2.7 CONCENTRATING REFLECTORS Several of the conversion systems investigated use concentrated sunlight with a unit size of several kilowatts or more. These systems require concentrating reflectors of several square meters or larger. This section describes the design of concentrating reflectors using lunar materials. The design is not highly detailed, since the details will depend on the conversion system. 2.7.1 Design Description Standard technology for large reflectors in space is vapor deposited aluminum on facets made of Kapton plastic film. The Kapton is supported by some open structure, typically made of a graphite epoxy composite. The facets are individually steered. A similar design which provides the same optical performance with almost no non-lunar material was developed. This design uses vapor deposited aluminum on 25 micron aluminum foil, with an aluminum supporting structure. The reflector always points at the sun, so the structure and the reflecting surface will be in a nearly constant thermal environment. There may be transient thermal warping at the beginning and end of eclipses, but the SPS does not produce power during those periods so warping at those times will have no effect on performance. Achievable specific mass of large reflectors is less than 0.05 kg/m*2 using Kapton facets and graphite epoxy structure.(1,ppl00-102) Conversion from graphite epoxy to aluminum increases mass by a factor of 3.02, so the specific mass of the reflector technology used in this study is assumed to be 0.15 kg/mA2, with no non-lunar mass. The facet steering devices were ignored in this mass estimate because of their small mass. Each steering device is only required to move a very lightweight facet with no atmospheric drag and insignificant gravity. Thus, each steering device should have mass of less than a kilogram. Concentration ratios used in this study would not require more than 7000 facets, so the total mass of the steering devices would not exceed 7 metric tons. 2.7.2 Concentrating Reflector Reference 1. Solar Power Satellite: System Definition Study, Parts 1&2, Vol. II, Technical Summary, D180-22876-2, Boeing Aerospace Comnany, Seattle, 1977.
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