Dynamics of a 4x6 Meter Thin Film Elliptical Inflated Membrane for Space Applications

and Background Dynamic characterization of a thin film inflatableelliptical structure is described in detail. A two-step finiteelement modeling approach in VISC/NASTRAN isutilized, consisting of (1) a nonlinear static pressurizationprocedure used to obtain the updated stiffness matrix, and(2) a modal "restart" eigen solution that uses the modifiedstiffness matrix.Unique problems encountered in modeling of this large4x6-meter lightweight inflatable arc identified, includingconsiderable difficulty in obtaining convergence in thenonlinear finite element pressurization solution. It wasfound that the extremely thin pol¢imide film material(.001 in or I mil) presents trerrendous problems inobtaining a converged solution when internal pressureloading is applied. Approaches utilized to overcome thesedifficulties are described.Comparison of finite element predictions for frequencyand mode shapes of the inflated structure with closed-formsolutions for a flat pre-tensionec_ membrane indicatereasonable agreement.tAerospace Technologist,_Professor of Mechanical Engi_leering; FellowASME*Structural Dynamics Lead Engineer; AssociateFellow AIAACopyright O 2002 by the American institute of Aeronauticsand Astronautics, Inc. No copyright is asserted in the UnitedStates under Title 17, U.S. Code. The J.S. government has aroyalty-free license to exercise all rigl_ts under the copyrightclaimed herein for Governmental purl:oses. All other rightsare reserved by the copyright owner.Inflatable structures have been the subject of renewedinterest in recent years for space applications such ascommunications antennas, solar thermal propulsion, andspace solar power (Figs. I-2). A major advantage ofusing inflatable structures in space is their extremely lightweight. An obvious second advantage is on-orbitdeployability and subsequent space savings in the launchconfiguration. A recent technology demonstrator flightfor inflatable structures was the Inflatable AntennaExperiment (IAE) that was deployed on orbit from theShuttle Orbiter. Although difficulty was encountered inthe inflation/deployment phase, the flight was successfuloverall and provided valuable experience in the use of suchstructures (Ref. 1).The Solar Orbital Transfer Vehicle (SOTV), discussedin Ref. 2, and Solar Thermal Upper Stage (STUS),described in Refs. 3-5, are possible technologydemonstrator flights for solar thermal propulsion. Thebasic concept behind solar thermal propulsion is to utilizesunlight or solar energy as a means of heating a workingfluid (propellant) to provide thrust at increased specificimpulse. As described in Ref. 6, thrust is produced byexpanding the heated propellant through a nozzle. Nocombustion occurs, and the thrust level is low. For thisreason, solar thermal propulsive systems are mainlyapplicable for orbital transfer vehicles. The engine systemenvisioned for the STUS is designed to utilize hydrogenpropellant to produce a thrust level of about 2 lbf. Twoinflatable parabolic collectors could be used that would berotated and gimbaled for focusing sunlight into anabsorber cavity (Fig. i, from Ref. 5). The collectorswould be inflated after separation of the upper stage fromthe launch vehicle.Many investigators have considered the use ofinflatable structures for space applications. Perhaps theearliest was Frei Otto (Ref. 7), who in 1962 publishedideas for inflated tubular frames for use in structures suchas orbiting platforms. A more recent proposed applicationinvolves the use of inflatable beam segments to replaceiAmerican Institute of Aeronautics and Astronautics