KSGC: Effects of Gravity on Ripple Configuration in Tensioned, Singly-Curved Parabolic Membranes Using Photogrammetry

This work is part of a larger project designed to quantify the effects of gravity and boundary support conditions op an orbiting Advanced Precipitation Radar Antenna, currently under development by Jet Propulsion Labs and ILC Dover. The singly-curved parabolic reflectorplanned for deploymentby the end of this decade is to be fabricated from I-mil metallized Kapton, and its RMS surface accuracy must be held below 0.17 mm to maintain the desired beam characteristics. However, it is well-known that membrane elements supported in tension are prone to out-of-plane rippling. To help predict the effects of gravity and boundary support on the full-scale surface rippling, a 1- m scale model of the parabolic reflector support system has been constructed to enable testing on NASA's KC-135 Weightless Wonder Aircraft. This airplane flies in a parabolic trajectory, and provides 25-second long periods of weightlessness, followed by slightly longer periods of 1.8-g conditions. Access to the KC-135 has been made possible through the NASA Reduced Gravity Student Flight Opportunities Program (RGSFOP). Our first flight took place during July 2003, and another has been approved for July 2004. Aboard the KC-135, a membrane mounted in the test fixture is tensioned using edge clamps that maintain the desired parabolic profile at the membrane boundaries. Targets for tracking the full-field surface deflection are provided by about 7000 2-mm dots stenciled on the membrane surface. In flight, the membrane configuration is monitored by four digital cameras mounted in the test enclosure. Wideangle lenses are used on the cameras, as they are mounted quite close to the membrane due to restrictions imposed by the size of the aircraft. Using photogrammetry, the highresolution digital images taken in-flight (at zero-g and 1.8-g conditions) and on the ground (at one-g) are processed to allow the three-dimensional location of each target (visible in at least three images) to be calculated. The preliminary results from last summer's flights indicate that for the same boundary conditions, surface rippling under zero-g conditions was significantly less pronounced than at one-g conditions for the membrane tested. In the follow-up experiments scheduled for this summer, a similar test set-up will be used to obfain zero- one- and 1.8-g data, with modifications to focus on repeatability and data accuracy. In addition, extensive ground-based experimentation will include rotation of the test article about its z-axis (horizontal) to reorient the position of the gravity field vector with respect to the membrane. Through analysis of data sets for several test positions and boundary conditions, 3-D surface representations of ripple configurations will be obtained under a variety of loading conditions at 0, 1 and 1.8 g. By comparison of the experimental surface contours with those generated using computational models (primarily finite element) currently under development, the validity ofthe computer models can be ascertained.