LARGE SPACE STRUCTURES Unlike solar arrays where a large amount of both space and terrestrial related experience is available, large space structures, with few exceptions, are mainly identified with studies and only very limited hardware verification. The ability to fabricate and assemble large space structures is of fundamental importance to the SPS. The scale of these structures is truly enormous, the reference design indicates an array of 10 by 5 km with an antenna of 1 km diam. However, the need to have large space structures will exist in the normal evolution of space activities, even without the SPS, a fact well recognised by ESA, NASA, and the aerospace industry. The building of large space structures must take place in space, i.e., on-orbit construction and in a zero# environment and will involve a high level of automation. The comments that follow can be applied generally to the space structure problem, although SPS specific needs will be identified. In their simplest form, structures consist of large numbers of interconnected beams, and therefore the approach has been to investigate means of producing beams automatically. Grumman Aerospace have made and demonstrated an automatic beam building machine which can produce a triangular section beam of 1 m side at a rate of about 1 m/min. The beam material is aluminium and the connections are made by spot welding caps. The machine can fit into the shuttle cargo bay and has the potential capability to continually produce beams some hundreds of metres long. A considerable amount of study work has been done on the structures and it appears that graphite reinforced composite materials show much promise. Other US companies, General Dynamics and McDonnell Douglas, are developing machines to form composite beams. The systems studies (1) have also given much attention to the actual construction procedure, because the SPS can be compared in magnitude to a very large civil engineering operation being undertaken in the space environment. Although the problems of weight are not present in space, Newton’s three laws are very much in evidence and much thought is required to be ever mindful of their effects. The basic major structural components characterised by Greenberg of Rockwell (2) are • Tension stabilised surfaces — solar cell blankets and their tensioning devices to the main frame • Rigid surfaces — antenna/klystron assemblies • Primary structure frameworks - platforms - compression frames - built-up trusses of triangular beams - triangular beams • Machine-made beams - open cap aluminium - open cap composite (graphite) - closed cap composite - composite geodetic • Beam to beam joints - lap joints - pin joints - fixed joints.
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