systems use liquid metals as a reactor coolant. Safety in launch requires that these systems be launched frozen and then melted in space. This requirement puts a formidable design task on any system, not only from having to perform the initial startup after launch, but also having to deal with the potential for an operational shutdown, freezeup and restart while in orbit. Heat rejection for all systems, open or closed, is a primary issue due to the constraints of weight and microgravity. The thermal hydraulic aspects of the systems must address the cooling of not only the reactor, but the power conversion and power conditioning equipment as well. This does not seem to be a big problem at this time for the SDI open cycle systems because of the availability of excess liquid hydrogen from the weapon cooling system. The design issues are mainly one of getting the appropriate amount of hydrogen to the particular component and then disposing of it in such a manner as not to interfere with operation of the weapon or other sensitive equipment on the platform. For systems utilizing superconducting generators, an additional thermal hydraulic task of dealing with a separate liquid helium system is required. Heat rejection for closed cycle systems is a far more difficult task, since all heat rejection must ultimately be made by radiation in space, and the present heat transfer equipment—heat exchangers, radiators, recuperators, etc.—are heavy and voluminous. Technology issues in space generally result from spacecraft (or base) requirements, microgravity, safety considerations and in some cases the use of new components or a component’s use outside the normal experience range. The problems differ greatly depending upon whether an open or closed cycle is being considered. Closed Rankine cycles, which utilize phase change fluids, provide a variety of thermal-hydraulic concerns that do not exist with Brayton cycles, either open or closed. Some of the major impacts of these items on closed and open cycle systems are highlighted below. Microgravity Considerations 1. The use of a phase change fluid in microgravity requires a means to manage the two phases. Thus separators are a part of the system. 2. Phase change power systems have condensers which must operate where shear forces control condensation since no falling film condensation can exist. 3. Obtaining adequate NPSH for the main feed pump in a closed Rankine cycle requires special techniques, i.e. rotary fluid management devices, jet pumps. 4. Hydrogen acquisition, NPSH development, and pumping are major concerns for open cycle systems since generally no acceleration will exist when the system is started. 5. Surface tension controlled circulation flows may exist in some closed thermal energy storage systems. Spacecraft Related Considerations 1. The SDI missions may require that a spacecraft accelerate during operation of the power system. This may have the effect of stalling or reducing flows in a component where the driving head is small, i.e. heat pipe, or reactor loop with very low pressure drop. Thus, maneuvers may cause flow and thermal transients in the system and must be considered in the design of components. 2. In open cycle SDI power systems the weapon systems (primarily accelerator and RF cooling paths) act as preheaters with respect to the power system. Thus, the
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