1991-09-01

Small Stirling Dynamic Isotope Power System for Multihundred-Watt Robotic Missions 912066

Free-piston Stirling engine (FPSE) and linear alternator (LA) technology is combined with radioisotope heat sources to produce a compact dynamic isotope power system (DIPS) suitable for multihundred-watt space applications which appears competitive with advanced radioisotope thermoelectric generators (RTGs).Unlike earlier DIPS concepts based on closed cycle Brayton conversion, the small Stirling DIPS is scaleable to multihundred-watt power levels or lower. The FPSE/LA convertor, which is not subject to the tip clearance to swept area scaling limitation of turbomachinery, remains a high efficiency convertor in sizes ranging from tens of kilowatts down to only a few watts. At multihundred-watt unit size, the FPSE can be directly integrated with the government-furnished General Purpose Heat Source (GPHS) via radiative coupling; the resulting dynamic isotope power system has a size and weight that compares favorably with the advanced modular (Mod) RTG, but requires less than a third the amount of isotope fuel. Thus the FPSE extends the high efficiency advantage of dynamic systems into a power range never previously considered competitive for DIPS. This results in lower fuel cost and reduced radiological hazard per delivered electrical watt.Having sucessfully flown on several earlier space missions, free-piston Stirling technology has the potential to achieve, as an isotope engine, the high reliability that is required for years of unattended remote operation. Hermetically sealed inside a container, there are typically only two moving parts and no sliding seals of any kind. Incorporating noncontacting gas bearings or flexures, there is no wear between the moving parts. Terrestrially, Stirling engine convertors have demonstrated virtually unlimited service life-in test, one radioisotope heated unit has achieved over 110 000 hr continuous operation.On Mars the small Stirling DIPS offers the benefit of being able to operate exposed to the atmosphere without degradation. RTG's must be hermetically sealed inside a container to survive.For these reasons the small Stirling DIPS appears an economical alternative to RTG's. When it is developed, it should prove to be a strong candidate to power the many multihundred-watt robotic missions anticipated within the next three decades for deep space and planet surface exploration.

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