the transportation vehicle which has attached to it a rectenna for receiving and converting the microwave energy into d.c. power which in turn is used for the electrical propulsion. The transportation vehicle meets with the Space Shuttle in LEO for its payload. It then travels in an equatorial orbit of increasing diameter as it receives acceleration from the ion propulsion engines. Near the Earth, the dwell time between the vehicle and the microwave beam is short as the vehicle passes into and out of the microwave beam which is limited to a total angular sweep of approximately 100°. However, the dwell time rapidly increases as the vehicle recedes from the Earth transmitter so that on the average the vehicle is illuminated approximately 15% of the time on its LEO to GEO voyage. The percentage of the time that the vehicle is illuminated by a microwave beam may, of course, be increased by employing two or more ground-based transmitters. One of the distinguishing features of such a transportation system is the large mass handling capability compared with existing systems. Although optimization studies are needed, it is evident that such systems are most economic with payloads of the order of 50,000 kilograms or more than ten times the size of present payloads. The scale of the system is consistent with that needed for the SPS, and may be suitable for carrying general cargo into geosynchronous orbit because of its potential to greatly reduce costs over current methods. A feature of the system that further distinguishes it from present methods of transporting material into geosynchronous orbit is that it is a shuttle system, traveling from GEO to LEO, as well as LEO to GEO. The development of the geosynchronous orbit obviously needs a two-way transportation system. Another distinguishing feature of the transportation system to be described is that both the microwave beam and the LEO to GEO vehicle must be located on or close to the equatorial plane so that an efficient microwave link between ground-based transmitter and the vehicle can be established each time the LEO to GEO vehicle orbits the Earth. It is known that the most efficient route to geosynchronous orbit is from an equatorial based launching site so that the proposed system is completely compatible with this already established advantage of the use of the equatorial plane. Several microwave technology developments, some of which were made quite recently, make the concept credible. An important one is the thin-film, etched circuit rectenna which simultaneously collects and converts the microwave power into de power with an efficiency of 80% and with a mass of less than one kilogram per kilowatt of d.c. power output (3,4,5). Its physical flexibility allows it to be rolled up into compact cylinders to be transported in the shuttle and to be deployed as shown in Fig. 1. Another development is contained in the completed study of a ground-based transmitter design that is based upon readily-available, low-cost components (6). DESCRIPTION AND PERFORMANCE OF THE SYSTEM To evaluate the performance of the system in terms and cost of time required to make round trips between LEO and GEO, it is necessary to make assumptions with respect to the composition of the vehicle and its propulsion, recognizing that these initial assumptions may be significantly changed by further system studies. The initial assumptions are outlined in Table 1. It is first assumed that the rectenna will deliver 10,000 kilowatts of power for most of the interorbital distance and will have a mass of 10,000 kilograms. This is well within the capabilities of existing technology. The ion engines are assumed to utilize
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