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NASA TECHNICAL NOTE

SKYLAB ATTITUDE AND POINTING CONTROL SYSTEM

by W. B. Chzlbb and S. M. Seltzer George C. Marshall Flight Center Marshall Space Flight Center, Ala. 35812

NATIONAL AND SPACE ADMINISTRATION WASHINGTON, 0. C. FEBRUARY 1971 I I TECH LIBRARY KAFB, NM

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-, - ___.. 0132813 I. REPORT NO. 2. GOVERNMNT ACCESSION NO. j. KtLIPlbNl'b LAIALOb NO. - NASA- __ TN D-6068 I I 1 1. TITLE AND SUBTITLE 5. REPORT DATE L February 1971 Attitude and Pointing Control System 6. PERFORMING ORGANIZATION CODE __- I 7. AUTHOR(S) 8. PERFORMlNG ORGANlZATlON REPORT # - W... -B. Chubb and S. M. Seltzer I 3. PERFORMING ORGANIZATION NAME AND ADDRESS 10. WORK UNIT NO. 908 52 10 0000 M211 965 21 00 0000 George C. Marshall Space Flight Center I' 1. CONTRACT OR GRANT NO. Marshall Space Flight Center, 35812 L 13. TYPE OF REPORY & PERIOD COVERED - _-- .. .- __ 2. SPONSORING AGENCY NAME AN0 ADORES5 National Aeronautics and Space Administration Technical Note Washington, D. C. 20546 14. SPONSORING AGENCY CODE

-. - - I 5. SUPPLEMENTARY NOTES Prepared by: Astrionics Laboratory and Directorate ~- 6. ABSTRACT

NASA's Marshall Space Flight Center is developing an -orbiting manned called Skylab. The purpose of Skylab is to perform scientific experiments in and earth resources and to study biophysical and physical properties in a zero environment. The attitude and pointing control system requirements are dictated by onboard experiments. These requirements and the resulting attitude and pointing control system are presented.

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TABLE OF CONTENTS Page SUMMARY ...... i

INTRODUCTION ...... 1

SKYLAB-A DESCRIPTION ...... i

MISSION IMPLEMENTATION ...... 2

SYSTEM REQUIREMENTS ...... 3

CONTROL SYSTEMS DESIGN PHILOSOPHY ...... 3

CONTROL SYSTEM DESCRIPTION ...... 4

SYSTEM OPERATION ...... 9

CONCLUSIONS ...... 11

APPENDIX ...... 11

REFERENCES ...... 13

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I. LIST OF ILLUSTRATIONS

Figure Title Page I. Sky1ab.A ...... 2

2 . CMGCluster ...... 4

3 . Momentum Vector Configuration ...... 5

4 . Distribution Law Principle ...... 5

5 . Combined Disturbance Impulse ...... 6

6 . TACS ...... 7

7 . Functional Block Diagram of the Attitude and Pointing Control System ...... 7

8 . Phase Plane Diagram (Nested Configuration) ...... 8

9 . Z-Local Vertical Maneuver in CMG/TACS Nested Configuration . 8

IO. Experiment Pointing System (EPS) ...... 9

I1. Earth Resources Maneuver Sequence ...... 10

LIST OF TABLES

Table Title Page I . EPS Control System Requirements ...... 3

11 . CMG Control System Requirements ...... 3

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

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.. . I DEFINITION OF SYIVIBOM - H momentum vector

total momentum vector

momentum vectors associated with each of the CMG's

principal moments of inertia about the X, Y, and Z axes, respectively

fine sensor gain KFS torquer gain Km rate gyro gain K~~ amplifier gain KCY

S Laplace transform

commanded torque

disturbance torque

axes

angle between normal and the jth momentum vector gj

gimbal rate about inner axis for jth CMG

gimbal rate about outer axis for jth CMG

rate gyro dumping ratio

fine sun sensor rise time 'FS amplifier rise time

rate gyro natural frequency SKYLAB ATTITUDE AND POINTING CONTROL SYSTEM

SUMMARY

NASA's Marshall Space Flight Center is developing an earth-orbiting manned space station called Skylab. The purpose of Skylab is to perform scientific experiments in solar astronomy and earth resources and to study biophysical and physical properties in a zero gravity environment. The attitude and pointing control system requirements are dictated by onboard experiments. These requirements and the resulting attitude and pointing control system are presented.

I. INTRODUCTION will provide the launch facilities and execute the launches. Major contractor support for Skylab-A is Thc Skylab-AI -3 is to be an experimental space indicated below: station dcvcloped by the National Aeronautics and Spnce Administration. The Skylab program is an (1) launch . Corp., estcnsion of the -Gemini- manned Corp., and McDonnell-Douglas Corp. space flight programs and makes extensive use of hardware and developed in those programs. (2) Skylab-A The purpose of the program is to increase our knowl- edge of manned space flight and to accomplish selected (a) Martin Marietta Corp. : cluster pay- scientific, technological, and physiological investiga- load integration and management tions. The objectives of the program are: (b) McDonnell-Douglas Corp. : orbital (1) To conduct solar astronomy scientific ex- workshop and module periments, emphasizing observations that cannot be obtained from earth because of absorption of certain (c) IBM: Apollo Mount (ATM) electromagnetic by the atmosphere. digital (primary and secondary) and work- shop interface unit (2) To conduct earth resources experiments designed to permit an evaluation of existing technology (d) Bendix Corp. : control moment and how it may be applied to aid in the solution of eco- and electronics and experiment pointing logical problems. The need for further development of electronic assembly sensor technology will also be determined. (e) North American Rockwell Corp. : (3) To conduct biomedical experiments to deter- modifications to the CSM mine the effect of long duration space flight on the crew. 2. SKYLAB-A DESCRIPTION The Skylab is being developed under the overall pro- gram responsibility of NASA's Office of Manned Space- flight. The Marshall Space Flight Center, Huntsville, The Skylab-A consists primarily of modules developed Alabama, has program management responsibility for under the and selected to meet the developing all Skylab hardware except the Command and operational requirements of the Skylab-A program. (CSM), for providing the launch vehicles, The modules will be launched aboard Saturn launch for flight evaluation, and for overall systems engineer- vehicles, placed in an earth orbit, and assembled there. ing tp ensure compatibility and integration of all hard- The Skylab-A (Fig. 1) consists of the following modules: ware. NASA's Manned Spacecraft Center in , , is responsible for modifying the CSM, for de- Orbital Workshop (OWS). The OWS is a modified empty veloping the spacecraft launch adapter (SLA), for de- S-IVB stage that will provide the with living veloping the experiments and crew support equipment, quarters while in orbit. It will contain necessary fd- for mission analysis and evaluation, and for preparation and waste-management facilities to support crew training. The in athree-man crew for the planned manned missions. ATM SOLAR ARRAY TACS ENGINE POD

