International Journal of Scientific & Engineering Research, Volume 7, Issue 4, April ISSN

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1 Intenational Jounal of Scientific & Engineeing Reseach, Volume 7, Issue 4, Apil ISSN Reduction at Payload Using Isolation System in a ypical Launch Vehicle Lekshmi Piya S,D. Sundaaajan, D.K Balakishna Rao 3 Abstact- Launch vehicles pesent an efficiently viable method fo placing satellites into obit. A typical launch vehicle consist of populsion modules eithe solid and/o liquid stages connected by intestages and payload module.dynamic analysis is used to find out the esponse of the stuctue, mainly at the satellite inteface due to vaious excitations encounteed duing flight. he basic objective of the wok is to educe the dynamic loads on the satellite by vibation isolation methods. In this wok analysis is caied out by incopoating an isolation system at vaious locations and the esults ae compaed to show the effectiveness of isolation system. Index tems- launch vehicle, shape, natual fequency, fequency esponse, acceleation, vibation isolation. I.INRODUCION Lage solid motos ae found to poduce vibations due to thust oscillations duing thei opeation. If the fequency of these oscillations matches with the lateal/longitudinal s of the stuctue, it would esult in lage esponses at satellites. Geneally isolation systems ae designed as a solution to such poblems. he pesent study is caied out to evaluate the equiement of isolation system and the its effectiveness.he geneal appoach fo dynamic solutions involving lage systems is to develop a mathematical l descibing the system s mass and stiffness to calculate s of vibation. In aeospace industy due to complex and complicated systems a finite element l is ceated to estimate the esponse due to diffeent excitations. his l should take into account the chaacteistics of the system design, the natue of the dynamic loading (type and fequency) and any inteacting media (fluids, adjacent stuctues). Fequency esponse analysis is caied out to estimate esponse due to the applied foces. he fist step in dynamic analyses is the fee vibation which detemines the stuctue s stuctual dynamic chaacteistics viz. natual fequencies and shapes. A launch vehicle stuctue can be idealized by beam-od l, quate-shell l,3-dimensional l o a M ech student,civil Dept.,Manipal Institute Of echnoogy,manipal,india Head,SMSD,Vikam Saabhai Space Cente,ivandum,INDIA. 3Poffesso,Civil Dept.,Manipal Institute Of echnology,manipal,india. lekshmipiya085@gmail.com combination of the above depending on the fequency equiements [4]. Due to lage L/D atio, a beam l is adequate fo captuing the pedominant esponses which will be dominated by fist few global s. Solid motos expeience sustained self-excited oscillations at the fequency of the fist longitudinal acoustic of the chambe [3]. he fequency of this half-wave is detemined by the length of the combustion chambe and the acoustic speed in the hot gas. Lage segmented solid ocket motos will exhibit pessue oscillations with coesponding thust oscillations. hese oscillations will inteact with the stuctual s of the launch vehicle. In this pape diffeent isolation schemes fo educing the vibation at the satellite due to solid moto pessue oscillation is attempted. Finite Element package MSC/NASRAN [5] was used fo ling and analysis of the stuctue. As pat of the evaluation pocedue, the launch vehicle is initially led without any isolation system and analyzed. hen equied modifications ae made to the system design by intoducing isolatos at vaious locations, and the esponses at the satellite base ae compaed..objecive A launch vehicle is subjected to vaious excitations duing its mission. Excessive dynamic loads duing its ascent can be detimental to satellite.one of the options is to educe the dynamic loads tansmitted to the base stuctue at which payload is attached. In this pape analysis is caied out by incopoating an isolation system eithe between the stapon and the 06

