skylab attitude and pointing control system

8/8/2019 Skylab Attitude and Pointing Control System

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N A S A T E C H N I C A L N O T E

SKYLAB ATTITUDE ANDPOINTING CONTROL SYSTEMby W. B . Chzlbb and S. M . SeltzerGeorge C. Marshall Space Flight CenterMarshall Space Flight Center, Ala. 35812

N A T I O N A L A E R O N AU T I CS A N D S PA CE A D M I N I S T R A T I O N W A S H I N G T O N , 0. C. FEBRUARY 1971


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TECH LIBRARY KAFB,NM

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- ___. 2. G O V E R N M N T A C C E SS I ON N O.I-,I. REPORT NO.NASA TN D-6068- - _1. T I T L E A N D S U B T I TL ES k y la b A t t it u d e a n d P o i n t i n g C o n t r o l S y s t e m

21. NO. OF PAGES 22. P R I C E *1 6 $3 .oo

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__-7. A U T H O R ( S )- W.. -B. Chubb and S. M . S e l t z e r

3. PERFORMING ORGANIZATION NAME AND ADDRESSG e o r g e C. M a r s h a l l S p a c e F l ig h t C e n t e rM a r s h a l l S p a c e F l i g h t C e n t e r , A l a b a m a 3 58 1 2

- -- . . _2. SPONSORING AGENCY NAME AN0 ADORES5N a t io n a l A e r o n a u t i c s a n d S p a c e A d m i n i s t r a t i o nW a s h i n g t o n , D. C. 20546

..

10132813j. KtLIPlbNl'b LAIALOb NO.5 . REPORT DATEFebruary 1971ILII

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10 . WORK UNIT NO. 908 52 10 0000M211 965 21 00 0000I' . CONTRACT OR GRANT NO.

L13. TYPE OF R EPOR Y & PERIOD COVERED

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- . - - I5. SU PPL EM EN T A R Y N OT ESP r e p a r e d b y: A s t r i o n i c s L a b o r a t o r y

S c i e n c e a n d E n g i n e e ri n g D i r e c t o r a t e~-6. ABSTRACTN A S A ' s M a r s h a l l S p a c e F l i gh t C e n t e r is d e v e lo p i n g a n e a r t h - o r b i t i n g m a n n e d s p a c es t a t i o n c a l l e d Sk y l ab . T h e p u r p o s e of S ky l a b is t o p e r f o r m s c i e n t i f i c e x p e r i m e n t s i ns o l a r a s t r o n o m y a n d e a r t h r e s o u r c e s a n d to s t u d y b i o ph y s ic a l a n d ph y s i c a l p r o p e r t i e si n a z e r o g r a v i t y e n v i r o n m e n t . T h e a t t it u d e a n d p o in t in g c o n t r o l s y s t e m r e q u i r e m e n t sa r e d i c t at e d by o n b o a r d e x p e r i m e n t s .a n d p o in t in g c o nt r o l s y s t e m a r e p r e s e n t e d .

T h e s e r e q u i r e m e n t s a n d t h e r e s u l t i n g a t ti t u de

S p a c e s t a t i o nC o n tr o l m o m e n t g y r oA t t i t ud e c o n t r o l

18.- 0 1ST R inUT O N S m T E M e N TU n c l a s s i f i e d - U n l i m i t e d

Fo r Sale by the Nation al Technical Inf orm ation Service, Springfield, Virginia 22151


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LIST OF ILLUSTRATIONSFigure Title PageI Sky1ab.A . . . . . . . . . . . . . . . . . . . . . . . . . . 22. CMGCluster . . . . . . . . . . . . . . . . . . . . . . . . 43. Momentum Vector Configuration . . . . . . . . . . . . . . . 54. Distribution Law Principle . . . . . . . . . . . . . . . . . . 55. Combined Disturbance Impulse . . . . . . . . . . . . . . . . 66. TACS Thrusters . . . . . . . . . . . . . . . . . . . . . . . 77. Functional Block Diagram of the Attitude and PointingControl System . . . . . . . . . . . . . . . . . . . . . . . 78. Phase Plane Diagram (Nested Configuration) . . . . . . . . . . 89. Z-Local Vertic al Maneuver in CMG/TACS Nes ted Configuration . 8

I O Experiment Pointing System (EPS) . . . . . . . . . . . . . . 9I 1 Earth Reso urces Maneuver Sequence . . . . . . . . . . . . . 0

LIST OF TABLESTable Title Page

I. EPS Control System Requirem ents . . . . . . . . . . . . . . 311. CMG Control System Requirements . . . . . . . . . . . . . . 3

III. Physical Characteristics of Skylab-A . . . . . . . . . . . . . 4

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DEFINITION OF SYIVIBOM-H

KFSKmK~~KCY

S

' S

momentum vectortotal momentum vectormomentum vectors associated with each of the CMG'sprincipal moments of inertia about the X, Y, and Zaxes, respectivelyfine sun ensor gaintorquer gainra te gyro gainamplifier gainLaplace transformcommanded torquedisturbance torquevehicle axesangle between or bit normal and the jth momentum vector gjgimbal r at e about inner axis fo r jth CMGgimbal r at e about outer axis fo r jth CMGra te gyro dumping ratiofine sun sensor r ise t imeamplifier rise timera te gyro natural frequency


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SKYLAB ATTITUDE AND POINTING CONTROL SYSTEM

SUMMARYNASA's Marshall Space Flight Center is developing an earth-orbi ting manned sp acestati on call ed Skylab. The purp ose of Skylab is to perform scientific experiments insolar astronomy and earth res ources and to study biophysical and physical pro perti es i na zero gravity environment. The attitude and pointing control system requirements aredictated by onboard experi ments.pointing control s yst em are presented.These requirement s and the resulting attitude and

I. INTRODUCTIONThc Skylab-AI - 3 i s to be an experimental spacestation dcvcloped by the National Aeronautics andSpnce Administration. The Skylab prog ram is anestcnsion of the Mercury-Gemini-Apollo mannedspace flight programs and makes extensive u s e ofhardware and technology developed in those programs.The purpose of the program i s to increase our knowl-edge of manned spac e flight and to accomplish selectedscien tifi c, technological, and physiological investiga-tions. The objectives of the prog ram are :

(1) To conduct solar astronomy scientific ex-periments, emphasizing observations that cannot beobtained from earth b ecause of abso rption of certainelectromagn etic waves by the at mosphere.(2) To conduct earth r esource s experimentsdesigned to permit an evaluation of existing technology

and how it may be applied to aid in the solution of eco-logical probl ems. The need for furt her development ofsen so r technology will also be determined.(3) To conduct biomedical experiments to deter-mine the effect of long duration space flight on the crew.

