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American Institute of Aeronautics and Astronautics1DETERMINING FUNCTIONAL RELIABILITY OF PYROTECHNIC MECHANICAL DEVICESLaurence J. Bement*NASA Langley Research CenterHampton, VirginiaHerbert A. MulthaupNew England, North DakotaABSTRACTThis paper describes anew approach forevaluating mechanical performance and predicting themechanical functional reliability of pyrotechnicdevices. Not included are other possible failuremodes, such as the initiation of the pyrotechnicenergy source. The requirement of hundreds orthousands of consecutive, successful tests on identicalcomponents for reliability predictions, usingthegenerally accepted go/no-go statistical approachroutinely ignores physics of failure. The approachdescribed in this paper begins with measuring,understanding and controlling mechanical performancevariables. Then, the energy required toaccomplishthe function iscompared tothat delivered b y thepyrotechnic energy source to determine mechanicalfunctional margin. Finally, the data collected inestablishing functional margin isanalyzed to predictmechanical functional reliability, usingsmall-samplestatistics. A careful application of thisapproach canprovide considerable cost improvements andunderstanding over that of go/no-go statistics.Performance and the effects of variables can bedefined, and reliability predictions can bemade byevaluating 20 or fewer units. The application of thisapproach to a pin puller used onasuccessful NASAmission is provided as an example.INTRODUCTIONAlthough pyrotechnics are required toperformcritical aerospace mechanical functions, currentlyaccepted test methods donot establish functionalmargins, nor are analyses statistically rigorous. Asdescribed inreference 1,few guidelines exist for theapplication of pyrotechnics. The current approach i st o treat these explosive and propellant-actuated*Senior Pyrotechnic Engineer, Testing and Development Branch,Associate Fellow, AIAACopyright ?bythe American Institute of Aeronautics, Inc. Nocopyright is asserted in the United States under Title, U. S. Code. TheU. S. Government has a royalty-free license to exercise all rights underthe copyright claimed herein for Government Purposes. All other rightsare reserved by the copyright owner.mechanisms as?black boxes? wi thout the ability tomeasure and quantify performance. The only quantifi-able performance attribute is "go or no-go;" iteitherdid or did not function as required. Thus, attributetesting is ?blind? toanyand all design variables (aswell as tolerances on these variables), which actuallycontrol functional failures; indeed, attribute tests areblind to the very existence of any and all physics offailure. Over 30 years ago on the Gemini Program,mechanical functional margin requirements wereimposed by conducting test firings with +/-15% pyro-technic loads. Although thisrequirementcontributednothing towards quantifying reliability predictions, itwasintroduced toprovide a qualitative assurance thatthe devices would function properly. Ifit still func-tioned with an 85% load, some functional margin isimplied. Or if itfunctioned wi thoutbursting with a115% load, some structural containment margin isimplied. As described in references 2 and 3,the+/-15% testing does not quantitativelydefine eitherthe relative effects of system parameters oractualmechanical functional margins. Consequently, whenpyrotechnic mechanical system failures occur4, themost frequently cited cause o ffailures was a lack ofunderstanding ofpyrotechnic component and systemfunctional mechanisms.Personnel in the Halogen OccultationExperiment (HALOE) Project Office at NASALangley Research Center were faced with just such afailure2. Afailure to properly stroke occurred in anewly manufactured lot (made under carefullytraceable ingredients and controlled procedures) ofpinpullers in a design that the project had selected for useon their mission. This lot of pin pullers was beingmanufactured bythe same source, usin g the samedrawings, as the units plannedfor use on HALOE.An investigation was conducted and adecision wasmade toredesign the pin puller, conductanotherqualification, and predict the reliability of theredesigned pin puller.The approach widely used inevaluating thereliability and confidence levels of pyrotechnically