Based partly on results from the referenced NASA studies, the cost estimates shown in Table 4.1-2 were developed. Data such as these are generated to yield results which give direction for future studies and are not intended to represent absolutes. The reader is encouraged to enter numbers of his own. The results will vary, of course, but the implication is clear: if such a concept is feasible, it is probably within the same cost-effective domain as the large payload vertical-launch boosters that have been deemed most applicable to the SPS program. The potential operational advantages of the concept are, in some instances, equally important. These are: Table 4.1-2. SPS Program Applicability - Cost Projections • Cost-effective across many program demands • Two-way logistics • Crew plus payload capabilities • Amenable to alternative launch/ landing sites • Aero-maneuver for injection to equatorial orbit • Reduced noise levels at launch • Reduced propellant requirements • Non-polluting, non-fossil propellants 4.2 ORBITAL TRANSFER Selection of an airbreathing HTO-SSTO concept as the reference ELV provides a unique opportunity to optimize the logistics between LEO and GEO. The capability of the ELV to perform aero-maneuvers for orbiter injection from an equatorial flight path results in two significant advantages: 1. Phasing requirements for rendezvous and/or staging are significantly reduced over inclined orbits. 2. GEO payloads can be sized to equal OTV stages if the LEO staging altitude is approximately 556 km (330 n. mi.) - equatorial. Each of these effects is discussed and its implications on the logistics scenario is delineated.