reasonably effective and is relatively insensitive to debris, but involves high-temperature moving parts. (5) Droplet radiator. This is perhaps the most interesting of the advanced concepts. It sprays the hot fluid from the heat exchanger directly into space in a fanshaped stream of fine droplets, collects them (at the apex of the fan), and pumps them as a liquid back into the heat exchanger. It has been ground-tested successfully in the laboratory, but the droplet generator and collector still need to be demonstrated at large scale and high temperatures for long-life applications. Liquid tin has proved the most effective fluid medium, offering temperature capability up to 1000 K. The droplet radiator offers promise of tenfold less mass than current heat-pipe radiators. Power Conditioning and Energy Storage Power Conditioning. Photovoltaic, thermoelectric, and thermionic power conversion systems are constrained by their nature to deliver DC output, so inverters are needed if the load requires AC power. Rotating machinery and Stirling engines, on the other hand, using rectifiers if necessary, can be used to deliver either DC or AC power at the desired voltage or frequency. Should the load or power transmission subsystem require very high voltage (e.g. ion propulsion devices or klystrons for microwave power generators), appropriate power conditioning subsystems will be needed to supplement either direct-conversion or dynamic power conversion systems. There are no technical problems or penalties involved in delivering conventional power modes; e.g. 24 V DC, 120 V DC, 400 Hz AC, 20 kHz AC, etc. A central-station power depot will be required to supply different customers with different power characteristics. These could range from conventional power needs (e.g. 400 Hz AC) to the very high DC voltages needed for microwave transmission using klystrons (see below) or for ion propulsion (e.g. 10-20 kV and 3-5 kV respectively). A careful market analysis will therefore be essential to determine the distribution of demand among the various power modes to determine whether or not it will be cost- effective to build, integrate, and launch any particular power conditioning subsystem. Energy Storage. An important market for the space-based central-station powerplant could develop in very short-term (e.g. an hour), very high power demand (e.g. 100-500 MWe), primarily for military space weaponry. It is almost certainly not economically sound to build that demand into the baseload capacity of the plant, but to satisfy it through energy storage using long-term low-power capacity. The available storage mechanisms are chemical batteries; chemical propellants driving either a turbogenerator, a magnetohydrodynamic channel, or a chemical laser; capacitors; inductors; and inertial storage in various rotating AC or DC machines. For the multi-hundred megawatt level only chemical propellants or rotating machinery (particularly homopolar generators and compulsators) are likely candidates, although chemical combustion systems may not be acceptable for certain demand modes due to contamination by the exhaust gases. The primary military need is for assured availability; the likelihood that these weapons will be used at all is small, and it is even less probable that they would be used more than once. Hence chemical propellants could be lifted and stored during central-station spacecraft deployment and are not likely to require replacement. Should cryogenic propellants be required, periodic electrolysis of water resupplied by infrequent cargo ferry flights could maintain the necessary inventory, using onboard long-term low-power powerplant capacity for the electrolysis process.
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