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{{short description|Radioisotope generator based on a Stirling engine powered by a large radioisotope heater unit}}
[[file:Stirling radioisotope generator head testing.jpg|thumb|Component of Stirling radioisotope generator is [[induction heating|heated by induction]] during testing]]

A '''Stirling radioisotope generator''' ('''SRG''') is a type of [[radioisotope generator]] based on a [[Stirling engine]] powered by a large [[radioisotope heater unit]]. The hot end of the Stirling converter reaches high temperature and heated [[helium]] drives the piston, with heat being rejected at the cold end of the engine. A generator or alternator converts the motion into electricity. Given the very constrained supply of [[plutonium]], the Stirling converter is notable for producing about four times as much [[electric power]] from the plutonium fuel as compared to a [[radioisotope thermoelectric generator]] (RTG).

The Stirling generators were extensively tested on Earth by [[NASA]], but their development was cancelled in 2013 before they could be deployed on actual spacecraft missions. A similar NASA project still under development, called [[Kilopower]], also utilizes Stirling engines, but uses a small [[uranium]] [[Nuclear reactor|fission reactor]] as the heat source.

== History ==

Stirling and Brayton-cycle technology development has been conducted at [[NASA Glenn Research Center]] (formerly NASA Lewis) since the early 1970s. The Space Demonstrator Engine (SPDE) was the earliest 12.5 [[kWe]] per cylinder engine that was designed, built and tested. A later engine of this size, the Component Test Power Converter (CTPC), used a "Starfish" heat-pipe heater head, instead of the pumped-loop used by the SPDE. In the 1992-93 time period, this work was stopped due to the termination of the related [[SP-100]] nuclear power system work and NASA's new emphasis on "better, faster, cheaper" systems and missions.

In 2020, a free-piston Stirling power converter reached 15 years of maintenance-free and degradation-free cumulative operation in the Stirling Research Laboratory at NASA Glenn.<ref>{{Cite web|title=Stirling Converter Sets 14-Year Continuous Operation Milestone|url=https://rps.nasa.gov/news/40/stirling-convertor-sets-14-year-continuous-operation-milestone|access-date=2021-10-25|website=NASA RPS: Radioisotope Power Systems|language=en}}</ref><ref>{{Cite web|date=2018-07-30|title=It keeps going and going: Stirling Engine test sets long-duration record at NASA Glenn|url=https://www.spaceflightinsider.com/space-centers/glenn-research-center/it-keeps-going-and-going-stirling-engine-test-sets-long-duration-record-at-nasa-glenn/|access-date=2021-10-25|website=SpaceFlight Insider|language=en-US}}</ref> This duration equals the operational design life of the MMRTG, and is representative of typical mission concepts designed to explore the outer planets or even more distant [[Kuiper Belt Object]]s. This unit, called the Technology Demonstration Converter (TDC) #13, is the oldest of several converters that have shown no signs of degradation. Since 2017, the NASA Radioisotope Power Systems Program at NASA Glenn has continued developing several candidate technologies for the first dynamic RPS to fly in space, including designs based on the record-setting TDC #13 and the gas-bearing-based Stirling converter that was used in the ASRG. A small turbo-Brayton system is also under technology development. Several viable generator designs in the range of 100-500 Watts have emerged from the ongoing dynamic conversion technology development effort. In the near-term, a lunar demonstration mission using a dynamic RPS as part of NASA's Artemis Program could be the first opportunity for a DRPS to be used in spaceflight. The use of DRPS in a lunar-landed payload would enable it to survive and operate productively during the frigidly cold, two-week lunar nights, or in permanently shadowed craters near the moon's poles.

