ALow-Shock StageSeparation Mechanism S.VaraPrasad Rao"andDeepak Agrawal* Abstract Alow-shock pyrobolt-actuated stageseparation mechanism formulti-stage aerospace vehicles hasbeen designed, developed andsuccessfully tested.Themechanism isfoundhighlyreliable, easytoassemble anddisassemble, andgenerates verylowoperational shock.Thisstageseparation mechanism isvery usefulfortheapplications inwhichtheseparation planeisveryneartoshocksensitive packages. Design, development andtestingexperiences, whichtheauthors havecomeacross duringthedevelopment of thislow-shock stageseparation mechanism, arepresented. Introduction Various mechanisms canbeemployed forstageseparation ofalargemulti-stage vehicle, depending on thespaceavailability, easeofintegration, andmostimportant ofall,thepermissible shocklevels generated duetostageseparation. Verylargevehicles employ flexible linear-shaped charge cords, wherelargeshocks (oftheorderof50,000to100,000 g)areproduced, dotolerate thesame,sincethe shockduration isoftheorderoffewtensofmicroseconds. Moreover insuchvehicles, shocksensitive devices likerelaysandcomputers areconfigured insuchaway(distance aswellaswithspecial mountings) thattheydonotremain vulnerable. Butforvehicles wheresuchhighshocks arenot permitted, separation mechanisms withverylowshockarespecially designed tosuitthespecific purpose. Onesuchlow-shock stageseparation mechanism isthe"pyrobolt-actuated stageseparation mechanism". Twostages arefastened together withsixoreightpyrobolts. Whenallthepyrobolts areoperated simultaneous, thestageseparates. Pyrobolt-Actuated StageSeparation Mechanism Thekeyelement inthisstageseparation mechanism isthepyrobolt, whichisspecially designed forthis purpose. Pyrobolt consists ofanelectro-explosive pyrocartridge, piston,cylinder, around-head release bolt,andfourcolletswithanarrestor asshowninFigure1.Thisassembly iskeptlockedbeforeoperation byalocking pin.Therelease boltheadisengaged inthefourcolletsandcancomeoutwhenthecollets areallowed tomoveradiallyoutwards. Thearrester stopstheradialoutward movement ofthecollets. The arrester islockedtothecasingbyalocking pin.Whenthepyrocartridge isfired,thearrestor movesup duetopyropressure aftershearing thelocking pin,hitsthechecknutandstops.Thearrestor thusmakes thecolletsfreetomoveout.Pyrocartridge pressure actsonthepistonalso.Thepistonpushes the release boltout.Pyrobolt release operation isshowninFigure1. Twostageshaveaflanged interface andarejoinedtogether withanumber ofpyrobolts. Thenumber of pyrobolts required tojointhetwostagesdepends upontheload,bending moment, andspaceavailable to mountpyrobolts. Morepyrobolts inthejointrequires morefiringcurrent. Lesspyrobolts inthejointneeds abiggerdiameter ofeachpyrobolt, andsoapyrobolt requires moremounting spaceandwiderinterface flange. Thedesigner hastoselectnumber ofpyrobolts tojointwostages basedonthefiringcurrent supplysourcelimitations andspaceavailable tomountpyrobolts. Interface BoltedJoint Loadsattheseparation interface aremainlyaxialforce,bending moment, andshearforce.Thebolted jointisdesigned fortheseloads.Tensile forceonapyrobolt isestimated fortheseloads.Thejointmay openintwopossible waysasshowninFigure3.Itmayopenabouttheneutralaxisasperbeambending theoryoritmayopenabouttheedgeofthestage.Pyrobolts atonesideoftheaxiswillexperience tensile "Defense Research &Development Laboratory, Hyderabad, India Proceedings ofthe35thAerospace Mechanisms Symposium, AmesResearch Center,May9-11,2001 145