Longer range, it might be reasonable to consider the use of asteroids to provide material for construction. Man-made material which is currently discarded in space, such as Space Shuttle external tanks, is another source of resources in space. No matter where the non-terrestrial resources come from - the moon, discarded launch vehicles, or asteroids - there is an interaction of its use with the assembly node for the big satellites to come later. If we use non-terrestrial materials, then we probably should not transfer the material to LEO for assembly of the satellite only to have to raise the mass to a higher orbit again. However, if you are using astronauts in the construction process, the radiation protection in LEO is preferred, even if this approach is more expensive. 4.3.3 Space Transportation The advancement of space transportation is critical to the realization of the Space Solar Power Program in going from demonstration of the technology to commercialization. Depending on the perspective, the trades for propulsion and transportation fall into three different categories. The first category focuses on the next 10-year period and includes incremental improvements of existing technology, such as lighter-weight upper stages and higher-temperature engines. It is also foreseen that low-power electric propulsion, at levels of a few tenths of kW, will be employed. When looking at the second category, the perspective is that of the period between 10 and 20 years from today. To the second category belong developments such as metallized propellants, technique for using aerobrake and aerocapture and reduction of propellant volume by using slush hydrogen. There are several different types of launchers which could become available during this period • SSTO - single-stage-to-orbit, e.g. Delta Clipper • TSTO - two-stage-to-orbit • NASP - National Aerospace Plane • HLLV - heavy lift launch vehicle, e.g. pressure-fed booster, so called Big Dumb Booster It is also possible that by then high-power solar electric propulsion of more than 100 kW could have become developed. The third category includes the developments which are considered as lying more than 20 years ahead. High energy density propellants and high-power nuclear electric propulsion of more than 100 kW belong to the third category. In the far future other types of propulsion, e.g. gun propulsion, mass drivers and laser propulsion, may become interesting alternatives. Examples of future trades that would relate to the Space Solar Power Program are • electric Orbital Transfer Vehicle (OTV) versus aerobrake OTV for transportation from LEO to GEO • use of mass drivers for transportation of materials from the lunar surface 4.4 Technology Development Plan The technology development plan presented in this section is derived from an examination of the program requirements, technology options and schedule. This plan is a chronological guide to the development of technologies that support the necessary projects and demonstrations which lead to a fully operational space solar power program (Figure 4.4).
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