Inertia/Momentum WheelDrive Typically, amomentum wheelisusedforattitude controland/orenergy storage. Itconsists ofaflywheel spinning athighRPMinanevacuated housing. Lowlossesandhighefficiencies arethedominant factors, sincethewheelwilloftenoperate continuously forupto15yearsorlonger. Normal speeds are 6,000to10,000 RPM.Cogging andrippletorquearesecondary factors sincethelargeinertiaandhigh speedtendtosmooth anyeffectonthespacecraft. Thebrushless motorhasproven tobethedriveofchoice. Inearlieryears,ACmotors wereusedbut advances inbrushless technology havemadethemmorepractical andefficient. Spinmotordesignisvery different fromthedrivemotorsusedindeployment mechanisms orfansandpumps. Themagnetic airgap islarge.Weight andsizearecompromised infavoroflowlossesandhighefficiency. Statorlamination material isselected forlowlosses(increasing cost)andisoperated atlowfluxdensities. Alternatively, the statormayonlyconsist ofawinding onanon-magnetic coilform.Thiswilleliminate anylossesdueto eddycurrents orhysteresis inthestatorlaminations (therearenone).Whilethisisthebestchoicefor minimum losses, themotormustbelargerinsizeandweight. Ultra-smooth Applications Themostcommon spacecraft areforcommunications, butspecial purpose satellites oftenrequire ultrasmooth operation ofmechanisms. Atelescope, earthresources satellite, orspecial experiment mayneed extreme attitude stability, minimum perturbations and/orsmooth operation asthespacecraft scansan area.Themotors required forthisapplication mustbeextremely smooth, withlowrippleandcogging. Examples arethegimbaltorquers oncontrol moment gyros.Also,theHubble SpaceTelescope needs smooth operation foracquisition andfinepointing (seeFigure3.) Again,theBDCmotoristhedesignofchoice, butitisnotinthesamedesigncategory asthespinmotor ordeployment drive.Itisveryspecialized foraparticular lowrippleapplication. Sizeandweightare secondary tosmoothness. Themotorwillbealargediameter, thinsection design. Everystepofthe designprocess mustconsider themaingoal.Theairgapwillbelarge.Magnet andtoothproportions are selected forlowcogging andripple.Thewinding isunique, withvarying turnsofwireineachslot.Thisis toproduce asmooth sinusoidal waveform. Commutation isalsosinusoidal tomatchthetorquesensitivity waveform. Aresolver orencoder isusedforposition information asdiscussed earlier(seeFigure2.) Miscellaneous Mechanisms Mostotherspacecraft mechanisms fallintothecategory ofoccasional actuation andthetypeofdrive motorisoftennotthekeytoproper operation. Applications maybeopening adoor,driving alatch, moving asunshieldoveracamera, orun-caging asystem. Selection cannowbebaseduponminimizing cost,complexity, size,weight, ortheutilization ofexisting electronics. Often, thedesign can accommodate existing motordrives thathavebeendesigned forotherapplications. Asimple stepper/gearhead maydothejobatminimum costandcomplexity. Brushmotors haveaplaceinthis category also. Conclusion GiventhevarietyofDCmotortypesthathavebeenusedinspacemechanisms, itisapparent thatthe application requirements, motordesignfeatures, andrelevant heritage motorlessons learned mustbe understood toassure anewmotormechanism willfunction acceptably. Table3,"DCMotorSelection GuideforSpaceMechanisms," provides anoverview ofthetypesofDCmotormechanisms andtheir special requirements asaguide,buttherearesuretobefuturespacemechanism casesthatwillnotbe clear-cut. Careful consideration andcoordination fromthesystems levelthrough themotormechanism designlevelshouldbesoughtasearlyaspossible insuchcasestodefinetheoptimum designapproach. Forfurtherstudy,thefollowing textsaresuggested: (1.)NASASpaceMechanisms Handbook. RobertL.Fusaro,NASA/TP-1999-206988. RefertoPartIII,Chapter 12on motortypes. (2.)SpaceVehicle Mechanisms, Elements ofSuccessful Desiqn. PeterL.Conley,Wiley,NewYork,1998.Referto Chapter 13onmotortypesandtesting. NASA/CP--2002-211506 102