3.Themoreexpensive anddifficultthetest,thelessnecessary itistoexecute 4.Addingaback-up component wiltnecessarily increase reliability 5.Successfully operating amechanism onceindicates highreliability Background Thespaceindustry today(2000)hasmuchincommon withtheaviation industry of1925,theautomobile industry of1890,thesteamengineindustry of1850,andtheshipbuilding industry of1500:production rates arelow,failures arecommon, product evolution israpid,experimentation iscommon, opportunities abound, andheroiceffortsarerequired tobesuccessful. Weareinthemidstoflearning howtobuildcost-effective spacevehicles well. Theuniqueenvironment ofspacemakesreliability challenging toattain.Theverydifferent environment (vacuum, largetemperature extremes, largetemperature gradients, longdormancy priortooperation, and highradiation) ofspacetravelprecludes directapplication ofterrestrial hardware, itonlyallowssome terrestrial concepts tobecopied. Failedhardware isusuallynotretrievable soalthough itispossible to determine whathappened tocauseafailure,itisusuallyimpossible tostudythefailedhardware and therefore determine thedefinitive rootcauseofafailure(broken automobiles andaircraftoftenconveniently present theirfailedhardware forstudyandsubsequent prevention). Afurtherdifficulty istheverylowrateof recurring production; mostspacecraft areoneofakind,evenwhentheyaresimilar,production ratesrarely exceed onepermonth. Evenspacecraft constellations haverelatively smallnumbers ofvehicles compared tootherindustries. Therelatively smallmarketforspacevehicles prevents thekindofcapitalization to provide theknowledge ofreliability thatotherindustries enjoy.Automobile manufacturers haveenjoyed this abilityforninedecades andwhilesomeautomobiles maynotbereliable, allofthedataisavailable toknow whatthereliability oftheautomobile is.Finally, duetoboththephysics andeconomics ofpresent day spaceflight, spacecraft engineers mustbalance therequired performance oftheirmechanisms withtheneed todesignproducible, reasonably priced,lightweight hardware. Thereareseveral misconceptions aboutattaining reliability thatkeepspacevehiclemanufacturers inthe darkages.Untilthespaceindustry embraces morerigorous testingtotrulydetermine thecapabilities of aerospace mechanisms, products willcontinue tofailatpresent rates. Misconception #1:Reliability ofaMechanism canbeCalculated fromthe Reliabilities oftheMechanism's Components. Theknowledge ofreliability ofanassembled mechanism cannotbedetermined byalgebraically manipulating thereliability ofthesubassemblies because thecalculation reliesonunproven assertions. For example: Amechanism engineer knowsthereliability ofapyrotechnic boltcutterfromthelottestdataand candetermine thereliability ofaretention boltbasedonTypicalmaterial strength dataand/orfracture mechanics. Fromstatistics, thereliability istheproduct ofthetwoitem'sindividual reliability. Theerroris madewhentheengineer makesuntested assumptions tocarryoutthatcalculation. Whencuttingabolt withapyrotechnic boltcutter,theseassumptions mayinclude: 1.Theboltfloatsfreelyintheboltcutterorrestsdirectly against theanvil 2.Thebolthasnomassorthesamemassasthelottestbolt 3.Theanviloftheboltcuttermustputsomeofitsenergyintocompleting thecuttingofthebolt 4.Thefrictionbetween theanvilandtheboltisnegligible 5.Thecutboltfragments willnothavesufficient energytorebound ordamage otherpartsofthespace vehicle 6.Theboltisunderpreload 7.Thepyrotechnic cutterissecurely clamped 8.Theflightconditions areidentical tothelottestconditions 52