findings indicate that the microwave transmission system will not interfere with the other users of the electromagnetic spectrum, and that there will be no serious biological hazards associated with the rectenna side lobes. The transmission problem may therefore reduce essentially to one of the economics of land usage. If it is accepted that the SPS is a desirable candidate energy system for relieving the world’s present almost total reliance on fossil fuels, then the question must be answered as to its competitive economic position relative to other candidate systems in the same time frame, such as nuclear. In considering this question, it is important to emphasize that the world’s energy needs in the next two decades will require a capital investment for new plants measured in the trillions of dollars. Consequently, the development and manufacturing costs for the SPS can only be rationally evaluated in the context of this major capital outlay, which is required if the world is to have any hope of maintaining its present standard of living in the face of a burgeoning population, or of satisfying the legitimate aspirations of the developing nations for an equal quality of life. Relative costs are therefore of more interest than absolute values: it is convenient to measure the economics of the SPS relative to the known costs of fission power plants, approximately $1000/kW for capital investment. However since the SPS does not use fuel the equivalent number for purposes of comparison is probably closer to $1500/kW to $2000/kW. Fusion remains a hope for the future, but too many scientific breakthroughs are needed to permit a rational evaluation of its economics at the present time. In evaluating the economics of the SPS many cost elements must be estimated, mostly on the basis of very little hard data. The approach which will be taken in this analysis will therefore consist of identifying the most sensitive cost parameters by considering the effect of their variations separately for a base case. It should then be possible to evaluate the economics of the SPS for any selected set of initial assumptions. 2. MAJOR COSTING PARAMETERS The driving costs for an SPS which will be addressed in this paper are: (a) transportation costs (b) industrial productivity in space (c) energy conversion element costs. Recent studies have clearly indicated that the costing algorithm is highly nonlinear, in that there is a strong interdependence between variables. Certainly, however, (a) has dominated the cost of space operations in the past. It is only with the advent of the reusable launch vehicle that it has become at all possible to consider the assembly of large systems in space. The dominating cost of space transportation has also led to two novel concepts which show great promise for reducing SPS costs: the use of lunar rather than terrestrial material for manufacturing (2), and the use of an electromagnetic reaction engine in place of chemical propulsion (3). A major purpose of this paper will be to project the potential costs of transportation to and in space, and to determine the conditions under which it appears that the use of more advanced concepts are desirable. The optimum scenario depends on the costs of transportation, including the costs of propellants at the various transportation origins and destinations. The moon is an
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