GAISUS-1 thermionic converter for the Integrated Solar Upper Stage

The Integrated Solar Upper Stage (ISUS) system is a compact orbital transfer vehicle which generates thrust to boost payloads from LEO to higher orbits. It does this by collecting and concentrating solar flux into a sensible thermal storage, graphite receiver which is used to heat hydrogen propellant to temperatures of up to 2500 K. The ISUS receiver also radiates heat into an array of thermionic converters which produce electrical power. The GAISUS-1 thermionic converter is a first generation planar converter designed to produce electrical power when coupled with the ISUS receiver. GAISUS-1 will deliver over 31 W/sub 0/ at 1900 K. A wrought Re hotshoe accepts radiant heat from the receiver. The back side of the hotshoe forms the emitting surface of the converter. Special attention was paid to optimize the electrical and thermal losses experienced through the sleeve. Triple and single sleeve geometries were thermally modeled and evaluated, resulting in the selection of a single sleeve design. A high temperature metal/ceramic seal isolates the emitter sleeve from the collector. A Nb collector is used and is an integral part of a Nb/Na heat pipe. The heat pipe transports reject heat from the collector surface to a thermal radiator (condenser) portion of the heat pipe. The converter utilizes an integral graphite Cs reservoir. This type of reservoir automatically produces a rise in Cs pressure in response to a rise in emitter/collector temperatures. This Cs pressure feedback mechanism insures adequate Cs coverage of the emitter over a broad range of operating conditions (temperatures).