accomplished through the use of integrated computer models that simulate not only the key design parameters but also their interactions. To date most of the effort for the photovoltaic SPS has been devoted to the solar array portion, since this is the most massive and costly element of the SPS. The subsystems and related elements simulated within the model are the solar array including structures and power distribution, the microwave antenna, the rotary joint, attitude control, the rectenna, and space transportation. Figure 6-1 gives an overview of elements included in the model. The primary outputs of the photovoltaic SPS model are total mass and unit cost. Secondary outputs include detailed mass and cost statements; blanket, reflector, and planform areas; number of HLLV launches; etc. Some of the design parameters included are: concentration ratio, temperature effects including passive radiator concepts, cell and reflector characteristics, and power distribution mast/feeder line mass and efficiency losses. Since all the major elements of the SPS are included, a more optimum overall SPS design can be established. For example, from an attitude control standpoint, it is desirable for the SPS to be long and slender when oriented perpendicular to the orbital plane. However, from a power distribution standpoint, a circular concept is preferred. With this model a concept can be chosen that gives the minimum total mass or cost. This model allows the study of variations in solar cell and reflector characteristics, concentration concepts, planform configuration shapes (e.g., rectangular, diamond, or elliptical), the impact of center-mounted versus end-mounted antennas, the impact on the number of HLLV launches based on the mass of the different segments of the SPS, payload density, mass and volumetric efficiency, HLLV payload capability, shroud size, and many other elements. Trade studies that have been accomplished to date include solar array orientation trades (perpendicular to orbit plane versus perpendicular to Sun), concentration ratios, power distribution efficiency, and planform length/width trades. Sensitivity studies have been conducted which include variations in the SPS efficiency chain from the solar cell to the ground power interface, power distribution voltage and efficiency, variations in the solar cell, and reflector characteristics. Results of these and other trade and sensitivity studies are contained within this report and were used in deriving some of the specific observations included in Section 11.0, Summary Baseline Definition. Planned future studies include the investigation of alternate concentration techniques, variations in planform configurations, variations in SPS output power levels, different power distribution and structural schemes, the impact of space fabricated versus collapsible structures, etc.
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