Damper Failure Investigation Pre-Mission Investiqation andFlightRationale Approximately twomonths beforetheeventual SRTMlaunchdatetheflightsparetorsion modedamping cartridge wasdiscovered tohaveseized, whilethesparebending modedamping cartridge appeared to function normally. Further quantitative evaluation ofbothdamping cartridges showed that:1)thespare bending modedamping cartridge function normally, and2)thetorsionmodedamping cartridge beganto strokeonlyafteraforceofgreater than440N(100Ibf)wasapplied. Normally, thetorsionmodedamping cartridge stroked once1N(0.25Ibf)wasapplied. Additional testingshowed thatoncethetorsioncartridge wascooledto0°Cthecartridge stroked readily. Atrendwasinferred basedonthesetwodatapointsinthe sensethatwhatever caused thecartridge seizure wasrelieved astheunit'stemperature wasdecreased. Basedonthistrend,adimensional interference between thelabyrinth sealinsidediameter andthepiston rodoutside diameter wassuspected tobethecauseofthecartridge seizure. Thepistonrodwas15-5 stainless steel.Whilethelabyrinth sealwasmadefromTorlonmaterial, andinstalled intoa302stainless steelhousing withamoderate interference fit.Basedonthematerials usedforthelabyrinth sealandthe pistonrod,aswellastheresidual stresscreated inthelabyrinth sealduetothepressfit,itwasknownthat theclearance between thelabyrinth sealandthepistonrodwouldincrease withadecreasing unit temperature. Hence, giventhetestdata,apossible failurecausewassuspected. Thetorsion mode damping cartridge wasdisassembled, andadimensional interference of0.005to0.007mm(0.0002" to 0.0003") wasmeasured between oneofthelabyrinth sealsandthepistonrod.Further itwasdetermined thatthepistonrodoutside diameter hadnotchanged andthattheinsidediameter ofthelabyrinth sealwas smaller thanexpected. Thedesign dataontherelevant assembly drawing showed thatthenormal clearance between thelabyrinth sealandthepistonrodshouldbe0.035+0.005 mm(0.0014" +0.0002"). Examination ofthequalityassurance paperwork foreachdamping cartridge setshowed thatafinal machining operation instruction, wheretheclearance between thesealandrodwasadjusted withinthe requirements, wasomitted ontheentiretorsion modedamping cartridge tot.Consequently itcouldnotbe verified thattheclearance between thesealandthepistonrodwascorrect. Therequired finalmachining operation andtheproperclearance wasverified onthebending modedamping cartridges. Yet,thetorsion modedamping cartridges wereverified tofunction properly duringacceptance testing10 months earlier, whichimplies thatadequate, ifnottherequired, clearance existed between thesealand pistonrod.Therefore, averyserious concern wasevident inthat,thelabyrinth sealwasdimensionally unstable withrespect totime.Twopossible physical mechanisms wereproposed toexplain thetemporal dimensional instability ofthelabyrinth sealmaterial: 1)swelling duetosilicone fluidabsorption, and2) stressrelaxation drivenbytheresidual stressassociated withthepressfit.Fluidabsorption intothe labyrinth sealmaterial wasnotconsidered credible giventhedissimilar chemistry ofthesilicone fluidand Torlon. Stressrelaxation ofthelabyrinth sealmaterial waspostulated asthephysical mechanism thatled totheseizure ofthetorsion modedamping cartridge. Included inthedamping cartridge assembly procedure isalabyrinth sealpost-press fitstressreliefheattreatment. Duringthisinvestigation itwas determined thattheheattreatment performed wasinadequate. Thisdetermination wasbasedonasetof testsrunonresidual labyrinth sealassemblies, whichhadbeenprevious heat-treated. Asecond heat treatment wasperformed ontheresidual hardware andtypically a0.0005" decrease inthelabyrinth seal insidediameter wasmeasured. Therefore, itwaspostulated thatthelabyrinth seallongtermdimensional change wasdrivenbyastressrelaxation mechanism. Theseizure ofthesparetorsion modedamping cartridge wasattributed totwofactors: 1)theclearance between thepistonrodoutside diameter andlabyrinth sealinsidediameter waslessthanrequired onthe assembly drawing; and2)theclearance between thesetwopartswasreduced toaninterference via stressrelaxation ofthelabyrinth seal.Therewasnojustification thattheflighttorsion modedamping cartridge wasanydifferent thanthespare,thusitwasconcluded thatflighttorsion modedamping cartridge waslikelytobeseizedaswell.On-orbit operation ofthetorsion modedamper wasnotrequired eitherforinstrument performance orforpayload safety.Ontheotherhand,properbending modedamper operation wasrequired forinstrument performance. Payload safetyconsiderations required thatthe bending modedamper function withinanexpected andpreviously assess envelope. Anacceptable for flightrationale wasgenerated forthebending modedamper basedon:1)qualityassurance paperwork verification thattheclearance between thelabyrinth sealandthepistonrodwasperthedrawing callout, and2)anadequate clearance between thesealandpistonrodwasestimated basedonaworsecase assumption ofthestressrelaxation drivendimensional change ofthesealinsidediameter. 92