9 Space Manufacturing, Construction, & Operations__________ 9.1 A Matter of Scale The task of this chapter is to examine how to better build a solar power satellite. This is by no means a trivial undertaking and probably represents the most daunting technical obstacle to the large-scale implementation of space solar power. According to the NASA/DOE reference system, a 5 GW SPS will weigh about 100,000 tonnes and will be equipped with a 10.7 km by 5.3 km array and a 1 km diameter transmitting antenna. To put the immensity of this structure in perspective. Space Station Freedom, which is currently planned to be the largest object put into orbit in the next ten years, will weigh just 305 tonnes, and its largest dimension is approximately 90 m. Thus the massive SPS contemplated by NASA/DOE will be at least a factor of 5000 times Freedom's volume and 300 times its mass. One might argue that it will not be desirable to build solar power satellites in geostationary orbit (GEO) on the tremendous scale of gigawatts—a multitude of smaller satellites in low Earth orbit (LEO) may be more suitable. However, there is a limit to how small one can productively make these satellites: they need to be at least on the order of several hundred megawatts if they are to produce significant amounts of energy on Earth. Therefore, only a factor of 10 or so can be saved in terms of size and mass. This savings still yields a satellite that is 500 times the volume of Freedom and 30 times its massOne might also argue that advances in collection efficiency, either directly through solar cell improvements and solar dynamic generators, or indirectly through solar concentrators and reflectors, could reduce the collecting area required. However, the efficiency is already assumed to be almost 20%, and since virtually no solar conversion systems claim to have theoretical (let alone practical) efficiencies greater than 60%, this reduction can only realistically save a size and mass factor of about three. Consequently, a best case scenario results in a satellite producing hundreds of megawatts with 60% solar conversion efficiency that is still well over 100 times the size and 10 times the mass of Space Station Freedom, which, incidentally, will take NASA several years to assemble. Therefore what one has, inevitably, is a very large space structure. The theoretical engineering considerations of large-scale structures are detailed in Section 9.2, which deals with the modelling and control theory of large space structures. Specifically, the topics of multibody dynamics, modal representation, linear/nonlinear control, and robust control are discussed. The actual construction of a large space structure is the topic of Section 9.3. The advantages and disadvantages of both erectable and deployable structures are outlined. Also addressed in detail is the assembly of Space Station Freedom, extravehicular activity (EVA) experiments involving erectable structures, packing efficiency, trusses, inflatables, and adaptive structures. The general conclusion of these sections is that the assembly and maintenance of solar power satellites, whatever their exact final size, will be of sufficient complexity to absolutely require, at least in our view, an assembly and maintenance oriented demonstration before any such system of satellites can be installed. The primary purpose of this last prototype would be to show not only the ability to build the solar power satellite, but also the related but yet quite different ability to maintain it, for a period of about 5 years (based upon a desired 30-year satellite lifetime). In the near term, the most important demonstrations would consist of small-scale assembly experiments (for example NASA's shuttle bay EASE and ACCESS assembly experiments). Also, small-scale demonstrations of inflatables, such as the inflatable rectenna proposed for the Arecibo 10 Million USS design example (see Sec. 10.1) could be of great importance. Then comes a possible watershed event for space solar power: the construction of Space Station Freedom. The successful manned assembly and manned and telerobotic maintenance of Freedom would represent a major confidence-inspiring milestone for large space structures in general and space solar power in particular. Failure in either of these two critical areas for Freedom could set back development of space solar power for over a decade. Once the first two space stations of the 21st
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