measureactuatoroutputtorque,apotentiometer onthetorsionshaft(flight)andanadditional temperature sensortobeusedfor"pulsedheating"oftheparaffinactuator.Thispulsedheating technique,inconjunction withpotentiometer readings,wouldpreventtheheatingoftheparaffinactuator tosheardiskfunction.Finally,arapidwarm-upandcool-downprocedurewasdevelopedand demonstrated toreduceO-ringstiction(oneofthepossiblecausesofthefailure)incasethepulsed heatingtechniquedidnotinitiallyopenthedoor. There-testoftheACISdoormechanism (August/September 1998)wasintendedtore-qualifythedoor mechanism underflightconditions. Itwasdecidedtore-testtheACISdoorasmountedtotheISIMonly, nottheentireobservatory. Thiswouldgreatlysimplifythetestingandalsoallowworkontheobservatory toproceedinparallelwithACIS/ISIMre-test.Akeydecisionofthefailureinvestigation teamandprogram management wasthatthetestwouldtakeplacewiththedetectorhousingwarm,aswasplannedforthe on-orbitopening.Thiswouldeliminateanyissuesoficeduringthere-test.Atthesametime,sincewater wasanissueinitselfduetopossibleVanDerWaals forces,waterwastobeinjectedintothe thermal/vacuum chambertosimulatetheoutgassing ofthecompleteobservatory's compositematerials. Someteammemberswouldhavepreferredtoduplicatealltestconditionsatthetimeoffailure(including detectortemperature), buttherewasconcernthatanotherfailure,evenundernon-flightconditions, wouldjeopardizeprogramschedule. There-builtACISdetectorwasshippedbackfromLockheedMartintoTRWinearlyAugust1998,and wasthenintegratedontotheISIM.TheISIMwithbothChandrafocalplanescienceinstruments (ACIS andHRC)wasthenmovedintotheTRWthermalvacuumchamber.Thesubsequent testthensimulated ascloselyaspossibletheprojectedon-orbitdooropeningscenario.Thepredictedtemperature profile wascloselyfollowed.TheACISdetectorhousingwaskeptcolduntilabout14daysintothetest,atwhich timethedoorwasopened.Priortodooropening,thedetectorhousingtemperature wasraisedto25°C usingtheACISbake-outheaters. Thedoorwasopened(andthenclosed)atotaloffivetimesduringtheISIMre-test,allusingthepulsed heatingtechnique. Thefirstthreeopeningsweredonewithabusvoltageof28voltsandthelasttwoat 32volts.Thedooranglepotentiometer anddriveshaftstraingagesweremonitoredduringthese openings.Allofthedatafromtheseopeningswasnominal.Thissuccessfulre-qualification oftheACIS doormechanism allowedtheprogramtoproceedtowardslaunch,butthefactthatthecauseoftheearlier failurewasstillnotknownwastroublingtoeveryone.TheACISfailureinvestigation teamwouldworkfor manymoremonths, stillattempting todiscover therootcauseoftheearlierfailure. Itshouldalsobenotedthattherewereadditional dooropenings inanambient condition scheduled and executed togainconfidence inthedooropening mechanism priortolaunch. Thiswasaccomplished through theuseoftheVGSEtore-pressurize theACISdetector housing cavitywithnitrogen toequalize thepressure priortoopening. Thiswasnotoriginally planned duetotheriskofcontamination entering thecavity,butamoredetailed lookatshowed thattheriskwassmallenough tobeacceptable giventhe circumstances. Ongoing Failure Investigations Theexpanded ACISdoorfaulttreeisshowninFigure3.Thistreeevolved overthecourseofthe investigation andguidedoureffortstoidentify thefailuremechanism. Asmentioned above,someofthe branches ofthetreewereaddressed byearlypost-failure inspections andtests. pr_ruptvr_l ShearDisk(FaulttreeItemD) Onepossible failuremechanism whichhadnotyetbeenruledout,wasanaccidental activation ofthe openactuator underambient pressure conditions priortotheTRWtest.Thiswouldhavetakenplace afterthelastopening atBall(January 98)andbeforetheopening attempt duringObservatory ThermalNacuum testing. Withavacuum insidethedetector, thesheardiskwouldhaveruptured ifthe openactuator werepowered whentheobservatory wasinanambient environment, sincethemechanism doesnothaveenough torquemargintoovercome oneatmosphere ofpressure against thedoor. Chandra flighttelemetry wasmonitored anytime thatChandra ortheISIMwerepowered up.An investigation oftheavailable telemetry datashowed notimeatwhichtheactuator temperature had 25