The four key performance parameters that dictate the selection of a space power conversion subsystem are: (1) Efficiency (2) System specific mass (3) Heat rejection temperature (since radiator mass dominates system mass at high power levels) (4) Reliability These parameters are of course closely interrelated; for example, increased efficiency reduces reactor specific mass but can increase radiator specific mass because it may reduce heat rejection temperature. Hence detailed trade studies are required to select the optimum power conversion system that meets technological practicability, cost, reliability, and availability criteria. Other factors that must also be considered are working-fluid properties, lifetime, vibration and torque, useful power range, susceptibility to modularization, startup and shutdown, compatibility with power conditioning requirements, and ability to withstand the space environment (radiation and debris). Ranking the various power conversion subsystems in order of reliability based on their test experience, thermoelectric converters are highest, Stirling engines lowest, and organic-fluid Rankine cycles, Brayton cycles, liquid-metal Rankine cycles, out-of-core thermionic converters, and in-core thermionic converters rank in that order. Radiators Although the optimum radiator temperature must be determined by thorough powerplant system analysis, it is clear that reducing the mass per unit radiator area and the mass growth per unit temperature increase will improve system performance, especially at higher plant output power levels. Hence there is a strong motive for replacing conventional fin-and-tube radiator designs by more efficient concepts. Heat-pipe radiators, for example, are now considered state-of-the-art (e.g. in the SP-100 system), but although considerably better than the primitive fin-and-tube designs they are still quite massive, particularly at high power levels. Advanced radiator concepts that have been proposed to achieve substantially better radiator performance (i.e. radiated power per unit mass) are as follows: (1) Membrane radiators. These are composed of thin metallic membranes that rotate to maintain their shape by centrifugal force and also to expose their hot surfaces to space frequently. Although low in mass, they are not robust and are limited in power dissipation capability by structural strength at high temperatures. (2) Curie-point radiators. These employ ferromagnetic particles which are heated in a fluid heat exchanger, are blown out into space where they radiate their heat away, and are recovered by the use of magnetic fields. Although efficient in principle due to the very large surface area of the particles, they have not yet been proven in the laboratory and are limited in peak temperature by the need for ferromagnetic properties. (3) Electrostatic radiators. This concept is similar to the Curie-point scheme, but uses electrostatic rather than electromagnetic forces to reattract the particles. The practicality of this concept also has yet to be demonstrated. (4) Moving belt radiator. The radiator surface area is a continuous belt which picks up heat from a rotating cylinder and rejects it to space. It can be designed to be
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