The limited payload capabilities of current launch vehicles impose severe restrictions on the mass and volume of space systems. Launch itself is also a major component of the total cost of these systems. In order to maximize the cost benefit ratio, there is a drive to use lower density materials for future space applications. Thin film inflatable structures fit this need due to their extremely low ratio of launch packaging volume to deployed useable area. This is an enabling feature that would allow spacecraft designers to develop systems many times larger than currently achievable with existing payload fairing sizes. Some examples include solar sails, sun shields, and inflatable optical surfaces such as reflectors, collectors, transmitters, and concentrators.
The scale of these structures makes terrestrial testing difficult. True free-free conditions can never be attained in practice. Gravitational sag and the inability to isolate airflow disturbances drives the need for accurate system models. A principle objective of this research was to further the development of modeling and simulation tools that could be used to support trade studies for future system development.
The most likely thin film candidate is Kapton, a DuPont polyimide that is commonly used in aerospace applications. Because Kapton is typically not used as a structural material, many relevant physical properties were not well quantified. A goal of this research was the determination of dynamic properties of commercially available Kapton films. Ultimately we would use our material and modeling techniques to design and fabricate a dynamically tailored demonstration structure. This work progressed through three primary tasks: characterization, component level modeling, and concluded after two and one-half years with the development of a larger scale demonstration structure.
Work initially focused on quantifying the basic stiffness and loss properties of Kapton. Next, considerable testing was performed to characterize the structural dynamics of thin film shapes. Samples included a variety of geometries: 1d strips, 2d scalloped flats, and 3d shells. Using laser-doppler velocimetry, we extracted mode shapes in air and at vacuum under various conditions– boundary, preload, net shape etc. and compared these data to theoretical models. A significant difference in dynamic behavior was observed between in-air and vacuum results. The hygoscopic properties of Kapton can have significant impacts for in vacuum tests. Loss of moisture will shrink the sample, drastically changing tension and increasing forces at the boundary.
From this basis, effort was then expanded to the design of a scaled structure and the demonstration of methods to influence or change its dynamic characteristics. This final structure was a 2 meter diameter torus comprised of a .003″ thick, Kapton. The walls of the torus were stiffened with small, hexagonal shaped, doubly curved domes. These domes increased structural stiffness by approximately 350\% without adding any mass. A scalloped reflective membrane was attached at hard points. Global vibration suppression techniques included tuned mass dampers as well as active and shunted piezoelectric patches.
A key result of this research effort is that despite being lightweight, thin film inflatable structures are relatively stiff dynamically. This property can greatly simplify the development of control systems making the overall concept of such structures more feasible. The fundamental conclusion from this work is that feasibility for large, high precision thin shell structures was firmly established and the tools needed for further development of such a system significantly advanced.