and retrieve waste water. The latter concept requires frequent resupply capability from Earth or an orbiting propellant depot, to replenish the OTV's propellant. Nuclear. Many reactor configurations have been proposed for space power supplies but to date only two have flown in space: the US SNAP 10A test flight in 1964 (about 500 We) and about 30 Soviet reactors during the past decade, powering Radar Ocean Reconnaissance satellites (RORSATS) at about 5-10 kWe. The USA is currently developing its SP-100 reactor-powered system, jointly sponsored by NASA, the Department of Energy, and the Strategic Defense Initiative Office (SDIO). A ground engineering test of this reactor is scheduled for 1995, after which a flight-rated system may be built to fly around 2000. The SP-100 uses a fastspectrum, compact lithium-cooled reactor employing uranium dioxide (UO2) fuel. Its peak power rating was recently increased from 1.6 MWt to 2.5 MWt delivered to the power conversion subsystem, which in the current test model is a bank of thermoelectric converters able to generate 100 kWe. The SP-100 reactor, however, with a more efficient power conversion subsystem (see below), could conceivably deliver up to 1 MWe. A previous US plan to obtain 300 kWe from even the relatively inefficient thermoelectric design was scrapped due to budgetary limitations. The only other current Western space nuclear reactor program is a technical predevelopment evaluation effort in France, jointly sponsored by the Commissariat de 1'Energie Atomique and the Centre National d'Etudes Spatiales. Their ERATO reactor, also a fast, compact lithium-cooled design, develops 1.1 MWt to power one to four pairs of counter-rotating Brayton-cycle (gas-turbine) power conversion subsystems delivering 100-400 kWe. The literature is replete with dozens of other prospective reactor types, but in view of their very long development cycle the only real prospects for the 2000-2010 period are likely to be SP-100, ERATO, and their derivatives. However, as with the early US SNAP series these basic reactor types can be used with a wide range of power conversion subsystems and radiator designs to provide electric power up to perhaps several MWe. Beyond the SP-100 class level, a number of ‘multi-megawatt' conceptual designs are being evaluated in connection with the US SDI program to deliver 5-25 MWe electric power long-term or 100-1000 MWe for short-term (e.g. 1 hour) ‘burst' requirements. It is not likely that these power systems will even begin to be developed until the late 1990s, if ever, and hence would not be available until well beyond 2010. Power Conversion Photovoltaic systems do not require separate power conversion subsystems; they deliver DC electric power directly. Solar thermal system designs have to date used Rankine (vapor) cycles employing several different types of organic working fluids. The chemical fuel cell, like the photovoltaic panel, is itself its own power conversion subsystem, delivering electricity directly. Rocket-type open-cycle chemically fueled systems employ high-temperature gas turbines (or possibly magnetohydrodynamic channels); no other conversion subsystem can obtain comparable power densities. If laser energy is desired, it can be obtained directly from chemical lasers. Nuclear reactors, on the other hand, can be used with a variety of power conversion subsystems. For open-cycle (‘burst' power) systems, gas turbogenerators or magnetohydrodynamic channels are the primary options.
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