It is also important to consider the location for the power delivery. For the precursor system it is believed that the first use will be in a subsidized context such as developing countries. The equatorial plane looks like a preferential location in this respect. As will be explained in the rectenna section, the equatorial plane also allows simple solution to the problem of rectenna configuration. From the discussion above, it can be concluded that GEO orbit could be a good choice, but this orbit is already a scarce resource and so it may be difficult to obtain a desired slot. There are also potential Electromagnetic Interference (EMI) problems to consider. In these respects, a lower orbit may be preferable. However, lower orbits have significant drawbacks in terms of period, visibility time from ground, aerodynamic drag and gravitational perturbations, hence precluding these orbits from efficient long term usage. An alternative to GEO could be: circular equatorial altitude: 20309 km, 12 h period, with satellite and ground station aligned at the local zenith sun transit, West to East rotation This orbit allows a visibility time from ground station of 6h24' per orbit, and is compatible with conservative energy storage devices such as fly wheels. This has the advantage of not requiring exotic high energy storage devices on the ground which store MW in a few minutes. An equatorial orbit coupled with a 12 h period allows flight over the receiving station each day at the same solar time. This will facilitate the ground operations. Furthermore, the orbits in the range of 20000 km present a very clean environment in terms of orbital debris and solar particles. All of these concepts require assembly in LEO (typically 350 km 28.5°) to allow heavy launchers to deliver the spacecraft to the assembly altitude with one launch. It is also necessary that the Shuttle be able to reach the same location in order to perform assembly operations and provide a life support base for the crew. The Extended Duration Orbiter will have an ability to remain in orbit for 30 days. At an inclination of 28.5°, the STS is capable of 40,000 lbm, or 18 Mt, which diminishes for a different orbital plane. There is thus a trade off between orbit (altitude and inclination) and the cargo capacity of the two vehicles launched from different launch sites. As an example, the Energia is capable of placing 70 Mt into a 28.5° orbit, which allows the Shuttle to remain for the maximum duration. A second Shuttle mission may be required for the larger, more advanced platform. The feasibility of construction of the system is discussed hereafter. 10.4.2 Platform Design/Sizing Manned vs. Automated Deployment The literature regarding structures of interest to the 1 MW-class was reviewed and the following points were developed. First, a large-scale platform of the 1 hectare area (100 m x 100 m) or larger would be very difficult to deploy automatically and yet exhibit the desired stiffness. It was noted that in 1990, the design of the Space Station Freedom was significantly changed (Fisher-Price Study) due in part to the perceived difficulty of orbital assembly of large structures. To attempt to construct a large platform automatically at this juncture would require a development and testing program, particularly with the use of a robotic-assisted system. This was thought to exceed the cost and scope of the current effort (other smaller platforms with deployable structures are covered in other sections of the report). The initial concept sizing used Space Station Freedom 5m truss which were originally intended for the full dual-keel design (an actual design would optimize the truss design and save, for example, 80% of the original development costs). The advantage of using this was that the mass, stiffness, construction times (the result of many hours of buoyancy tank experiments as well as an STS flight during which a large element was constructed) and other parameters are well understood. From this, it was argued that the most conservative and therefore least expensive large structure suitable for an early Solar Power Satellite demonstration was the result of this strategy. Examples of erectable structures are given in the following Figures (Mikulas, 1988; Katzberg, et al, 1990). Figure 10.4.1 shows 5 m truss assembly inside of the Shuttle cargo bay. In Figure 10.4.2, the truss assembly can be seen as it is gradually constructed. In previous on-orbit experiments (which correlated well to buoyancy tank experiments), the truss elements could be assembled at approximately 1 tubular element per minute. This results in truss assembly of 30 m per hour, such that a 100 m section requires over 3 hours for assembly. Examples of large scale elements are given in Figure 10.4.3, showing the ‘dual-keel' space station in which the dimensions of the dual keel are close to 100 m x 100 m. The last example, shown in Figure 10.4.4, illustrates the construction of a Mars mission construction bay. Large truss segments, once assembled, are then moved into place with a manipulator arm system. The 100 m x 100 m planar array, followed by the prismatic array, are
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