Never mind that it is an inept analogy. The early shuttle test flights have been very successful, and point the way toward a sure, if lengthy, evolution toward the kind of low-cost space transportation that will be necessary for an SPS program to be practical. New Concepts for Orbit-to-Orbit Transportation Transportation options for getting SPSs to GEO orbit included all-propulsive conventional rocket propulsion and the electric propulsion systems described in the reference system. The electric systems held about a twenty percent cost edge. Since that time, practical concepts for aeroassisted return of conventional upper stages to low Earth orbit have been devised (17). Aeroassist was not evaluated in the SPS studies. Present indications are that it would equal the cost potential of the electric systems without the high risk. Preliminary evaluations of the supply of oxygen to low Earth orbit from the Moon for upper stage refueling, again using aeroassist techniques, show a significant payoff for large-scale programs like SPS. Results obtained thus far indicate that the Earth launch operations needed for SPS installation would be less than for the electric propulsion option, and that the cost would also be less (18). Without the need to deal with low-drag systems, the low Earth orbit station could be placed at lower altitudes, e.g., 300 km. These concepts suggest major mitigation of all the space transportation environmental issues: [1] use of depressed launch vehicle trajectories to minimize water injection into the mesosphere; [2] elimination of electric propulsion systems; and [3] reduction by a factor of two or more in rocket effluents deposited in the F-region of the ionosphere. At the same time, reduction in the reference system transportation costs is deemed likely. Initial studies of launching payloads from the Earth’s surface directly to GEO by electromagnetic accelerator indicate feasibility for payloads up to a ton or so, provided that the payload can withstand accelerations of several thousand g’s. Costs would likely be less than $10 per kilogram. What contribution such a technology would make to an SPS program is presently unclear, but the low cost potential is very interesting. WHAT SHOULD WE BE DOING? The research program proposed as a part of the reference system reports included some action for all relevant space technologies. The recommendations were made rather in isolation from related program activities. Many of the advancements recommended will occur without an SPS program. What we must ask is “are there any that will not?” For at least the next ten years, space transportation and space operations technology will be pushed by other applications such as rapid growth in space communications and the emerging field of microgravity processing. These same applications will push the technology of the design, construction, and control of large space structures, and of electrical power generation and use in space. If current NASA plans for a manned space station are pursued, the need for station autonomy will push the state of the art in federated and distributed data management architectures and software. SPSs pose no particular challenges in communications system design. SPSs will need electric propulsion for orbit stationkeeping and attitude control but
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