WORKSHOPSOLAR

INSTRUMENT UNIT AIRLQCK MODULE

MULTIPLE DOCKING ADAPTER

e- COMMAND MODULE

FIGURE 1. SKYLAB-A

Certain experiments and supporting facilities for their each side of the OWS and four panels are attached to operation also will be contained in the OWS (see Appen- the ATM. Their purpose is to provide power for the dix). Attitude control commands will be implemented Skylab-A system. The average power output of these by a -gas thruster attitude control system (TACS) combined arrays is approximately 7.2 kW. located on the OWS. Two solar array panels attached to the sides of the OWS will provide some of the power Instrument Unit gv). The IU is used only required by the Skylab-A. The OWS will remain in during launch and for the subsequent seven and one- orbit in unmanned storage modes and will be reactivated half hours of orbital operation while the CMG's are be- during subsequent manned revisits. ing brought up to operational speed (i49 rou/a). Once in orbit, it provides sequencing commands to actuate Multiple Docking Adapter (MDAL ang Airlock Module (Ae$). and control the deployment of the Skylab-A elements. It The MDA andvdockinp.~orts for the CSR.1 and also provides an IU digital command system and telem- provide a pressurized passagewaybetween the CSM and etry with the ground. the living quarters in the OWS. The MDA contains the ATM control and display (C&D) panel. Experiments Command and Sey:ce Module (CSM). The CSM provides housed in these two modules are described in the the crew with transportation between the earth and the Appcndix; of the experiments are included in the Skylab-A. It also contains food, , and other Earth Resources Experiment Package (EREP). essentials for crew support when the CSM is detached from the Skylab-A. The CSM provides some of the communication, instrumentation, and thermal control -Mount (ATM). The ATM is to be a capability. It has the capability of providing attitude developed primarily to collect control to counteract transient responses to docking data on to increase man's knowledge maneuvers through use of its of the solar environment. It provides a mounting (RCS) which utilizes hypergolic storable propellants. structure, or rack, to which are attached the control moment gyroscopes (CMG's), attitude control computers Eqeriments. The experiments are one of the pnmaw and sensors, and associated electronics. Within the reasons for development of the Skylab-A and dictate rack 811 experiment spar is located to which are attached the attitude and pointing control requirements. A solar experiments and spar control sensors. The spar description of these experiments and their locations gimbaled about axes with respect to the rack. is two are provided in the Appendix. The ATM provides a capability for film retrieval and installatton by astronaut (EVA). 3. MISSION IMPLEMENTATION Solar Arrays~. Two solar array panels are mounted on The unmanned Skylab-A, less the CSM, is placed in a

2 near-circular 435-km orbit with a nominal inclination package with respect to the LOS. The requirements of 0.87 rad by a two-stage Saturn V . for the 2-local vertical (2-LV) mode of operation (e.g., Within the seven and one-half hours of IU lifetime, the during cnrth resources experiments) are the same as Skylab-A ATM rack is oriented toward the sun, and the shown in Table II except that a error of solar panels are deployed. The Skylab-A interior is a3.5 x -‘ rad is acceptable. then pressurized to make it habitable for the crew. 5. CONTROL SYSTEMS DESIGN PHILOSOPHY Approximately one later the CSM, carrying a three- man crew, is placed into a temporary 150 x 222 lan The ATM pointing and control system that has evolved orbit by a Saturn IB launch vehicle. Using its own to date has been influenced by a number of factor^.^ propulsion system, the CSM achieves a rendezvous with The prime requirement is to meet the high accuracy the remainder of the Skylab-A and docks to an axial system pointing specifications in the presence of exter- port of the-MDA. It is planned for the crew to remain nal disturbance torques. The significant disturbance onboard the Skylab-A for 28 days to conduct experiments. Torques of interest are those caused by earth-orbital They then will prepare the Skylab-A for orbital storage environmental influences (gravity gradient and aero- and return to earth in the CSM on the 29th day. Two dynamic disturbances) as well as internal movements of subsequent launches, similar to the secod launch, are the astronauts on board. Because of these earth-orbital planned. Manned mission durations of no greater than environmental influences, the vehicle attitude must be 56 days are anticipated. The first two flights are held to a fixed position relative to the orbital plane. To planned for the last quarter in 1972. meet the pitch and yaw pointing accuracies, a two-axis gimbaled EPS with a maximum range of *3.5 x IO-’ rad is 4. SYSTEM REQUIREMENTS required. The primary requirement for the EPS is to provide experiment package isolation from the relatively The Skylab-A attitude and pointing control system has large vehicle perturbations that can result because of been developed to meet the high accuracy requirements astronaut effects. established by the desired experiment conditions. These conditions must be maintained by the control system The CMG control system of the rack was chosen pri- under the influence of external and internal disturbance marily because of performance benefits with respect to torques, such as gravity gradient and aerodynamic dis- both dynamic response and compensation of cyclic ex- turbances and onboard .aut motion. The Skylab-A ternal disturbance torques caused by gravity gradient attitude control system provides a stable base about and aerodynamic effects. Most passive control schemes which the ATM spar attitude control system may re- (gravity gradient, for example) would not have the re- spond to meet higher frequency attitude control system quired accuracy and could not develop sufficient torque demands. Hence, a fine pointing system, i. e., the to meet the dynamic performance requirements. During experiment pointing system (EPS), for directing the data gathering intervals when experiment optics are ex- experiment package on the ATM spar has evolved to posed, use of CMG’s prevents optics contamination that meet the stringent experiment pointing requirements. would result from reaction control thruster exhaust. The design requirements for each of the systems are listed in Tables I and II. Roll is defined as the angular The TACS (and the RCS, if necessary) is available to provide coarse attitude control and CMG momentum de- TABLE I. EPS CONTROL SYSTEM REQUIREMENTS saturation capability (if needed). The TACS and CMG system must be c&pable of maneuvering the vehicle to a desired experiment observation ; i. e. , 2-LV Command I Stability and solar inertial. It was also necessary to meet the System Pointing for Axis 15 Min maneuver requirements of the vehicle using the CMG Uncertainty system as much as possible to minimize TACS propel- lant consumption. EPS X (pitch) ai.i x i0”rad *i.i x i0-5rad EPS Y (yaw) *i.i x i0-5rad *i.i x i0-5rad Control of Skylab-A as differentiated from control of the experiment spar mounted Skylab-A may be accom- x iO-’rad on EPS 2 (roll) k2.9 CMG control plished by the CMG’s alone, the CMG’s in conjunction system with the TACS (nested system), or the TACS alone. In addition, in an emergency situation and as a -up for iZ-LV maneuvers, the CSM reaction control system may TABLE II. CMG CONTROL SYSTEM REQUIREMENTS be utilized. Command I System Stability for Major control system design philosophy considerations Pointing 15 Min Axis Uncertainty were:

