over-center, anyresistiveforcewouldbeovercomebythepreloadedspringinthelockfuse(especiallyon theTDLAthathasasignificantly largerpreload(5338N(1200lb)vs.2002N(450Ib)). Duringthetroubleshooting testruns,itwasdiscoveredthattherewasarelationship betweenthetime elapsedonthelastpulseintothehardstopatthetimeofendofcommandandtheamountof"kickback" experienced. Theamountofpotentialenergyexistingatthemid-pointofthepulsefaroutweighed the energyattheverybeginningorveryend.Sincethenumberofstepsrequiredtoreachthehardstopisnot exactlythesameforeachrun(thetolerancestackistoolargetotryandcontrolthis),theamountof kickbackfromruntoruncanvarysignificantly. Thisphenomenon wasfirstdiscoveredduringhottestingbut canberepeatedatambientaswell.Itwasalsofoundthattheamountofkickbackwasrelatedtothe amountofcurrentdrivingthestepper.Highercurrentmeantpotentialforgreaterkickback. Atthispointseveraloptionswereconsidered forpreventingoratleastreducingthekickback.Theoption chosenwasazero-velocity commandthatallowedanyenergythathadbeenbuiltuptobleedoffpriorto poweringdownandsignificantly reducethekickback. Lesson: -Useofastepper motorincertainapplications mayrequireadditional commands inordertoavoid kickback. 3)ResoIver Interference DuringthecoldcycleofThermal Vacuum testingonaparticular unit,themotorfailedtorotateafterseveral revolutions. Thechamber wasbrought backuptoambient andthemotorwasinspected withavideoprobe through aremovable portonthemotorhousing. Theformation ofsomedebriswasdiscovered andthemotor wassubsequently opened up.Further investigation revealed indications ofaninternal interference withinone ofthemotorcomponents. Thisparticular vendorassembly contained apartthatwasnotreflected inthe controldrawing asitwasaddedlateinthedesignphase. Itprevented axialmisalignment inonedirection andwassignificant enough tostallthemotorduringcoldtesting. Theproblem wasresolved bymodifying thematinghardware toensuresufficient axialgrowthduetothermal conditions. Lesson: -Ensure Controldrawings accurately reflectcondition ofhardware. 4)Lockbindina TheDLA,oncefullytestedatacomponent level,isinstalled ontotheSARJandundergoes system-level testing. Duringthecoldcycleofsystem thermal vacuum testing(-42.7°C (-45°F)), anattempt tolockthe DLAfailed.Afterseveral attempts togettheDLAtorespond failed,thechamber wasbrought backto ambient andadditional troubleshooting wasperformed. Theunitresponded nominally atambient anduseful datawasgathered duringthereturntoambient astheEDMwasintentionally leftinthefailedposition (commanded tolock)andatapproximately -23°C(-9°F),thelockpopped intoposition. Sincethedata collected pointed toathermal issue,theareasoftightfitsontheEDMmechanism cameunderclose scrutiny. Theunitwasremoved fromtheSARJandunderwent athorough inspection. Itwasdiscovered thatthe bypass linkontheEDMcameincontact withatie-wrap attached totheframeasthemechanism cycled through theneutralposition. Itwasthought thatatambient thecablewasmorecompliant andwould compress enough toallowthemechanism topushthrough theinterference, butatcoldthecablewould stiffenandprevent mechanism motion. Theharness wasthenre-routed toensuresufficient clearances at temperature andtheunitwasreturned tothermal conditions atacomponent level.Duringthisretest,the failurewasrepeated, revealing eitheraninadequate repairoramisidentification oftherootcause. 7O