As important, however, is the consideration of its applicability to programs of much lesser demand than SPS. Additionally, the versatility of the concept is particularly desirable for future missions. Using the airbreathirig HTO-SSTO vehicle as a reference, a fully reusable common-stage, LO2/LH2 orbital transfer vehicle (OTV) was selected from NASA-contracted studies and sized for compatibility with the ELV. Since the reference ELV can return with payloads, the empty OTV stages are returned to earth for subsequent refurbishment, thus extending their operational life cycles. The integration of cargo payloads was investigated and payload "mixes" were defined which satisfied the massdensity capabilities of the ELV and the construction sequence demands of the SPS. A traffic model which meets these requirements is shown in Figure 1.3-3. Figure 1.3-3. SPS Cargo Traffic Model Cumulative Cargo Masses to Orbit The first three tasks are directed toward establishing technical feasibility; however, they were conducted with a constant awareness of the significance of the fourth task, Programmatics (Section 5). Results of this task— the "bottom line" of economic viability—have yielded insights into the practicability of SPS as an economic venture. Cost estimates for the elements of the program are shown in Figure 1.3-4 based on the 1985 technology projections and construction of 120 5-Gw satellites over a 30-year period at a rate of four per year with a first-unit IOC date of 1995. The total program cost estimate, including $57.7 billion for DDT&E, is $850 billion. This equates to an average capital investment requirement of $1,400 per kilowatt. Under these investment conditions, user chargers could range from 30 to 50 mills/kwh.
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