The radioisotope thermoelectric generator (RTG) is a flight-proven, capable source of power that reliably converts heat into electricity. NASA and the Department of Energy (DoE) have developed a new generation of such power systems that could be used for a variety of space missions. The newest RTG, called a Multi-Mission Radioisotope Thermoelectric Generator (MMRTG), has been designed to operate on Mars and in the vacuum of space. However, shorts between the internal electrical power circuit and chassis frame of the MMRTG have been observed in the engineering unit, qualification unit, and flight unit. The internal shorts seemed to appear and sometimes clear spontaneously. A root cause has not been determined for these internal shorts, and their resistance, power rating, and energy rating are largely unknown. A mitigation and measurement technique is needed.

Planed life testing provides an opportunity to test a measurement technique that could characterize the shorts occurring inside the MMRTG. In the event that a short has formed inside the MMRTG, this technique would consist of applying another short (active short) between the internal electrical power circuit and chassis frame of the MMRTG. This measurement technique will attempt to address two main areas of concern: (1) if the internal short of the MMRTG can be cleared in the presence of an active short, and (2) the amount of energy required to clear the internal short.

The active short will consist of a current- sensing resistor that will indicate the amount of current flowing through the internal short of the MMRTG. The circuitry of the active short will allow an individual to control when and where to apply the active short on the MMRTG, and to potentially capture the amount of time required to clear the internal short. The active short circuit will also be equipped with multiple safety features, including a fuse and current-limiting resistor, in order to protect the internal components of the MMRTG from exceeding specified current ratings.

Currently, if an internal short forms within the engineering unit or qualification unit, the action is to do nothing other than record the time a chassis short has occurred. The active short circuit is a low-risk, simple addition to an already existing performance test setup that will potentially characterize and mitigate the internal shorts forming inside the MMRTG. Data from a characterization test can eventually be incorporated into a mitigation technique for future missions of the MMRTG.

This work was done by Gary S. Bolotin and Nicholas R. Keyawa of Caltech for NASA’s Jet Propulsion Laboratory. NASA is seeking partners to further develop this technology through joint cooperative research and development. For more information about this technology and to explore opportunities, please contact Dan Broderick at This email address is being protected from spambots. You need JavaScript enabled to view it.. NPO-49738

NASA Tech Briefs Magazine

This article first appeared in the July, 2016 issue of NASA Tech Briefs Magazine.

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