is that its relatively high coefficient of thermal expansion could lead to problems in maintaining the proper "flatness" required for supporting the solar collecting and microwave transmitting elements. Another possible disadvantage for using aluminum, especially in the 1995 time frame, is that electric power is the primary energy source in converting aluminum containing ores to pure aluminum metal. 2. Graphite Composites Recent development and promising future development of high modulus and high strength graphite fibers tend to favor the use of graphite composites for the major structural elements of SPS. Graphite composites with an epoxy matrix for "normal" environmental temperature exposure, or polyimide matrix for high temperature use, are being used more frequently in aerospace primary structure applications. Graphite has a favorable low coefficient of thermal expansion, can be packaged in "prepreg" tape form with its matrix material, transported to orbit, and used to fabricate almost any required structural beam shape during SPS assembly. An alternate to the use of uncured "prepreg" tapes is to use cured graphite reinforced thermoplastic rolls or sheet materials which offer possible advantages in fabrication and joining techniques (see Appendix V-A-2). Two concerns which probably require additional evaluation are possible radiation damage (to matrix) and low temperature effects on composite mechanical properties, although existing development programs indicate neither concern is unsurmountable. A potential concern is the availability of carbon-base raw materials for use in the production of the graphite and matrix materials in the 1995 time period. Presently, graphite composites are produced from petroleum base raw materials, although recent advances in low cost "pitch" raw materials for graphite promise to be suitable substitutes. Additional study/evaluation is required in this area to fully assess the possible impact on graphite composite materials used in the SPS construction. 3. Cable Materials The primary potential use of stainless steel in the SPS would be for tension cables. Stainless steel wire can be cold drawn to very high tensile strengths and woven into cables to meet design requirements. The major disadvantages of steel (high density and moderately high coefficient of thermal expansion) tend to negate any real advantage of using steel cables in the SPS. Kevlar 49 fiber (polyimide) is considered to be an alternate candidate for the tension cable because of its high tensile properties. However, its moderately high negative coefficient of thermal expansion could cause serious problems if not properly corrected by design. Kevlar 49 also has only moderate resistance to radiation damage; therefore, a useful life of thirty years is doubtful. Protective coatings with sleeving materials would probably be required, or periodic cable replacement could be necessary. A potential use of graphite fiber or graphite composite tension cables is being considered. Since the previous use of graphite in cable systems is limited, development programs will probably be required to establish fabrication and application characteristics. 4. Meta! Matrix Composites Although not studied in detail, boron filament/aluminum matrix composites should be considered for use in SPS construction. Boron/aluminum composites perhaps best resist all of the environmental extremes, including radiation and low temperature effects. Another advantage of its use is the possibility of using the boron/aluminum structure for conducting electricity from the solar cells to the antenna, thereby eliminating nonstructural conducting
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