the6.8N-m(60in-lbf)specification. Materialtestingshowedthatboththeopenandclosesheardisks weresimilarinthicknessandshearproperties,soitislikelythattheSTARSYSrotaryactuatoroutputwas around10.2N-m(90in-lb)atthetimeofthefailureatTRW.Completedisassembly andinspectionofthe mechanism andactuatorsdidnotrevealaproblem.Nothingwasobstructing themechanism andtheonly discrepancy wastherupturedsheardisk.Thisclosedoutnumerousbranchesonthefaulttreesince everythingappearedandfunctionednominally. Thesheardiskrupturepointvarieslinearlydependingontheamountofextensionoftheactuatorlinear outputshaft,whichcanbeanywhere from0to19mm.At0mmoflinearextension, testdatashowed the actuator wouldreachanequivalent pressure toachieve 6.8N-m(60in4b)ontheoutputshaftataround 90°C.Therupture temperature wouldbehigheratanequivalent outputtorqueof10.2N-m(90in4b). Foranextension of19mm,thesheardiskwouldrupture atanestimated 154°C. Subsequent discussions willshowthatwebelieve theflightunitsheardiskruptured ataround 132°Cwhichalso supports theevidence thatthedoormechanism hadrotated20-25degrees (5.7mmoflinearextention) at thetimeoffailureandwasnotlockedup.Theclearances inthebushings andthewindup inthe mechanism components couldallowtheactuator linearoutputpintoextendasmuchas5.7mmeven withastuckdoor. NASA/Industry TeamFormation andFailure Investigation Approach FailureInvestiaation Team Doormechanism failurediagnostics wereinitiated bythesmallteamofLockheed Martin, NASAMSFC andSTARSYS personnel asdiscussed above. Theirgoalwastodetermine thecauseofthefailure,fix theproblem, andthenreturntheACISdetector toflightconfiguration sothatitcouldbereinstalled on Chandra andstillmeetthe(thenprojected) January 1999launchschedule. Thisteamwasinitiallygiven twoweekstocomplete theseactivities, basedonthe(optimistic) assumption thatthefailuremechanism wouldbeeasilyidentified andrectified. Asiteventually became clearthatthefailurewasnotgoingtobe quickly resolved, NASAMSFCformed alargerACISDoor"Tiger"teamwhichincluded members from NASAMSFC,theChandra external reviewcommittee, MIT,Lockheed Martin, STARSYS, TRWandSAO. Theteam'sfunction wastooversee andfacilitate theongoing ACISdoorfailureinvestigation andthe decision making process. Intheend,theteamwastooperate foralmostoneyear,ascircumstances dictated, withweekly telecons andmeetings asnecessary. Thesituation atthetimethelargerteamwasformed, inearlyJuly1998,wasnotapromising one.As described above,initialworkbythesmaller teamhadruledoutmanyoftheproposed failure mechanisms. Also,program schedule pressure waslarge,eventhoughthelaunchdatehadbeenslipped tomid-summer 1999duetootherfactors. Theteam,alongwithseniorNASAandprogram management, decided topursueparallel pathsofoff-linefailureinvestigation alongwiththere-build andre-testofthe ACISdetector andthenre-integration withtheChandra Observatory. Itwasbelieved thatthere-test effortwouldalsosupplyvaluable datatothefailureinvestigation team.Asafurtherbackup possibility, NASA-MSFC evendeveloped analternate actuator thatcouldhavebeensubstituted intoACISshould subsequent problems withtheabovepathsoccurdownstream closertolaunch. Withmoreresources available now,manyactivities couldproceed inparallel, eventhoughthepdmary activity remained atLockheed Martin. Theseactivities included theexpansion ofthefaulttreetoinclude itemsnotinitiallyconsidered, investigation intotheissueoficeformation, moredetailed investigations into thestiction properties ofViton,asearchofthepriortestdatatolookforapossible inadvertant powering ofthedooractuator, development ofadetailed finiteelement modelofthedoormechanism toprovide additional designverification and,ofcourse, there-build oftheflightACISdetector forare-testatTRWin August of1998. ACISDetector Rebuild andRe-Test IntherebuildoftheACISdetector, muchattention wasgiventopossible waystogaininformation on performance ofthedooractuator. Instrumentation tomonitor doormechanism performance wasadded priortothere-testwhichstartedinAugust, 1998.Thisinstrumentation included non-flight straingagesto 24