7. CONCLUSIONS & RECOMMENDATIONS 7.1 CONCLUSIONS It is possible to design an SPS which contains less than 12 as much non- lunar material as the earth baseline design, with an increase in total mass of less than 82. This implies a cost reduction of over 972, assuming a 50:1 cost ratio favoring lunar materials. The design uses mostly existing technology and is suitable for automated manufacture in space. Photovoltaic SPS concepts are clearly preferable to turbine engines for lunar resource utilization. Besides the great advantage in non-lunar mass required, photovoltaic systems are much better suited to automated manufacture in space. Photovoltaic designs, with the partial exception of TPV, contain large numbers of identical simple components. Turbine engines have a great variety of parts, many of them complex, most of which are used in limited numbers. Stirling engines occupy the middle ground in both complexity and volume of components. Whether or not repeated annealing of solar cells heals radiation damage is an important but not critical question. Without annealing, the total mass of the silicon planar SPS would increase by 502, but the mass of the GaAs SPS design would increase less than 12. Neither would require a significant increase in non-lunar mass, so thermal systems would still have no advantage over photovoltaics and silicon planar would still be preferred over GaAs concentrator designs. This study determined that significant mass savings can be realized in the silicon planar design by canting the panels at 12 degrees and rotating the SPS at each equinox, i.e., twice a year. This reduces the solstice cosine loss from 8.292 to 2.192, without requiring the satellite’s attitude to be continuously adjusted to face the sun squarely. Heat pipe radiators are preferable to liquid droplet radiators. A possible exception to this conclusion would be the temperature range where sodiumpotassium could be used as droplet fluid. This technology is speculative at present, and the recoverability of lunar potassium is questionable. Moving belt radiators are very promising, but remain to be demonstrated in space. Aluminum with passive thermal control is suitable for most structural applications in the SPS. An exception is the microwave antenna structure, which must remain very flat despite large temperature excursions. A lunarbased material, foamed glass, meets this requirement, but has not been demonstrated. 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. A new type of microwave lens proposed in this study, or a microwave reflector, might allow economic construction of SPSs as small as 200 MW.
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