The Skylab i s being developed under the overal l pro-gram responsibility of NASA's Office of Manned Space-flight. The Mars hall Space Flight Cent er, Hunt sville,Alabama, has program management respons ibility fordeveloping a l l Skylab hardware except the Command andServ ice Module (CSM), for providing the launch vehicles,fo r flight evaluation, and fo r overall s yst ems engineer-ing tp ens ure compatibility and integration of all hard-ware. NASA's Manned Spacecra ft Cen ter in Houston,Texas, is responsib le fo r modifying the CSM, for de-veloping the sp acecra ft launch adapter (SLA), or de-veloping the experiments and crew support equipment,fo r mission analysis and evaluation, and for astronautcrew training. The Kennedy Space Center in Florida

will provide the launch facilities and execute thelaunches.indicated below:Major contr actor support fo r Skylab-A is

(1) Satu rn launch vehicles. Boeing Corp .,Chr ysl er Corp., and McDonnell-Douglas Corp.(2) Skylab-A

(a) Martin Mari etta Corp. : clust er pay-load integratio n and management(b) McDonnell-Douglas Corp. : orbitalworkshop and airlock module(c) IBM: Apollo Telescope Mount (ATM)digital computers (primary and secondary) and work-shop computer interface unit(d) Bendix Corp. : control moment

gyroscop es and electro nics and ex periment pointingelectronic assembly(e) North Americ an Rockwell Corp. :modifications to the CSM

2. SKYLAB-A DESCRIPTIONThe Skylab-A con sis ts prim ari ly of modules developedunder the Apollo program and selected to meet theoperational requirements of the Skylab-A program.The modules will be launched aboard S aturn launchvehicles, placed in an earth orbit, and assembled there.The Skylab-A (Fig. 1) consists of the following modules:Orbital Workshop (OWS). The OWS is a modified emptyS-IVB stage that will provide the astronauts with livingquarter s while in orbit. It will contain necessary f d -preparation and waste-management facilities to supportathree-man crew for the planned manned missions.


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ATM SOLAR ARRAY TACS ENGIN E POD

WORKSHOPSOLARI N S T R U M E N T U N I T

AIRLQCK MODULEMUL T IP L E DOCKING ADAP T E R

e- O M M A N D M O D U L EFIGURE 1.

Certain experiments and supporting facilities for theiroperation also will be contained in the OWS (se e Appen-dix). Attitude contro l command s will be implementedby a cold-gas thr ust er attitude control sy ste m (TACS)located on the OWS. Two sol ar arr ay panels attachedto the sides of the OWS will provide some of the powerrequired by the Skylab-A. The OWS will rem ain inorbit in unmanned storage modes and will be reactivatedduring subsequent manned rev isits.Multiple Docking Adapter (MDAL ang Airl ock Module (Ae$).The MD A a n d v d o c k i n p . o r t sor the CSR.1 andprovide a pressurized passagewaybetween the CSM andthe living quarters in the OWS. The MDA contains theATM control and disp lay (C&D) panel. Expe rime ntshoused in these two modules are described in theAppcndix; most of the experiments ar e included in th eEarth Resources Experiment Package (EREP).-Moun (ATM). The ATM is to be asolar observatory developed primarily t o collectdata on so la r phenomena to increase man's knowledgeof the sol ar environment. It provides a mountingstructure, or rack, to which ar e attached the controlmoment gyroscopes (CMG's), attitude control comput ersand sen sor s, and associated electronics. Within therack 811 experiment sp ar is located to which are attachedsolar experiments and spar control sensors . The spa ris gimbaled about two axes with respect to the rack.The ATM provides a capability for film retrieval andinstallatton by astro naut extravehicul ar activ ity (EVA).Solar Arrays~. Two solar a rray panels are mounted on

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SKYLAB-A

each side of the OW S and four panels are attached tothe ATM. Their purpose is to provide power for theSkylab-A sys tem. The averag e power output of thesecombined arr ay s is approximately 7.2 kW.Saturn V Instrument Unit gv). The I U is used onlyduring launch and fo r the subsequent seven and one-half hours of orbi tal operation while the CMG's are be-ing brought up to operational speed (i49rou/a). Oncein orbit, it provides sequencing commands to actuateand control the deployment of the Skylab-A elements. Italso provides an IU digital command system and telem-etr y link with the ground.Command and Sey:ce Module (CSM). The CSM providesthe crew with trans port ation between the ea rth and theSkylab-A. It also contains food, water , and othe ressentials for cre w su pport when the CSM is detachedfrom the Skylab-A. The CSM provides some of thecommunication, instrument ation, and therm al controlcapability. It ha s the capa bili ty of providing attitudecontrol to counteract transient responses to dockingmaneuvers through use of its reaction control system(RCS) which utili zes hypergolic s torab le propellan ts.Eqer iments . The experiments are one of the p n m awrea son s fo r development of the Skylab-A and dic tat ethe attitude and pointing control requir ements. Adescription of these experiments and their locationsare provided in the Appendix.

3. MISSION IMPLEMENTATIONThe unmanned Skylab-A, less the CSM, is placed in a


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near-circular 435-km orbit with a nominal inclinationof 0.87 rad by a two-stage Saturn V launch vehicle.Within the seven and one-half hour s of IU life time , theSkylab-A ATM rack is oriented toward the sun, and thesol ar panels a re deployed.then pressurized to make it habitable for the crew.Approximately one day la te r the CSM, carr yin g a three-man crew, is placed into a temp orar y 150 x 222 lanorbit by a Saturn IB launch vehicle. Using its ownpropulsion system, the CSM achieves a rendezvous withthe rema inder of the Skylab-A and docks to an axialpo rt of the-MDA. It is planned f or the crew to remainonboard the Skylab-A for 28 days to conduct experiments.They then will pre par e the Skylab-A f or orbital sto rag eand return to ear th i n the CSM on the 29th day.subsequent launches, similar to the secod launch, areplanned.56 days ar e anticipated. The fir st two flights ar eplanned for the last quarter in 1972.

