1 COMPOSITES FOR MISSILE AND SOME AIRCRAFT APPLICATIONS G. BALASUBRAHMANYAM, Associate Director, Advanced Systems Laboratory, Hyderabad 500 058 EXTENDED ABSTRACT CompositeshavefoundextensiveapplicationsinAerospaceareamainly becauseoftheirhighspecificstrengthandstiffnesspropertieswhichin-turnhave offered substantial weight saving and improved performance.In line with this concept numberofCompositeProductshavebeensuccessfullydevelopedatAdvanced SystemsLaboratoryforvariousMissileProgrammessuchasRe-EntryVehicle Structures, Rocket Motor Casings, Canisters, Ablative Components, Flex Nozzles, J et Deflectors etc., and also some Aircraft components such as Radomes, Brake Discs and Drop Tanks for Light Combat Aircraft. This paper deals with some of the details regarding design, development and qualification testing of above composite products.Furtheraimofthispaperistohighlightsolutionstosomeofthedifficultareas encounteredduringabovedevelopmentwork.Allthesecomponentshavemetthe requirements fully during ground and flight tests. 1.RE-ENTRY VEHICLE STRUCTURE Re-Entry Vehicle Structure, consisting of a nosetip and conical sections is the payloadforthemissionandthusbecomesahighlycriticalstructure.Thisis subjected to severe aerothermal environment during Re-entry Phase of Flight and its performance governs the Vehicle accuracy and survival to a large extent.Excessive erosionoccurringonnosetipandotherpartscaneffectVehiclestabilityand asymmetric erosion on these parts will generate control problems.Another significant performance requirement is inside temperature of the structure should not rise beyond 60oC for smooth functioning of the electronic components placed inside.In order to satisfyaboverequirements,nosetipismadeoutofCarbon-Carbon(C-C)material using 4D-Preform structure.The 4-D structure offers quasi isotropic properties in aProceedings of the International Conference on Aerospace Science and Technology 26 - 28 June 2008, Bangalore, India INCAST 2008 - IT102 plane perpendicular to the Missile Axis, to have almost uniform erosion.Coming to conical sections, two layer composite construction is employed to satisfy the two main functionalrequirements.The inner Carbon-Epoxy layer out of CNC filament winding takes care ofstructuralloadsandtheouterCarbon-Phenoliclayerusingtapewindingsatisfies the thermal protection requirements. Regarding design aspects, initially thermal analysis is carried out to determine theoreticallythetemperaturedistributionandsurfaceablationforeachcomponent.The problem is basically that of heat transfer in a solid domain with time dependent heatfluxattheoutersurfaceandtemperaturedependentmaterialproperties.The shape change due to ablation of surface adds complexity to the problem.A computer codeisdevelopedforheattransfercalculationswithconvectiveboundarywhere reactionoccursand2-Dablationmodelusingfiniteelementsinspaceandfinite differenceapproximationinthetimedomain.FurtherFEManalysis,usingANSYS package, is carried out for the following cases:- (i)Static analysis for zero and non-zero angle of attack conditions. (ii)Free vibration analysis for natural frequencies and mode shapes. (iii)Forced vibration analysis for stage separation amplification factor. (iv)Cut-out analysis for stress concentration and reinforcement requirements. (v)Buckling analysis against axial compression, external pressure and thermal loads. It is essential to have safer margins in all the above cases for the finalizeddesign parameters. 2.CANISTERS Canisters are used for storage, transportation, handling and launch of Missiles.This forms an essential requirement of modern Missiles where the launcher has to be highly mobile to minimize enemy threat.Different types of launch platforms such as ground, ship and sub-marine are considered for usage and the sub-marine launch Proceedings of the International Conference on Aerospace Science and Technology 26 - 28 June 2008, Bangalore, India 3 becomes critical design consideration if same canister has to be used for all platforms.It consists of a long Composite cylindrical tube with a metallic dome at the bottom and a short metallic cylinder in the front.Critical area of Canister development turns out to be leak proof joint between metal and composite portions of the tube.The joint not only experiences large direct stresses, but also is subjected to large stresses due to stiffnessmis-matchbetweenthemetallicandcompositeshellsinbothhoopand longitudinal directions.Basically two types of joint concepts, namely, radial joint and axial joint are tried out simultaneously to arrive at a positive conclusion.The axial joint configurationcertainlyprovidesmoreeffectivehermeticalsealing.However,it requires a minimum thickness of shell to accommodate axial fasteners.On the other hand, for radial joint there is no thickness limitation. Thecompositecylinderconsistsoffourlayers.Theinnermostlayeristhe thermal protection layer against gas generator combustion gases.The second layers is the impervious leak proof layer.The third layer is the structural layer and the outer mostfourthlayeristhefireprotectionlayeragainstexternalfireinthevicinityof platform.The two inner most layers do not contribute to the strength of the Canister.However, the fire protection layer, because of different construction with fire resistant properties, will have lower strength properties. The Principle Load cases for which a Canister has to be designed are: 1)Launch Load 2)Emergency Dive Load 3)Transportation and Handling Loads The Missile is launched by firing gas generator housed at the bottomdome of the Canister.The pressure of the gas generator will act in the volume below thebottomoftheMissile.SincetheMissilewillmovewithtime,thevolumeand corresponding height of the canister over which the pressure