START-3: Operational evaluations of the ISUS engine ground demonstration thermionic power system

START-3 was a test program conducted in order to demonstrate and characterize the operational performance of the prototype Integrated Solar Upper Stage (ISUS) thermionic power system. The test device consisted of a graphite thermal storage unit, multilayer foil insulation, and sixteen thermionic converters electrically connected in a series array. Several thermal input conditions were achieved during the test, which resulted in measuring converter performance at average converter hot shoe temperatures in the range of 1600 K to 2000 K. Results indicate that the thermionic converters did not perform as well as expected in the array configuration. Follow-on testing of the individual sixteen converters is currently being performed.