Twoballscrewconfigurations areavailablewiththisdesign,differinginthelengthoftheball-nuttravel. Thetravelrequirements forthetwoapplications are50mmand6.5mm.Thegeneralballscrewdesign wasselectedtoprovidetheappropriate screwpitchanddiameter toachieve therequired speedof actuation, thehelixanglefortheneeded efficiency andtelemetry resolution, andforloadhandling capability. Thebail-nut alsocarriesthepotentiometer wiperassembly, whichisrequired forthecoarse potentiometer located inparallel withthemovement trackoftheball-nut. Providing lubrication fortheballscrew/ball-nut pairwasespecially challenging sincetheactuator configuration didnotallowforanenclosed ballscrew andball-nut assembly, andthepossible useofwet lubrication. The53Rchardness oftheballscrew material (440Cstainless steel)provided thesubstrate on whichdrylubrication wouldperform andendure best.Anadditional designobstacle waspresented by theimmediate proximity ofthelinearpotentiometer totheballscrew, whichcouldleadtodeposits ofloose drylubricant onthepotentiometer track.Thedrylubrication deposit wouldbethedirectproduct of intimate contact generated between theballsandtheballscrew surface, whichwasimpregnated withdry lubricant. Basedonprevious experience withusingdrylubrication onintimately contacting surfaces, the decision wasmadetouseTungsten Disulfide drylubricant (Dicronite). Aseparate qualification testwasconducted todetermine theviability ofthistypeoflubricant inthese specific operating environments. Theballscrew wasexposed to-50°Ctemperature witharesistive force of93N(2IIb)andwasactuated for1,800cycles(1cyclewasfromfullretracttofullextendandreturnto fullretract), whichexceeded theperformance requirements oftheactuator specification. Atthe completion ofthistest,itwasdetermined thattherewerenodeposits ofloosedrylubricant induced from normalwear. Theselection oftheincremental stepper motoroverabrushiess DCmotorwasdrivenbytheincremental positioning andunpowered holding capability ofthestepper motor. Thestepper motormagnetic design features anumber ofequally spaced permanent magnets ontherotor.Theinteraction oftherotor magnets andthetoothstructure ofthestatorcoreresultsinastrongattractive force,independent ofany electrical excitation ofthestatorwinding. Thisholding forceismagnified byafactorof100,duetothe 100:1harmonic drivegearratio. Theballscrew selected forthisapplication wastestedanditsback-driving moment wasmeasured tobe 0.18N(1.6in4b)attheworstcaselaunchandground transportation induced loads.Incomparing this valuetotheactuator holding capability of0.85(7.5in4b)itwasconcluded thattheactuator wouldnot back-drive undertheworstcaseloading condition. Thestepper motorwasatwo-phase 15-degree stepper, chosen oversimilarly sizedthree-phase stepper motors. Thechoicewasbasedonitssuperior performance observed aftermotorcharacterization testing. The15-degree stepping increments provided asufficiently smalllinearsteptomeettheproduct requirements. Theoutputrotarystepattheballscrew wasreduced to0.t5degree afterpassing through a100:tharmonic drivespeedreducer. Thisthenresulted ina1.75IJm(0.000069 inch)linearstepfor everymotorstep. Theharmonic driveselected forthisprogram wasoneofthe"miniseries"unitsfromHarmonic Drive Technologies, whichalsooffered ahighdegree ofpositioning accuracy andrepeatability, zerobacklash, relatively hightorsional stiffness, andhightorquecapability. Thesmallsizeofthisharmonic driveallowed ittobeusedin-linewiththemotor,andproviding astreamlined andcompact assembly. Thelubrication oftheharmonic driveassembly aswellasthemotorandoutputbearings was accomplished usingBraycote 815ZoilandBraycote 601grease. Thislubricant wasusedtomeetthe demanding coldoperation temperatures of-50°C. Twoco-molded carbonfilmtyperedundant rotary(fine)andlinear(coarse) potentiometers weredesigned toprovide therequired telemetry. Therotarypotentiometer wasattached totherotaryoutputofthe 164