Spacecraft 8 Attitude Control Anomaly Report

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1NASA PROGRAM GEMINI WORKING PAPER NO. 5050

SPACECRAFT 8 ATTITUDE CONTROLANOMALY REPORT

DISTRIBUTION AND REFERENCINGThis paper is not suitable for general distribution or referencing. It may be referencedonly in other working correspondence and documents by participating organizations.


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NASA PROGRAM GEMINI WORKING PAPER NO. 5050

SPACECRAFT 8 ATTITUDE CONTROL ANOMALY REPORT

Prepared by: James L. Gibbons, Test Evaluation OfficeVictor P. Neshyba, Test Evaluation OfficeTest Operations, Gemini Program Office

Approved by:

S. H. SimpkinsonManager, Office of Test Operations

Authorized for Distribution:

Charles W. MathewsManager, Gemini Program Office


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FOREWORD

The attitude control anomaly which occurred on the Gemini VIIImission became the focus of attention of many Manned Spacecraft Centerand vehicle contractor personnel. The authors sincerely acknowledgethe contributions of the personnel who participated on the MissionEvaluation Team, and in the testing, analyses, and planning of designmodifications and procedural changes for subsequent flights. To a con-siderable measure, this document is a report of their contributions.In addition to the basic work accomplished by a large number of people,the following personnel assisted directly in the documentation of thisreport:

William H. Douglas, Test Operations OfficeGregory P. Mclntosh, Test Operations OfficeLonnie W. Jenkins, Propulsion and Power DivisionJohn E. Williams, Test Operations OfficeEdward P. Gammon, Test Operations OfficeCharles S. Pinch, Guidance and Control Division


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TABLES

Table PageI SUMMARY OF POSTULATED FAILURES 12

FIGURES

Figure1 OAMS Propellant consumption during TCA 8

quiescent period 162 Electrical circuitry associated with TCA 8 173 Circuit conditions necessary in order for a single

high-resistance short circuit to allow the oxidizersolenoid valve only to be open 18


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1.0 PURPOSE

The purpose of this report is to document (l) the characteristicsof the Spacecraft 8 attitude control anomaly; (2) the analyses whichwere undertaken to identify the cause of the anomalyj (j) the resultsof the analyses; (k) the action taken to avoid a recurrence of theanomalyj and (5) the procedures established to effect rapid vehiclecontrol "by the crew in the event of a recurrence of this anomaly onfuture spacecraft.

2.0 SUMMARY

The Spacecraft 8 Orbital Attitude and Maneuver System (OAMS) thrustchamber assembly (TCA) 8 started to fire in an uncontrolled and inter-mittent manner while the spacecraft was docked with the Gemini AgenaTarget Vehicle (GATV). By undocking the two vehicles, the crew isolatedthe problem to the spacecraft, and they regained control by opening thecircuit breakers to remove power from the OAMS and by activating andusing the Reentry Control System (RCS). Analysis reveals that theprobable cause of the thruster firing was a short circuit to spacecraftground .which allowed current to flow through the thruster valve solenoidcoils, thus causing them to open the poppet valves. The intermittentnature of the fault has not been clearly established, but it may haveresulted from an intermittent short circuit or a combination of short-circuited and open-circuited wires.

3.0 DESCRIPTION OF EVENTS

The Gemini VIII attitude control anomaly occurred during revolu-tion 5, approximately 26 minutes after docking, and while the spacecraftwas out of range of network stations and tracking ships. Prior to theanomaly, the GATV was commanded by the flight crew to yaw the dockedvehicles 90s. This maneuver was successfully completed in 55 secondsand with a yaw rate very close to the normal 1.5 deg/sec.At 7:00:00 ground elapsed time (g.e. t.), the docked spacecraft-GATVcombination had been configured for the platform parallelism test, theAttitude Control System (ACS) was in flight control mode 3, and the GATV


