addingprofiledaluminumstripstothedrumsurface.Thewheelswererunfor124,000 cyclesoversteps rangingfrom1mmto2.5mm.AsnotedinTable1,twowheelcandidates wereeliminated afterthistest because theyprovedtobeabrasive totheanodized aluminum drumsurfaceandathirdwaseliminated duetoevidence ofstresscracking. Thenexttestintheserieswasthelifetestinambient airwheretheremaining wheelswererunfor 2,300,000 drumrevolutions. DuringthistesttheE-CTFE material wasdropped because itexhibited significant coldflowatthehighlystressed notchareaandtwoothermaterials wereeliminated dueto cracksdeveloping inthenotcharea.Thesurviving PEEKmaterial showednoevidence ofcracking and suffered onlyminorwear. ThePEEKtestwheelthenunderwent thermal-vacuum testingwherethewheelswererunfor800,000 drumrevolutions attemperatures rangingbetween 60°Cand-54°Covera5.3-dayperiod.Frictiontests weremadeatthestartandendofallplateaus bylockingthewheelhubandmeasuring thetorque neededtocausethedrumtoslip.Theresultsshowthatthecoefficient forslidingfrictiondidnotchange fromambient airtovacuum, didnotchangesignificantly withtemperature, butdidconsistently increase overtheduration ofthetest.Thecoefficient ofslidingfrictionatthestartofthetestwas0.26(airand vacuum) climbing to0.42attheendofthetest(vacuum). Itisnotpossible toresolvefromthisdataifthe increase infrictionislargelyduetorunningeffects,suchaswearofthetireandtransferofmaterialtothe anodized aluminum drum,orifextended exposure tovacuum hasastrongeffectonfriction. Insummary, thePEEKidlerwheeltirematerial survived anextremely demanding setofdevelopment tests,including veryhighimpactloadsduringaccelerated testing. FortheMTflightdesignoftheRSU andLDUidlerwheels,thestress-concentrating notches inthetireandlugsonthehubwereeliminated andthetireisallowedtosliponthehub. DriveWheelMaterial Inparallelwiththematerials development program fortheidlerwheeltire,asimilarselection processwas undertaken tofindanappropriate tirematerial fortheLDUfrictiondrivewheel.Thegeneralrequirements dictated acompliant, temperature andvacuum stable,long-wearing, non-abrasive material withahigh frictioncoefficient overthethermal-vacuum environment. Areviewofmaterial properties narrowed the tirematerial tophenyl-substituted siliconeelastomers, fromwhichsevencandidate materials werepicked fordevelopment testing. Twodifferent manufacturers specializing inrubberprocessing wereselected toproduce 7.6-cm-diameter testwheels,twoforeachcandidate material, bycompression moldingtheelastomer tiredirectlyonto wheelhubs.Unfortunately thereisstillasignificant "art"toprocessing elastomers. Onlyoneofourtwo vendors hadtherubbergrinding technology neededtomeetourdimensional requirements, whileonlythe othervendorhadtheprocess neededtoincludereinforcing beltsintothetires.Inpreparing ourtest wheels,oneofthevendors successfully bondedthetirestothehubs,buttheothervendorhadaprocess problem, whichledtodebonding oftheelastomer tirefromthealuminum hubforanumberoftest samples. Testwheelsforalloftheelastomer materials wereprepared withoutanyreinforcing material addedtothe tire.Inaddition, threematerials werealsotestedwithanembedded beltofAramidfabricreinforcement moldedintothetire.Theinitialseriesofselection testscomprised compression set,jointcrossing, and life/performance attemperature extremes (-54°Cto66°Cinair).Allofthematerials passedaninitial compression settestwheretherecovery oftheelastomer wasmeasured afterbeingloadedfor24hours. Thejointcrossing testssimulated theloadingconditions experienced whencrossing gapsandoffsetsin therailswhilethelife/performance testranthetestwheelsunderapreloadof311Nagainsta30.5-cm drumatambient andtemperature extremes foratotalof120,000 rotations. Atallphasesoftesting, wheelfrictionwasmeasured bylockingthetestwheelanddetermining theforcerequired tomakethe drumslipunderthewheel. 124