acts will also depend on time.Simultaneously, reaction from Missile shoes act on the Canister as concentrated load.Thisdependsonthelaunchconditions,likeangleoflaunch,vibrationofthe platformincaseofsurfacelaunch,hydrodynamicpressureincaseofsub-marine launch.Thus,atanyinstantoftime,gasgeneratorpressureactonpartofthe Canister and shoe reactions at appropriate locations. Proceedings of the International Conference on Aerospace Science and Technology 26 - 28 June 2008, Bangalore, India 4EmergencyDivesituationarisesinasub-marinelaunchedMissilewhenthe sub-marinehastodivetomaximumdepthwithoutclosingthecontainerliddueto some emergency.In this case, hydrostatic pressure corresponding to the depth of the sub-marine acts on the Canister. ThefirstlayerismadebyTapeWindingof100mmwidthcarbon-phenolic prepreg tape using a pressure roller for consolidation and staggering of joints.The secondlayerisfabricatedusing100mmwidthpolyesterflimsytapebytapelay-up andsubsequentconsolidationwithhoopwinding.Thestructurallayerismadeby CNC filament winding using Prepreg Rovings for helical and hoop thicknesses.The fourthlayerisagaintapewoundusing100mmwidthglassepoxyfabricprepreg.Further, number of tests are carried-out at ground level to check the Canister for all loading conditions before going for Flight Test. 3.CARBON-CARBON BRAKE DISCS Carbon Carbon Composites are the state-of-the-art materials for aircraft Brake Discs.All most all the fighter and passenger aircrafts are using Carbon Carbon Brake Discsinviewofsignificantweightsavingsandcostwithrespecttolandingper overhaul (LPO) over the conventional metallic discs with ceramic pads.Higher levels of heat absorption per unit mass is feasible with Carbon Carbon Brake Discs.Aircraft BrakeDiscshavetoabsorbtremendousamountofheatenergygeneratedinthe process of braking action due to friction between the rotors and stators. Thefibrearchitectureandthematrixmicrostructurehavetobesuitably selected to meet the thermo-structural and tri-biological design considerations.High specificheat,highthermalconductivity,lowco-efficientofthermalexpansionand stable coefficient of friction over the temperature of operation with low wear are the key characteristics of Carbon Carbon Brake Discs. Carbon Carbon Brake Discs have been developed both for Airforce and Navy versionsofLCA.Thekeytechnologiesdevelopedincludepreformingwithwoven fabrics,choppedstrands,spunyarnfabrics,densificationwithhighpressureliquid pitchimpregnation,carbonization,(HIP)andGraphitization.Squeezerivetingof metallic clips with Carbon Carbon rotors has been established.Non-rubbing zones of CarbonCarbonDiscsareprotectedwithspeciallyformulatedceramicpaintfor oxidation resistance. Proceedings of the International Conference on Aerospace Science and Technology 26 - 28 June 2008, Bangalore, India 5 4.RADOMES Primaryfunctionoftheradomeistotransmitelectro-magneticenergywith minimum distortion and loss.Normally it forms as nose part of the vehicle structure and is subjected to flight loads as well as environmental conditions corresponding to thevehicleonwhichitismounted.Electro-magneticdesign,basedonoperating frequency range, is the critical design requirement for radomes and for this purpose, a versatilein-housesoftwarecalledEMPEROR(Electro-magneticperformanceof radomes) is developed based on plane wave spectrum analysis.This software has shownmatchingpredictionsforproblemsincomparisonwithothersoftwares available.Different types of radomes based on E-glass, D-glass and Kevlar contour woven socks together with low loss polyester, epoxy, phenolic and cyanate ester resin systemshavebeendevelopedsuccessfullyforvaryingapplications.Closecontrol overdesignthicknessanduniformdistributionofthematerialsystemaretheprime requirementsforthiscomponent.Incasetheperformancerequirementsonloss, power transmission, side lobe level and bore-sight error are very stringent as in the case of Light Combat Aircraft, it becomes necessary to go in for a variable thickness radome, in place of uniform thickness radome, which is normally used.Also for large sizeradomes,anInsertionPhaseDelay(IPD)measurementandcorrectionsystem willbeemployedtomakefinercorrectionsafterfabricationsothatrejectionsare almost eliminated on production requirements. 5.ROCKET MOTOR CASINGS Rocket Motor Casing is basically a pressure vessel with specially designed end fittings namely polar bosses and skirts.The polar bosses are provided with matching interfacestoigniterandnozzleassembliesoneitherside.Theskirtstakecareof attachmenttotheforeandaftendsectionsoftheMissile.Thepolarbossesare structurallydesignedbyconsideringacantileverconfigurationsubjectedtointernal pressure load and nozzle ejection load.A rubber encapsulation is provided between the metal polar boss and the composite Proceedings of the International Conference on Aerospace Science and Technology 26 - 28 June 2008, Bangalore, India 6 casing to absorb the differential expansions.The skirt is designed by considering it as asimplecylindricalcompositecolumnsubjectedtoaxialinertialforceandbending moment.CNC filament winding is an obvious choice to realize the casing and non-geodesic helical winding is resorted to.Doilies of re-inforcing fabric are laid-up at the enddomestoprovidelongitudinalstrengthnearpoleopeningsandcircumferential strength near the equator.Finally, efficiency of a rocket motor casing can be judged in terms of performance factor which is of the order of 5 for metallic casings and it can be upto 20 for Composite Casings with advanced material systems. Number of other components are also tried out with Composites and in short, Composites are the best materials for Aerospace applications. Proceedings of the International Conference on Aerospace Science and Technology 26 - 28 June 2008, Bangalore, India