(d) individual mass of crew of 100 kg (e) modular habitats, 30 tons, supporting 25 people each (f) 80% learning curve (g) no discounting (h) consumables usage of 5 kg/person/day if oxygen is not available, 1 kg/person/day when oxygen is a by-product of space refining (7) (i) all space industrial operations are lumped into four steps: mining, refining, manufacturing, and assembly (j) mining site is on lunar surface, final destination of products is geosynchronous orbit (k) crew rotation 4 times/year from all sites (1) crew productivities of 2500 kg/man hr in mining, 20 kg/man hr in both refining and manufacturing, and 250 kg/man hr in assembly (m) machine productivities of 1 kg/hr kg at all sites except manufacturing, which is .2 kg/hr kg (n) 10% of products in manufacturing and assembly steps are brought from earth (o) 90% of material brought to mining and refining sites is discarded. The method of solution programmed to obtain the results of Figs. 6-12 is outlined in Appendix C, together with the calculated values of the parameters for a particular case. 5. ENERGY PAYBACK Several studies have been conducted to determine the time required to pay back the energy expended in manufacturing and positioning an SPS system. A recent and exhaustive study (8) indicated that the energy payback time was dominated by the rectenna system. An aluminum rectenna structure would result in a payback time of 1.20 to 1.60 years for the complete SPS system, whereas a treated wood structure could reduce this to .60 year. Comparison was made on the hours of electric power out and it was postulated that the conversion efficiency of thermal energy to electric energy was 33%. The study was based entirely on the use of terrestrial materials. Since the rectenna system represents such a large part of the energy expenditure and since the system must be located on earth it is evident that reducing the use of terrestrial materials by using lunar resources will not have a major effect on the energy payback time. In the case of the wood structure, the energy represented by the space systems and propellants was about half of the total including the rectenna. Thus, if most of this energy is produced in space through a space manufacturing facility, the payback time could be halved. Since the payback time is short, even for the worst scenario, this parameter does not appear to be significant in determining the choice of SPS manufacturing scenarios. 6. CONCLUSIONS The use of lunar material for the construction of SPS will result in a substantial savings in cost as compared to the use of terrestrial material if more than five SPSs are to be built and if electromagnetic propulsion is available for transportation of raw material from the surface of the moon to the refining site in space. If the refining
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