Advanced Stirling Radioisotope Generator EU2 Anomaly Investigation

Abstract

The Advanced Stirling Radioisotope Generator (ASRG) Engineering Unit 2 (EU2) is the highest fidelity electrically-heated Stirling radioisotope generator built to date. NASA Glenn Research Center (GRC) completed the assembly of the ASRG EU2 in September, 2014 using hardware from the now cancelled ASRG flight development project. The ASRG EU2 integrated the first pair of Sunpower's ASC-E3 Stirling convertors (ASC-E3 #1 and #2) in an aluminum generator housing with Lockheed Martin's Engineering Development Unit (EDU) 4 controller. After just 179 hours of EU2 generator operation, the first power fluctuation occurred on ASC-E3 #1. The first power fluctuation occurred 175 hours later on ASC-E3 #2. Over time, the power fluctuations became more frequent on both convertors and larger in magnitude. Eventually the EU2 was shut down in January, 2015. An anomaly investigation was chartered to determine root cause of the power fluctuations and other anomalous observations. A team with members from GRC, Sunpower, and Lockheed Martin conducted a thorough investigation of the EU2 anomalies. Findings from the EU2 disassembly identified proximate causes of the anomalous observations. Discussion of the team's assessment of the primary possible failure theories, root cause, and conclusions is provided. Recommendations are made for future Stirling generator development to address the findings from the anomaly investigation. Additional findings from the investigation are also discussed.convertors and larger in magnitude, up to 9.4 W. Early power fluctuations were less than 2 seconds in duration, while later drops in power lasted for hours. Eventually the EU2 was shut down in January, 2015.Given the flight-like fidelity of the EU2 hardware, with the ASC-E3 convertors and the EDU 4 controller, and the flight-like rigor that had been used to assemble the generator housing assembly (GHA), such an early failure was both unexpected and disconcerting. The EU2 was intended to be part of a broader program to establish a reliability database for future Stirling-based power systems, 3 but this failure was a setback to that initiative. An anomaly investigation was chartered to determine root cause of the power fluctuations, to the extent possible, so as to learn from this event and then apply insights gained to future dynamic power conversion systems. The EU2 anomaly investigation team included members from GRC, Sunpower, and LM. Various GRC technology experts were brought in as needed to support different phases of the investigation.This paper chronicles the EU2 anomaly investigation process that took place in 2015. Findings from the disassembly and inspection of the GHA and ASC-E3 convertors are discussed, along with the proximate cause of the power fluctuations. Extensive analysis, modeling, and follow-on testing was conducted, leading to possible failure theories and an assessment of likely root cause and conclusions. Recommendations are provided for future Stirling generator development. A. The ASRG EU2This section provides the reader with a ...