2 Intenational Jounal of Scientific & Engineeing Reseach, Volume 7, Issue 4, Apil ISSN coe vehicle o at the satellite inteface, thus potecting the satellite fom the thust oscillation of the stapons.hus the main objective of the wok is to find the esponse eduction at the payload inteface afte the intoduction of isolatos. 3. MODELLING Launch vehicle is led using 3D beam element. he popellant tanks, solid popellant moto and inte-stages connecting them ae led using equivalent aea (A), bending moment of inetia (Ixx and Izz) and tosional moment of inetia (J) [5]. he stuctual mass is smeaed ove the appopiate subsystems. he solid popellant mass is smeaed along the moto case mass. Fo liquid popellant, slosh ling is adopted by simulating the igid mass with inetia and slosh masses at appopiate location fom the tank bottom []. Longitudinal dynamics of the liquid popellants ae epesented using equivalent esonatos. he spacecaft is simulated by its mass lumped at its cente of gavity. Figue-[] shows finite element l of a typical launch vehicle. he coe to stapons ae connected using equivalent beam elements. he connections ae made in such a way that the thust tansfe is at the foe end attachment. Finite element ls ae geneated incopoating stiffness coesponding to the isolato at coe-stapon connection foe end as well as between spacecaft and payload adapte. Scala sping element is used to l isolatos. Geneal pupose finite element softwae MSC/NASRAN is used fo dynamic analysis. 4. FREQUENCY RESPONSE ANALYSIS A fequency-domain l is a set of input-output tansfe functions with espect to the independent vaiable fequency (ω). Fequency domain methods ae most efficient fo andom vibations and peiodic loadings. As the amplitude and the fequency of the excitation ae vaied, the esponse also changes. In this manne, the esponse of the system ove a ange of excitation fequencies is detemined. When subjected to dynamic foces, a stuctue s total esponse is the sum of the esponses of its s of vibation [6]. he physical displacements ae expessed in tems of modal coodinates η(t) as q = φη( t) () he equation of motion unde constaints given in equation [] M ]{ q } + [ C ]{ q } + [ K ]{ q } = { } () [ Q Eliminating q fom Eq.() and pe-multiplying Figue []: Finite Element l of a typical launch vehicle by the tanspose of the modal matix Φ, decoupled equations will be of the fom: M η + C η + Kη = {Q} (3) Whee, M = φ Mφ, C = φ C φ, K = φ Kφ, Q = φ Q Both M and K ae diagonal matices. If columns of Φ ae mass nomalized, then M = I, K = I whee = Equation () educes to η + C η + η = Q he steady state solution is obtained by assuming that the esponse is hamonic with fequency ω. he deivation is same as that of single-degee of feedom system, whee e iω t is factoed to obtain the following elations. [ ω ] η = Q η = [ ω ] Q q = φ [ ω ] u(iω) = H(iω) F(iω) 06

3 Intenational Jounal of Scientific & Engineeing Reseach, Volume 7, Issue 4, Apil ISSN Whee, H(iω) = φ[ ω ] he tansfe function is n H jk(iω)= = + iγ ω ω ϕ j ϕ In ationalized fom, H jk(iω) k φ n ϕ jϕk ( ω ω ) = ( ) + (γ ω ω) n ϕ jϕk (γ ωω) i = ( ) + (γ ω ω ) Fig[] : Longitudinal shape fo natual fequency of.6hz 5.RESULS AND DISCUSSIONS Finite Element package was used fo ling of the stuctue.once the launch vehicle is led, nomal modal analysis was caied which gave fequencies and associated shapes of the stuctue. able [] gives the natual fequencies of the l without isolatos at coe to stapon joint as well as at space caft inteface. Since the aim is eduction of the axial esponse, the fist few longitudinal s ae given in able-[]. Fequencies.6 Hz and 8.59 Hz ae the dominant s up to 30Hz. Natual Fequency ABLE [] AXIAL FREQUENCIES Remaks of the shapes.63 Stapon balancing coe 8.60 Stapon dominated by st axial Stapon dominated by nd axial 5.85 Stapon dominated by 3 d axial Coe axial and stapon 4 th axial Fig[3] : Longitudinal shape fo natual fequency of 8.59Hz Figue [-3] shows the longitudinal shape of the coe and solid boostes coesponding to.63hz and 8.59 Hz without isolatos, espectively. he s ae solid stapon dominated axial s. Usually the lage segmented solid ocket motos the pessue oscillation fequency will be less than 30Hz. Fequency esponse analysis was caied out to find out the esponse at the satellite base due to the thust oscillations of the two solid boostes. Foce is applied at the head end of solid stapons.hee diffeent cases whee consideed to find out the esponse eduction. Without isolatos With isolato at satellite inteface With isolato at coe stapon inteface he excitation foce of N (unit foce) was applied fom 0.Hz to 00Hz.Fequency esponse analysis was done to find out the esponse (acceleation) due to unit foce at solid stapon head end. he esponse at the location coesponding to the satellite inteface was the main aea of inteest. he vaiations in the acceleation of the thee diffeent cases, at the above efeed location ae shown in Figues [4-6]. 06

