Propellant costs for the SSTO are twice the two-stage value because of the required larger flow and the greater amount of LOX/LH2 used. Maintenance and program support costs are major cost elements for both vehicles. Stage maintenance cost is slightly higher for the SSTO than for the two-stage vehicle, because maintenance cost is a function of stage cost which is weight dependent. The SSTO is heavier; therefore, its stage maintenance cost is higher. SSTO engine maintenance cost is much higher than the two-stage engine cost, because the total installed thrust is higher and the number of engines is greater. Maintenance and overhaul ground rules were patterned after airframe and engine overhaul specifications for shuttle, with some modifications to differentiate between water and land landing stages. Ground operations and program support costs have shuttle as their basis. Modifications were made to allow for the absence of crew and payload related functions. Operations and support costs for HLLV use approximately the same number of people located at Kennedy Space Center per flight as shuttle, but the man-hours were adjusted linearly with the HLLV turnaround time compared to shuttle. Major technology requirements are those associated with minimizing cost and improving performance. Cost is minimized by eliminating expendable hardware such as kick stages, shrouds, or ablative heat shields. Payload improvements may be made by eliminating an actively cooled heat shield in favor of a passive system that will withstand the high heating rates and yet be salt water compatible. Each of the vehicles requires the development of new engines. A large LOX/LH2 engine and new LOX/hydrocarbon engines are required for the SSTO. A new high chamber pressure LOX/hydrocarbon engine is used for the two-stage booster. Landing modes for both land surface and water need development. These should include implications of surface atmospheric environment. The influence of salt water on the hot structure or engines must be determined. These investigations should include both parent metal effects and those of coatings and finishes. The evaluation of the two concepts provides the following observations: 1. The development schedule and cost for each are about equal. 2. Operationally, the SSTO has shorter recovery and processing time and requires fewer launch pads.
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