The Skylab-A int eri or i s

TwoManned mission durations of no greater than

4. SYSTEM REQUIREMENTSThe Skylab-A attitude and pointing contr ol syst em hasbeen developed to meet the high accuracy requir ementsestablish ed by the desir ed ex periment conditions.conditions must be maintained by the control sy stemunder the influence of extern al and internal distu rbancetorques, such as gravity gradient and aerodynamic dis-tur bances and onboard astron.aut motion. The Skylab-Aattitude control system provides a stable base aboutwhich the ATM spar attitude control system may re-spond to meet higher frequency attitude control systemdemands. Hence, a fine pointing sy ste m, i. e., theexperiment pointing system (EPS), for directi ng theexperimen t package on the ATM s pa r has evolved tomeet the stringent experiment pointing requirements.The design requirements for each of the sys tems arelisted in Tables I and II. Roll is defined as the angular

These

TABLE I. EPS CONTROL SYSTEM REQUIREMENTS

SystemAxisEPS X pitch)EP S Y (yaw)EP S 2 (roll)

CommandPointingUncertaintya i . i x i0 rad* i . i x i0 -5radk2.9 x iO-rad Itability for15 Min*i . i x i0-5rad*i . i x i0 -5radCMG controliystem

TABLE II. CMG CONTROL SYSTEM REQUIREMENTS

SystemAxisCMG X @itch)CMG Y &aw)CMG Z (roll)

CommandPointingUncertaintyii.2 x i0-3rad*i.2 x iO-radi2.s ~ o - ~ r a d

Itability for15 Min*2 . 6 x iO-rad*2.6 x i0-3rad

rotation about the line of sight (LOS) from the experi-ment package to the cent er of the sun, and pitch andyaw are defined as angular deviations of the experiment

package with respect to the LOS.fo r the 2-local verti cal (2-LV) mode of operatio n (e.g.,during cnrth resources experiments) are the same asshown in Tab le II except that a navigation error ofa3.5 x io- rad is acceptable.

The requirements

5. CONTROL SYSTEMS DESIGN PHILOSOPHYThe ATM pointing and control s ystem that has evolvedto dat e has be en influenced by a n umber of factor^.^The prime requirement is to meet the high accuracysys tem pointing specifica tions in the presence of exter-nal dist urba nce torqu es. The significant disturb anceTorques of intere st are those caused by earth-orbitalenviro nmental influences (gravity gradient and aero-dynamic disturbances) as well as internal movements ofthe astro naut s on board. Because of these earth -orb italenviron mental influences, the vehicle attitud e mus t beheld to a fixed position relative to the orbital plane. Tomeet the pitch and yaw pointing accu racie s, a two-axisgimbaled EPS with a maximum range of *3.5 x IO- r ad isrequired. The pr ima ry requirement for the EPS is toprovide experiment package isolation from the relativelylar ge vehicle perturbations that can resu lt because ofastronaut motion effects.The CMG control sys tem of the rack was chos en pri-marily because of performance benefits with respect toboth dynamic resp ons e and compensation of cyc lic ex-ternal disturban ce torques caused by gravity gradientand aerodynamic effects. Most passive control schemes(gravit y gradient , fo r example) would not have the re-quir ed accu racy and could not develop sufficient torqueto meet the dynamic performance requirements. Duringdata gathering i ntervals when experiment optics are ex-posed, use of CMGs prevents op tics contamination thatwould result from reaction control th ruste r exhaust.The TACS (and he RCS, if necessary) i s available toprovid e co ar se attitude control and CMG momentum de-satu ratio n capability (if needed).sys tem must b e c&pable of maneuvering the vehicle to adesired experiment observation orientation; i. e. , 2-LVand sol ar inertial. It was also necessary to meet themaneu ver req uire ment s of the vehicle using the CMGsystem as much as possib le to minimize TACS propel-lant consumption.Contro l of Skylab-A as differentiated from contro l ofthe experim ent spa r mounted on Skylab-A may be accom-plis hed by the CMGs alone, the CMGs in conjunctionwith the TACS (nested sy stem) , or the TACS alone. Inaddition, i n an emergency situation and as a back-up forZ-LV m aneu vers, the CSM reaction control syst em maybe utilized.Major control system design philosophy considerationswere:

The TACS and CMG

(1) To use the CMG system to meet vehiclecontrol and maneuver requirements whenever possible.(2) To minimize the action of noncyclic torq ues@ias torqu es) upon the vehicle. Thes e torq ues ar is efrom two sources. The fir st is that the major princi-pal moments of inertia of the vehicle are not identicaland the inter action of the gravity field upon these in-ertias produces a bias momentum accumulation of

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approximately 600 Nms about the vehicle X axis ove r anorbital period. The second is a res ult of vehicle vent-ing of waste gases and accounts for a bias momentumaccumulation of from 500 to 1000 NmS about a non-specified vehicle axi s.(3) When in the so la r inertial mode, to hold thataxis (nearly the X principle ) about which b ias momen-tum accum'ulation is minimum in the orbit plane. This

significantly reduces g ravity gradie nt bias torques.

syst em. The momentum exchange devices are threeorthogonally-mounted double-gimbaled CMG's; eachhas a sto red momentum capability of 2700. Nms. ?$eCMG cluster is shown i n Figure 2 where 6,g)and 6,~)are the gimbal rates about the inner and ou r axes,respectively, for the jt h CMG; j - 1,2,3. The Skylab-A(Fig. 1)principal momenta of inertia and mass data arepresented in Table m.TABLE III. PHYSICAL CHARACTERISTICS OF SKYLAB-

5( = 0.8862 x 10'kg m24) Consistent with the longmission lifetime of 240days, td eliminate all single point failures in the design.(5) To provide maximum sys tem o perating flexi-bility in regard to sensor and computer selection.(6) To provide automatic a le rt and caution andwarning signals to the astr onau ts and the ground con-trollers of abnormal system operation.(7) To provide through digital compu ter softw arethe capability of test ing and switching out malfunction-ing equipment and switching i n backup equipment.

= 0.5835 X 107kg - m2= 0.5753 x 107kg m2

'Y

1Mass = 0.830 x 1o5kgOverall Length = 36.1 mS-IVB Diameter = 6.58 m

6. CONTROL SYSTEM DESCRIPTIONThe attitude and pointing control system (APCS) con-sists of the three basic sys tems : the CMG, the TACS,and the EPS. The f irst two syste ms may control theSkylab-A ei the r separat ely o r together in a nested con-figuration. The EPS ystem is used only fo r experi-ment s par control.The CMG system is a momentum exchange contro l

O U T E R GIMBAL \

The use of CMG's in the pointing and cont rol of a largemanned space station is new, and the prob lems asso ci-ated with that type of s yste m a r e unique and were un-solved. Some of the problems encountered in the devel-opment of the CMG control system include the following:(1)An acceptable control law for use of the CMG's(2) A mean s for preventing the CMGIs from "falling

in c ontr ol of th e Skylab-A.

