of an SDS on Space Station Freedom would represent a major milestone for space-based power generation. Other novel types of power collection offering competitive efficiencies include thermoelectric and thermophotovoltaic systems. Development of new radiators, such as the liquid droplet radiator which can be up to 10 times lighter than conventional radiators, will also increase the overall competitiveness of SDS. Transportation Space transportation is usually the single greatest expense of a space project. Considering that about 25 to 40% of the cost for a mission that lasts several years is spent on the provision of a service that may take as little as ten minutes, it is clear that special attention must be given to this topic when considering a project on the scale of space solar power. The high cost of space transportation has been a stumbling block for previous studies of space solar power, such as the NASA/DOE study, and those studies have in fact been overly optimistic about future launch costs. Space transportation will affect the development of space solar power in both the demonstration and final system phase. Early demonstrations can easily be delivered to orbit by current launchers, and some may in fact be launched via “piggyback” options. One such scenario is the proposed Arecibo Earth to space US $5 Million design example, which plans to make use of the Ariane Structure for Auxiliary Payloads (ASAP) ring on Ariane 4. On the other hand, demonstrations scheduled for the next ten to twenty years which are larger in size— 1 MW or more— would profit from the use of systems presently under development or updated versions of current launchers. The long-term space transportation needs of a large space solar power system have been considered. A basic scenario has been presented which assumes that different types of payloads will have different criteria for selecting launch services, depending on how they prioritize such factors as cost, reliability, availability and resiliency. Hence we conclude that a system of solar power satellites is likely to require a fleet consisting of at least three classes of vehicles: personnel, priority cargo, and bulk cargo. For Earth to orbit delivery: the personnel transports developed will be highly reliable Single-Stage-To-Orbit vehicles; the priority cargo vehicles will be Heavy Lift Launch Vehicles; and the extremely low cost bulk cargo vehicles may be either electrical mass drivers or chemical accelerators. For inter-orbit transfers, the personnel and priority cargo transports developed may be LOX/LH2-fueled Orbital Transfer Vehicles (OTV). Bulk will likely be handled by low thrust electrical propulsion OTV. Since certain solar power satellite construction scenarios involve lunar resource utilization, lunar transports are also considered. Because most of the technology is now available, a cislunar transportation system based on chemical propulsion could be realized in less than a decade. However, in the future it may be beneficial to make use of electric propulsion, nuclear thermal propulsion, and especially electromagnetic mass drivers. The central question concerning future space transportation, though, is not its type but its cost. Though difficult to predict, an overall one order of magnitude cost reductions seems reasonable, particularly when taking into account the effects of scale. On the order of 20,000 tonnes payload mass have to be launched for every 1 GW of power delivered to Earth. This is hundreds of times more than the current global annual launch rate. Therefore significant price reduction can be expected as the launch rate increases. We believe that the reduction in transportation costs that a space solar power program would bring about beneficial side-effects to all global space-related activities. Lastly, we recognize the importance of developing alternatives to traditional chemical propulsion. One such alternative that may be of particular relevance to space solar power is the development of electric propulsion, which is in fact one of the ““high leverage” issues discussed in Section I. Not only can electric propulsion reduce the transportation cost of space solar power satellites (as in the 1 MW class design example) but it represents a possible market for beamed power. Electric propulsion Orbital Transfers Vehicles are potentially made more effective by using beamed power. Utilization of a remote power source would increase the mass efficiency of electric propulsion relative to competing technologies and would significantly reduce vehicle trip times compared to standard electric vehicles. Consequently, it might be worthwhile to attempt a power beaming demonstration to an electric propulsion vehicle as soon as this is technologically feasible. Manufacturing, Construction, & Operations The most daunting technical obstacle facing space solar power will probably be the assembly and maintenance of the large space structures which they require. We found that even in a best case scenario, where the satellite is transmitting 500 MW and is converting solar energy to electricity at an efficiency of 60%, the satellite will still be well over 100 times the size and 10 times the mass of
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