2.85 System Weights A weight summary for the orbital Brayton cycle system is presented in Table IV-B-l-c-2. 2.9 Evaluation of the Boeing Brayton Concept The two most significant challenges associated with the overall SPS concept are found in transporting the system to its ultimate geosynchronous destination and assembly of the system in space. As the subjects of transport and assembly are covered in other sections of this report, and since the Boeing concepts in these areas are still evolving, no evaluation of the Boeing transport and assembly concepts, except as related to the conversion system, is presented in this section. As such, the evaluation is essentially confined to the conversion system and its components. Of all the reflector-concentrator concepts considered adaptable for SPS purposes (see Figure IV-B-l-c-9), individually steered flat facets such as Boeing has proposed appear to be the most feasible from the combined standpoints of efficiency, attitude control, redundancy, and life requirements. The system analysis is not yet sufficiently advanced to know with certainty the optimal number of individual facets required. From a thermal efficiency standpoint, the larger the number of facets, the better, as this more closely approximates a parabola; however, there are other considerations such as weight, packaging, assembly, cost, etc., which must be included in the final analysis. These same considerations, coupled now with reflector/concentrator inertial and attitude constraints, are important in the selection of the optimum number of modules (Boeing has chosen four). Thus, while it is too early for a quantitative decision on these items, qualitatively it is that the concept is viable. A particular area requiring further investigation is the three-dimensional aspect of the Boeing reflector/concentrator, particularly in regard to structural rigidity. Reflector efficiency is a function of the reflectivity of the surface, geometrical imperfections, faceted construction, alignment, and degradation with time due to meteoroid punctures, high-energy particle sputtering, and low energy proton entrapment leading to bubble formation. While the reflectivity of a highly polished silvered mirror surface approaches 1.0, with polished aluminum somewhat lower at about 0.95, the aluminized plastic film proposed for the SPS has a reflectivity of between 0.85 and 0.91 (the Boeing study assumes 0.89, 1984 technology). The assumed reflectivity is reasonable, but very little is known about degradation over the 30-year lifetime of the system. Boeing states that "fortunately geosnychronous orbit lies within the bowshock of the earth, so that the low energy protons of the solar wind are not a problem. The high energy protons of the Van Allen belts will probably pass through the reflector with no effect other than sputtering. Surface erosion rates between
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