Figure 4.1-3. Cost per Flight Comparisons The guidelines for this study require placement of 120 satellites in orbit, of approximately 60 million pounds each. If an orbital burden factor (pounds of propellant required in LEO to lift a pound of payload from LEO to GEO) of 2 for an OTV can be achieved, then the total program mass needed to be placed in LEO is 21.6 x 10$ pounds. Using cost numbers provided in the referenced studies for the two ELV's under discussion, and applying a Wright production learning curve of 0.85 for the HTO-SSTO, the costs shown in Table 4.1-1 result. Note the lack of sensitivity in DDT&E costs to the totals for an SPS program, yet the high degree of sensitivity to payload mass (i.e., in Cp and Co). Rockwell and other industries were engaged in the early 1960's in studies of reusable orbital carriers investigating the applicability of air-breathing engines. In the pre-Phase A studies of Space Shuttle, Rockwell began an effort on a smaller version of the air-augmented rocket concept shown in Figure 4.1-4. Preliminary calculations indicated that the concept was feasible; however, before full substantiating data could be developed, the decision was made that the Space Shuttle would be of the type that could be launched from KSC. The vehicle depicted is an uprated version of that earlier concept sized for a payload capability of approximately 91,000 kg (200,000 lb). In-house effort is underway to establish mass and performance data for this ELV but it is presented here primarily to illustrate the potential of an HTO-SSTO which has a greater payload capability than 65,000 pounds.