KSGC: Static Analysis of Tensioned Gossamer Membrane Structures Using Photogrammetry in Microgravity

Substantial progress has been made in the past several years towards the use of ultra-light-weight (gossamer) designs for the next generation of space structures. Chief among their advantages is their light weight and compactness in launch configuration. However these same attributes also currently serve as barriers to their near-tenn implementation and deployment in space. The light weight and flexibility of gossamer structures means that ground testing in a one-g, earthbound environment poses difficulties, as it is difficult to correlate behavior in one-g to perfonnance in orbit, in zero-g. Given the nature of gossamer structures, they are susceptible to large amplitude, out-of-plane defonnations due to in-plane (tension or shear) forces, as well as small, out-of-plane forces. While wrinkling of these structures can occur solely on account of in-plane tensioning (used for support), weight loading due to gravity in one-g environments is known to significantly affect the final wrinkle configuration of these structures. Many of the potential applications of thin-film gossamer structures under study by NASA require precision control of surface contour, hence it is vital to predict surface contour as a function of support tension in the absence of gravity. One challenge encountered when designing an approach to surface measurement of gossamer membranes is their extreme light weight and compliance. Contacting sensors cannot be used for this, as the weight of such sensors would significantly affect the response of the membrane surface. For this reason it is necessary to use a non-contact means of measuring these surfaces to analyze their characteristics. . Photogrammetry, used in conjunction with extremely high-resolution digital cameras, is a proven, accurate, and cost-effective means of recreating the topographical characteristics of these structures. Bec~use of the problems presented by ground testing gossamer structures in a one-g environment, it is necessary to develop a strategy for eliminating the effects of gravity loading on the wrinkle configuration attained by the thin-film test article. One means of doing this is to test in a micro gravity environment, such as that offered on-board NASA's KC-135. The Paducah Weightless Wildcats (PaWWs), a group of students from the University of Kentucky Extended Campus in Paducah, has been selected by NASA to study the impact of microgravity on wrinkling, as part of NASA's Reduced Gravity Student Flight Opportunities Program in July 2003. Aboard the KC-135, the students will study the wrinkling configuration due to only the tension provided by the support members, as well as imperfections in the assembly and membrane fabrication, without the complicating effects of the gravity load. An experimental apparatus is being designed that will allow wrinkle patterns to be analyzed using photogrammetry. The test article will be enclosed in a Lucite@box to prevent membrane movement caused by air currents within the aircraft, and to protect the fragile membrane from contact with crew members and other equipment that may come loose during the zero-gravity portion of the flight. Photographs of the test article's surface configuration will be taken simultaneously from four different locations around the test article, using four on-board digital cameras mounted around the perimeter of the enclosure. The digital photographs' Will latei be analyzed using' Photomodeler@ 4.0 to recreate the surface contour via photogrammetry, and will allow the detennination of wrinkle pattern as a function of tension. These data will by compared against ground testing data in order to quantify the effects of gravity on surface contour.