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pressure. In the following discussion, it should become clear that thecause of the inadvertent firing was a short circuit to spacecraft groundin the circuitry between the Orbital Attitude and Maneuver Electronics(OAME) package and the thruster valve solenoid coils.The following information is presented as being pertinent to theanalysis of the problem:(a) At 7:00:26.785 g.e.t., TCA 8 turned on for U.9 seconds, turned

off for 4.0 seconds, and then turned on again. The thruster on-off telem-etry data and the rate-gyro telemetry data verified this condition. At7:00:38.285, the OAMS was activated to stabilize the docked spacecraft-GATV combination. The telemetry data indicated that TCA 8 stayed on forthe remainder of the mission. However, the rate-gyro telemetry dataindicate that at 7:02: 37. 4 TCA 8 either stopped thrusting or the thrustlevel was sharply reduced. There was evidence of a continuing low levelof constant thrust. TCA 8 remained in this mode until 7:07:20.3 g.e.t.,at which time the rate-gyro data indicate that TCA 8 resumed operationwith a full thrust and that it continued operating until the circuitbreaker was opened at approximately 7-18:15 g.e.t., although some vari-ations in its thrust level occurred immediately subsequent to undocking.During the 4-minute 42.9-second interval that TCA 8 was inoperative or wasthrusting at a very low level, the rate-gyro telemetry data show periodsof roll-left and yaw-left acceleration without any other thruster beingactive. This acceleration is in the range of that which could be causedby TCA 8 expelling only oxidizer.

(b) Figure 1 depicts for the period of degraded thrust, a compari-son of propellant usage calculated from propellant temperature and pres-sure parameters with propellant usage calculated from thruster activi-ties during the same period. Three curves are portrayed. The firstcurve is based on the measured thrust activity of TCA's 1 through 7 andassumes that both the fuel and the oxidizer valves of TCA 8 were closed.The second curve is based on the same activity of TCA's 1 through 7 plusa continuous flow of oxidizer only from TCA 8 during this period. Thethird curve includes flow of both oxidizer and fuel and is included toaid in the interpretation of the other two curves. (Based on rate data,the third curve is not a possible mode of TCA activity.) The effect oftemperature on flow rate has been included in the calculation of thedata. Based on the data, it is evident that no conclusion may be drawn


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and grounding the other terminal through the GAME. Each OAMS thrusteris operated by simultaneously energizing the fuel and oxidizer solenoidvalves with one transistorized valve driver in the GAME package. Thegrounding circuit is shown in figure 2.Enclosures 1 through k are extracted sections of the Gemini VTIIMission Report and provide a detailed analysis of the respective opera-

tions and systems related to the anomaly in support of the contents ofthis report:(a) Pilots Report - Appendix A(b) Guidance and Control - Appendix B(c) Propulsion System - Appendix C(d) Electrical System - Appendix DThe possible causes of the control anomaly have been carefullyweighed and considered in a detailed study of all available data in-cluding the results of investigative action taken as detailed in Sec-tion 5.0 of this report. A set of postulated failures has been derivedand table I is a listing of these postulated failures in relation tothe known factors and the sequence of events for the anomaly. Most of

the possible causes are ruled out on the basis of inconsistency with allknown factors, although each could provide a solution to some of thefactors involved. These possibilities are separately listed intable I a. Based on evidence uncovered to date, the most probable causeis that a short circuit occurred either at the four-wire Junction or be-tween this junction and the fuel-valve solenoid. This was probably fol-lowed with a rupture of wire so that the wire from the solenoid becameopen circuited while the other end of the ruptured wire remained shortcircuited to ground during the degraded thrust period. Finally, theopen-circuit wire was again recontacted and remained short circuitedat least until the circuit breaker was opened.

It is noted that if the low-level continuous thrust during thequiescent period is attributed -to some cause other than the flow of


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5.0 ACTION TAKEN

The following is a summary of investigative actions taken as a re-sult of the spacecraft anomaly and the results of each action. Alsoincluded is a summary of preventive measures which have been accomplishedand a listing of procedures which have "been formulated to assure recoveryof spacecraft control in the event of a similar occurrence.

