SPS project can pass all the political, technical, and economic problems and more than one SPS is to be built then a large-size launcher with some 500 Mg payload capability (GEO) is the most economical solution. The high development effort will be easily recovered in short time. The cost differences shown for the three vehicle sizes in Fig. 5 are relatively small since a minimum fleet size has been assumed. The cost for smaller vehicles will increase since a larger fleet size would probably be required in reality. The number of launch vehicles required for one SPS launch depends strongly on the average turnaround time per vehicle (from one launch to the next one) and the program duration desired for the SPS launch operations. Figure 6 illustrates the fleet size required with the program duration and the turnaround period as parameters. It is clearly desired from the economic standpoint to keep the vehicle fleet at a minimum size, in order to minimize the vehicle cost, refurbishment cost and direct operations cost. Therefore, under favourable conditions (6 year program, 3 week turnaround time, or 3 year program, 1.5 week turnaround time) a 200 to 300 Mg payload vehicle would be the optimum size, since only 2 to 3 vehicles are required. Under less favourable conditions, i.e., a 3 year program goal and 3 week turnaround time, a 400 to 600 Mg launch system is required or the fleet size would grow to 5 or 6 vehicles, doubling the vehicle and operations cost. The same applies if only 50 reuses prove to be more economical than 100 because of a too large refurbishment effort. As already shown 1972 in the MBB Space Tug Study there exists an optimum number of reuses, where the total of vehicle fabrication and refurbishment costs reaches a minimum.
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