the reference orbit. In the comparison of chemical versus electric COTV options, the chemical option has a shorter trip time and requires significantly less technology development for SPS application; but due to the lower performance, significantly more propellant, i.e., HLLV flights, is required to support the chemical transfer. Consequently, the best approach at this time appears to be an electric propulsion COTV augmented by a chemical propulsion system to overcome gravity gradient torque at low altitude (<2500 km) and to provide attitude control during occultation. Personnel Orbital Transfer Vehicle (POTV) - The functions of the POTV are to deliver personnel and supplies from LEO to GEO and to return personnel from GEO to LEO at 90-day intervals. For this orbital transfer function, the electrically propelled option is not viable due to its extended trip time. Therefore, a LOX/LH^ fueled vehicle which can make the trip in the order of one day was chosen for the crew rotation function. For the option of electric propulsion for cargo transfer, a dedicated LOX/LH^ OTV with two common stages is chosen as the most cost effective POTV configuration. The propulsion vehicle with associated modules is shown on Figure A-25. The vehicle transports 160 personnel and crew supplies for 480 man-months from LEO to GEO and returns 160 personnel to LEO for rotation. The propulsion vehicle has a start burn weight of 890 tons and a payload up of 151 tons and a payload down of 55 tons. The payload up consists of 160 personnel in a passenger module, 480 man-months consumables in a resupply module, and a flight control module piloted by a crew of two. The payload down is identical except the resupply module returns empty to LEO. For the option of chemical propulsion for cargo transfer, only one stage of the two stage LOX/LH^ COTV (Figure A-19) would be required due to the lower mass of the crew rotation module and flight control module as compared to the cargo payload delivered to GEO by the common stage COTV. The use of this approach minimizes the DDT&E expense, as well as maximizing the utilization of the vehicles.
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