In the early 21st century, a major project using this concept was undertaken: the [[Advanced Stirling Radioisotope Generator]] (ASRG), a power source based on a 55-watt electric converter.<ref>{{cite report|archive-url=https://web.archive.org/web/20080923011723/http://www.grc.nasa.gov/WWW/TECB/RPS_ASRG_%20Handout.pdf |title=ASRG data sheet|url=http://www.grc.nasa.gov/WWW/TECB/RPS_ASRG_%20Handout.pdf|publisher=NASA Glenn|archive-date=September 23, 2008|access-date=May 9, 2023}}</ref><ref>{{cite report|url=http://www.ne.doe.gov/pdfFiles/factSheets/SpaceRadioisotopePowerSystemsASRG.pdf |title= Space Radioisotope Power Systems Advanced Stirling Radioisotope Generator|archive-url=https://web.archive.org/web/20110927225836/http://www.ne.doe.gov/pdfFiles/factSheets/SpaceRadioisotopePowerSystemsASRG.pdf |archive-date=2011-09-27 }}</ref><ref>{{cite report|url=http://gltrs.grc.nasa.gov/reports/2007/TM-2007-214806.pdf |title=NASA Technical Memorandum TM-2007-214806 |archive-url=https://web.archive.org/web/20090605101702/http://gltrs.grc.nasa.gov/reports/2007/TM-2007-214806.pdf |archive-date=2009-06-05|date=May 2007|access-date=May 3, 2011}}</ref> The thermal power source for this system was the [[General Purpose Heat Source]] (GPHS). Each GPHS contained four [[iridium|iridium-clad]] [[Pu-238]] fuel pellets, stood 5&nbsp;cm tall and 10&nbsp;cm square, and weighed 1.44&nbsp;kg. The hot end of the Stirling converter reached 650&nbsp;°C and heated helium drove a free piston reciprocating in a linear alternator, heat being rejected at the cold end of the engine. The [[alternating current]] (AC) generated by the alternator was then [[Electric power conversion|converted]] to 55 watts [[direct current]] (DC). Each ASRG unit would use two Stirling converter units with about 500 watts of thermal power supplied by two GPHS units and would deliver 100-120 watts of electric power. The ASRG underwent qualification testing at NASA Glenn as a power supply for a future NASA mission.

The ASRG was designed into many mission proposals in this era,<ref>[https://archive.today/20140215161242/http://www.spacenews.com/article/civil-space/36485discovery-mission-finalists-could-be-given-second-shot B. Berger - Discovery Mission Finalists Could Be Given Second Shot (2014)]</ref> but was cancelled in 2013,<ref>[http://futureplanets.blogspot.com/2013/12/the-asrg-cancellation-in-context.html The ASRG Cancellation in Context] Future Planetary Exploration</ref> due to NASA budget constraints.<ref>{{Cite web|date=2013-12-08|title=Future Planetary Exploration: The ASRG Cancellation in Context|url=http://futureplanets.blogspot.com/2013/12/the-asrg-cancellation-in-context.html|access-date=2021-10-25|website=Future Planetary Exploration}}</ref>

== See also ==

* [[Advanced Stirling radioisotope generator]]
* [[Radioisotope heater unit]]
* [[Radioisotope thermoelectric generator]]

== References ==

{{refs}}

== External links ==

* [https://rps.nasa.gov/news/?page=0&per_page=40&order=publish_date+desc%2Ccreated_at+desc&search=&blank_scope=Latest NASA's Radioisotope Power Systems Program News]
* [https://web.archive.org/web/20060616124949/http://newfrontiers.larc.nasa.gov/PDF_FILES/09_NF_PPC_Schmidt.pdf Radio Isotope Power Systems for the New Frontier]
* [https://web.archive.org/web/20080916094600/http://www.ne.doe.gov/pdfFiles/Stirling.pdf Why Develop a Stirling Radioisotope Generator?]
* [https://www.nasa.gov/feature/glenn/2020/stirling-convertor-sets-14-year-continuous-operation-milestone Free Piston Stirling Converter sets record at NASA Glenn Research Center]
* {{Cite journal |url=https://www.scientificamerican.com/article/stirling-in-deep-space/|journal=Scientific American |title=Stirling in Deep Space |first=Mark |last=Wolverton |date=March 2008|volume=298 |issue=3 |page=14 |doi=10.1038/scientificamerican0308-22 |pmid=18357812 |bibcode=2008SciAm.298c..22W |access-date=May 9, 2023}}
* {{YouTube|75wUYbVyTeY|title=NASA Stirling Engine Based Nuclear Power Plant For Lunar Use}}

[[Category:Nuclear technology]]
[[Category:Electrical generators]]
[[Category:Nuclear power in space]]
[[Category:Stirling engines]]