*2.6 x iO-’rad (1) To use the CMG system to meet vehicle CMG X @itch) ii.2 x i0-3rad control and maneuver requirements whenever possible. CMG Y &aw) *i.2 x iO-’rad *2.6 x i0-3rad (2) To minimize the action of noncyclic torques CMG Z (roll) i2.s ~o-~rad @ias torques) upon the vehicle. These torques arise from two sources. The first is that the major princi- about the line of sight (LOS) from the experi- pal moments of inertia of the vehicle are not identical ment package to the center of the sun, and pitch and and the interaction of the gravity field upon these in- yaw are defined as angular deviations of the experiment ertias produces a bias momentum accumulation of

3 approximately 600 Nms about the vehicle X axis over an system. The momentum exchange devices are three . The second is a result of vehicle vent- orthogonally-mounted double-gimbaled CMG's; each ing of waste gases and accounts for a bias momentum has a stored momentum capability of 2700. Nms. ?$e accumulation of from 500 to 1000 NmS about a non- CMG cluster is shown in Figure 2 where 6,g) and 6,~) specified vehicle axis. are the gimbal rates about the inner and ou r axes, respectively, for the jth CMG; j - 1,2,3. The Skylab-A (3) When in the solar inertial mode, to hold that (Fig. 1) principal momenta of inertia and data are axis (nearly the X principle) about which bias momen- presented in Table m. tum accum'ulation is minimum in the orbit plane. This significantly reduces gravity gradient bias torques. TABLE III. PHYSICAL CHARACTERISTICS OF SKYLAB-A

(4) Consistent with the mission lifetime of 240 long = 0.8862 x 10'kg m2 days, td eliminate all single point failures in the design. 5(

(5) To provide maximum system operating flexi- 'Y = 0.5835 X 107kg - m2 bility in regard to sensor and computer selection. 1, = 0.5753 x 107kg m2 (6) To provide automatic alert and caution and warning signals to the astronauts and the ground con- Mass = 0.830 x 1o5kg trollers of abnormal system operation.

(7) To provide through digital computer software Overall Length = 36.1 m the capability of testing and switching out malfunction- ing equipment and switching in backup equipment. S-IVB Diameter = 6.58 m

6. CONTROL SYSTEM DESCRIPTION The use of CMG's in the pointing and control of a large manned space station is new, and the problems associ- The attitude and pointing control system (APCS) con- ated with that type of system are unique and were un- sists of the three basic systems: the CMG, the TACS, solved. Some of the problems encountered in the devel- and the EPS. The first two systems may control the opment of the CMG control system include the following: Skylab-A either separately or together in a nested con- figuration. The EPS system is used only for experi- (1) An acceptable control law for use of the CMG's ment spar control. in control of the Skylab-A. The CMG system is a momentum exchange control (2) A means for preventing the CMGIs from "falling

OUTER GIMBAL \

OUTER GIMBAL TORQUER

OUTER GIMBAL CMG MOUNTING PLANE

FIGURE 2. CMG CLUSTER

4 I

intoff an undesirable orientation such as that which would not allow the use of the CMGfs to control the spacecraft even though the CMG cluster was not in ita saturation momentum state.

