A solar bimodal system provides separate propulsion and electrical power modes for space missions. The Integrated Solar Upper Stage (ISUS) program provides for the design, construction and testing of a bimodal system using thermionic conversion for the electrical conversion and hot hydrogen for propulsion. The thermionic converters and the graphite solar absorber/receiver/heat storage device comprise the Receiver Absorber Converter (RAC) module. The absorber is a thick-walled graphite cylinder equipped with many small flow holes for heating the hydrogen propellant and is surrounded by a MULTI-FOIL® assembly. In the propulsive mode of operation the graphite is heated up to 2,500 K while the thermionic converters, which surround the graphite, are shielded thermally by the MULTI-FOIL system. For the power generation mode, part of the insulation system is retracted to expose the thermionic converters to the hot graphite heat source. The insulating package consists of 35 layers of tungsten foil and 55 layers o...