ORBIT-TO-ORBIT TRANSPORTATION by R.P. Bergeron Rockwell International, Space Operations and Satellite Systems Division The Satellite Power System (SPS) program necessitates the transfer of significant cargo mass and personnel from low earth orbit (LEO) to geosynchronous earth orbit (GEO). The SPS transportation costs represent a major portion of program funding requirements and therefore require a most cost-effective approach toward LEO-GEO transfer. Orbital transfer vehicle propulsion options include both chemical (COTV) and electrical (EOTV) options. The chemical options evaluated included single- and two-stage liquid oxygen/liquid hydrogen propulsive elements. The electric propulsion options considered alternate power sources (i.e., silicon or gallium aluminum arsenide solar arrays), propellant type (mercury, argon, cesium, etc.), low and high current density thrusters, methods of maintaining attitude hold during periods of shadow (chemical or electric), and programmatic impact of LEO-GEO trip time. The proposed EOTV construction method is similar to that of the SPS and, by the addition of a transmitting antenna, may serve as a demonstration or precursor satellite option. The results of the studies to date have led to the tentative selection of a single-stage COTV for crew and priority cargo transfer (the COTV is refueled in GEO for return to LEO). The size of the propulsive element is dictated by the estimated crew transfer requirement. An EOTV concept is favored for cargo transfer because of the more favorable orbital burden factor over chemical systems. Although it is highly desirable to maintain a maximum degree of commonality between the SPS and EOTV, the gallium arsenide solar array is favored over the silicon array because of its self-annealing characteristics of radiation damage encountered during multiple transitions through the Van Allen radiation belt. Transportation system operations are depicted in Figure 1. A heavy-lift launch vehicle (HLLV) delivers cargo and propellants to LEO, which are transferred to a dedicated EOTV by means of an intra-orbit transfer vehicle (IOTV) for subsequent transfer to GEO. The Space Shuttle is used for crew transfer from earth to LEO. At the LEO base, the crew module is removed from the Shuttle cargo bay and mated to a COTV for transfer for GEO. Upon arrival at GEO, the SPS construction cargo is transferred from the EOTV to the SPS construction base by IOTV. The COTV with crew module docks to the construction base to effect crew transfer and COTV refueling for return flight to LEO. Crew consumables and resupply propellants are transported to GEO by the EOTV. Transportation requirements are dominated by the vast quantity of materials to be transported to LEO and GEO (Figure 2). The average annual mass to orbit is in excess of 100 million kilograms, with over 100 personnel transfer flights per year. Figure 1. Transportation System Orbital Operations Figure 2. Orbital Mass Flow Requirement
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