(3) A means of desaturating the CMG cluster periodically without the use of mass expulsion TACS. b A method was needed to permit torques produced by an fl3 external force field such as gravity gradient to be used ANTI- PARALLEL CONDITION to effedt CMG momentum desaturation. I

(4) The optimal orientation of the vehicle to minimize external bias torques which would tend to saturate the CMG cluster; i. e., the placement of the minimum principal axis of inertia into the orbital plane. An acceptable control law for use of the CMGIs in con- trol of the ATM cluster was developed. It can concep- tually be broken down into three parts: the steering law, the distribution law, and the rotation law. A de- WITH DISTRIBUTION LAW tailed derivation of these laws is given in Reference 6. The function of the control law is to utilize three nor- FIGURE 3. MOMENTUM VECTOR CONFIGURATION malized torque commands together with the present orientation of the CMGIs (in terms of their direction ORBIT NORMAL cosines) to generate inner and outer gimbal rate com- mands on each of the three CMG's. The steering law generates gimbal rate commands in such a way that (assuming the actual gimbal rates are equal to the commanded gimbal rates) the torques resulting on the vehicle are identical to the desired torques in direction and magnitude. Only when the maximum gimbal rate capability is exceeded will themagnitude of the result- ing torque be less than commanded, but the direction will still be that of the command.. No crosscoupling is inherent in the control law.

Only three degrees of freedom are utilized by the steering law. The remaining three are used by the other two laws. Because the bulk of the CMG momen- tum change is along the orbit normal, the distribution ROTATE iil AND k2 ABOUT 5+E2 UNTIL law tries to make the components of the CMG vectors EOUAL COMPONENTS OF i?l AND iiz LIE along the orbit normal equal to each other. This has ALONG THE ORBIT NORMAL the effect of spreading the vectors, which in turn re- tu. a, =a2) duces the angular velocity required of the vectors to meet the needed momentum change. It also prevents FIGURE 4. DISTRIBUTION LAW PRINCIPLE the CMG% from falling into the undesirable anti- parallel orientation illustrated in Figure 3. The distribution is made by rotations about vector sums of To desaturate the CMG cluster periodically, without pairs of individual CMG momentum vectors and thus the use of the mass expulsion TACS, a method was does not affect the total momentum; i. e. , no disturb- developed utilizing the earth's gravity-gradient force ance torques are transmitted to the vehicle as a result field to effect CMG momentum desaturation. This of application of the distribution law. Figure 4 is a method is described in detail in Reference 6. The simplified diagram illustrating this principle. No dis- basic concept is best described with the aid of Figure tribution is necessary for two-CMG operation. 5 which depicts the per orbit momentum build-up of the CMG cluster caused by gravity gradient and The rotation law utilizes only rotations about pair sums aerodynamic torques. This figure reveals that if and total is not disturbed (no periodic CMG desaturation was not provided, the CMG torque on the vehicle). The angular velocities for the cluster would be saturated for progressively larger rotations are generated such that the largest gimbal portions of an orbit after the . The axis of angles are reduced, thus avoiding (as much as possible) saturation would be roughly the vehicle X axis. This httting of the gimbal stops. The gimbal angles are means that after complete saturation, the CMG cluster weighted by their fifth power such that the distribution could not compensate for a disturbance torque about the law ia predominant for small gimbal auglea aad the axis of saturation. rotation law is predominant for large gimbal aoglea. For twoCMG operation (no distribution law) the first An ipvestigation of the causes of the predominant non- power of the tuglea is wed. cyclic torques (i. e. , gravity gradient and aerodynamic)

5 the center of the solar disk. This was accomplished by sampling the vehicle monientum at specified times during the daylight orbital period nnd comparing it with the pre- vious day’s samples. The compared samples indicated whether the bias momentum components about the var- ious vehicle axes were increasing or decreasing. This information was then translated into appropriate angle MIDNIGHT position commands about the vehicle 2 axis to ensure minimization of bias momentum accumulation.

-600 (2) The saturation effects of the remaining non- - 800 cyclic disturbance torque were nullified by periodically producing controlled bias torques which would tend to desaturate the CMG cluster. The controlled.hias tor- ques are produced by employing rectified components Y(Nms) of the gravity gradient torques encountered during the night portion of the orbit to desaturate the CMG cluster. 6000 The rectification of the gravity gradient torques is made possible by maneuvering the vehicle about two axes dur- ing the night side of the orbit. The magnitude of the maneuver angles is a function of the momentum accumu- lation during the daylight portion of the orbit.

The TACS is composed of six cold-gas thrusters and the necessary logic to select and the proper thruster. -6000 The thrusters are mounted as shown in Figure 6. - 8000 The thruster force is dependent upon the cold-gas tank and will be approximately 45 kg at the be- ginning of the mission and diminish to about 4-1/2 kg Z(Nmr) at the end of the mission. The minimum impulse bit will be maintained at a constant level within the hounds of the selectable thruster firing time (40 to 400 ms). 4000 2134 2014 A functional block diagram of the CMG system and TACS is shown in Figure 7. System information is- available from two sources, the acquisition sun sensors or the strapdown equations which utilize rate gyro in- -4 000 4 4 formation as their basic input. Sensors as well as the NOON MIDNIGHT basic digital computer are redundant to provide increased - 60001 system reliability over the 240-day mission. Rate in- -80001 formation is provided by rate gyros. The gyros have a coarse scale of *ti .7 x io-‘ rad/s and a fine scale of *i .7 x io-’ TOTAL MAGNITUDE (Nms) rads. The scale is selected by logic within the operating digital computer. 8ooo+- - -3CMG - - CAPABILITY- - - - - 8100- - -Nmr - -- Thc ATR? digital compiitcr (ATAIDC) is the primary data processing, compuhitionnl, and logic generation facility in the control system. Fast loop computations (e. g., ““h/\m rate gyro proccssing and CMG control laws) arc pcr- 2000 formed at n rate of five times per second, and slow loop 0 computations (c. g., orbital navigation) are performed at $. + a rntc of onc tinic pcr second. The computer memory NOON MIDNIGHT capacity is lci,300 words with lengths of 16 bits. Present estimates of computcr memory requirements are approx- FIGURE 5. COMBINED DISTURBANCE IMPULSE imntcly 15,500 words. The ATMDC is the “brains” of the system and performs the following primary APCS reveals that with the given vehicle configuration and functions. mission requirements (i. e. , point the vehicle 2 axis at radiometric center of sun every daylight period), it (1) Orbital navigation and timing is impossible to eliminate the noncyclic torques, but (2) Control of operational modes it is possible to minimize them. (3) Maneuver generation commands This problem of CJIG momcntum management was (4) CMG and TACS control attacked in two scp:irate !va!.s. (5) CMG momentum desaturation commands (1) 1ne noncyclic disturbance torques were mini- (6) System redundancy management (test and selec- mized by finding nn optimal vehicle orientation while still tion of alternate vehicle components in the case of off- meeting the requirement that the vehicle 2 axis point to nominal component or system operation).

