Low-cost in-space transportation also represent a critically-iinportant technology area for SSP. This area includes diverse technologies, many being addressed by NASA’s Advanced Space Transportation (AST) Program. An important issue will involve the degree of commonality that can be achieved between early SSP systems (such as the SunTower concept) and in-space transportation systems that support later, GEO-based SSP (such as SolarDisc) at relatively small scales. Wireless Power Transmission. A significant difference between the 1979 SPS Reference and the concepts examined in the present study is the technology and approach used for wireless power transmission. The SPS Reference Study used 2.45 GHz klystron devices, each with a 70 kW output power capability, while the 1996 SolarDisc concept uses solid state devices operating at 5.8 GHz, each with a 10W output power capability. Furthermore, the solid state devices are tightly packed in the transmitting array to allow electronic beam steering which provides improved ground site accessibility. This explains some of the reason for the large mass differences, but there are many others including how the transmitting array is coupled to the power collection system, pointing requirements, and device operating efficiencies. The higher production cost per Watt for the SolarDisc power transmission manufacturing is due to the assumption that the design complexity of the solid state device will add production costs versus the klystron power amplifier. SSP scenarios would have a major market impact on either power transmitting device, klystrons or solid state. The only power transmission device considered which is currently mass produced is the magnetron, used mostly in microwave ovens. Although there are certain advantages to using magnetrons, the most significant of which is a low manufacturing cost, the ability to operate at higher transmission frequencies (5.8 GHz versus 2.45 GHz) is a major factor in selecting a solid state power transmitting device. Future study should address again the issue of using magnetrons at 2.45 GHz for GEO-based SSP approaches. Energy Storage. Energy storage systems may be utilized either on the ground or in-space as a part of the architectural approaches examined by the “fresh look” study. Probably the essential requirement is for sufficient energy storage and related PMAD to assure uninterrupted, high-quality electrical power delivery to terrestrial customers during transitions from one down link power beam to the next — in particular in the case of the LEO and MEO SunTower concepts. On-board SPS energy storage was not examined extensively in the ‘Tresh Look” study. Solar Conversion. Dramatic advances have been made during the past 20 years in terrestrial photovoltaic (PV) systems for the conversion of sunlight into electrical power. These advances have not by any means been matched by space PV systems. Two principle concepts for solar conversion were incorporated in this study: thin film PV and concentrator PV. The first of these is one in which the terrestrial state of the art is fast approaching the parametric goals of the present study. Long-life in the presence of space radiation, including the Earth’s Van Allen radiation belts, is a key requirement for any SSP concept. Solar energy conversion is a critical area for SSP and requires both additional study and early technology investments. Guidance, Navigation & Control. The 1979 SPS Reference Concept relied on very conventional approaches to GN&C, including three-axis stabilization of the primary large platform (achieved through control moment gyros and attitude control propulsion) and use of a large mechanical gymbal for the pointing of the transmitter array. The current study relies on more advanced concepts, primarily gravity-
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