ACISDoorMechanism Design Desian Reauir_ments ThedesignoftheACISDetector Assembly wasperformed byLockheed MartinSpaceSystems Company undercontract toMIT(contract #SC-A-124624), whointurnwasundercontract toNASAMSFCforthe ACISInstrument. Designworkstarted in1994withaprototype unitbuiltpriortoPreliminary Design Review (PDR). Aqualification unitwasbuiltandtestedinearly1995withtheflightbuildandteststarting inearly1996.AnEngineering Unitwasalsobuiltandwasdelivered toMITinearly1995.Therewere manyrequirements whichcontributed tothefinaldesignoftheACISDetector Assembly Door Mechanism. ThekeyACISdoorrequirements arelistedbelow: 1.DesignLife 2.Operating Temperature Range 3.FlightOperating Temperature 4.Survival Temperature Range 5.Operating/non-operating Pressure 6.Operating Voltage Range 7.Actuator Min.Operating Torque 8.Actuator Min.StallTorque 9.On-orbit Operation 10.Ground Operation 11.Cleanliness Requirements 12.Internal Vacuum 13.SealCapability253Cycles -60°Cto+45°C(Qualification) 20°C -76°Cto+45°C 800TORRto0TORR 24VoltsDCto36VoltsDC >5.1N-m(45In-lb) >6.8N-m(60In-lb) Oneopencycle <100Cycles Class100AandmeettheRequirements of MSFC-SPEC-1238 andMIL-SPEC-1443 UptooneatmPressure difference AcrossDoor <1Torrleakage perDay Contamination controlwasasignificant designdriverforthedoormechanism assembly andforcedthe useofmaterials andcomponents thatwereknowntobecompatible withtheserequirements. The requirement foraninternal vacuum (inthedetector housing) forallground processing andlaunch , environments, combined withtheverylowallowable leakrateforsuchasmallinstrument volume, demanded anearlyperfecthermetic doorseal.Anunlubricated VitonO-ringwaschosen asthesealing material because ofitsvacuum compatibility andlowoutgassing properties. Thetorquerequirements for thedooractuator werederived fromsomeearlydevelopment testswhichshowed arequirement forat least2.3N-m(20in-lb)oftorquecapability fromtheactuator whenusingadryVitonO-ring. Actuator tradestudies performed earlyinthePDRdesignphaseresulted inadecision toproceed withaRotary Paraffin Actuator builtbySTARSYS Research todrivethedoormechanism openandclosed. This actuator (Part#HL4570B), basedonadesignfromaprevious NASAprogram, hadredundant built-in limitswitches andheaters withapositive latchatbothextremes oftravel.Theminimum stalltorque capability forthisactuator was6.8N-m(60in-lb),whichwasthreetimestherequired torque. DqqrMechani_;m Aqtuator andLinkage Therotaryactuator usestwolinearparaffin actuators, oneforeachrotational direction. Uponactuation, heaters insideacartridge warmtheinternal paraffin. Astheparaffin changes phaseataround70°Cits volume increase forcesasteelpinoutward; thislineartravelisconverted intorotarymotionoftheoutput shaftandcoupled ACIStorsiondriveshaft. Thedriveshaft andafoldingtwo-bar linkage rotatethedoor approximately 92degrees andoutofthefieldofviewuponopening; intheotherdirection thelinkage is driventoanearlystraightened lockingposition whenthedoorisclosed. Thelinkage contains an adjustable turnbuckle, settoprovide acalibrated forceagainst theVitonO-ring. Thisforce,determined through development testing, provides adequate sealing forevacuation oftheinstrument without additional applied forces. A"swinglink"supporting thedoorhingeaxisallowsthedoortoseatfullyonthe O-ringsothatO-ringcompression isuniform uponpreload andevacuation. Aninternal sheardiskin eachactuator prevents excessive pressure insidetheactuator fromcausing acatastrophic failureofthe paraffin actuator cartridge &protects theothercomponents inthedevicefromdamage. However, a sheardiskrupture doesrendertheactuator permanently disabled. Figure1showsapictureoftheACIS doormechanism intheopenposition, withtheaccess coverremoved andtheSTARSYS actuator onthe 17