5.1 Spacecraft TestsA series of tests to investigate the OAMS thruster anomaly were con-ducted on Spacecraft 8 during the postflight evaluation at the contrac-tor "s facility. These tests were conducted in response to SpacecraftTest Requests (STR's) written during the evaluation. Additional testswere conducted on equipment similar to that installed in the Spacecraft 8adapter equipment section which was not recovered.5.1.1 Propulsion (STR's 8050 and 851$).- Six solenoid valves weredissected and visually inspected to evaluate the quality of the wiringinsulation and potting. Four of these valves were removed from TCA's 5Band 3A of the Spacecraft 8 RCS. TCA 5B was selected "because it was man-ufactured at approximately the same time as the OAMS TCA 8. The othertwo valves inspected were selected at random from scrapped valves loca-ted at the Manned Spacecraft Center and at the spacecraft contractor's

plant.The valves and the electrical risers were sectioned and inspectedand no indication of poor quality or any other abnormality that wouldcontribute to a solenoid-valve malfunction was found.Two TCA propellant valves with the same configuration as those onSpacecraft 8 were subjected to a series of functional and high-potential

dielectric-strength tests to determine the effect of usage on dielectriccharacteristics. The valves tested had accumulated a total of 1230.6 sec-onds of firing time and 25 100 valve cycles. These valves proved to beunaffected by their previous usage. In addition, a 1000 V ac dielectrictest was employed rather than the normal 500 V ac test.


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attitude control system. The cover of the ACE package was removed be-fore starting the test. Evidence of salt-water corrosion and arcingwere visible. The tests were performed using the Aerospace GroundEquipment (AGE) connectors to the maximum extent possible. Some abnor-mal readings were encountered, but later these were resolved as havingbeen caused by the condition of the ACE package. During the next tests,the flight wires were disconnected from the ACE and their continuity,to the spacecraft wiring was checked. No abnormal readings were found.The evidence of arcing in the ACE indicated that the ACME control 1 and2 circuit breakers may have been on at landing. The results of thesetests indicated no anomalies in the spacecraft reentry-section wiring.5.1.3 Electrical wire bundle clamps.- A detailed evaluation ofthe wiring was conducted to Investigate the condition of cable clampsand wire-bundle chaffing protection. Ten areas were investigated wherewire-bundle clamps were close to wire-bundle branch segments. Wirebundles with bent radii under clamps were particularly selected. The

areas were photographed and the clamps were then removed to inspect fordamage to wire bundles caused by clamping. No damage was found. Clampcushions were also examined for cold flow and the results were negative.

5. 2 Systems Analyses5.2.1 Propulsion system.- A review of valve design, testing pro-

cedures, and system tests was conducted. Acceptance test data forTCA 8, and the history- of- failure records of all Gemini thruster sole-noid valves during manufacturing, development, qualification, and reli-ability testing were examined. The probability that the failure can beattributed to a short circuit within the valves is considered remote.The coil windings are insulated from the bobbin by two layers ofmica and a silicon varnish in addition to the wiring insulation. Thecoil is insulated from the valve case by fiberglass wrappings andsilicon varnish. The coil lead wires are insulated by Teflon sleevingin addition to the normal insulation and are also potted with Sylgardin the valve riser to within a very short distance of the solder pins ;on the electrical receptacle. Soldered connections to the receptacleare protected by Teflon sleeves extending from the receptacle into the


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conducted again during engine pre-delivery acceptance and in PIA testing.During all 500-V rms dielectric-strength tests, the potential was ap-plied for 1 minute with an allowable maximum current of 500 |jA. Theminimum allowable insulation resistance is 500 Mf> During dielectric-strength testing performed in PIA tests, the flow of current betweenthe pins of the electrical receptacle and the oxidizer valve case was47 pA, and similarly for the fuel valve, the current was 39 |jA- Thecoil resistances were 43.1 ohms and 42.7 ohms on the oxidizer and fuelvalves, respectively, and were within the 42 (2) ohm limits. X-raystaken during manufacturing of the fuel and oxidizer valves for TCA 8show that these valves were assembled normally in the riser to the re-ceptacle, the most suspected area.

Relevant valve failures encountered on all configurations (25 and100-pound thrusters) and modifications (Spacecraft 2 through 6 types)were reviewed by the spacecraft contractor, the thruster manufacturer,and the NASA. These revealed only one failure which could have producedthe anomaly. A metal chip between the coil and solenoid case caused ashort circuit which would have had the effect of either grounding thepower supply or of actuating the valve, depending upon attachment ofthe coil lead. This type of failure would normally be detected by meansof the dielectric-strength testsj however, this particular valve did nothave the chip and burr controls that were exercised on the valves usedfor Spacecraft 4 and subsequent spacecraft. The sample size investi-gated was in excess of 2000 valves. During pre-delivery acceptancetesting, a total of 11 thrusters were rejected for failure to pass thedielectric requirements previously discussed. None of these thrusterswere ever close to the point of indicating an incipient short circuitto ground.

