Microwave power is radiated from slotted waveguides arranged in a planar array. The shape and position of the waveguides must be precisely controlled to maintain the microwave beam's quality. Because the waveguides are subjected to large temperature changes, they must have a very low coefficient of thermal expansion. The General Dynamics design used a conductive coating of aluminium in a foamed glass waveguide. Because the waveguides do not bear a structural load, foamed glass was deemed acceptable for this application. Research by the SSI may develop a lunar-derived material which could be used in SPS waveguides. Conclusions The SSI/SRA Study concluded that it is possible to design an SPS which contains less than 1% as much non-lunar material as the Earth baseline design, with an increase in total mass of less than 8%. This implies a transportation cost reduction of over 97%, assuming a 50:1 cost ratio favouring lunar materials. The design uses mostly existing technology and is suitable for automated manufacture in space. Whether repeated annealing of solar cells heals repeated radiation damage is an important but not critical question. Without annealing, the total mass of the silicon planar SPS would increase by 50%, but the mass of the GaAs SPS design would increase by less than 1%. Neither would require a significant increase in non-lunar mass. Flywheels composed of lunar glass appear to be the most promising SPS energy storage technology. Glass flywheel technology is still uncertain, however. The klystron is the preferred microwave amplifier for photovoltaic SPSs. Although less efficient than the magnetron, the klystron's low non-lunar mass is a clear advantage with either silicon or GaAs conversion systems. Recommendations There should be a strong effort to find and characterize a structural material which has very low coefficient of thermal expansion and which can be made from lunar material. Such a material is vital to construction of the SPS microwave antenna, and would be useful in other structural applications. Recent work on lunar concrete is promising. A glass-glass composite (being pursued by SSI) or glass-metal composite might be ideal. A GaAs concentrator design should be thoroughly optimized. By tapering the radiator disk and using higher concentration ratios, it may be possible to reduce both the total mass and the non-lunar mass by more than half. This would make the GaAs design more economical than silicon planar, if silicon cannot be repeatedly annealed or if manufacture of silicon cells in space proves to be very costly. REFERENCES [1] Bock, E. et al. (1979) Lunar Resources Utilization for Space Construction, Vols. 1-3, General Dynamics Convair, San Diego, CA., NAS9-15560, April. [2] Gillett, S.L. (1984) Elements on the Moon. [3] Horne, W.E. (1985) Boeing Aerospace Company, personal communication, 20 June. [4] BOEING aerospace company (1976) Systems Definition: Space Based Power Conversion Systems, Final Report, Boeing Aerospace Company, Seattle, WA, NAS8-31628, November.
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