DYNAMIC CHARACTERIZATION OF AN INFLATABLE CONCENTRATOR FOR SOLAR THERMAL PROPULSION
An inflatable structural system that is a technology demonstrator for solar thermal propulsion and other applications is characterized for structural dynamic behavior both experimentally and computationally. The inflatable structure is a pressurized assembly developed for use in orbit to support a Fresnel lens or inflatable lenticular element for focusing sunlight into a solar thermal rocket engine. When the engine temperature reaches a pre-set level, the propellant is injected into the engine, absorbs heat from an exchanger, and is expanded through the nozzle to produce thrust. The inflatable structure is a passively adaptive system in that a regulator and relief valve are utilized to maintain pressure within design limits during the full range of orbital conditions. Modeling and test activities are complicated by the fact that the polyimide film material used for construction of the inflatable is nonlinear, with modulus varying as a function of frequency, temperature, and level of excitation. Modal vibration testing and finite element modeling are described in detail in this paper. The test database is used for validation and modification of the model. This work is highly significant because of the current interest in inflatable structures for space application, and because of the difficulty in accurately modeling such systems.
Dynamic Testing of an Inflatable, Self-Supporting, Unpressurized Thin-Film Torus
Dynamic testing of an inflatable thin-film torus structure with regular formed convex dome surface features has been performed to evaluate structural natural frequencies, mode shapes, and modal damping ratios. The structure presented two unique challenges with respect to modal testing. First, it is extremely lightweight, flexible, and highly damped. Second, the thin film that provides the integrity to the structure is not smooth and flat, but has a pattern of hexagonal dome structures formed into it that increases the local stiffness of the thin film to the point that the structure is self-supporting in the gravity environment of Earth when the internal pressure is released. The dynamic testing was performed in this self-supporting state. In the modal test a loudspeaker provided acoustic excitation, and a laser displacement sensor was used to measure the vibration response at various points on the torus surface. The acoustic excitation and the laser displacement measuring technique were chosen because they are all noncontact methods that avoid mass loading of the structure as would happen if accelerometers and a shaker were used. Three in-plane and three out-of-plane modes were extracted using this approach. The experimental results indicate that the noncontact modal data-acquisition approach for extremely lightweight structures is suitable and effective. The modal identification procedure found modes that are analogous to elastic ring modes as well as widely spaced repeated modes.
Deflections of inflated cylindrical cantilever beams subjected to bending and torsion
In the aeronautical field, there has been from time to time an interest in inflatable structures, ranging from their consideration for use in space to the design and manufacture of an inflatable wing for a man-powered aircraft and their use in remotely piloted vehicles . This interest stems from the fact that such structures have relatively low weight; are suitable for reacting light loads and can be packed into a small volume. One particularly useful structural component is the inflated circular cylinder acting as a cantilever and a number of such cylinders are used in Ref. 3 to form the main load carrying structure of a wing. In Ref. 1 an expression is derived for the collapse load of a single cylinder with the assumption that the root behaves like a plastic hinge, and in Ref. 5, Comer and Levy go on to consider the beam deflections and stresses between incipient wrinkling and final collapse.