5.2.2 Attitude control and maneuver electronic (ACME).- The OAMEcircuitry was reviewed and it was determined that a short circuit in thedriver transistors or in the circuitry between the driver transistorsand the output or AGE terminal connectors (connectors 5J2 and Jl seefig. 2) could cause TCA 8 to fire. A malfunction in other portions ofthe ACME could cause inadvertent thruster firings, but would fire a pairof thrusters rather than a single thruster.

This analysis limited the area of possible failure within the OAMEto the following:


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(d) A short circuit in the wiring leading to AGE terminal Jl(e) A short in the driver transistors.The crew reported they had switched from the primary to the sec-ondary drivers. If this action were accomplished while the OAMS powerswitch was turned on,' it would eliminate the driver transistors and thecircuit "between the driver transistors and relay K106 as suspect.Any of the five possible short--circuits previously listed wouldnormally activate "both the oxidizer and fuel solenoids, and would simu-lanteously cause a telemetry indication of thruster ON. An interruptionof this short circuit would cause "both solenoids to drop out and closethe fuel and oxidizer poppet valves. The same interruption would causea telemetry thruster-off indication.If it were possible to have a high-resistant short circuit in theGAME package that would cause the fuel solenoid to drop out withoutaffecting the oxidizer solenoid, the resultant voltage at the input tothe telemetry system high-level multiplexer would "begreater than the5- to 11-V minimum required to cause a telemetry indication of TCA 8off. This is illustrated in figure 3- There were periods positivelyestablished during the anomaly, when the telemetry indicated a thrust-ing and no acceleration resulted. Because the telemetry data are not

consistent with the known factors, the only possibility'that the GAMEpackage caused the Gemini VTII anomaly would have to include a simul-taneous and separate failure in the telemetry system high-level multi-plexer or associated wiring which resulted in the continuous telemetryindication of TCA 8 being on. The failure history of the componentswere reviewed and no failures were recorded which could result in ashort circuit to ground. The Spacecraft 8 GAME had no failures duringbuildup, or pre-deli very acceptance, or PIA testing. There were no ACMEproblems during Spacecraft Systems Tests (SST). Component installationdrawings and procedures were reviewed in detail to determine if a proba-bility for misinterpretation existed. All drawings and procedures werefound to be adequate.The system was then reviewed to determine if improvements were re-


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The design of the modules and printed circuit "boards surroundingthe parameter module in question was investigated to determine the pos-sible existence and characteristics of ground circuitry or mechanicalstructure in the proximity of the signal circuits.The TCA input is isolated from the instrumentation system by a105 000-ohm resistance. The cable area containing the wiring from the

input connectors to the Microdot connectors was investigated. Allshielded wiring within this section has an outer cover of transparentinsulation. The entire cable area is filled with high-density foampotting which presents any relative motion between wires, connectors,and structure. The modules are also foam potted to the individualmother boards, and the mother boards are separated with a layer of ap-proximately l/l6 inch of mica-mat and mylar insulation board.A review of the failure history of the connectors involved revealedthat six failures of this type connector had occurred. In one of thefailures, two pins broke off within the connector; this condition wasdetected when the connector was demated. The remaining five failureswere caused by misalignment of the connector pins when the connectorwas mated. As a result of this misalignment, the affected pin wascrushed within its individual pin well. None of the recorded failuremodes of these connectors displayed any tendency to expose the faultypin to other circuitry in the vicinity or to ground. Any failure whichexisted prior to launch and which would result in an open-circuited orshort-circuited connector pin will be detected by normal testing.Nothing was found in the design of the, instrumentation system whichwould be suspected of causing an anomaly such as that experienced onSpacecraft 8.