OUTER G I M B A LT O R Q U E R

O U T E R G I M BA L C M G M O U N T I N G P L A N EFIGURE 2. CMG CLUSTER

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intoffan undesirable orientatio n such as that whichwould not allow the use of the CMGfs to control thespa cecr aft even though the CMG clu ste r was not in itasaturation momentum state.(3) A means of desatu rating the CMG clust erperiodically without the use of mass expulsion TACS.A method was needed to per mit torques produced by an

external force field such a s gravity gradient to be usedto effedt CMG momentum desaturati on.(4) The optimal orien tation of t he vehicle tominimiz e ex ternal bias torques which would tend tosaturate the CMG cluster; i. e., the placem ent of theminimum principal axis of inertia into the orbital plane.

An acceptable control law for use of the CMGIs in con-tr ol of the ATM clu st er was developed. It can concep-tually be broken down nto three parts: the steeringlaw, the distributio n law, and the rota tion law. A de-tailed deriva tion of these laws is given in Referen ce 6.The function of th e control law is to utilize three nor-malized torque commands together with the pres entorie ntat ion of the C M G I s (in term s of their directioncosines) to generate inner and outer gimbal rate com-mands on each of the three C MG 's . The steerin g lawgenerates gimbal rate commands in such a way that(assuming the actual gimbal rat es a re equal to thecommanded gimbal rates) the torques resulting on hevehicle ar e identical to the desired torques in directionand magnitude. Only when the maximum gimbal ratecapability is exceeded will themagnitude of the result-ing torque be less than commanded, but the directionwill still be that of the command.. No crosscoupling isinherent in the control law.Only thre e deg rees of freed om ar e utilized by thesteering law. The remaining three are used by theother two laws. Because the bulk of the C M G momen-tum change is along the orbi t normal, the distributionlaw trie s to make the components of the CM G vectorsalong the orbi t normal equal to each other. This hasthe effect of spre ading the vectors , which in turn re-duces the angu lar velocity requi red of the vectors tomeet the needed momentum change. It also prevent sth e C M G % from falling into the undesi rable anti-parallel orientation illustrated in Figure 3. Thedistribution is made by rotations about vector s ums ofpairs of individual C M G momentum vectors and thusdoes not affect the total momentum; i . e. , no disturb-ance torques a re transmitted to the vehicle as a resultof application of the distri bution law. Figu re 4 is asimplified diagram illustrating this principle. No dis-tribution is nece ssar y for two-CMG operation.The rota tion law utilizes only rotations about pai r sumsand total angul ar momentum is not disturbed (notorque on the vehicle). The angular velocities for therotationsare generated such that the larg est gimbalangles are reduced, thus avoiding (asmuch as possible)httting of the gimbal stops. The gimbal angles areweighted by the ir fifth power suc h that the distributionlaw ia predominant for small gimbal auglea aad therotation law is predominant fo r large gimbal aoglea.For w o C M G operation (nodistribution law) he f i r s tpower of the tuglea iswed.

fl3 bI ANTI- PAR ALL EL CONDITION

WIT H DIS T RIBUT ION L AWFIGURE 3. MOMENTUM VECTOR CONFIGURATION

ORBIT NORMAL

ROTATE iilAND k2 A B O U T 5+E2UNTILE OUAL COMP ONE NT S OF i?l AN D i iz LIEAL ONG THE O R B I T N O R M A L tu. , =a2 )

FIGURE 4. DISTRIBUTION LAW PRINCIPLE

To desaturate the CMG clust er periodically, withoutthe use of the mas s expulsion TACS, a method wasdeveloped utilizing the earth' s gravity-gradient forcefield to effect CMG momentum d esatura tion. Thismethod is described in detail in Reference 6. Thebasic concept is bes t desc ribed with the aid of F igure5 which depicts the pe r or bit momentum build-up ofthe CMG cluster caused by gravity gradient andaerodynamic torques. This figure reveals that ifperiodic C M G desa turat ion was not provided, th e CMGclus ter would be satura ted for progressiv ely larg erportions of an orbit a fter the first orbit. The axis ofsatu ratio n would be roughly the vehicle X axis. Thismeans that after complete saturation, the CMG clustercould not compensate f or a disturbance torque about theaxis of saturation.An pvestigation of the causes of the predominant non-cyclic torques (i. e. , gravity gradi ent and aerodynamic)

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-600- 800Y(Nms)

6000

MIDNIGHT

-6000-8000Z(Nmr)

2134 201400 0

41 NOON-4 000- 6000 4MIDNIGHT-80001

TOTAL MAGNITUDE (Nms)3CMG CAPABIL IT Y 8100 Nm r8ooo+- ------ - - - -- - --h/\m

20000 $. +NOON MIDNIGHT

FIGURE 5. COMBINED DISTURBANCE IMPULSEreve als that with the given vehicle configuration andmission requirements (i. e. , point the vehicle 2 axisat radiometric center of sun every daylight period), itis impossible to eliminate the noncyclic torques, butit is possible to minimize them.This problem of CJIG momcntum management wasattacked in two scp:irate !va!.s.

(1) 1 e noncyclic disturbance torques were mini-mized by finding nn optimal vehicle orienta tion while st illmeeting the requirement that the vehicle 2 axis point to

the center of the sola r disk.sampling the vehicle monientum at specified times duringthe daylight orbital perio d nnd comparing it with the pre-vious days samples. The compared samples indicatedwhethe r the bias momentum components about the var-ious vehicle axes wer e increasing o r decreasing. Thisinformation was then translated into appropriate angleposition comm ands about the vehicle 2 axis to ensureminimiz ation of bias momentum accumulation.

This was accomplished by

(2) The satur atio n effects of the remainin g non-cycl ic disturbance torque we re nullified by periodicallyprod uci ng controlled bias tor que s which would tend todesa turate the CMG cluster. The controlled.hias tor-ques are produced by employing rectified componentsof the gravity gradient torques encountered during thenight portion of the orbit to desaturate the C M G cluster.The rectification of the gravity gradient torques is madepossib le by maneuve ring the vehicle about two axes dur-ing the night si de of the orbit.maneuver angles is a function of the momentum accumu-lation duri ng the daylight portion of the orbit.The TACS is composed of six cold-gas thr ust ers andthe necess ary logic to select and fir e the prope r thruster.The thrusters are mounted as shown in Figure 6.The thruster force is dependent upon the cold-gas tankpre ss ure and will be approximately 45 kg at the be-ginning of the mis sio n and dimi nish to about 4-1/2 kga t the end of the mission . The minimum impulse bitwill be maintained at a constan t level within th e houndsof the selecta ble thrust er firing time (40 to 400 m s).A functional block diagram of the CMG system andTACS is shown in Figure 7. System information i s -available from two sources, the acquisi t ion sun senso rsor the strapdown equations which utilize rate gyro in-formation as thei r basi c input. Sens ors as well as thebasic digital computer are redundant to provide increasedsys tem reliability ove r the 240-day miss ion. Rate in-formation is provided by r ate gyros. The gyros have acoarse scale of *ti .7 x io- rad/s and a fine sca le of *i 7 x ior a d s . T he s c al e is selected by logic within the operatingdigital computer.Thc ATR? digital compiitcr (ATAIDC) is the primary dataprocessing, compuhitionnl, and logic generation facilityin the control syste m. Fas t loop computations (e. g. ,rat e gyro proccssin g and CMG control laws) ar c pcr-formed at n ra te of five tim es pe r second, and slow loopcomputations (c.g. , orbital navigation) are performed ata rntc of onc tinic pcr second. The computer memorycapacity is lci,300 word s with length s of 16 bits. Presentestimates of computcr memory requirements are approx-imntcly 15,500 words. The ATMDC is the bra ins ofthe syste m and per for ms the following pri mar y APCSfunctions.

