as given below:(a) Starting with a preliminary autopilot design with the three rotational channels (pitch, yaw and roll) uncoupled. The aerodynamics also neglects coupling at this stage. (b) The development then progresses to an investigation of the coupled channels, which requires aerodynamic data describing both stability and control couplings. (c) At a later stage in the design process, the aerodynamic input to the autopilot may be modied to include special requirements associated with the particular missile design being developed, such as aeroelastic and o-design propulsion eects on aerodynamics. (d) The nal tune-up of the autopilot can best be made with a six-degree-of-freedom missile system simulation that requires a three-dimensional description of the aerodynamics of the conguration.

1.3

Subsystem Interrelationships

The aerodynamicist should be aware of the ways in which other subsystems are affected by the aerodynamics of the proposed conguration. The most closely related subsystems are as listed below: (a) Guidance Subsystem (Instructions to autopilot : Sensors, Computer, Filters). This measures the error between the missiles actual and desired courses, computes the corrections necessary to reduce or null the error according to a chosen guidance 2

law and gives commands to the autopilot to activate the controls to achieve the corrections. The commands may be for lateral accelerations, angular rates etc. (b) Depending on the mission, the guidance subsystem may make some demands on the aerodynamic characteristics of the missile. For example, (i) If the mission calls for a long duration cruise phase, it may be desirable to have a statically stable aerodynamic conguration to minimize demands on the control system. (ii) On the other hand, if the mission is short range requiring large acceleration to attack a maneuverable target, neutral stability is preferred. (c) Autopilot Subsystem (Instructions to Controls : Accelerometers, Gyroscopes, Filters, Ampliers). The AP receives the guidance commands and processes them into commands to the controls such as deections or rates of deection of control surfaces or jet controls through action of servomechanisms. (i) To provide the deection at a desired rate, the servomechanism motors must contend with the inertia of the control device and the torque about its shaft. In any case the maximum value of resulting hinge moments determine the size of the servo actuators so that reducing this maximum value is important. (d) Controls Subsystem (Action to maneuver airframe : Aero surfaces, jet controls, Control Servos) (e) Airframe Subsystem (Response to control action : Aerodynamics, Propulsion, Structure)

1.4

Aerodynamic Control

Three basic types of aerodynamic control are in use namely: (a) Canard Control (small surfaces forward on the body), (b) Wing Control (main lifting surfaces near the body center of gravity), and (c) Tail Control (small surfaces far aft on the body).

3

The tail steering controls initially give an acceleration in a direction opposite to the intended direction. For example if an upward maneuver is desired, the aft control produces a downward force in order to turn the missile body to a positive angle of attack that will develop a resultant upward force. This is an important characteristic of non-minimum phase systems as will be discussed later. The canard control has a force in the direction as does the wing control, which generally produces most of the resultant force, with the body usually at a relatively low angle of attack.

1.5

Autopilot Requirements

Three of the principal requirements of a good autopilot are quick response with minimum acceleration error, stability, and robustness. (a) When the autopilot calls for a control deection to achieve a lateral maneuver, it takes time to move the surface into the called-for position against the possible resisting aerodynamic torque and the inertia of the surface. (b) It also takes time for the control moment to move the missile to the required angle of attack for the maneuver. (c) These are the factors that aect the capability of the autopilot to achieve a quick response. (d) The time that it takes for the airframe to achieve 63 percentage of its called-for maneuver is generally referred to as the eective rst-order time constant of the autopilot-airframe subsystems. This response represents the transient response to a step input in control

1.6

Integrated Guidance and Control

Integrated guidance-control systems have the potential to improve missile system performance by taking advantage of the synergism existing between subsystems. These systems allow the designer to impose unusual performance requirements on the missile. 4

Such requirements may arise out of the new sensor and warhead technologies that may require complex maneuvers at target interception.

1.7

Conclusion

5

Chapter 2 Missile Instruments2.1 Introduction

While the missile is moving in space, forces and moments produce accelerations and hence velocities and displacements, with respect to the earth or any other reference frames. Hence the missile control system needs to measure accelerations, velocities and displacements in space. Conventional potentiometers and tacho generators cannot do these measurements. Gyroscopes or gyros, accelerometers are generally used as sensors in short range and medium range missiles. Long-range missiles use GPS, INS or GPS/INS as sensors or navigational aids.

2.2

Gy

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