conditions. Onorbit,theconditionsatthetimeofdeployment canberead,andtheappropriate power-on timedetermined byasoftwaretablelook-up.However,temperature andvoltageatthetimeofdeployment wouldnotbeavailable.Temperature sensorswerenotpossiblesincenoadditionalwiringwasavailable. Evenifwiringwereavailable,theactuatorswereondifferentsidesofthespacecraft, whichwouldsee differentoperatingenvironments. Severaltemperature sensorswouldhavebeenrequired.Andtherewas noplantoreadbusvoltage.Thevoltageattimeofdeployment couldvarybetween22and36V,or11 wattsofinputpowertoalmost30watts!Wedidnotknowifasingletimedpowerpulsecouldworkacross suchawidevoltageandtemperature range,yettherewaslittleflexibilityintheserequirements. Analternativeconceptwaspresentedasasolutionduringanearlydesignmeeting.Starsyscoulddesign aninternalpower-disconnect, whichwouldopenthepowercircuitwhenthepinhadpulled,providing inherentcontrol.However,tokeepoverallprogramcostdown,development workwasdiscouraged. Emphasiswasplacedonutilizingproven,existingdesigns.Toourcustomer,therisk,costandschedule ofthislevelofdesignworkdidnotfittheprogramphilosophy. Starsysagreedtodesigntheactuatorsuch thatthesingletime-outapproachwouldwork. Tosummarize, thedesignchallengeofthetime-outcontrolstrategyisasfollows:selectasinglepowerontimethatguarantees fullactuationcanoccurfromthelow-temperature, low-voltage condition,andthen verifythattheactuatorcansurvivethissamepowerdurationwiththehighvoltageconditionfromthehigh temperature. Failureofeitheroneofthesewouldjeopardizeamajorspacecraftfunction. Strateqy forTime-out Control First,theheaterwassizedforoperation undertheworstcaseconditions. Thespecification required full pinretraction within12minutes inthecoldest environment (-65°C) atthelowestvoltage (22VDC),witha partially failedheateroperating at75%ofcapacity. Usingamodelthatbalances heatinputwithconductive lossesatactuation temperature, andaddingappropriate margin, itwasfoundthat8wattswassufficient to meettherequirement. Thisdictated anominal heaterresistance of44ohmsoverall. Theheaterdesignwasalsodrivenbythe2-wirelimitation. Normally, 4heaterwiresaredesirable fortwo separate, redundant heatercircuits. Thedesignchallenge wastobuildsomedegree ofredundancy into the2-wirecircuit.Thiswasachieved through thelayoutoftheresistive elements withintheKaptonlaminated heater.Fourseparate heatertraceswereplacedbetween two"bus-bar" traces, alloperating in parallel withanoverallresistance of44ohms.Intheory,asingleelement failurewouldstillleave3other tracesintact(or75%ofheatercapacity), allowing fullretraction undertheworst-case conditions. Theheaterisacriticalcomponent intheactuator, andmethods forverifyirlg properheaterperformance hadalready beenwelldeveloped. Forexample, 100%incoming inspection isperformed, including an electrical test.In-process inspections existfortheapplication oftheheatertothebody.Electrical verification testsareperformed regularly fortheremainder ofassembly andtestingprocess. Giventhese procedures, andtherobustheaterdesign, thesacrifice offullheatercircuitredundancy wastradedfora simpletwo-wire powercircuit. Withtheheatersizedproperly forworstcaseoperation, theissueofactuator survival wasaddressed. The actuator wouldneedtosurvive fortheduration ofthepowersignalunderhot,highvoltage conditions. The operating pressure insidetheactuator, undernominal 222-N(50Ibf)load,isapproximately 20.2MPa (3000psi).Toprevent extrusion failureoftheVitonseals,internal pressures mustbekeptbelow68.9 MPa(10,000 psi).Someparaffin actuators incorporate ashearfeature toachieve this.Thisisapart, usuallyaflatdisk,whichfailsinpureshearduetoexcess internal pressure, allowing expansion ofthewax chamber volume. Theexpansion reduces thehydraulic pressure tobelowcriticallevels,thereby reducing theriskofsealfailureorrupture ofthebody.Asheardiskwasnottobeincluded inthepinpuller. Thepinpuller shaftwasgivenalmost 100%excess travelbeyond therequired stroketocreatemore internal volume forwaxexpansion. Afterthe0.635-cm (0.250-in) strokerequired forrelease, thepinwould continue toretractanother 0.635cm(0.250in)toaninternal hardstop.Toreachcriticalpressures with thepinathardstopwouldrequirethatthewaxbebrought totemperatures abovethesurvival rangeofthe heater. Thetemperature required toexpand theparaffin tosealextrusion pressures (68.9MPaor10,000 279