6

I I

THRUSTER 2

THRUSTER1 , THRUSTER 3

DETAIL 0

+X 4-

+Y

DE

I I I I I I I I I I I I I I I I I I SENSOR 12) I I I NESTED CONTROL I SYSTEM I r--1 I THRUSTER I c FIRING I I COMMANDS I in--@TCSA L I - I I ', I- I IU / LVDC I' UL I CMGEA:CMG ELECTRONIC ASSEMBLY CWGIA CMQ INVERTER ASSEMBLY I TCSA THRUSTER CONTROL SWITCHING ASSEMBLY I I READOUT ASSY. 1 I LVDC. LAUNCHINGVEHICLE DIGITAL COMPUTER pC------CONTROL SUISYSTEM ------FIGURE 7. FUNCTIONAL BLOCK DIAGRAM OF THE ATTITUDE AND POINTING CONTROL SYSTEM

7 \ CMG+TACS TACS NO FIRE REGION 1 FULL TORWE REGION

- -. '\ ATTITUDE ERROR

CMG ZERO TORQUE REGION ( SHADED ) CMG+TACS FULL TORQUE REGION

FIGUEW 8. PHASE PLANE DIAGRAM (NESTED CONFIGURATION)

While the CAIG system and TACS may operate independ- TACS ently of each other, the nominal mode of operation is in -IMPULSE- !67.0 a "nested" configuration. Figure 8 is a phase plane dia- gram of this type of system operation. A11 control is delegated to the CMG system as long as it has the capa- bility of maintaining the attitude and rate error within the "no fire region" of the phase plane. If the attitude or rate error exceeds this region, the TACS supplies sup- 2700 J,I plemental control authority until the vehicle attitude and rate errors are again within the "no fire region. A CMG inhibit region has been established on the phase plane to prevent the situation where the TACS and the CMG's would produce opposing torques. If the state of the vehicle lies in both the CMG zero torque region and the TACS no fire region, the attitude rate (which -2700 t I rehains constant in this zone) will cause the attitude -5400 4 I error to decrease until the vehicle state leaves the zone. The TACS, in addition to bounding the magni- 73.0 Na -8100 tude of the rate and position errors, also performs a 99.6 Nr

CMG momentum desaturation function in the %ested1' SRT: SUNRISE TERMINATOR mode of operation. If the CMG total momentum vector SST: SUNSET TERMINATOR excceds 95% of the CMG capability, the digital computer directs those TACS engines to fire which will most efficieutly cause the total momentum vector to fall FIGURE 9. Z-LOCAL VERTICAL MANEUVER IN below 95%. An example of this type of operation is CMG/TACS NESTED CONFIGURATION illustrated in Figure 9. The EPS operates independently from the CMG system and TACS. It has its own sun sensors and rate gyros meet positioning requirements about the third axis. The for position and rate control. Control signals are gen- flex-pivots allow about *3.5 x IO-' rad of rotation of the X erated in the experiment pointing electronic assembly and Y axes while the roll positioning device allows for a (EPEA), an analog device. rotation of i2.i rad about the experiment package Z axis. Figure 10 shows a block diagram of this system. The EPS utilizes flex-pivot* gimbal bearings for control While the EPS provides automatic control of the experi- about two axes and an open-loop positioning device to ment package X and Y axes, manual positioning of these

*A flex-pivot gimbal bearing is made of a pair of flat cross-leaf springs and is welded to and supported by rotating sleeves. It has no backlash and provides limited angular travel.

8 SOLAR VECTOR *- BI I

SAME AS ABOVE

FIGURE 10. EXPERIMENT POINTING SYSTEM (EPS) two axes is provided for the purpose of offset pointing. positioned, the RPM will hold the location until a repositioning Fine sun sensors (FSS) are used for sensing spar atti- command is received. The astronaut repositions the tude crrors, with rate gyros sensing spar rates. The spar in accordance with experiment demand require- EPEA conditions the sensor's signals to provide rate ments. The spar roll position is displayed on the C&D plus dlsplacenient command signals to the flex-pivot panel. actuators (dc torque motors).