5 3 Quality Evaluation5. 3-1 Test histories.- A list of postulated failure modes was de-rived as part of the anomaly investigation. The test histories of com-ponents which could contribute to postulated failure modes were reviewedto determine if there was any history of failures similar to the postu-


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determine the adequacy of the quality of the wiring harness, the wireprotective techniques, the Inspection criteria, and the effectivenessand timeliness of the wiring-harness inspections. The following areexamples of some of the discrepant conditions that were found:(a) Wire "bundles laying against structure edges, rivet heads, nuts,"bolt heads, and coaxial-cable fittings. In these cases, neither the wire"bundle nor the edge, et cetera, was covered to prevent contact and con-

sequent wire insulation damage.(b) Excessively long ground wires(c) Wire "breakouts from wire "bundles where the wire "broken out wasunder tension(d) Excessive harness lengths.As a result of this evaluation, the following actions were taken:(a) Reworked discrepant conditions in Spacecraft 9 adapterassemblyC"b) Established more stringent criteria for wire installation andinspection(c) Established a formal physical spacecraft inspection, prior toshipment, to reinforce in-process inspection during spacecraft fabri-cation(d) Established a more rigid spacecraft receiving inspection atKennedy Space Center and implemented formal shakedown inspections atspecific milestones in the preflight test plan(e) Established a special NASA spacecraft acceptance inspectionteam to perform detailed spacecraft inspections with the results ofthese inspections being reported at the Spacecraft Acceptance Review(SAR) Board meetings.Ground rules covering critical wiring installation aspects were


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11(a) CAMS thruster disabling: A single switch which will providethe crew with a rapid means of disabling "n QAMS thrusters has beenincorporated into the OAMS circuitry.(b) Circuitry protection: All exposed electrical circuitry in theOAME package has been coated with a soft conformal epoxy to protect

against possible inadvertent electrical short circuits.(c) Wiring configuration: Grommet material has been added to thestructure in six general areas to increase clearance of wire bundles.The Z-10J fairing has been trammed to increase clearance between thefairing and the wire bundle. Engineering orders have been issued toremove the tape from the TCA heater and solenoid wiring.(d) Wire-bundle changes: Specially selected clamps are being usedexclusively for Gemini wire-bundle retention.5.3'^ Flight procedures.- Flight procedures have been re-evaluatedand revised to provide the crew with optimum operating procedures fora.n. known possible flight emergencies. A copy of these procedures isattached as Appendix E.


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APPENDIX A

PILOT'S REPORT(Excerpted from Section 7.1.2 of theGemini Program Mission Report for Gemini VIII)


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A - 37.1.2.9 Control system problem.- At approximately 7 hours g.e. t.,the two spacecraft were configured for the platform-parallelism test,which was to have provided a comparison of the spacecraft and GATVattitude reference systems. The GATV Attitude Control System (ACS) wasactive, and the TDA L-band transponder was off. The spacecraft attitude-control power switch and maneuver-control switches were off. The radarwas off, and the control mode switch was in PULSE.Shortly after sending encoder command C4l (recorder OK), roll andyaw rates were observed to be developing. No visual or audible evidenceof spacecraft thruster firing was noted, and the divergence was attrib-

uted to the GATV.Commands were sent to de-energize the GATV ACS, geocentric rate,and horizon sensors, and the spacecraft Orbital Attitude and ManeuverSystem (OAMS) was activated.The rates were reduced to near zero, but began to increase uponrelease of the hand controller. The ACS was commanded on to determine

if GATV thruster action would help reduce the angular rates. No im-provement was noted and the ACS was again commanded off. Plumes from aGATV pitch thruster were visually observed, however, during a periodwhen the ACS was thought to be inactivated.

After a period of relatively stable operation, the rates once againbegan to increase. The spacecraft was switched to secondary bias power,secondary logics, and secondary drivers in an attempt to eliminate pos-sible spacecraft control-system discrepancies. No improvement beingobserved, a conventional troubleshooting approach with the OAMS com-pletely de-energized was attempted, but subsequently abandoned becauseof the existing rates.

An undocking was performed when the rates were determined to below enough to preclude any recontact problems. Approximately a 3 ft/secvelocity change was used to effect separation of the two vehicles.Angular rates continued to rise, verifying a spacecraft control-system problem. The hand controller appeared to be inactive. TheReentry Control System (RCS) was armed and, after trying ACME-DIRECTand then turning off all OAMS control switches and circuit breakers, wasfound to be operative in DIRECT-DIRECT. Angular rates were reduced to


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APPENDIX B

GUIDANCE AND CONTROL(Excerpted from Section 5.1.5 of theGemini Program Mission Report for Gemini VIII)

