The SNAP-19 Radioisotopic Thermoelectric Generator Experiment

The Nimbus III/SNAP-19 launch on April 14, 1969, represented the first flight of a nuclear power supply on a NASA observatory and the culmination of six years of interagency effort between NASA and the AEC. The SNAP-19 consists of two generators, two dc-dc converters, and a telemetry signal conditioning unit. Power is produced by converting the radioactive heat of decay of the isotope Pu-238 in a static thermopile consisting of 90 PbTe 2n-3p couples per generator. SNAP-19 S/N 009 produced 49.02 watts of usable power at launch and has degraded to 44.03 watts after 7½ months, which represents a significantly higher degradation rate than predicted prior to launch from endurance testing of the same technology in electrically heated generators. Leakage of generator fill gas has resulted in a pressure drop from 15.5 to 1.32 psia. Generator currents have decreased as generator internal resistances have increased with time. Hot-cold junction temperatures have risen about 5°F during this period. while fin root tenperatures have risen 2-3°F