Fig. I. Concept of a microwave-beam powered transportation mode from low-Earth to geosynchronous orbit. Microwave power is beamed from a ground station to the vehicle which has a large rectenna to absorb the microwave power and convert it into d.c. power to energize the electric propulsion engines. The vehicle is transported in modular form within the cargo bay of the Shuttle and assembled in low-Earth orbit. Each module contains ion engines together with their supporting rectenna sections which are stowed in rolled up form during transportation in the Shuttle. $1000 to put a kilogram into low-Earth orbit, while the current cost to put a kilogram into geostationary orbit is about $30,000. It is estimated that an improved chemical upper stage to be developed for use with the Space Shuttle will reduce this cost to approximately $13,000 per kilogram (2). A major reason for an inactive program on an electrically propelled interorbital vehicle has been the lack of a suitable source of electric power for the ion engines. Solar cell arrays have been considered unsuitable for an interorbital application, as distinguished from operation in deep space, because of their high ratio of mass to power output and their inability to get through the Van Allen belt without an excessive amount of shielding. The cost of solar cell arrays and the problem of constructing them in the very large areas required for a meaningful LEO to GEO transportation system have also been a deterrent. It now appears that the concept development phase of the SPS evaluation program has stimulated microwave technology development to the point where it can be considered a viable approach to solving the need for an economic source of electric power. This microwave technology originally developed to transfer energy from space to Earth can be turned around and adapted on a much smaller scale to beam energy from the Earth to an electrically powered LEO to GEO transportation vehicle. Microwave power transmission technology has been further adapted to an interorbital vehicle by the recent development of the thin-film, etched-circuit rectenna which is considered a major breakthrough in the receiving portion of the system for both space and aerospace applications (3,4,5). The concept of the microwave powered interorbital transportation system is shown in Fig. 1. The microwave energy is beamed from a point near the equator to
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