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5-1.5-3 Control system performance evaluation.- ContinuedAt 7:00:26.7 g.e.t., approximately 27 minutes after docking, the

telemetry signal from thruster 8 indicated ON for .9 seconds, OFF fork.O seconds, then ON for the remainder of the flight. The spacecraft/GATV combination was being controlled by the GATV Attitude ControlSystem (ACS) at this time in Flight Control Mode 3. The system wasgyrocorapassing, in-plane, with geocentric (GEO) rate ON. The OAMSattitude control power was OFF, the ACME mode select switch was inPULSE, and the IMU was in 0KB RATE. In this configuration, the ACMEis incapable of transmitting valid firing commands to the thrusters.Figure 5.1.5-19 contains the sequence of significant events asthey occurred during the anomaly plotted in relation to spacecraft rollrate. As indicated, the initial telemetry firing indications fromthruster 8 were correct, in that the dynamic response matched the dis-turbance which should have been present. The first corrective actionwas taken, with the ACME in pulse mode, 11.5 seconds after the anomalyoccurred. This mode was ineffective due to the short firing timesassociated with pulsed operation; therefore, DIRECT and then RATE

COMMAND were selected with more success. In fact, while in the rate-command mode, the rates were essentially reduced to zero. At7: 02:37 ^ ge.t., the dynamic responses indicate that thruster 8 stop-ped firing, although the telemetry indication remained ON. Low gradeaccelerations were present which were representative of those which canresialt from a thruster expelling oxidizer only. Accelerations of thisorder could also have been obtained from the GATV ACS (for which notelemetry data are available), but in a very unlikely set of conditions.During this period, several firing commands were sent to thrusternumber 8 with no response. At 7*07*20.3 g.e.t., after an interval ofk minutes ^2.9 seconds, the original disturbance returned, indicatingthat thruster number 8 was again operating at or near full thrust. Fromthis time until the spacecraft was separated from the GATV at7: l^:12.3 g . e t., the disturbance was present and, as seen in fig-ure 5.1.5-20, was controllable in the direct mode. The pitch and yawrates were held to low values during this period; however, the rollrate did exceed 10 deg/sec for a total of approximately 100 seconds insix 15-to-20 second intervals. Each time the roll rate exceeded10 deg/sec, it was quickly brought back to near zero using the directcontrol mode, and did not exceed 20 deg/sec at any time prior toundocking. The status of the GATV ACS throughout this period is un-:


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momentarily at 7:13:38. 8 g-e.t. There was no telemetry channel toindicate the utilization of the yaw/pitch roll-logic switch or themotorized fuel shut-off valves; however, by analyzing the combinationof thruster firings in response to roll hand-controller commands, itwas determined that the pitch logic was not selected for roll controlduring the anomaly period.Separation from the GATV occurred at 7:15:12.3 g.e.t. with thrus-ters 11 and 12 firing for 6.6 seconds. Rates at this time were +3;-5, and -2 deg/sec in pitch, roll, and yaw, respectively. Afterseparation, moderate hand-controller activity was present, althoughthe direct mode was not sufficient to contain the roll rate. At

7:15: 7 g.e.t., the ACME bias power was inadvertantly removed,dis-abling the control system, and the roll rate increased to 296 deg/secover the next three minutes, due to the uncontrolled firing ofthruster 8, although short periods of intermittent or degradedthruster 8 performance appeared to exist. It is clear that the crewwas not aware that ACME bias power was off because significant hand-controller activity is evident during this period. As noted in fig-ure 5.15-19> the RCS squib valves were actuated at 7;l6:25.1 g.e.t.,but no RCS thrusters were fired until 7:19:03.8 g.e.t., probably be-cause the ACME-DIRECT switch was in the ACME position with the ACMEbias power off. When the ACME-DIRECT switch was apparently placed inthe DIRECT position, RCS control was normal. The disturbance torquefrom thruster 8 ceased at 7:18:15-7 g.e.t. when the CAMS attitude-thruster circuit breakers were opened. Control was regained using theRCS in DIRECT-DIRECT. Subsequent checks of the CAMS thruster 8 cir-cuit breaker and the RCS using ACME modes indicated correct ACME per-formance; in addition, telemetry indications and fault characteristicslead to the conclusion that the malfunction probably was external tothe control system. (See sections 5-1.7 an(i 5-1.8 for further discus-sions of the flight-control anomaly.)

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Spacecraft 8 Attitude Control Anomaly Report

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Transcript of Spacecraft 8 Attitude Control Anomaly Report