commercial airline practice. A nominal packing density of 160 kg/m^3 (10 1b/ft^3 ) is assumed for resupply consumables. The resupply modules are to be exchanged each mission. While at GEO, the resupply module may be used as the consumables storage module. Thus, multiple access aisles are included in the sizing. A gross packing density of 93 kg/m (6 1b/ft ) allows for a large growth factor. The POTV is a common-stage chemical (two stages having the same propellant capacity) vehicle, utilizing LOX/LH^ propellant. (See Figure R-26.) The propellant tankage for both stages is the same, but the propulsion system for the first stage has twice as many engines as the second. This allows the initial thrust/weight for both stages to be kept at about 0.15 g. The engines have a specific impulse of approximately 470 s at a mixture ratio of 6:1. K. Natural Resources Table R-6 identifies the type and amount of materials required in the construction of one satellite power system using GaAs solar cells at a concentration ratio of 2. NOTE: Totals do not reflect growth allowance. L. Operations a. Construction This section describes the space-related aspects of SPS operations. The overall operations scenario is first described. This is followed by a description of the construction approach, including the construction base and the approach to handling cargo. Overall Operations Scenario - Figure R-27 summarizes, by location (i.e., earth, LEO and GEO), the space-related operations required to support the construction and operation of the SPS satellites subsequent to establishment of the necessary ground and space bases. The right hand side of the figure identifies the four types of transportation vehicles used in the space operations: the HLLV transfers cargo and crew to LEO; the electric cargo orbital transfer vehicle (EOTV) transfers cargo between LEO and GEO; the personnel orbital transfer vehicle (POTV), which is a two-stage chemically propelled vehicle, transfers crew and priority cargo between LEO and GEO; and the intra-orbitai transfer vehicle
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