simplealuminumbracketryforeseenfortheGimbalUnit,combinedwithhybridbearingswasnotlonger feasibleduethefollowingreasons: •Inthelowtemperature case(?Tabout130°C)the?CTEbetween theAIbearing seatandtheouter bearing ringcaused unacceptable highstresses inthealuminum part. •Thehighstresses intheAI-bracket caused inaddition unacceptable radialdeformations oftheouter bearing ring. •Thisnegative effectisevenincreased duetothesmallCTEoftheceramic balls,keeping their diameter almostconstant whilecooling downthesteelrings. •Theuseofliquidlubricant attheexpected lowtemperature regimeisnotadequate andtheuseof MoS2lubrication hadtobeconsidered. Inordertosolvetheproblem, atrade-off between different potential solutions wasperformed. Inafirst approach, titanium brackets instead ofaluminium wereconsidered inordertodealwiththeCTE mismatch, however, asignificant masspenalty hadtobetakenintoaccount. Theuseofstainless steel forthebrackets wouldhavealsosolvedthethermal mismatch problem, however, itwasconsidered outof scopefromthemasspointofview.Table5showstheresultsofthethermal deflection analysis. Table5:Thermal Deflections fordifferent Material Combinations Material ofbracket AT ADIn ADout AD ADBall ADres [K] [i.u'n] [#rn] [pro] [l._'n] [pro] Aluminum -110 Titanium 70 Dispal$225 70 Assumptions andExplanations:-39,4E -79,96 -40,50 -5,34 -29,82 25,1C 28,40 3,30 3,40 -3,50 25,1C 31,30 6,20 3,40 -0,60 Maxallowable bearingdiameter change<10pm Bearing ballsandbearingringsin440C Nopre-stresses causedbybearingintegration tolerances andbearingpreloadareassumed AT=Max.temperature difference between integration (ambient) andworstcaseoperational temperature AI_,=Bearing innerdiameter ADout=Bearing outerdiameter AD=Resulting diameter changebetween thebearingrings ADBaN=Diameter changeoftheball(multiplied by2forbearingdiameter change) z_Dr_=Resulting negative bearingplay Basedontheseresultsandthefactthatthedetailed designdrawings werealready finished, theonly quickandreliable wayoutoftheproblem wastoswitchfromhybridbearings tostandard 440Csteel bearings ofthesamedimensions, tochoose drylubrication asabaseline, andasaveryimportant point, touseDISPAL S225Aluminum alloy(AISi)asabaseline forthegimbalbrackets instead ofAluminum 3.4364alloy.DISPAL S225hasthebigadvantage toprovide aCTEthatisclosetosteelatthespecific weightofstandard Aluminum. Themajorconcern onthematerial wasitspotential stresscorrosion cracking susceptibility expected duetothehighamount ofSiinthealloyandthesintermanufacturing process oftherawmaterial. Thematerial wasalready usedinaspaceproject(MHS)byDSSandinthe frameofthisprojectastresscorrosion cracktestwasperformed. Basedonthistest,amaterial usage agreement wasestablished byNASAfortheproject. Themostimportant material properties ofDISPAL S 225(produced byPEAK)compared toalternative materials aresummarized inTable6. Arl_enna toAPMondAPMtoSatellite Interface Themechanical I/Fbetween Antenna andAPMisrealized byaholepattern ontopoftheframebrackets thatcaneitherdirectly accommodate theantenna structure orallowstheantenna toattachviaa dedicated I/Fadapter. Intheactualdesign, thelatterpossibility isusedtoreducethethermal interference between themetallic structure oftheAPMandtheCFRPantenna structure toaminimum. Byusingthe ringshaped I/Fplate,asymmetric radialstressdistribution canbeachieved between the 291