mainline operational power satellite program. The Space Shuttle is designed to operate only to low Earth orbit, i.e., 500 km circular orbit altitude and below. Consequently, the transportation of personnel and material to the geosynchronous orbit will require the development of new systems. Earlier NASA plans called for development of a vehicle called the Space Tug. This vehicle was designed for unmanned deployment and retrieval of payloads from geosynchronous orbit based from the Shuttle orbit of approximately 200 kilometers. The Space Tug, as envisioned in these earlier studies, is not adequate to fulfill the needs of a Satellite Power Station Program, hence new systems must be defined to fulfill these far larger requirements. The mass of material necessary to be placed in low Earth orbit for the Power Satellite Program includes the mass of the satellite itself, the necessary equipment and material to perform the construction process, the structures and systems for housing the personnel to be involved in the construction activity and the orbital transfer systems and the propellants for them necessary to transfer the power satellite and supportive elements from low orbit to geosynchronous orbit. Perhaps the most important two drivers upon the launch system are the specific mass of the power satellite (KG/KW) and the propellant needs of the orbit transfer vehicle. The term "orbit burden factor" will be utilized to describe the low Earth orbit payload requirements, in excess of those required for placement of the satellite elements themselves. The "orbit burden factor" is defined as the ratio of those masses necessary to provide the orbit transfer function and to support the construction and operation activities associated with the satellite power station program to the mass of the satellite itself. Conventional chemical propulsion, if utilized for transfer of all of the necessary mass to geosynchronous orbit, will require that propellants be supplied to low Earth orbit in amounts of twice or more the mass of the satellite materials and parts to be transported. This obviously constitutes a heavy burden upon the launch vehicle fleet and results in increased costs for the transportation. Consequently, it is of interest to examine more efficient propulsive schemes than conventional chemical rocketry. Electric propulsion devices of several forms have been defined and, in a few instances, reduced to practice. These devices offer significant improvement in specific impulse levels achievable and hence significant reduction of the "orbit burden factor" associated with supplying their propellants to low Earth orbit. The electric propulsion devices require, however, that electrical energy be supplied, in addition to a working fluid, in order to produce the impulse necessary to effect the transfer. Electrical power may be produced by an independent power supply that is an integral part of the transfer system itself or may, in the xase of low Earth orbit construction, be fulfilled by drawing power from the satellite power station module being transported. This latter possibility may result in a more efficient and more cost effective orbit transfer system. It now appears the more effective choice is to utilize expendable rather than reusable propulsive devices if payload-supplied power can be made available. System requirements for low Earth orbit
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