On-Orbit FailureDiaqnosis AspartoftheSRTMon-orbit checkout procedure, flightrulesrequired thatthenaturalfrequencies ofthe deployed mast'sfirstvibration modes bemeasured. Therationale behind thisrequirement wasthatthe stability oftheShuttle's reaction controlsystem isassured byproperplacement andsizingofnotchfilters whichthenservetomasklowfrequency dynamics. Thissystem identification wasperformed withthe dampers locked, andunlocked. Thesystem response wasidentical forbothdamper states. Further, duringthepulsetestwiththeunlocked damper, zerodamper relative displacement wasmeasured viathe displacement sensor mounted oneachmechanism. Basedoncomparison ofthesystem identification resultsfromthedampers lockedversusunlocked testsitwasconcluded thedampers wereinoperative. Thedampers werere-locked, andthemission continued toasuccessful conclusion. Proper instrument operation wasachieved without functional dampers byutilizing overlapping design performance margin contained withintheotherSRTMsub-systems. Post-Mission FailureInvesti.qation Following themission afailureinvestigation wasconducted inordertodetermine therootcauseofthe SRTM damping mechanism failures. Theconclusion ofthisinvestigation wasthatbothdamping assemblies failedduetoacommon modefailureattributed tothedamping cartridge mechanical design. Specifically, itwasfoundthatalldamping cartridges assembled forSRTMhadseized. TheSRTM damping cartridge seizure wastracedtoadimensional interference between thepistonrodoutside diameter andthelinearbushing insidediameter. Itwasfurtherdetermined thattheinsidediameter ofthe linearbushing, madefromTorlon, hadchanged dimensionally; i.e.,theIDhadreduced, thereby eliminating therequired clearance between thebushing andpistonrod.Asdiscussed above, thetwo possible physical mechanisms whichexplain thetemporal instability ofthebushing insidediameter are:a) silicone fluidabsorption bythelinearbushing andb)longtermcreepofthelinearbushing duetoresidual stress. Priortothelaunch ofthemission thefluidabsorption ofthelabyrinth sealmaterial wasnot considered acredible explanation forthedimensional change ofthelabyrinth sealmaterial. Itisnotedthat allthedamping cartridges assembled forSRTMultimately seizedduetolabyrinth sealinterference with thepistonrod;ontheotherhand,alabyrinth sealtestunitthathasnotexperienced longtermfluid exposure hasnotseized. Therefore, itispossible thattheassumption that,thedimensional instability of thelabyrinth sealduetofluidabsorption isincredible, isnotvalid.Theconclusion reached todateisthat thedimensional change ofthelabyrinth sealmaterial isattributable tooneprobable causeandan additional possible cause,thatisstressrelaxation andfluidabsorption, respectively. Afinaltechnical conclusion reached hereisthatthematerial usedforthelabyrinth sealisinappropriate for thisapplication. Anunderstood butoverlooked requirement forthelabyrinth sealisthatitmustretainlong termdimensional stability giventheverytightclearance requirement between thisdynamic sealandits matingpart.Agreater conclusion isreached regarding thismechanism failurewhenthedesignheritage of thedamping cartridge isexamined. Itturnsoutthat;amaterial otherthanTorlonwasusedinprevious versions ofthedamping cartridge designforthelabyrinth seal.Therefore thetruerootcauseforthe SRTMdamping mechanism failureisthatthedamping cartridge designheritage wasvoided. Anadditional comment worthconsidering isthataprotoflight development approach wasfollowed withthissystem basedonprojectschedule andcostconstraints, andconsequently specific engineering models werenot developed. Hence, inordertomeetshelfliferequirements, similarity toexisting designs wasrequired. Unfortunately, therequired similarity waslostduetoaseemingly innocuous designchange. Conclusion Themastvibration damping system implemented forSRTMwasdiscussed herein. Thesystem design followed astraightforward approach andusedofftheshelfcomponents modified tomeetspecific performance requirements. Further, thissystem metallacceptance andperformance testrequirements, yetfailedin-service, i.e.,on-orbit. Thetechnical rootcauseofthesystematic failureofthissystem was identified anddiscussed. Additionally, amoregeneral failurerootcausewasdiscussed. Thisdatais presented inordertobenefitfuturedamping system applications. 93