Zero- and One-g Comparison of Ripple Amplitude in Single-Curved Parabolic Membranes Using Photogrammetry

This experiment was designed to quantify the effects of gravity and boundary support conditions on a scale model of an orbiting, singly-curved parabolic thin-film reflector planned for deployment by the end of this decade. As it is well-known that membrane elements supported in tension are prone to out-of-plane rippling, a 1-m scale model of the parabolic reflector support system was constructed and tested on NASA's KC-135A Weightless Wonder microgravity aircraft. Two membranes were tested: a fabric-backed metallized membrane (76.2 µm Nylon, 38.1 µm Mylar, 0.1 µm Al) and a 127 µm metallized Kapton membrane. Each membrane was placed in the fixture, and tensioned using edge clamps that maintained the desired parabolic profile at the membrane boundaries. Targets for tracking the full-field surface deflection were provided by about 7000 2-mm dots placed on the membrane surface. In flight, the membrane configuration was monitored by four digital cameras mounted in the test enclosure. Wide-angle lenses were used on the cameras, as they had to be mounted quite close to the membrane due to restrictions imposed by the size of the aircraft. Using photogrammetry, the high-resolution digital images taken in-flight (at zero-g conditions) and on the ground (at one-g) were processed, and the three- dimensional location of each target visible in at least three images was calculated. It was found that for the same boundary condition, surface rippling under zero-g conditions was less pronounced than at one-g conditions for the fabric-backed Mylar test article.