Analytical and experimental investigation of the dynamics of polyimide inflatable cylinders

Abstract

An investigation of the dynamics and load-deflection behavior of an inflatable cylindrical strut constructed of Kapton polyimide film is described Such a structure has considerable practical application and potential as a component of inflatable solar concentrator assemblies, antenna structures, and space power systems. Non-rigidized inflatable structures for potential space applications are adaptive systems in that a regulator and relief valve would be required to maintain pressure within design limits during the full range of orbital conditions. Further, the polyimide film material used for construction of the inflatable (and the filmto-pressurized-air interaction) is highly nonlinear, with modulus varying as a function of frequency, temperature, and level of excitation. In view of these characteristics, a series of tests and analytical modeling efforts are described for determining the inflatable cylinder’s response at various pressures and loading conditions. A number of interesting phenomena have been discovered during this investigation and related studies. It has been found that the inflatable cylinder exhibits (1) decreasing effective modulus of elasticity as function of frequency in free-free and cantilevered configurations, (2) wrinkling near a cantilever end in response to increased static tip load, and (3) bifurcation phenomena or resonant peak splitting with increased dynamic force amplitude. Introduction and Backaround In recent years, inflatable structures have been the subject of renewed interest for space applications such as communications antennae, solar thermal propulsion, and space solar power. A major advantage of using inflatable structures in space is their extremely light weight. This makes inflatables a perfect match for solar thermal propulsion because of the low thrust levels available. An *Doctoral Candidate; Student Member of ASME **Aerospace Technologist, Structural Dynamics; Senior Member AIAA Copyright Q 1999 by the American Institute of Aeronautics and Astronautics, Inc. No copyright is asserted in the United States under Title 17,U.S. Code. Be U.S. government has a royalty-free license to exercise all rights under the copyright claimed herein for Governmental purposes. All other rights are reserved by the copyright owner. Michael L. Tinker ** Structural Dynamics and Loads BrancWED23 Structures and Dynamics Laboratory NASAlMarshall Space Flight Center Huntsville, AL 35812 olvious second advantage is on-orbit deployability and subsequent space savings in the launch configuration. A recent technology demonstrator fligh: for inflatable structures was the Inflatable Antenna Experiment (IAE) that was deployed on orbit from the Shuttle Orbiter. Although difficulty was encountered in the inflation/deployment phase, the flight was successful overall and provided valuable experience in the use of such structures (Ref. 1). The Solar Orbit Transfer Vehicle (SOTV), discussed in Ref. 2, is a planned technology demonstrator flight for solar thermal propulsion. The basic concept behind solar thermal propulsion is to utilize sunlight or solar energy as a means of heating a working fluid (propellant) to provide thrust at increased specific impulse. As described in Ref. 3, thrust is produced by expanding the heated propellant through a nozzle. No combustion occurs, and the thrust level is low. For this reason, solar thermal propulsive systems arc mainly applicable for orbital transfer vehicles. Another technology demonstration program for solar thermal propulsion is the Solar Thermal Upper Stage (STUS), which is described in Refs. 4-6. The engine system envisioned for the STUS is designed to utilize hydrogen propellant to produce a thrust level of about 2 lbf. Two inflatable parabolic collectors could be used that would be rotated and gymballed for focusing sunlight into an absorber cavity (Fig. 1. from Ref. 6). The collectors would be inflated after separation of the upper stage from the launch vehicIe. In Fig, 2, a prototype inflatable solar concentrator (Refs. 7-8) is shown that consists of a torus/lens assembly supported by three struts. This concentrator is constructed of Kapton polyimide film, with epoxy as the primary adhesive for joints. In practical applications, the Fresnel lens of such a concentrator assembly would focus sunlight into a collector near the fixed ends of the struts. Solar energy stored in the collector could be utilized to heat a propellant as described previously. The inflatable struts shown in Fig. 2 are attached to a base plate by means of three cylindrical appendages. These hollow appendages also allow inflation of the concentrator assembly through air hoses connected at each strut. It can be seen from Fig. 2 that inflatable cylindrical struts are critical components of structural assemblies for practical applications. In view of their importance, structural dynamic and static behavior of typical inflated polyimide struts are investigated in detail in this paper. The remainder of the paper is structured as follows: 1. Review of previous work in the literature dealing with inflatable beam-type structures, 2. Detailed description of the geometry and materials for cylinders treated in this study,