4 Intenational Jounal of Scientific & Engineeing Reseach, Volume 7, Issue 4, Apil ISSN Fig [4]: cuve of l without isolatos.00e-04m/s occued at a fequency 8.60Hz due to the application of unit foce (N)excitation at thust location without isolatos.it got educed by almost 50% when the isolatos whee installed at coe stapon inteface. ables [-3] shows the pecentage eduction in axial acceleation fo the dominant s up to 00Hz afte the intoduction of isolatos at satellite inteface as well at coe stapon inteface espectively. ABLE[] AXIAL ACCELERAION FOR UNI FORCE (N) A HRUS LOCAION WIH ISOLAOR A SAELLIE INERFACE Without Isolatos fequency anslational Acceleation (m/s ) Isolato at Satellite Inteface fequency anslational Acceleation (m/s ) % Reduction in Acceleati on.63.50e-04.3.e % e e-04.5% e e-04 inceased e e-05 inceased Fig [5]: cuve of l with isolatos at payload inteface e e-05 inceased ABLE [3] AXIAL ACCELERAION FOR UNI FORCE (N) A HRUS LOCAION WIH ISOLAOR A CORE SRAPON INERFACE fequency Without Isolatos anslational Acceleation (m/s ) Isolato at Coe Stapon Inteface fequency anslational Acceleation (m/s ) % Reduction in Acceleati on.63.50e e-05 86% Fig [6]: cuve of l with isolatos at coe stapon inteface he Figues [4-6] clealy shows the effect of isolation on the acceleation at the satellite inteface. It is evident that the esponse is least possible in case when the isolatos wee intoduced in between coe stapon than fom the othe two cases. At fequency of.63hz the axial esponse got educed by 9.33%, when isolatos at satellite inteface. While it got educed by 86%, when the isolatos whee at the coe stapon inteface. A maximum axial acceleation of e e-04 50% e e-05 44% e e-05 37% e e-05 9% Significant eduction in the esponses at the satellite base is obseved in case when the isolatos whee installed at coe stapon inteface. 06

5 Intenational Jounal of Scientific & Engineeing Reseach, Volume 7, Issue 4, Apil-06 7 ISSN CONCLUSION hee is a need to educe dynamic loads on launch vehicle so that spacecaft and thei instuments can be designed with moe concentation on obital pefomance athe than launch suvival. A softe ide to obit will allow moe sensitive equipment to be included in missions, educe isk of equipment o component beakdown, and possibly allow the mass of the spacecaft bus to be educed. Fom the diffeent cases analyzed, it is clea that the vibation isolation systems between coe and stapon pefomed vey well to educe stuctue vibation levels tansmitted to the satellite. he isolation system was designed specifically to educe the effects of solid moto esonant bun in the 0 Hz to 30 Hz fequency ange, which it did vey well, when installed at coe stapon inteface. REFERENCES [] G. B. Paddock, Dynamic Stability of Space Vehicles Volume I Lateal Vibation Modes, NASA CR-935, Novembe 967. [] R. Gieseke, R. Schuett and D. Lukens, Dynamic Stability of Space Vehicles Volume III osional Vibation Modes, NASA CR-937, Novembe 967. [3] Dynamic Envionmental Citeia, NASA echnical Handbook, NASA-HDBK-7005, Mach 00 [4] P. J. Gimes, L. D. Mcigue, G. F. Riley,and D. I. ildelt, Advancements In Stuctual Dynamic echnology Resulting Fom Satun V Pogams,NASA CR-540,June 970 [5] MSC/NASRAN Quick Refeence Guide, he MacNeal Schwendla Copoation, USA. [6]S.R.Damodaaswamy and S.Kavitha, Stuctual Dynamics And Aseismic Design,PHI Leaning Pivate Limited,DelhI