The magnitude of the

(1) Orbital navigation and timing(2) Control of operatio nal modes(3) Maneuver generation commands(4 ) CMG and TACS contr ol(5) CMG momentum desaturation commands(6) System redundancy management (te st and sele c-tion of alternate vehicle components in the case of off-nominal component or system operation).

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+ X 4-

THRUSTER1 ,THRUSTER 2

THRUSTER 3

DETAIL 0

+ Y

DE

I IIIIIIIIIIII SENSOR 12)I N EST EDC O N T R O LSYSTEM r--1

THRUSTER IF I R I N G IC O M M A N D Sin--@I

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',I U /' U LVD CI

I I READOUT ASSY. 1 Ip C O N T R O L SU I SYST EM- - - - - - - -------I C M G EA : C M G EL EC T R O N I C A SSEMB LY

C W G I A C M Q I N V E R TE R A SS EM BL YI T C S A THRUSTER CONTROL SWITCHING ASSEMBLYL V D C . L A U N C H I N G VEHICLE DIGITAL COMPUTER

IIIIIIIIIII

FIGURE 7 . FUNCTI ONAL BLOCK DI AGRAM O F THE ATTI TUD E AND POI NTI NG CONTROL SYSTEM7


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CMG+TACSF UL L T O RWE REG IO N\1ACS NO FIR E REGION

--. '\ ATTITUDE ERRORCMG+TACSFULL TORQUE REGION

CM G ZERO TORQUEREGION ( SHADED )

FIGUEW 8. PHASE PLANE DIAGRAM (NESTED CONFIGURATION)While the CAIG syste m and TACS may operate independ-ently of each other, the nominal mode of operation is i na "nested" configuration. Figure 8 is a phas e plane dia-gram of thi s type of sy ste m operation. A11 cont rol isdelegated to the CMG sys tem a s long as i t has the capa-bility of maintaining the attitude and rat e er r o r withinthe "no fire region" of the pha se plane. If the atti tude orrat e e rr o r exceeds this region, the TACS supplies sup-plemental control au thority until th e vehicle attitude andrat e er ro rs a re again within the "no fi re region. ACMG inhibit region has been established on the phaseplane to prevent the situation where th e TACS and theCMG's would produce opposing torq ues. If the stateof the vehicle li es in both the CMG zero t orqu e regionand the TACS no fire region, the attitude rate (whichrehains constant in this zone) will cau se the attitudee r r o r to decrease until the vehicle state leaves thezone. The TACS, in addition to bounding the magni-tude of the rate and position err or s, also perform s aCMG momentum desaturation function in the %ested1'mode of operation. If the CMG total momentum vecto rexcceds 95% of the CMG capability, the digital computerdir ects those TACS engines to fir e which will mostefficieutly cause t he total momentum vector to fallbelow 95%. An example of this type of operation isillustrated in Figure 9.The EPS ope rates independently from the CMG systemand TACS. It has it s own sun sensors and rate gyrosfo r position and rate control. Control sign als ar e gen-erat ed in the experim ent pointing electronic assemb ly(EPEA), an analog device.The EPS utilizes flex-pivot* gimbal beari ngs f or controlabout two axes and an open-loop positioning dev ice to

2700J , I-2700 t I-5400 4 I

TACS- M P U L S E - !67.0

73.0 Na99.6 Nr8100

SRT: SU N R I SE T ER M I N A T O RS S T : SU N SET T ER M I N A T O R

FIGURE 9. Z-LOCAL VERTICAL MANEUVER INCMG/TACS NESTED CONFIGURATION

meet positioning requi rement s about the third axis. Thflex-pivots allow about *3.5 x I O - ' ra d of rotation of thand Y axes while the r oll positioning device allows forrotation of i2.i rad about the experiment package Zax i s . Figure 10 shows a block diagram of this sys tem.While the EPS provides automatic control of the experment package X and Y axes, manual positioning of the

*A flex-pivot gimbal bearing is mad e of a pa ir of flat cross- leaf spr ing s and is welded to and supported by rotatingsleeves. It has no backlash and provides limi ted angular travel.

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SOLARVECTOR *-B ISAME AS A B O V E

FIGURE 10. EXPERIMENT POINTING SYSTEM (EPS)two axes is provided fo r the purpose of offset pointing.Fine sun sens ors (FSS) ar e used for sensing spa r atti-tude crrors, with rat e gyros sensing spa r rates. TheEPEA conditions the sensor's signals to provide rateplus dlspl acenient command sig nals to the flex-pivotactuators (dc torque motors).The experimen t package can be offse t pointed in the Xand Y axes over a rang e of*5.8 x lO-'rad, with the cen terof the so lar disk being the z ero position.disk measures approximately*9.3 x i ~ - ~ r a dro m limb tolimb,optical wedge located in each FSS.mounted in the path of the sunlight passing through theFSS optics and can be rotated to re fra ct the sunlight afixed angle in a controlled direction. The wedges arepositioned by a drive mechanism controlled by theastronaut via the manual pointing contro ller. The wedgedrive varies from+6.5 x lo-' r a d s nea r ze r o of fsetpositions to 53 . I x+5.8 x lO-'rad sun offset position. A wedge offset producesan FSS output err or voltage that ca uses the s pa r torotate about the appropriate axis (X o r Y) and point theFSS, and thereby the experiment package, in a direc-tion that will driv e the FSS output voltage to null.Stability is then automatically maintained by the EPS.The experiments ar e aligned to the FSS.tion of each FSS wedge is displayed on the C&Dpanel and corresponds to the experiment package offsetposition fr om the cente r of the sun in the X or Y axis.The panel also contains televisio n disp lays of the sun,as viewed through expe rimen t tele scop es, and experi-ment readout displays to a ss is t the astronaut in pointingthe experiment package.The roll positioning mechanism-(RPM) is used to rotatethe sp ar about the 2 axis.manded by the astro naut via the manual pointing con-troller (rate switches) located on the control and displaypanel . Spar r ol l r a te s of i l .Zx lO- ', i 6 . i ~ l O - ~ , + i . 2 x i O - ~ ,and *6.1 x lo-' r a d s can be commanded. Once the sp ar is

