G. Natural Resources In a program as large as the solar power satellite program which utilizes advanced technology, the selection of materials is often influenced by availability of a particular material. The material availability may be limited by total natural resource or limited by the amount of yearly production. The first program materials list was published by NASA in 1976 (ref. 2). This list is shown in column 1 table A-7. However, before that time several modifications had been made to informal working materials lists based on design changes required for supply reasons. The following is a list of significant changes prior to 1976. 1. Original concepts for photovoltaic cells utilized gold for electrical contact; however, size and number of cells required for each satellite precluded the use of gold. Designs are now using silver, copper, and aluminum in various amounts. 2. The rotary electrical joint as originally designed contained large amounts of silver. The joint was redesigned to reduce the requirement for this precious metal to less than 40 metric tons per rotary joint. 3. Both klystrons and amplitrons have been considered for use as microwave generators in the satellite. Each has advantages and disadvantages. One disadvantage to the use of amplitrons as presently designed is the 7 to 10 metric tons of platinum required (15 to 20 percent of U.S. production) in the cathode. This demand for platinum would restrict the use of amplitrons for use in the solar power satellite program. Alternative designs to replace platinum with thoriated tungsten are being pursued. Subsequent to the mid-1976 materials lists, two other significant adjustments to design have occurred because of material demand. These are as follows: 1. A redesign of the rectenna structural members eliminated over 75 percent of the aluminum from the rectenna design. The aluminum was replaced by steel--a more abundant natural resource. Further detailed design studies are being pursued to obtain the best minimum resource design for the rectenna.
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