the self-propelled mode, the solar cells are self-annealing of the radiation damage occurring during transit and consequently, the dedicated OTV can be reused. The OTV uses an ion bombardment thruster with an aperture diameter of 100 cm and argon propellant. The power conditioners of the SPS propulsion system process only the low-voltage fixed power (278 W input per thruster). The other supplies are taken directly from solar arrays. The beam power is obtained from the OTV solar array. To avoid significant power loss from plasma discharge, the array voltage is maintained at 2000 V; this is stepped up to the beam voltage by dc-dc converters before collection by the main solar array power distribution lines. The accelerator and discharge power sources are small arrays near the thrusters. This location reduces cabling mass. Because only 50 kW per thruster is generated, thermally induced voltage transients can be regulated by voltage limiters. An auxiliary power unit (APU), charged by the discharge supply solar array, furnishes 278 W to the thruster low-voltage supplies. Because of the desire to minimize propellant requirements, the OTV design was based on a high specific impulse of 13,000 s. Personnel Transport Systems (POTV) - The construction sequence developed for the SPS required a crew rotation every 90 days for crew complements in multiples of 48. A crew and resupply module (CRM) was synthesized on this basis. Based on previous Rockwell studies of passenger modules, a parametric sizing curve for passenger modules was developed. For a crew complement of 48 persons, the module would weigh approximately 200 kg (440 lb) per man, or 9,600 kg. Comparable data were extracted from these studies for consumables, passenger/ personal effects, in-transit consumables, crew module, resupply module, and on-orbit habitable module spares. A conceptual layout of the CRM is shown in Figure R-25. A command module area is provided to monitor and control OTV performance during crew rotation flights. Spacing and layout of the passenger module is comparable to current
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