The solarOffset pointing i s accompl ished by positioning anThe wedge i s

r a d s for wedge positions near the

The posi-

The mechanism is com-

positioned, the RPM will hold th e location until a repositio nincommand is received. The astronaut repositions thesp ar in accordance with experiment demand require-ments.panel. The spar roll position is displayed on the C&D

7. SYSTEM OPERATIONThe APCS may be required to oper ate in any one of sixmutually exclusive pri mzr y modes.configured to allow the APCS to perfo rm it s requ iredtasks.one of hree sources: (1) via switches on the flightcrew's C&D pane l, (2 ) by way of a n onboard digitaladdress system (DAS), and (3) from the ground-operated digital command system (DCS). Selectedmode changes are als o made within the digital com-puter as a function of s en so r information and com-ponent redundancy managem ent consider ations. Theprimar y operational modes are a s follows:Stand-By Mode : Jn his mode the ATMDC will not beproc essi ng output commands to the CMG syst em o r theTACS. This mode will be used if no vehicle control isreq uir ed of the APCS. With the exception of inhibitingthe comma nds to th e TACS and CMG syst em however,all normal ATMDC functions will be enabled. If thestand-by mode exis ts at the time of a computer switch-over, this mode will be maintained.Solar Incrtial Mode: In tlus mode the "ncsted" CMGsystem nnd TACS will be used for vehicle control. Thevehicle 2 nsis is pointed toward the center of the solardisc.but i s orientcd so that it i s cons istent with minimumbias momentum ncciimulation considerations and pointsin thc direction of thc carth a t the morning tcrminator.Thc vchiclc Y nsis forms a right-11:mdcd triad.ATMDC will obtain attitude information for inputs to thecontrol law as follows:

These modes areThe request for mode changes may come from

Thc vchiclc X Rsis l ies n early in the orbital plane

The

9


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(1) Orbital Day: The ATMDC will u s e theacquisition sun sensor forX and Y axes attitude infor-mation ana the strapdown computation for 2 axis atti-tude information. The ra te gyro outputs will be usedfo r rat e stabilization in all three axes.(2) Orbita l Night: The ATSZDC will use thestrapdown computatioj for X , Y, nd 2 axes attitudeinformation and the rate gyros for r ate stabilization.

Momentum management desaturation maneuvers will beperformed dhrin g the night periods. Automatic entryinto the sol ar ine rtial mode occur s when the A P C S is inthe ex perim ent pointing mode (day operation) and thecrew does not exit the mode manually prio r to orbitalsunset. This mode will al so be entere d at the time ofa computer switchover, unless the mode existing at thattime is the stand-by mode.Experiment Pointing M-odg:mode is identical to the day portion of the sol ar inertialmode, with respect to vehicle control.however, the EPS will be activated. Normally, theexperiment pointing mode will be ente red manually,with automatic exit at orbital sunset. Automatic entryis also provided as an option to be activated by thecre w via DAS command. The cre w will be able to en-able o r inhibit automatic experiment pointing modeentry at will. If the cr ew has enabled automatic entry,the ATMDC will activate the mode each orbital sunrise,if and only if the system is in the sol ar inertial mode thepreceding night period.

The experiment pointingIn this mode,

SOLAR

CMG Ncstcd Attitude Hold Mode: The vehicle will beunder ncsted CMG/TACS control in this mode and w i l lbe maintained in an ine rti al hold. The ATMDC will usethe strapdown computation fo r attitude information i nX, Y, nd Z, and the rate gyro outputs fo r rate stabili-zation. The momentum management maneuv ers will beinhibited in this mode. No provisions exist fo r automaentry to or exist from this mode. Manual vehicle attitucommands may be entered in this mode via DAS. Theatt itude i s not limited, but the maneuver rate is limitedto a5.2 x r ads .TACS Attitude Hold Mode: This mode will be an ine rtiahold mode, random attitude, with the TACS in control.A s in the at titud e hold mope, the ATMDC will obtainattitude and rate information from the strapdown compution and the rate gyros. No provisions e xist for automaentry to or exit from th is mode; and the attitude is notlimite d, but the maneuver ra te is limited to * 5 . 2 x iO-raZ-Local Vertical (Z-Lv) Mode: T his mode will beused f or e arth pointing exper iments during the mannedperiods and for rendezvous d uring the unmanned periodsThe vehicle will be un der CMG/TACS control in thi smode. The mode will have two sources of activation.For ear th resources e~ e r i m en t s , he crew wi ll com-mand the ATMDC via the C&D panel switch to en te rthe Z-LV mode. At that time the ATMDC should st ar tthe maneuver to Z-LV. The elapsed time from initia-tion of the maneuv er until the Z-LV attitude i s reachedwill be a stored value in the ATMDC, addressable bythe crew via the DAS. Figur e 11 illustrates a typical

VECTOR R EP OPER A T I ONOVER T A R GET

S E T UP E R E PEXPER I M EN T S

TERMINATORM A N EU VER

\ \\ \ \ \

REFERENCE WHEN SUNCOMES INTO VIEWCMG CONTROLM OM EN T U M M A N A GEM EN T

\---. E R E P : E A R T H R E S O U R C ESEX P E R IM E N T PACKAGEORBIT MIDNIGHT

FIGURE 1 1 . EARTH RESOURCES MANEUVER SEQUENCE10


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Z-LV earth resources maneuver sequence.dezvous. both the time fo r initiation and the time forcompletion of the Z-LV acquisition will be specified.In thin case the ground will command the mode change.Leaving the Z-LV mode will be similar; i t may becommanded ei the r by crew command (C&D switch)or ground (relative to time).Mode Options: Sev era l somewhat unrelat ed ''sub-modes" will be available, pr ima rily for the crew, foraltering certain operational aspects of the APCS.Again, the stat es of these submod es may be changedby cr ew command, ground command, or ATMDC pro-gram control. The submodes include inhibiting auto-matic gravity gradient desaturation maneuvers, in-hibiting CMG or TACS control, switching the navigationtiming sequence from a computational procedure to onebased on sun prcsc nt disc rcte s, and the capability ofpcrforniing a preprogramcd set o f maneuvers torcacquirc thc sol ar vector should the system suffera tcmpornry loss of atti tude inCormation.Ilybrid simulations of thc Skylab nttihitlc and pointingcontrol system a1 M:irshall Spnc'c Flight Ccntcr havebccn in operation Cor Llic 1):tsl csightcvn months. Ingcmc,rnl, simuI:iti*~l' c , s i i l t s h:tvc, I)c*cm n :Igrc.c~mcmlwith 1hcwrclic:tl i ) r c - i l i c . t c s t l rcLsiills. 111 Lliosr inst:inc'cswhcrc pix*dictctl :inti a c . t i i : i l rcsulls Ii:ivc tliffr~rctl, licsimul:ilion motlrls hnvc I)wn usrd :is dcs ibn 1001s tororrcct digiti1 logic to rliminntc or minimize thcsctlifl crcwcs . Onc :ircn whcrc inconsistent result swcrc nolrtl conccrnctl gimbal nnglc stops. Inncr gimlJal