The experiment package can be offset pointed in the X 7. SYSTEM OPERATION and Y axes over a range of*5.8 x lO-'rad, with the center of the solar disk being the zero position. The solar The APCS may be required to operate in any one of six disk measures approximately*9.3 x i~-~radfrom limb to mutually exclusive primzry modes. These modes are limb, Offset pointing is accomplished by positioning an configured to allow the APCS to perform its required optical wedge located in each FSS. The wedge is tasks. The request for mode changes may come from mounted in the path of the passing through the one of three sources: (1) via switches on the flight FSS optics and can be rotated to refract the sunlight a crew's C&D panel, (2) by way of an onboard digital fixed angle in a controlled direction. The wedges are address system (), and (3) from the ground- positioned by a drive mechanism controlled by the operated digital command system (DCS). Selected astronaut via the manual pointing controller. The wedge mode changes are also made within the digital com- drive varies from+6.5 x lo-' rads near zero offset puter as a function of sensor information and com- positions to 53. I x rads for wedge positions near the ponent redundancy management considerations. The +5.8 x lO-'rad sun offset position. A wedge offset produces primary operational modes are as follows: an FSS output error voltage that causes the spar to rotate about the appropriate axis (X or Y) and point the Stand-By Mode: Jn this mode the ATMDC will not be FSS, and thereby the experiment package, in a direc- processing output commands to the CMG system or the tion that will drive the FSS output voltage to null. TACS. This mode will be used if no vehicle control is Stability is then automatically maintained by the EPS. required of the APCS. With the exception of inhibiting The experiments are aligned to the FSS. The posi- the commands to the TACS and CMG system however, tion of each FSS wedge is displayed on the C&D all normal ATMDC functions will be enabled. If the panel and corresponds to the experiment package offset stand-by mode exists at the time of a computer switch- position from the center of the sun in the X or Y axis. over, this mode will be maintained. The panel also contains displays of the sun, as viewed through experiment , and experi- Solar Incrtial Mode: In tlus mode the "ncsted" CMG ment readout displays to assist the astronaut in pointing system nnd TACS will be used for vehicle control. The the experiment package. vehicle 2 nsis is pointed toward the center of the solar disc. Thc vchiclc X Rsis lies nearly in the orbital plane The roll positioning mechanism-(RPM) is used to rotate but is orientcd so that it is consistent with minimum the spar about the 2 axis. The mechanism is com- bias momentum ncciimulation considerations and points manded by the astronaut via the manual pointing con- in thc direction of thc carth at the morning tcrminator. troller (rate switches) located on the control and display Thc vchiclc Y nsis forms a right-11:mdcd triad. The panel. Spar roll rates of il.ZxlO-', i6.i~lO-~,+i.2xiO-~,ATMDC will obtain attitude information for inputs to the and *6.1 x lo-' rads can be commanded. Once the spar is control law as follows:

9 (1) Orbital Day: The ATMDC will use the CMG Ncstcd Attitude Hold Mode: The vehicle will be acquisition sun sensor for X and Y axes attitude infor- under ncsted CMG/TACS control in this mode and will mation ana the strapdown computation for 2 axis atti- be maintained in an inertial hold. The ATMDC will use tude information. The rate gyro outputs will be used the strapdown computation for attitude information in for rate stabilization in all three axes. X, Y, and Z, and the rate gyro outputs for rate stabili- zation. The momentum management maneuvers will be (2) Orbital Night: The ATSZDC will use the inhibited in this mode. No provisions exist for automatic strapdown computatioj for X, Y,and 2 axes attitude entry to or exist from this mode. Manual vehicle attitude information and the rate gyros for rate stabilization. commands may be entered in this mode via DAS. The attitude is not limited, but the maneuver rate is limited Momentum management desaturation maneuvers will be to a5.2 x rads. performed dhring the night periods. Automatic entry into the solar inertial mode occurs when the APCS is in TACS Attitude Hold Mode: This mode will be an inertial the experiment pointing mode (day operation) and the hold mode, random attitude, with the TACS in control. crew does not exit the mode manually prior to orbital As in the attitude hold mope, the ATMDC will obtain sunset. This mode will also be entered at the time of attitude and rate information from the strapdown computa- a computer switchover, unless the mode existing at that tion and the rate gyros. No provisions exist for automatic time is the stand-by mode. entry to or exit from this mode; and the attitude is not limited, but the maneuver rate is limited to *5.2x iO-’rad/a. Experiment Pointing M-odg: The experiment pointing mode is identical to the day portion of the solar inertial Z-Local Vertical (Z-Lv) Mode: This mode will be mode, with respect to vehicle control. In this mode, used for earth pointing experiments during the manned however, the EPS will be activated.’ Normally, the periods and for rendezvous during the unmanned periods. experiment pointing mode will be entered manually, The vehicle will be under CMG/TACS control in this with automatic exit at orbital sunset. Automatic entry mode. The mode will have two sources of activation. is also provided as an option to be activated by the For earth’resources e~eriments,the crew will com- crew via DAS command. The crew will be able to en- mand the ATMDC via the C&D panel switch to enter able or inhibit automatic experiment pointing mode the Z-LV mode. At that time the ATMDC should start entry at will. If the crew has enabled automatic entry, the maneuver to Z-LV. The elapsed time from initia- the ATMDC will activate the mode each orbital sunrise, tion of the maneuver until the Z-LV attitude is reached if and only if the system is in the solar inertial mode the will be a stored value in the ATMDC, addressable by preceding night period. the crew via the DAS. Figure 11 illustrates a typical

SOLAR VECTOR

€REP OPERATION OVER TARGET

SET UP EREP EXPERIMENTS

TERMINATOR MANEUVER

CMG CONTROL REFERENCE WHEN SUN \ MOMENTUM MANAGEMENT COMES INTO VIEW \ \ \ \ \ \ ---. EREP: EARTH RESOURCES EX P E R IM E N T PACKAGE

