1. Vacuum exposure 2. Radiation effects 3. Temperature extremes (6°K to 500°K) 4. High fabrication loads during construction; low operational loads after construction 5. Orbital fabrication techniques 6. Thirty-year operational life In general, materials used in the SPS fall into two general categories, structural and nonstructural. The structural materials make up the framework, trusses, cables, etc., which support the solar cell arrays and antenna elements. The nonstructural materials include solar concentrators (reflectors), solar cells, antenna wave guides, etc., which are necessary for the operation of the system. In the SPS configurations evaluated in this study, the structural materials actually account for less than 3 percent of the total SPS weight. The general characteristics of both structural and nonstructural materials evaluated in this study are discussed below in more detail. Environmental Effects: In evaluating materials for the SPS, environmental effects arising from vacuum, radiation and high and low temperatures are of primary consideration. For the screening of generic classes of materials, changes in properties were considered acceptable if the change was reasonably well characterized for the environment, if the material retained useful engineering properties with the effects of the environment considered, and if the change was reversible. An example of an environmentally-caused change would be loss in ductility with low temperatures. Aluminum alloys are well characterized at temperatures as low as liquid helium (4°K). Several alloys are considered acceptable for use in structural applications at these temperatures even though the loss of ductility at the cryogenic temperature is significant. These alloys are considered acceptable because: 1. The change has been quantitatively determined with a reasonably good data base. 2. The remaining ductility is enough to be useful in an efficient structural design. 3. The change is reversible; i.e., on warming, the ductility returns to the material and the degradation is neither additive nor accumulative. Another example is in the area of plastic films. Here, the polyimide films such as Kapton (H-film) become brittle at low temperatures; but, again, enough ductility remains at temperatures as low as 77°K to make the material useful for this temperature range. The ductility of this material also returns with increasing temperature and no permanent damage would be anticipated. Using criteria similar to these examples, various candidate materials were examined for application in the SPS environment. Structural Materials: 17 Aluminum Alloys Approximately 4 to 6 million pounds of structural materials are required in the fabrication of one SPS. Aluminum alloy, because of its availability and low cost, would probably be the best choice if the SPS was required for construction today. Aluminum can be packaged in sheet rolls, transported to orbit, and fabricated to meet most structural design requirements. Aluminum exhibits excellent stability for the space environments, and its mechanical and physical properties are well characterized. One potential problem with aluminum
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