For ren- CMG stops ar c locatcd at 1-1.4 rad and outer stops ape at+3.8 and -2.3 rad. These physical conatraint s Iimitedtotnl CMG momentum utilization whencvcr thc slopswcr c cncuuntcrcd. Special digital logic was dcvclopedto minimi zc the number of sit uati ons when C M G gimbalstops would be encountered.A documcnt' dcsc ribi ng in dctail thc ATM digi talcomputcr program rcquircmcnts has bccn prcparcd byNASA and rcccntly ul,tlatcdo fo r NASA by IBM. InclQdedin tlus document arc detailed rcquirrments and imple-mentation schenics in analytical form for functions suchas nionicnlum management, CMG rriomcnhun dist ribu tion ,CMG control laws, TACS logic, vehicle altitude reference(strapdown), and APS moclc contro l.

8. CONCLUSIONSBased on the mission and high accuracy experimentpointing requirements, the design of the Skylab-Aattitude and pointing control system has beendeveloped. A significant portion of the syst em i scapable of manual operation. enabling the astronautsto perfo rm functions requiring human judgment, suchas choosing scientific targets and selecting and point-ing appropriate experiments toward these tsrget s. Tokeep the number of tasks to be perfoinied by the astro-nauts within reason , many functions such as attitudepointing stability ar e perfo rmed using closed loop auto-matic control.

A P P E N D I XSKYLAB E X P E R I X I E N T S *

...q>crimcnts Locatcd in the Apollo Telescope MountWhite-Light Coronograph (S052): Use an extcrn allyocculted coronogrnph to monitor, in the 4000 to G O O 0 Arnngc, the brightnes s, form, and polarizatio n of thesolar corona from 1. 5 to G solar radii.X-Ray Spectrographic Telescope (S054): Recordspectra of sol ar flarc X-ray cmission in the 2 to 10 Awavelength rang e wilh a re solu tion of 0 . 5 A.U V Scanning Polychromator Spectrohel iometer (S055A):Photoelectrically record high resolution so lar im agesin six spcctr al lines simultaneously.Dual X-Ray Tclcs copc (S05G): Obtain high resolu tion(5 arc SCC) photographs of the sun's coronal X-rayemission in the 3 to 60 A wavelength region.Extrcme UV Coronal Spectroheliograph (S082A):Obtain high resolution (5 arc sec) spectroheliogramsof the sol ar atmosph ere in the 150 to 650 A wavelengthrange.Extreme U V Spectrogr aph (S052B): Record spect ra ofthe solar disk i n the 900 to 3900 A wavelength regionwith a 0.08 to 0.16 A spectral resolution.

Expcrimcnts Located in t l icCl~b$~lWoiksliopSpccimen Mass Mcnsurerncnt (M0 7 4 ) : Demonstratethe fcensibility of mass mcnsurcmcnt Wit . lKJ i l t gravityto asses s food intake, uri nar y output, and bone andmuscl e changes during flight.In-Flight Lower Body Ncgativc Pressure (M092):Record hcart ratc, blood pressure, and clcctrocardi-ogram data during flight with negative pres su re on thelower body to evaluate s pncc flight cardiov ascul ardeconditioning.Vc ct or ca di og " (M093): Monitor clcclrical actionsof the henrt during space flight, using senso rs and signalconditioners to obtain vectorcenrcliograms.Hmnn Vest ibul ar Function (M131): Evaluate thecondition of the crcw during flight to determine angularncccleration co mfort zone and to identify vestibu larchnngcs.I'inic Rnd Motion Study (M151): U s e time nnd motionstudlcs of standardiz ed mcchnnicnl ta sks tr, evaluatethc rclativc consistency bctween ground-based and in-flight astronaut pcrformance.

- -* Compllod by M m t l n Merlclts C o p .11


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Metabolic Activity (M171): Evaluate man's metaboliceffectiveness in space to determine long-durationmission requirements fo r logistics resupply, environ-mc nt d control, and task planning.Body Mas s Measur eme nt (M172): Validate a m a s smeasurement device large enough to contain a ma n andto provide data for bone and tissue studies.Gravity Substitu te Workbench (M507): Access the useof aerodynamic and electrostatic force fields as an ai din the manipulation of loose object s i n zero-g.Ast rona ut EVA Hardware Evaluation (M508): Evaluateman's capability to perfo rm work under the conditionsimpo sed by sp ace flight and develop quantitative des igncriteria applicable to future missions in space.Ast rona ut Maneuvering Equipment (M509): Obtain dat aon the mechanical and human-factor problems encoun-tered by ma n using maneuve ring devices .UV Ste lla r Astronomy (SO19): Pe rfo rm a partial skysurvey of Milky Way star fields to obtain UV spectrausing a Ritchey-Chrctien objective-prism spectrograph.X-Ray U V Solar Photography (S020): Obtain X-ray/UVsolar spect ra by a grazing incidence spectrog raph tosupport development of solar flare prediction techniques.Gegenschein Zodiacal Light (5073): Me asu re the inte nsi tyand polarization of the night sky light in the zodiacal "3Gegenschein region.Pa rti cl e Collection (S149): Study flux, size, compo-sition, and vclocity of mic rome teo roid s in the nea r-eart h environment.In-Flight Aerosol Analysis (T003): Determine theaerosol p article concentration and s ize distribution inthe spacecraft atmosphere as a function of time.Crew/Vehiclc Disturb,uce s (T013): Measur e the effectsof crew motion on the dynamics of th eir sp acecraft anddcter mine how these motions affect high acc urac y point-ing experiments.Foot-Controllcd Mancuvering Unit ("020): Determinethc Cecnsibility of the maneuvering unit for astronauttranslatio n and rotntional maneuvers i n space.Coronograph Contamination Measurement (T025):Monitor the presence of particulate matter in the nearvicinity of the spac ecra ft and provide mea sure ments ofthe solar f-corona.ATM Conhminatio n Measure ment (T027): Measure thesh y brightnes s background caused by s ol ar illuminationof contamination parti cles around a spac ecraf t anddetermine the effect of contamination on the opticalproperties of lenses and mirr ors.Experim ent s Loca ted in-the-command Servic_e_Mod@?Radiation in SDacecraft 03008): Measure and record the

