but as the densities decrease, payload shroud weights increase and costs rise. In the comparisons to follow, the smaller payload shroud was assumed. Shown on the same illustration is the most cost-effective of four ELV concepts developed by the Boeing Aerospace Company under NASA Contract NASI-13944, ''Technology Requirements for Advanced Orbital Transportation Systems." This study was constrained to a Space Shuttle payload capability and orbiter cargo compartment size. Also, the program required only four vehicles to conduct some 1,710 sortie flights. In short, the vehicle was not intended to be employed as a cargo ELV for SPS; however, it is of interest to compare cost-per-flight estimates of the two vehicles, on the assumption that the same cost model (BAC) was used. Figure 4.1-3 shows the comparison. Based on the premise that increasing vehicle size will reduce dollars per pound to orbit, the obvious question arises: How much increase in payload mass can be achieved with an HTO-SSTO vehicle? The necessity of finding an answer to this question lies in the realization that the DDT&E cost of an all-new ELV will be high and that, lacking a definition of other programs that require large and heavy payloads, the development of an HLLV booster of the ballistic/ ballistic type shown could be justified only in the context of SPS program demands. Thus the development of a new ELV that is applicable to programs of lesser payload demands and yet cost-effective for an SPS program would be highly desirable. Furthermore, early development of such a vehicle could significantly reduce the transportation costs of demonstration and prototype power systems which will be required before total commitments are made to an SPS program. Costs of an ELV as it pertains to a particular program can be expressed in a variety of ways. It has been convenient to show costs in dollars per pound to orbit; the constituents of such costs are as follows:
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