Structure The structural requirements of the silicon planar system and the GaAs concentrator system are similar. The silicon planar structure is used here as an example. A rectangular configuration was chosen. This is one of the common shapes chosen in the previous studies for a silicon array satellite. In this configuration, the structure bears a compression load caused by tension in the solar cell array. Lunar materials which are abundant and suitable for structural applications are glass, aluminium, iron, titanium, and magnesium. Foamed glass was selected by General Dynamics because of its low coefficient of thermal expansion. SRA rejected this material for primary structure because its properties were only hypothetically known and because of the general inadvisability of building with brittle materials. Other options based on glass were dropped from consideration for the same reasons or because they require a large fraction of non-lunar material. However, foamed glass was used for the microwave transmitter structure because of the stringent flatness requirements of the antenna. Aluminium was selected for the primary structure because of its high specific strength and its low non-lunar alloying requirements. Due to its high coefficient of thermal expansion, some parts of the aluminium structure may require passive thermal control or active length compensation. A lunar composite material might be developed which would combine the low coefficient of thermal expansion of glass with the superior qualities of metals. If developed, such a material might be ideal for SPS structural applications. The SSI is beginning research aimed at developing such a material. Power Distribution The Power Distribution System (PDS) controls and conditions power transmitted from the conversion system to the microwave transmission system. It also provides energy storage and handles fault detection and isolation. The major mass components of the PDS are: conductors, voltage conversion units, switchgear, energy storage facilities for powering essential subsystems during occultations and shutdowns, and the electrical rotary joint which carries power between the sun-facing and the earth-facing parts of the SPS. Four options were considered for the main power buses: sheet aluminium conductors, refrigerated buses, superconducting buses, or microwave transmission. The sheet aluminium option was clearly indicated because it requires essentially no non-lunar material and has lowest net losses. Previous SPS studies used oscillator-transformer-rectifier systems for DC-DC conversion at the microwave antenna, with an overall conversion efficiency of about 96%. Modern power electronics provide DC-DC conversion at over 98% efficiency. This increased efficiency reduces non-lunar mass needed for cooling and reduces the mass of the power conversion system. Solid-state voltage converters were selected. The SSI/SRA study estimated that 90% of the material in liquid metal plasma valve switchgear could be replaced with lunar material. Stored energy is required during eclipses to maintain essential systems and to keep cathode heaters hot in microwave amplifiers. For a 5 GW SPS, the required energy storage is 6 MW-hrs [6]. Storage alternatives considered by SRA are compared in Table 5.
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