is to determine whether the magnets degrade at this temperature level and affect alternator efficiency. The next step is to replace the 650 K shell-and-tube hot end of the SPRE with a heat-pipe heater capable of 1050 K operation. After this upgrade the SPRE will have evolved into a 12.5 kWe Component Test Engine (CTE). When this configuration develops the desired power and efficiency, it only need be scaled up by a factor of two to become a single-cylinder SSE. During the MTI upgrading of SPRE, NASA will provide whatever technology enhancements—be they in code development or component development—that have been developed during the course of SPRE upgrading. Depending upon the progress in fabricating and joining some of the superalloys, the CTE engine may be fabricated from more conventional superalloys thereby sacrificing life to 10000 hr capability. However, the 25 kWe SSE will be designed for 60000 hr life with high-strength superalloys. Supporting Research and Technology As part of the CSTI High Capacity Power Element, NASA is conducting advanced Stirling technology research—some of which is being done in-house. One area under consideration is the Stirling hot-end heat exchanger. Whether the heat source is solar or nuclear, there is a strong probability that heat pipes will be required to transport heat to the engine heat exchangers. Currently there is a paucity of test data relative to liquid-metal heat-transport systems coupled to Stirling engines. An inexpensive 1 kW engine was designed using many existing components from a previously tested engine. This engine incorporated three modular heat exchangers with integral sodium heat pipes. Calculations showed that each of the modules on this small engine would operate at the same conditions as a similar type heat-pipe module on the SSE. To date the heat pipes have operated up to about 975 K with temperature variations of less than 10 K and no known heat pipe problems have occurred. All results indicate that the heat-pipe modules are viable candidates for Stirling cycle heat exchangers. Each heat-pipe module was designed to produce up to 2 kW of power. Details of the heatpipe modules are given in Ref. [7]. The 1 kW heat-pipe Stirling is shown in Fig. 7. SSE Engine The Stirling Space Engine is an engine design based upon eventual 1300 K operation, but one that will be fabricated initially from superalloy materials and run at 1050 K to confirm design features. Sunpower provided the initial SSE conceptual design. Currently MTI is conducting the preliminary engine design as part of the NASA-awarded, $15.4 million, 4-year contract. This 1050 K engine (see Table I for goals and specifications) will serve as a transition from demonstrated Stirling technology feasibility at 650 K peak temperature to 1050 K. The transition will incorporate advanced heat exchangers using sodium heat pipes. Hydrodynamic gas bearings will be used for the piston. The gas bearing method for the displacer has not been determined. An improved alternator design will be used to reduce the losses encountered during SPDE testing. The success of the 1050 K engine may play an important role in various spacepower programs. For example, a successful Stirling at 1050 K may necessitate that Stirling be given a closer look for future Space Station missions. Also, depending upon the SP-100 reactor and thermoelectric converter progress, Stirling may deserve another look as a near-term option as well as a growth consideration. However, the main thrust
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