ORBIT MIDNIGHT FIGURE 11. EARTH RESOURCES MANEUVER SEQUENCE

10

1 Z-LV earth resources maneuver sequence. For ren- CMG stops arc locatcd at 1-1.4 rad and outer stops ape at dezvous. both the time for initiation and the time for +3.8 and -2.3 rad. These physical conatraints Iimited completion of the Z-LV acquisition will be specified. totnl CMG momentum utilization whencvcr thc slops In thin case the ground will command the mode change. wcrc cncuuntcrcd. Special digital logic was dcvcloped Leaving the Z-LV mode will be similar; it may be to minimizc the number of situations when CMG gimbal commanded either by crew command (C&D switch) stops would be encountered. or ground (relative to time). A documcnt' dcscribing in dctail thc ATM digital Mode Options: Several somewhat unrelated ''sub- computcr program rcquircmcnts has bccn prcparcd by modes" will be available, primarily for the crew, for NASA and rcccntly ul,tlatcdo for NASA by IBM. InclQded altering certain operational aspects of the APCS. in tlus document arc detailed rcquirrments and imple- Again, the states of these submodes may be changed mentation schenics in analytical form for functions such by crew command, ground command, or ATMDC pro- as nionicnlum management, CMG rriomcnhun distribution, gram control. The submodes include inhibiting auto- CMG control laws, TACS logic, vehicle altitude reference matic gravity gradient desaturation maneuvers, in- (strapdown), and APS moclc control. hibiting CMG or TACS control, switching the navigation timing sequence from a computational procedure to one based on sun prcscnt discrctes, and the capability of pcrforniing a preprogramcd set of maneuvers to 8. CONCLUSIONS rcacquirc thc solar vector should the system suffer a tcmpornry loss of attitude inCormation. Based on the mission and high accuracy experiment pointing requirements, the design of the Skylab-A Ilybrid simulations of thc Skylab nttihitlc and pointing attitude and pointing control system has been control system a1 M:irshall Spnc'c Flight Ccntcr have developed. A significant portion of the system is bccn in operation Cor Llic 1):tsl csightcvn months. In capable of manual operation. enabling the astronauts gcmc,rnl, simuI:iti*~li'c,siilts h:tvc, I)c*cm in :Igrc.c~mcml to perform functions requiring human judgment, such with 1hcwrclic:tl i)rc-ilic.tcstl rcLsiills. 111 Lliosr inst:inc'cs as choosing scientific targets and selecting and point- whcrc pix*dictctl :inti ac.tii:il rcsulls Ii:ivc tliffr~rctl,tlic ing appropriate experiments toward these tsrgets. To simul:ilion motlrls hnvc I)wn usrd :is dcsibn 1001s to keep the number of tasks to be perfoinied by the astro- rorrcct digiti1 logic to rliminntc or minimize thcsc nauts within reason, many functions such as attitude tliflcrcwcs. Onc :ircn whcrc inconsistent results pointing stability are performed using closed loop auto- wcrc nolrtl conccrnctl gimbal nnglc stops. Inncr gimlJal matic control.

APPENDIX SKYLAB EXPERIXIENTS*

..E.q>crimcnts - Locatcd in the Expcrimcnts Located in tlicCl~b$~lWoiksliop -Light Coronograph (S052): Use an extcrnally Spccimen Mass Mcnsurerncnt (M074): Demonstrate occulted coronogrnph to monitor, in the 4000 to GOO0 A the fcensibility of mass mcnsurcmcnt Wit.lKJilt gravity rnngc, the brightness, form, and polarization of the to assess food intake, urinary output, and bone and solar corona from 1. 5 to G solar radii. muscle changes during flight. X-Ray Spectrographic Telescope (S054): Record In-Flight Lower Body Ncgativc Pressure (M092): spectra of solar flarc X-ray cmission in the 2 to 10 A Record hcart ratc, blood pressure, and clcctrocardi- wavelength range wilh a resolution of 0.5 A. ogram data during flight with negative pressure on the lower body to evaluate spncc flight cardiovascular UV Scanning Polychromator Spectroheliometer (S055A): deconditioning. Photoelectrically record high resolution solar images in six spcctral lines simultaneously. Vcctorcadiog" (M093): Monitor clcclrical actions of the henrt during space flight, using sensors and signal I Dual X-Ray Tclcscopc (S05G): Obtain high resolution conditioners to obtain vectorcenrcliograms. (5 arc SCC) photographs of the sun's coronal X-ray emission in the 3 to 60 A wavelength region. Hmnn Vestibular Function (M131): Evaluate the condition of the crcw during flight to determine angular Extrcme UV Coronal Spectroheliograph (S082A): ncccleration comfort zone and to identify vestibular Obtain high resolution (5 arc sec) spectroheliograms chnngcs. of the solar atmosphere in the 150 to 650 A wavelength range. I'inic Rnd Motion Study (M151): Use time nnd motion studlcs of standardized mcchnnicnl tasks tr, evaluate Extreme UV Spectrograph (S052B): Record spectra of thc rclativc consistency bctween ground-based and in- the solar disk in the 900 to 3900 A wavelength region flight astronaut pcrformance. with a 0.08 to 0.16 A spectral resolution.

- - * Compllod by Mmtln Merlclts Cop. 11 Metabolic Activity (M171): Evaluate man's metabolic Effects of Zero-g on Human Cells (S015): Study the in- effectiveness in space to determine long-duration flucncc of zero-g on living human cells and determine if mission requirements for logistics resupply, environ- the absence of gravity has a significant effect on their mcntd control, and task planning. metabolism. Body Mass Measurement (M172): Validate a mass Potato Respiration (SOG1): Determine whether removal measurement device large enough to contain a man and from the earth's rhythmic geophysical environment will to provide data for bone and tissue studies. affect a well-known biorhythm. Gravity Substitute Workbench (M507): Access the use Circadian Rhythm, Pocket Mice (S071): Determine the of aerodynamic and electrostatic force fields as an aid effects on the physical functions of pocket mice when in the manipulation of loose objects in zero-g. removed from gravity and the geophysical 24-hour period. Astronaut EVA Hardware Evaluation (M508): Evaluate Circadian Rhythm, Vinegar Gnat (S072): Determine the man's capability to perform work under the conditions effects on the physical functions of vinegar gnats when imposed by space flight and develop quantitative design removed from the gravity and the geophysical 24-hour criteria applicable to future missions in space. period.

