II. CONCLUSIONS The scope and complexity of the satellite power concept coupled with the limited depth of the present study would make it inappropriate to draw absolute conclusions. However, the SPS concept appears to be technically feasible in that no design or operational problems were encountered that did not appear amenable to solution. The economic viability of the system appears promising but is obviously dependent upon a combination of technology advancement and/or the costs of competitive sources. Within the limitations of the study and based on a variety of assumptions and/or estimates, the following preliminary conclusions are presented. 1. The maximum power output of an individual microwave transmission link to Earth is about 5 GW and the transmitting antenna diameter is about 1 km, based on the following assumptions: a. An operating frequency of 2.45 GHz 2 b. A maximum allowable power density at the ionosphere of 23 mW/ciri c. A maximum allowable antenna waveguide temperature of 485 K 2 resulting in a power density at the antenna of 21 kW/m d. A 1O-dB Gaussian taper of the microwave beam 2. The estimated mass of a 10-GW SPS (incorporating solar energy converters sufficient for two 5-GW microwave power transmission systems) 6 6 is between 47 x 10 and 124 x 10 kg, based on the following assumptions: a. Silicon cell arrays with an efficiency of 15 to 17 percent at 30° C and a concentration ratio of 2 b. An overall system conversion and transmission efficiency range of 4.2 to 8.0 percent c. A weight growth of 50 percent over present estimates 2 The resulting solar array areas ranged from 96 to 183 km . 3. The silicon solar cell arrays make up well over half the weight and cost of the satellite. Consequently, additional effort on solar arrays offers the most potential for overall system improvement, particularly with respect to new approaches that could result in significant weight reduction. 4. Considerations of the structure indicated that minimum weight can be achieved if design loads are limited to those encountered on orbit and after construction. If this is done, the structure can be held to a very small percentage (~5 percent) of the SPS weight. The major factor in design will not be weight but the development of techniques for automated on-orbit construction and for conducting large electrical currents.
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