The stiffness requirements for the MPTS are dictated by its pointing requirements (~arc minute) and a separation of its natural frequency from that of the array. The reference system meets both of these requirements with a frequency margin of more than an order of magnitude. Materials - Early SPS concepts employed aluminum as an efficient structural material with a wealth of aerospace experience. As the thermal/structural and thermal/structural/dynamic interactions became apparent, however, the desirability of a structural material which was insensitive to the thermal environment also became apparent. Since this insensitivity can be readily obtained by the use of graphite composites (more than two orders of magnitude lower coefficient of thermal expansion than aluminum), this material has been considered as the prime candidate for an SPS structural material. The graphite composite materials have a higher Youngs modulus-to-density ratio than aluminum; however, its raw material costs are an order of magnitude higher than aluminum. The trend of graphite composite material costs is downward due to an expanding market; however, for the raw material cost to approach that of aluminum would require a major market acceptance such as the replacement of steel in the automobile. Energy investment requirements for the production of graphite composites are on the order of one-half to one-quarter that of an equal mass of aluminum. Raw material costs are only one facet of the structural system costs. The ease by which a material can satisfy overall system requirements or conversely restrict system performance could have a much greater influence on the final cost of electrical energy. For example, the Reference System material is a thermophastic resin which provides ease of forming. It should be emphasized, however, that this is a preliminary selection based on the current level of understanding of the structural material requirements. Development Features - Our lack of experience with a space structure such as required for an SPS was pointed out in the beginning of this section. There are other unique features which should also be mentioned.
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