The choice of a liquid metal heat transport fluid over a conventional organic fluid in the radiator is a necessary one due to the inherent life limitation of the conventional fluids, a problem which is aggravated by temperature. The temperature of the fluid in the radiator must be held fairly high [350 to 550F) to reduce this major weight component. Although liquid metals should be used only if no other fluids are adequate, this choice seems best at present for the thermal engine. The corrosion problem does not exist at the low-temperature end of the cycle as it does at the upper end. The final fluid choice need not be made now, although if freezing turns out not to be a problem during periods of eclipse, the familiar NaK-78 may be a good selection. If freezing is a problem, some mixture such as Cesium-Nak could be used to lower the freezing point of the fluid. The Boeing study to date makes no mention of thermal storage requirements (if any) for eclipse periods, and this problem must be addressed. During the semi-annual eclipse periods, the rotating turbomachinery would probably be kept spinning to avoid start-up and shut-down damage to the bearings. The turbogenerator set would thus act as a flywheel energy storage system whose stored energy would be released to supply bearing and windage losses in a no-load condition. Spinning reserve for a large gas turbine system such as this is expected to be below 4% of output power. If this spinning reserve is not sufficient for the maximum 75 min/day eclipse period, then supplementary heat storage would have to be provided. During the no-load condition, unless the radiating surfaces of the heat rejection subsystem are covered in some manner, the radiator will continue to reject large amounts of energy to space. If it is determined that freezing in the radiator cannot be tolerated, then significant thermal storage material will have to be included as an integral part of the radiator design. If freezing can be permitted, then by judicious choice of the heat transport fluid and careful consideration of the thermostructural design of the heat rejection system, the radiator can be made much lighter and more compact. Since the radiator is the heaviest component of the system, its optimization is very important. The Boeing work available to date makes no mention of any of these important aspects of radiator design. Furthermore, it is felt that the conventional fluid pump in the radiator loop is not adequate. For lifetimes on the order of the SPS requirement, conventional pump components are much inferior to the electromagnetic (EM) pump. Although the EM pump has a much lower efficiency than that of a conventional fluid pump, the life requirement in this case would be foremost, and the EM pump, with no moving parts, is considered the only type capable of meeting the life requirement. A final consideration in evaluating the Boeing study is its lack of mention of the use of cryogenically cooled superconducting generators and superconducting cables for power transmission to the microwave system. While still in a very early development state, a closed-cycle superconducting system for both electrical power generation and transmission, even with its associated liquefaction plant, has an enormous weight-saving
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