11.0 SUMMARY BASELINE DEFINITION As the satellite power systems are studied, more knowledge will be obtained as to the interactions, limitations, and impacts of such systems. Along with this study activity, many baseline concepts will be developed to aid in the understanding of the SPS program. These baselines will naturally change and evolve as study iterations are made. The final SPS design will probably be significantly different than can currently be envisioned. When a concept is defined and studied, problem areas can be identified and techniques to improve the design will be noted. Since the baselines of this study were established, a great deal of analysis has been completed and several areas of potential improvement have been noted. The next iteration baselines will be forthcoming when additional analysis is completed. 11.1 PHOTOVOLTAIC One photovoltaic concept that was used in this study was a 10 GW SPS of elliptical planform with two 1 km diameter end-mounted antennas. Further study of this configuration identified some unfavorable traits, such as the elliptical configuration is not best suited for end-mounted antennas both from a structural and power distribution standpoint. It was noted much earlier in the study activity that for an SPS with a center-mounted antenna, the elliptical configuration offered several advantages over the rectangular configuration. These included less structure and power distribution mass and less attitude control propellant. However, these advantages do not apply to the end-mo unted antenna. It appears that the rectangle is better suited for the end-mounted antenna. For the center-mounted antenna the diamond configuration may be the most favorable for the power distribution and attitude control requirements. The impact on the structure and assembly has yet to be completed. Most of the previous SPS study activity assumed that the SPS solar array would be maintained perpendicular to the Sun (Z-solar). After a trade study was completed which included both mass and cost analyses, it was concluded that from a cost standpoint there was only a slight difference in cost for Z-solar versus pointing perpendicular to the orbital plane (X-POP). The initial cost for the X-POP concept was slightly greater than that for Z-solar because the larger solar array is required to offset not always being pointed directly at the Sun. The total cost for the X-POP concept was slightly lower than Z-solar primarily because of the lower propellant requirements for the 30 year lifetime. The rotary joint requirements are also less severe for the X-POP orientation since a two-axis rather than a three-axis gimbal is required. Therefore, the next baseline which is established may be oriented X-POP rather than Z-solar.
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