of the SDR configuration influences the drag acting on the space station. Based on the estimated area and masses of each SDR option a reboost fuel mass was calculated for two different types of fuel having different specific impulse. These were hydrazine and a gaseous hydrogen/oxygen mixture. In Table II, two cases are shown: the baseline configuration and the ‘bow-tie' configuration. The reboost fuel mass for the “butterfly' configuration is less than and comparable to the reboost mass for the ‘bow-tie' configuration. The specific impulse of the hydrogen/oxygen mixture is almost twice as much as the hydrazine fuel, making it a more desirable alternative. Regardless of the fuel used, the reboost fuel mass needed in the case of the ‘bow-tie' and ‘butterfly' configurations is larger than for the baseline and modified baseline options since the former have surfaces which present a large cross section in the space station direction of flight during some portion of each orbit. Concentrator Shading In the baseline configuration, the SDR deploys vertically in front of the parabolic concentrator and casts a shadow on its reflective surface during the entire insolated portion of the orbit. This shading, which can be as large as 3% of the concentrator's surface, complicates the design of the receiver in terms of its cavity flux distribution. One of the most important advantages of the ‘bow-tie' and ‘butterfly' configurations is their location with respect to the concentrator reflecting surface which precludes this shading. Mass In order to meet the specified first model frequency, tension rods have been added to all three SDR alternatives. However, the increased stiffness is achieved at the expense of a corresponding increase in mass. In the case of the ‘bow-tie' and ‘butterfly' configurations, there is a further mass increase due to an extra radiator supporting plate and deployment motor. Specifically, the increment in mass is 176 kg (389 lb) for the modified baseline design, 263 kg (581 lb) for the ‘bow-tie' configuration and 246 kg (543 lb) for the butterfly configuration. Summary of Results Table III summarizes the results. All three alternative designs to the SDR baseline configuration present a number of advantages and disadvantages. No physical con-
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