The Department of Energy, Lockheed Martin, Infinia Corporation (formerly known as the Stirling Technology Company), and the NASA Glenn Research Center are developing a Stirling Radioisotope Generator (SRG) as an alternative to radioisotope thermoelectric generators for NASA space science missions. The 20- to 25-percent efficiency of the SRG system would reduce the required amount of radioisotope by a factor of 3 or more in comparison to radioisotope thermoelectric generators. This would significantly reduce radioisotope cost, radiological inventory, and system cost, providing efficient use of the scarce domestic supply of radioisotope resources.
In addition to the highly focused SRG program, NASA is spearheading an effort to develop an even higher efficiency, lower mass Stirling convertor for use with a radioisotope, reactor, or solar concentrator heat source. One of the goals for this upgraded version of the SRG is to improve performance by increasing the Carnot efficiency of the convertor. To achieve this improvement, researchers must maximize the hot-end temperature and the pressure of the working gas. Although increasing both the temperature and pressure of the working fluid will place an increased burden on all components of the convertor, the heater head will be the most severely tested. The current heater head is made from the nickel-base superalloy 718 and operates at 650 °C, which is this alloy’s maximum use-temperature. Further increases in temperature will require the use of a more advanced high-temperature material.
At the NASA Glenn Research Center, over 200 material candidates were initially screened with regard to the following properties: (1) creep resistance, (2) fabricability, (3) helium gas containment, (4) long-term stability and compatibility, (5) ability to form a hermetical closeout seal, and (6) ductility and toughness (to assist in fabrication, handling, and resistance to foreign object damage). In the end, the list of candidates was narrowed to five superalloys. These five alloys were then subjected to additional testing, including creep testing under a wide variety of conditions. From these experiments, the nickel-base superalloy MarM-247 was the clear choice as the primary material for the new heater head.
Creep properties of the conventional heater head material, IN718 compared with the more advanced material, MarM-247. The data are plotted using a Larson-Miller parameter, which allows test data collected at different temperatures to be plotted on a single curve.
Long description of figure 1.
One critical feature of the heater head is its thin wall thickness (<1 mm in some regions). Since thin walls are known to affect the creep behavior of superalloys, it was necessary to produce several heats of material with different grain sizes and, thereby, determine the optimum microstructure. Further analysis of the creep data allowed a detailed reliability assessment to be performed, and it established the maximum operating temperature and stress that could be tolerated with a high confidence of success for the intended mission duration.
In additional to the mechanical properties of the heater head, some method of hermetically sealing the MarM-247 to the convertor body is required. Unfortunately, one negative consequence of MarM-247’s high strength is that it cannot be joined by traditional fusion welding techniques. Also, any joining technique that requires heat input into the heater head must be compatible with the overall assembly and with the material heat-treatment procedure that is required to develop the material’s high strength.
Many joining techniques were explored at Glenn, but eventually two methods emerged as the most promising. One is a brazing process that uses a braze filler metal and brazing cycle that is completely compatible with the heat-treatment schedule. The other is electrospark deposition, which is a nonfusion technique normally used for surface repair. The brazing concept was demonstrated and proven not to adversely affect the properties of the MarM-247, even after long-duration missions. The electrospark deposition technique is undergoing detailed evaluation.
The ASC developed jointly by Sunpower and Rocketdyne, complete with a fully fabricated MarM-247 heater head.
Long description of figure 2.
Independent of the in-house effort, NASA awarded a NASA Research Announcement (NRA, NAS3-03128) to a Sunpower/Rocketdyne team to develop an Advanced Stirling Convertor (ASC). Upon the completion of Glenn’s MarM-247 optimization program, the results of the material screening and optimization efforts were presented at the Space Technology and Applications International Forum. A detailed summary of Glenn’s findings also were presented to the NRA team at their request. After reviewing the data, the NRA team accepted the results of Glenn’s study and adopted MarM-247 as the material of choice. Since then, Glenn and the Sunpower/Rocketdyne team have collaborated on development of the ASC. In late September 2005, the team successfully operated an ASC testbed convertor using the MarM-247 heater head. The head temperature was limited to 650 °C for the first run, but subsequent runs have been performed at the design temperature of 850 °C, which is 200 °C higher than for the previous SRG design.
Find out more about this research:
Glenn’s Thermal Energy Conversion Branch:
http://www.grc.nasa.gov/WWW/TECB/
Power and In-Space Propulsion:
http://www.grc.nasa.gov/WWW/5000/pep/
Glenn contact:
Dr. Randy R. Bowman, 216-433-3205, Randy.R.Bowman@nasa.gov
Author:
Dr. Randy R. Bowman
Headquarters program office:
Science Mission
Programs/Projects:
RPS
Last updated: October 16, 2006
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