Environmental Loss Characterization of an Advanced Stirling Convertor (ASC-E2) Insulation Package using a Mock Heater

The U.S. Department of Energy (DOE) and Lockheed Martin Space Systems Company (LMSSC) have been developing the Advanced Stirling Radioisotope Generator (ASRG) for use as a power system for space science missions. This generator would use two high-efficiency Advanced Stirling Convertors (ASCs), developed by Sunpower, Inc., and NASA Glenn Research Center (GRC). The ASCs convert thermal energy from a radioisotope heat source into electricity. As part of ground testing of these ASCs, different operating conditions are used to simulate expected mission conditions. These conditions require achieving a particular operating frequency, hot end and cold end temperatures, and specified electrical power output for a given net heat input. While electrical power output can be precisely quantified, thermal power input to the Stirling cycle cannot be directly measured. Traditionally, the environmental losses have been characterized based on testing a non-operating convertor. However, the thermal profile of a non-operating convertor does not adequately characterize the thermal profile of an operating convertor and, therefore, does not accurately predict net heat input. In an effort to improve net heat input predictions, numerous tasks have been performed which provided a more accurate value for net heat input into the ASCs , including testing validation hardware, known as the Mock Heater Head , to provide a direct comparison to numerical and empirical models used to predict convertor net heat input. The Mock Heater Head was primarily constructed of existing test hardware which had the same relative thermal paths as a convertor. The dominant conduction path is a removable rod that interfaces the interior dome of the Mock Heater Head. Two different diameter rods were used to vary the thermal load on the heater. While the rod and heater head cylinder wall provided conductive paths from the hot end to the cold end, the resulting thermal profile did not accurately simulate that of an operating convertor . The Mock Heater Head also served as the pathfinder for a higher fidelity version of validation test hardware, known as the Thermal Standard. This paper describes how the Mock Heater Head was tested and utilized to validate a process for the Thermal Standard t hrough test configuration, inst rumentation, calculations performed, test methods, and data reduction.