hardware,athotandcoldtemperature extremes,withnominalresults.Atthecompletion ofthistest,the integratedACISwasdeliveredtoMSFCforanadditionalonemonthcalibrationtestintheMSFCX-Ray Calibration Facility(XRCF)thermal/vacuum chamber.DuringX-raycalibration, severaladditionaldoor operationscycleswereperformedunderflight-likeoperatingconditionswithnominalperformance. After calibrationactivitieswerecomplete,theunitwasvibrationtestedaspartofthefinalacceptance testby MSFC.Preandposttestmechanism performance, whichincludeddoorcycling,wasnominal,thus addingfourmoredooropen/closecycles.TheintegratedACISwasthenshippedtoBallAerospace in BoulderforintegrationontotheIntegratedScienceInstrumentModule(ISIM).TheDetectorAssembly waskeptundervacuumfor5monthsuntiltheISIMwasfullyintegrated. ISIMlevelthermal/vacuum testingstartedinOctober 1997.Duringtheseteststhedoorwasopened andclosedtwomoretimes underflight-like conditions. Oneadditional successful dooropening occurred atambient temperature and pressure inJanuary of1998.TheISIMwasthenshipped toTRWforintegration ontotheChandra telescope. Periodic pumpdownsoftheACISDetector kepttheinternal pressure between 0and10torr forallofthesystem integration andtestactivities (including system Acoustic test). Related Testing Another STARSYS HL4570B rotaryactuator (samemodelasfordoor)wasusedintheACISLargeVent Valvemechanism. Afailureofthisactuator occurred duringtheACISsystem thermal vacuum testatMIT Lincoln LabsinAprilof1997.Thisfailurewasduetoanoperator errorwhichcaused boththeopenand closeactuators tobepowered simultaneously. Underthiscondition, oneofthesheardisksruptured and required theremoval andreplacement oftheLargeVentValverotaryactuator withaflightspareactuator. Corrective actionwastofollowprocedures toprevent simultaneous powering ofopenandclose actuators. Thelargeventvalvewascycledhundreds oftimesduringtestandintegration andwasnever anissuebeforeorafterthisevent.However, thisfailureonthelargeventvalvepointed toapotential weakness intheactuator design. Asdiscussed earlier, diskrupture willnormally occuratacase temperature ofabout154°C,andthePSMCwillcut-offpowerwhentheactuator temperature reaches about135°C. However, itwasdiscovered thatthis154°Crupture temperature wasbasedonalinear actuator inwhichtheoutputpinisallowed toextendnormally tofullextension. Therefore, therotary actuator mustoperate withstalltorques lessthan6.8N-m(60In-lb)ontherotaryoutputshaft.Ifthe outputpinisrestrained fromextending, thenthereisnoadditional volume fortheparaffin toexpand into andthesheardiskwillrupture atamuchlowertemperature ifpowerisnotremoved promptly. Thus,in thecaseofarestrained actuator, thetemperature sensors donotprovide protection fromrupture except atend-of-stroke. Asitturnsout,instead ofaconstant temperature shut-off point,eachparaffin actuator couldbeprotected byavariable shut-off temperature dependent ontheamount oflinearextension ofitsoutputshaft (transformed intorotaryactuator shaftrotation). Atzerodegrees ofrotation fortherotaryactuator mechanism, theshut-off temperature shouldhavebeenaround 90°Crisingto150°Catfullrotation or end-of-stroke. However, sinceanormaloperation ofthedoororlargeventvalvewouldresultinan actuator temperature oft05-110°C, wecouldnotsimplylowertheshut-off temperature without affecting theabilitytooperate normally. Itwasalsotoolateintheprogram tore-design theelectronics toprovide a variable shut-off temperature. Itwasevident, however, thatinthecaseofaboundactuator shaftearlyin thestroke,wewouldnotachieve ourgoalofopening thedoor.Thispotential newprotection wouldhave allowed ustotryopening thedoormanytimesratherthanjustonce.Thedecision nottoimplement this protection wasbasedonourhighconfidence inourabilitytoopenthedoor,sincewehadneverobserved astiction torqueabove2.3N-m(20in-lb)inover3yearsoftesting. Tosummarize thecomponent andsystem leveltesting: theACISflightunitdoorwassuccessfully cycled atleast23timespriortotheopening attempt atTRWundervarious operating temperatures and pressures. TheACISteamhadconfidence inthedoormechanism performance forsystem levelthermal vacuum testandforflight.Therehadbeennoanomaly inanyoftheprevious dooropening attempts. In fact,closingthedoorhadalwaysbeenofgreater concern sincethedoorwasclosedagainst afrozen (stiff)O-ringseveral timesduringthesetests.Measured paraffin actuator casetemperatures werealways 20