Astronaut Maneuvering Equipment (M509): Obtain data Experiments Located in the Multiple Docking Adapter on the mechanical and human-factor problems encoun- tered by man using maneuvering devices. Zero-g Flammability (M479): Determine the effects of zero-g on the flammability of nonmetallic materials in UV Stellar Astronomy (SO19): Perform a partial sky a spacecraft environment. survey of fields to obtain UV spectra using a Ritchey-Chrctien objective-prism spectrograph. Material Processing in Space (M512): Demonstrate and evaluate molten-metal flow characteristics under zero-g X-Ray UV Solar Photography (S020): Obtain X-ray/UV and space vacuum conditions. solar spectra by a grazing incidence spectrograph to support development of prediction techniques. Nuclcar Emulsion (SOO9): Investigate the physical and chemical characteristics of primary cosmic Gegenschein Zodiacal Light (5073): Measure the intensity incidcnt on the earth's atmosphere. and polarization of the night sky light in the zodiacal "3 Gegenschein region. W Airglow Horizon Photography (S063): Secure phota- graphs of the UV emission from the airglow layers of the upper atmosphere. Particle Collection (S149): Study flux, size, compo- sition, and vclocity of in the near- Multispectral Photographic Facility (Si 90): Determine earth environment. the cxtcnt to which multiband photography may be applied In-Flight Aerosol Analysis Determine the to earth sites using six Ifasselbald electric (T003): cameras with synchronized shutters. aerosol particle concentration and size distribution in the spacecraft atmosphere as a function of time. Spectrometer (S191): Manually acquire and track ground truth sites Crew/Vehiclc Disturb,uces (T013): Measure the effects to obtain spectrometer data to evaluate earth resources sensing from orbital of crew motion on the dynamics of their spacecraft and dctermine how these affect high accuracy point- altitudes in the visible to infrared spectral regions. ing experiments. Ten-Band Multispectral Scanner (5192): Secure quantitative radiance values simultaneously in ten Foot-Controllcd Mancuvering Unit ("020): Determine thc Cecnsibility of the maneuvering unit for astronaut spectral bands, from visible to infrared, using imagery translation and rotntional maneuvers in space. scanning with automated data processing techniques. Coronograph Contamination Measurement (T025): E-xperiments Located in the Airlock Module Monitor the presence of particulate matter in the near Expandable Airlock Technology (DO21): Demonstrate vicinity of the spacecraft and provide measurements of the feasibility of employing expandable structures in the solar f-corona. an earth-orbital environment. ATM Conhmination Measurement (T027): Measure the Thermal Control Coatings (D024): Determine the shy brightness background caused by solar illumination effects of the near-earth environment on thermal of contamination particles around a spacecraft and control coatings to gain new into the determine the effect of contamination on the optical mechanisms of degradation. properties of lenses and . , Altimeter, and Radiometer Experiments Located in-the-command Servic_e_Mod@? (S193): Obtain active and passive microwave data from Radiation in SDacecraft 03008): Measure and record the space for application to earth resources disciplines. absorbed radiation inside the spacecraft to assure astronaut Experiments Located in-the Instrument UniJ nwnreness of any dangerous increase in radiation levels. Galactic X-Ray Mapping (S150): Perform a high Mineral Balance (M071): Precisely measure the input and sensitivity survey of a portion of the celestial sphere output of calcium and by the astronaut to quantify to dctermine galactic X-ray sources and to develop an r:itcs of gain or loss (also conducted in the OWS and upon undcrstanding of the apparent phenomenon of X-ray rcturn). background radiation.

12 Precision Optical Tracking (T018): Track the Saturn Cytogenetic Studies of Blood (M111): Determine the space vehicle with a radar system during the preflight and postflight chromosome aberration early iaunch phase to determine liftoff motions. Crequencies in the peripheral blood leukocytes of the Preflight and Postflight Experiments crew. Bone Densitometry (M072): Make a densitometric Man’s Immunity in Vitro Aspects (M112): Assay comparison of preflight X-rays of selected bones of humoral and ccllular immunity as reflected by the the body to evaluate bone demineralization under plasma concentrations of the major immunoglobulin prolonged . classes, study the functions of blood lymphocytes. and assay selectcd coagulation factors. Bioassay of Body Fluids (M073): Compare plasma and urine samples taken before and after flight to assess the Blood Volume and Red Cell Span (M113): Document metabolic changes in man as a result of space flight. changes in red cell mass, red cell survival, and plasma volumes occurring as a result of space flight. Lower Body Negative Pressure (M091): Apply negative Rcd Blood Cell Mebbolism (M114): Determine the pressure to the lower half of the astronaut’s body before and after flight to ascertain the cardiovascular effects of space flight on red cell metabolism and function changes resulting from space flight. membrane integrity.

1. Chubb, W. B., Schultz, D. S., and Seltzer, S. hl., 5. Kennel, H. F., A Control Law for Double-Cimhaled Attitude Control and Prwision Pointing of Apollo Control hloment Gvros L-sed for Space Vehicle Attitutlc Telcscope hIount, Journxl of Spacecraft and Control, NASA TM X-64536, July 20, 1970 Vol. 5, No. 8, Aug. 19GS. 6. Kennel, H. F., Angular ?rIomenturn Desnturation 2. Chuhb, W. B., and Egstein, LIichacl, Application Using Gravity Gradient Torques, SASA TM X-53748, of Control Moment Gyros in the Xttitudc Control of Mav 27, 1968. the Apollo Telcscope LIount, AI.\.-\ Paper No. 68- 866. Xug. 1968. 7. Skylab-A ATM Digitnl Computer Program Require- ments Document (PRD), 50M-37941, George C. .>. Chubb, W. B, Stabilization and Control of the Apollo Marshall Space Flight Center, July 1, 1970. Telescope Mount, NASA TM X-53831, May 6. 19r;9. 1. Seltzer, S. &I., Developing an Attitude Control 8. Skylab-A Apollo Telescope Mount Digital Computer System for the Apollo Telescope LIount, Second Flight Program, Program Definition Document, Asilomar Conference on Circuits and Systems, IBM #70-207-0002, NOV. 4, 1970. Pacific Grove, Calif., Oct. 30- Sov. 1, 1968.

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