PHOTOGRAMMETRIC MEASUREMENT OF GOSSAMER SPACECRAFT MEMBRANE WRINKLING

Photogrammetry methods are being developed for measuring the shape and dynamics of future gossamer spacecraft structures, which characteristically contain large areas of thin-film membranes. Examples include solar sails, large membrane antennas, telescope sun shields, inflatable solar arrays, space solar power collectors and transmitters, radar arrays, and planetary balloons and habitats. Membrane wrinkling (and slackening)--caused by various sources including seaming and folding, nonuniform tensioning, thermal effects, and creep--can potentially reduce performance or cause overheating or structural fatigue in space. This paper documents an experimental photogrammetric technique capable of measuring membrane wrinkle patterns and amplitudes using thousands of projected dots of light and multiple synchronized cameras. Typical results are presented for a 0.5 m x 1.0 m area of interest on a 2m square solar sail model. In this application, the photogrammetric measurement precision was approximately 25 microns (0.001 inches). The paper closes by mentioning a promising extension of existing techniques using a laser-induced fluorescence approach. INTRODUCTION NASA is giving renewed attention to the topic of large, ultra-lightweight space structures, also known as “gossamer” spacecraft. Revolutionary concepts for large antennas and observatories, solar sails, inflatable solar arrays and concentrators, and inflatable habitats, among others, are being studied [Ref. 1]. These structures characteristically contain large areas of thin-film membranes and can be tens or even hundreds of meters in size. Complete gossamer systems are generally too large and flexible for meaningful ground testing, so increased reliance must be placed on analytical methods validated by representative scale-model test articles [Refs. 2-3]. Figure 1 shows several examples of gossamer ground test articles. The delicate nature of these structures requires non-contacting, optical measurement techniques. Photogrammetry has proven to be an effective and robust technology for this purpose. Photogrammetry is the science of measuring three-dimensional object coordinates with photographs [Ref. 4]. Research to develop effective photogrammetry and videogrammetry methods for shape and dynamic measurements of gossamer structures began about three years ago. Several earlier publications discuss related and complementary aspects of this work [Refs. 5-8]. This paper has two sections. The first section summarizes an experimental photogrammetric technique for measuring membrane wrinkles using thousands of projected dots of light and multiple synchronized cameras. The second section mentions a promising extension of this technique using a laser-induced fluorescence approach. Membrane wrinkling (and slackening)--caused by various sources including seaming and folding, nonuniform tensioning, thermal effects, and creep--can potentially reduce performance or cause overheating or structural fatigue in space and therefore must be well understood both analytically and experimentally. TEST PROCEDURES AND RESULTS Figure 2(a) shows a lightly tensioned 2m square solar sail model. This structure was designed using constant-thickness scaling laws [Ref. 9] and displays considerable wrinkling in a vertical orientation in gravity. The exact shape of the wrinkles can change easily with slight air currents or other environmental disturbances, so a full-field, rapid measurement technique is required. A grid of about 5000 dots was projected onto the indicated area, which is about 0.5 m x 1.0 m in size. Four Olympus E-20 digital cameras located as shown in Fig. 2(b) photographed the dot pattern simultaneously. Several sets of photographs were taken in a darkened room at