pressure (both solar and microwave) and a sun-following rotation with a period of one year. Because standard structural design was judged to be sufficient, no design or analysis of spin-stabilized or spin-stiffened solar arrays was done in this study. However, this option merits further consideration if conventional structures prove to be uneconomical. The selected structural material should be lunar and structurally sound. Abundant lunar materials suitable for structural applications are glass, aluminum, iron, titanium, and magnesium. Foamed glass was selected by General Dynamics because of its low coefficient of thermal expansion (CIE). It was rejected for primary structure in the current study because its properties are 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 assumed for the microwave transmitter structure because of the stringent flatness requirements of the antenna. Magnesium was unlikely to be needed in non-structural parts of the SPS, so it was eliminated to avoid the need for additional refining processes. Titanium was not considered because, as with magnesium, there is only limited experience using it in large structures. Titanium should be reconsidered when more experience with it has accumulated or if aluminum proves to be unsuitable. Aluminum was found to be the most acceptable structural material because it has less mass for equivalent strength than iron and because it requires less non-lunar alloying agents. Due to its relatively high CTE, some parts of the aluminum structure may require thermal coatings, shielding, insulation, or active length adjustment, as discussed in section 3.5. In high temperature applications, steel may have to be used because of its higher melting point. An aluminum-glass composite might be developed which would combine the low CTE of glass with the superior qualities of aluminum. If developed, such a material would probably be an ideal structural material for SPS applications. 3.3 SELECTED DESIGN The structure selected has a rectangular shape. This structure, with dimensions of 10710 m by 5348 m (aspect ratio 2), weighs about 5.98X10E6 kg. It is made of 4 modules each containing 32 bays as shown in Figure 3.3-1. This rectangular structure was chosen mainly because it has been studied extensively in ref. 1 for a 10 GW solar power satellite. The design parameters of this 10 GW sattelite are used to approximate the configuration of a 5 GW satellite. Due to the limited scope of the project, detailed investigation of other possibilities for the structure was not undertaken.
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