Forthesetests,thefrictioncoefficient wasmeasured bylockingthewheelandmeasuring thetorqueto rotatethedrum.Thetrendsevidentinthedynamic frictiondataare: •Thefrictioncoefficient increases invacuum overair(20%and13%respectively forLT50andSE6660) •Thefrictioncoefficient ishigherathotthanatcold(16%dropbetween 60°Cand-54°CforbothLT50 andSE6660) •Thefrictioncoefficient decreases overlife(8%and9%droprespectively forLT50andSE6660) Sincethefinalflightdesignofthedrivewheelhasa30.5-cm diameter withatitanium hubratherthanthe 7.6-cmdiameter withanaluminum hubasusedinthedevelopment testing,bonding testsoftheelastomerwheelmaterials totitaniumwereperformed. Testsperformed withtheDowCorning LT50elastomerbondedtometalcoupons indicated thattheDowCorning S-2260primerusedwiththetestwheels waseffective ontitanium aswell.Inordertoqualifythedrivewheeltiremanufacturing process, afull-size 30.5-cm-diameter drivewheelwasmanufactured withtheDowCorning LT50elastomer. Thematerial testwheelsweremanufactured inthefirstquarterof1992.Theinitialsetoftestsonthese wheelswascompleted inSeptember 1992withtheDowComing LT50material beingselected asthe primecandidate material fortheLDUdrivewheelwiththeGESE6660 material beingheldasabackup. A wheelstiffness test,startedinAugust1995,oftheLT50testwheelidentified alossofwheelfriction coefficient duringthe3yearsthatthewheelhadbeenheldinstorage(underambient conditions). The frictionlosswasfoundtobepermanent andcouldnotberestored bycleaning, abrading orvacuum baking.TheSE6660 testwheelstilldemonstrated acceptable performance. Notethat,although thetwo elastomer formulations arebothphenylmethyl-dimethyl siliconepolymers, thecatalysts aredifferent for thetwoproducts (DowCorning -TforDCLT50andVaroxTMforGESE6660) andthequantity offree siliconeoilusedtodisperse thecatalystwas7timesgreaterintheDCLT50material thantheGESE6660 material. Theunpolymerized freeoilissuspected tobethecauseofthelossoffriction. Toaddress theconcern thatthesefrictiontestsstillreliedonafixedwheelandwereperformed inair,a moreflight-representative setoftestswereinitiatedfortheSE6660 material. Fortheseteststhewheel waspreloaded at396Nagainstafiatrailsurfaceandthefrictioncoefficient wasestablished bymeasuringthetorqueonthewheelrequired tosustainslipping. Because frictioncoefficients weremostlya concern athottemperatures, thesetestswereconducted attemperatures rangingfrom21°Cto70°Cfor atotalof6daysinvacuum. Frictioncoefficients, measured inairbeforeandafterthevacuum testing,did notsignificantly differ.Although thetestresultsinitiallyshowedadropinfrictioncoefficient invacuum, the frictioncoefficient didrecoverandcontinued toincrease overtheduration ofthetestinvacuum, witha finalvalueof0.82foranetincrease of22%forvacuum overairvalues. Theoverallbehavior isindicative ofmoisture, absorbed bytheelastomer whileinair,compromising thefrictioncoefficient untilthemoisture isdrivenoff.Aninteresting resultwasthatwhentheelastomer wheelslippedontheanodized aluminum railsurfaceinvacuum, aluminous discharge wasproduced wherethetirematerial wasleavingthetire-torailcontactpatch. Extreme Temperature Exposure Thepresence oftheMTtendstomoderate thetemperatures ofthespacestationrailsonwhichitruns. ButtoensurethatMTwheelmaterials willnotbedamaged bytheMTtranslating ontoexposed railsthat arebeyondnormaloperating temperatures, aseriesofextreme temperature exposure testswereperformedforboththePEEKplasticidlerwheelmaterial andtheGESE6660 etastomer drivewheelmaterial. Forthehottemperature extreme, aSE6660-tired testwheelwasheatedto64°Candthenrununder preloadona93°CrailwhileaPEEKtestwheelwasheatedto129°Candthenrununderpreloadona 93°Crail.Forthecoldtemperature extreme, theSE6660 testwheelwascooledto-68°Candthenrun underpreloadona-73°CrailwhilethePEEKtestwheelwascooledto-76°Candthenrununderpreload ona-76°Crail.Bothwheelspassedthisextreme temperature testingwithnoevidence ofdamage. 126