An initial assessment of the performance and operating cost of such a system has been made®. The assessment has been made on the basis of scenarios, without any attempt to optimize the system from the viewpoint of cost. However, the scenario chosen indicates that the cost of ground- based, 60 cycle, electric energy for a microwave power transfer system that is only 3% efficient is less than $1000 (at 10 cents per Kw-hr.) for each kilogram of payload delivered to geosynchronous orbit. Such a system can be made much more efficient, by a factor of at least fifteen as determined from SPS studies, if it is economically justified by a corresponding reduction in earth to low-earth-orbit launch costs and by subsequent heavy traffic to GEO. The scenario that was investigated in some detail also assumed a high specific impulse of 7000 which minimizes the propellant mass but maximizes the electric energy required for a given propulsive effort. It also assumed a large vehicle with the consumption of 10,000 kilowatts of DC power, and payloads ranging from 25,000 to 100,000 kilograms, representing payload fractions of 40% and 72%, respectively. The assumptions and specifications for the interorbital vehicle are given in Table 1. Table 2 provides performance data for various payload fractions in terms of DC energy consumed per kilogram of payload, propellant mass fraction, and two-way transit times. Figures 5 and 6 give altitude and accumulative electric energy consumption as a function of total elapsed calendar time for a scenario, slightly modified from that in Table 2, that uses a reduction in electric power absorbed by the satellite after an altitude of 15,000 kilometers is reached. This modification reduces ground transmitter costs, at the expense of increasing transit times and reducing overall transmission efficiency. It is evident from Figure 5 that the time spent in low earth orbits is lengthy because of the low fraction of time that the microwave beam engages the satellite and the large increments of energy needed to make significant changes in altitude. Figure 6 indicates that the energy consumed in low earth orbit is very low because of the high transmission efficiencies between the ground-based transmitter and the satellite. On the basis of Figure 6 the total 60 cycle energy
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