Effects of Zero-g on Human Cells (S015): Study the in-flucncc of zero-g on living human cells and determin e ifthe absence of gravity has a significant effect on theirmetabolism.Potato Re spir atio n (SOG1): D ete rmi ne whether re mova lfrom the earth's rhythmic geophysical environment willaffect a well-known biorhythm.Circadia n Rhythm, Pocket Mice (S071): Determin e theeffects on the physical functions of pocket mice whenremoved fro m gravit y and the geophysical 24-hour periodCirc adia n Rhythm, Vinegar Gnat (S072): Det ermi ne theeffects on the physic al functions of v inegar gnats whenremoved from the gravity and the geophysical 24-hourperiod.Experiments Located i n the Multiple Docking Adapte rZero-g Flammabi lity (M479): Determin e the effects ofzero-g on the flammabili ty of nonmetallic mat eria ls i na spacecraft environment.Material Proc essi ng in Space (M512): Demonst rate andevaluate molten-metal flow cha rac ter ist ics under zero-gand spa ce vacuum conditions.Nuclcar Emuls ion (SOO9): Inves tigat e the phys ical andchemical characte ristic s of primary cosmic radiationincidcnt on the earth's atmosphere.W Airglow Horizon Photography ( S 0 6 3 ) : Secu re phota-graphs of the U V emission from the airglow layers ofthe upper atmosphere.Multispectral Photographic Facility (Si90): Determinethe cxtcnt to which m ultiband photography may beapplied to earth s ite s using si x Ifasselbald electriccameras with synchronized shutters.Infra red Spect rom ete r (S191): Manually acqu ire andtrack ground truth site s to obtain spectromete r datato evaluate earth re source s sensing from orbitalaltitudes in the visible to infrared spectral regions.Ten-Band Mu ltis pect ral Scan ner (5192): Se curequantitative radianc e v alues simultaneously in tenspectral bands, from visible to infrared, using imageryscanning with automated da ta pr ocessing techniques.E-xperiments Located in the Airlock ModuleExpandable Airlock Technology (DO21): Demonstratethe feas ibility of employing expandable struc tur es i nan earth-orbital environment.Ther mal Control Coatings (D024): Determine theeffects of the near-earth environment on thermalcontrol coatings to gain new insight into themechanis ms of degradation.Microwave Scatterometer, Altimeter, and Radiometer(S193): Obtain active and pass ive microwave data f romspace for application to earth resou rces disciplines.

absorbed radiation inside the spacecra ft to ass ur e astron aut Experime nts Located in-the Instrument UniJnwnreness of any dangerous increase in radiation levels. Galac tic X-Ray Mapping (S150): Pe rfo rm a highMineral Balance (M071): Precis ely meas ure the input and sensitivity survey of a portion of the celestial sphereoutput of calcium and nitrogen by the astrona ut to quantify to dctermine galactic X-ray sourc es and to develop anr:itcs of gain o r los s (also conducted in the OW S and upon undcrstanding of the a ppar ent phenomenon of X-rayrctu rn). background radiation.

12


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Preci sion Optical Trackin g (T018): Trac k the Saturnspace vehicle with a laser radar system durin g theearly iaunch phase to dete rmine liftoff motions.Preflight and Postflight E xperimen tsBone Dens itom etry (M072): Make a densitometriccomparis on of preflight X-rays of selected bones ofthe body to evaluate bone demineralization unde rprolonged weightlessness.Bioassay of Body Fluids (M073): Com par e pla sma andurine samples taken before and after flight to assess themetabolic changes in ma n as a res ult of space flight.Lower Body Negative Pr es s ur e (M091): Apply negativepressure to the lower half of the ast ronauts bodybefore and after flight to ascertain the cardiovascularfunction changes re sulti ng from spac e flight.

Cytogenetic Studies of Blood (M111): De ter min e th epreflight and postflight chromosome aberrationCrequencies in the peripheral blood leukocytes of thecrew.Mans Immunity in Vitro Aspects (M112): Assayhumoral and ccllular immunity as reflected by theplas ma concentrations of the maj or immunoglobulincl as se s, study the functions of blood lymphocytes.and assay selectcd coagulation factors.Blood Volume and Red Cell Life Span (M113): Documentchanges in red cell mass, red cell survival, and plasmavolumes occurring as a res ult of space flight.Rcd Blood Cell Me bbo lis m (M114): Dete rmin e theeffects of space flight on red ce ll metabol ism andmembrane integrity.

1.

2.

.>.

1.

Chubb, W . B. , Schultz, D. S . , and Seltzer, S. h l . , 5.Attitude Control and Prwision Pointing of ApolloTelcscope hIount, Journxl of Spacecraft and RocketsV o l . 5, N o. 8, Aug. 19GS.Chuhb, W. B . , and Egstein, LIichacl, Applicationof Control Moment Gyros i n the Xttitudc Control ofthe Apollo Telcscope LIount, AI.\.-\ Paper N o. 68-

6.

866. Xug. 1968. 7.Chubb, W . B, Stabi lizat ion and Control of the A p o l l oTelescope Mount, N A S A TM X-53831, May 6 . 19r;9.Seltzer, S. & I . , Developing an Attitude ControlSystem fo r the Apollo Tele scope LIount, SecondAsilomar Conference on Circuits and Systems,Pacific Grove, Calif ., Oct. 30- Sov. 1, 1968.

8.

Kennel, H . F . , A Control Law for Double-CimhaledControl hloment Gvros L-sed for Space Vehicle AttitutlcControl, N A S A TM X-64536, J u l y 20, 1970Kennel, H. F. , Angular ?rIomenturn Desn turat ionUsing Gravity Gradient Torques, S A S A TM X-53748,Mav 27, 1968.Skylab-A ATM Digitnl Computer Program Require-men ts Document (PRD), 50M-37941, George C.Marshall Space Flight Center, July 1, 1970.Skylab-A Apollo Tele scope Mount Digital Com puterFlight Pro gra m, Pro gra m Definition Document,IBM #70-207-0002, NOV. 4, 1970.

George C. Mar sha ll Space Flight CenterNational Aeronautics and Space AdministrationMarsha l l Space Fl igh t Cent er , AlabamaSeptember 1970Cos t Code: 908 52 01 0000965 21 00 0 0 0 0

NASA-Langley, 1910- 1 M211 13


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skylab attitude and pointing control system

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