Reference E4 is the most definitive document on orbital assembly even though it considers only the support structure. It reports on a Martin-Marietta study which was based on the Raytheon/Grumman design as a baseline. Reference E4 states that "we found that the structure was not designed for easy assembly in orbit and not totally compatible with the presently defined space transportation system." The reference then describes a re-design which would utilize such techniques as thermite- type fusion welding for fastening, triangular trusses, and telescoping tubular tension members. An automated mobile assembler was also conceived for the highly repetitious operations, and it is reported that this would minimize the number of manned, direct activities. With two mobile assemblers, it was estimated that it would take nine months at 24 hours per day to complete a support structure. Selection of the actual construction period will require trade-offs of costs, facilities requirements, manpower, capital issues, and so forth to establish an optimal period. 2. Economic Viability Economic viability is generally defined as the ability of the proposed systems to provide (or, in the case of the PRS, to transmit) electrical energy at unit cost competitive with projected costs associated with terrestrial systems of known technology. Any determination of economic viability therefore rests on three main elements: • Estimates of system cost in dollars (both recurring and non-recurring) • A systematic and explicit methodology for reducing these costs to an estimate of mills/kilowatt-hour that may be meaningfully compared to terrestrial costs • Estimates of competitive power generation (or transmission) costs at the time the proposed system becomes operational. Throughout the literature, the estimates of system costs explicitly exclude the indirect costs or savings associated with differential environmental or resource depletion effects, although these effects are noted in several of the references. Similarly, the impact of the proposed
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