Stirling Engines For Space Power

Given the elementary but irrefutable logic above it is difficult to understand the virtual total neglect of Stirling engines by NASA during the 1960s. This was the era of prodigious expenditures on hydrogen-oxygen fuel cells for the Apollo missions. 011 the Rankine cycle SNAP8 reactor power-plant, and on various Bray ton turbines. So are as is known, no NASA funds atid only minor Air Force funds were expended on Stirling engines.

One possible explanation for NASA neglect of Stirling engines is that the confidentiality clauses in the Philips licence agreement foredad full-hearted participation by General Motors in Government contracts which required full disclosure. Other companies known to have been interested in the possibilities of the Stirling engine were daunted and discouraged by the impressive thorn-fence of patents that Philips built around the new technology and the exclusive licence with General Motors.

At any event only one program of substance on Stirling engines for space power-systems was carried out in the 1960s, and another small program is presently in progress.

The earlier program, sponsored by the U.S. Air Force, was executed by the Allison Division of General Motors and was directed to the design and development of a 3 kW (4 hp) solar space power-system. The Stirling engine, shown in Fig. 18.1, was a single-cylinder piston-displacer machine with rhombic drive and antecedents that were recognizably Philips. The engine used helium as the working fluid and was heated by solar energy concentrated by a large Fresnel lens. A comprehensive report (Parker and Malik 1962), in ten volumes, was prepared on the project and even at this late stage remains an interesting and worthwhile reference source. Volume I deals with the design of the engine and Volume 10 the test and evaluation of the prototype machine. So far as is known the unit never went beyond a first prototype and was never developed to the flight hardware stage.

This engine was particularly notable because it represented the first application to Stirling engines of an intermediate heat-transfer liquid-metal healing system. A sodium/potassium eutectic mixture was used to

Rhombic Stirling Engine

Fto. IS. I Allison Type PD46 Stirling engine for 3kW solar-heated space power-system

transfer heat from the solar energy absorber to the heater head of the engine. 'Hie use of an intermediate heating fluid is now becoming commonplace in advanced Stirling engines for various applications. Further details of (he Allison space power-system are given in Chapter 13.

Another application of Stirling engines lor space power-plants was initiated in 1975 by the U.S. Energy Research and Development Administration. now the Department of Energy. This is a 1 kW (1.4 hp) isotope power system (KIPS) being executed by Mechanical Technology Incorporated, Latham. New York.

Goldwater and Morrow (1977) have described the concept and the early phases of the development. The unit combines a single-cylinder Beale displacer-typc frcc-piston engine with an MTI linear generator as shown in Fig. 18.2. The engine is heated by a radioisotope energy source. To establish a system in perfect dynamic balance an opposed-piston arrangement is proposed for the ultimate flight engine.

The well-publicized but unscheduled descent in Northern Canada in early 1978 of the Russian satellite with an isotope power-system (thermoelectric generator) aboard casts some clouds on the political acceptability of the KIPS system for the future. It is likely that, in civil applications at least, solar-powered systems will be emphasized. These

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