Solar thermal OTV—Applications to reusable and expendable launch vehicles

The Solar Orbit Transfer Vehicle (SOTV) program being sponsored by the U.S. Air Force Research Laboratory (AFRL) is developing technology that will engender revolutionary benefits to satellites and orbitto-orbit transfer systems. Solar thermal propulsion offers significant advantages for near-term expendable launch vehicles (ELVs) such as Delta IV, mid- to farterm reusable launch vehicles (RLVs) and ultimately to manned exploration of the Moon and Mars. Solar thermal propulsion uses a relatively large mirrored concentrator to focus solar energy onto a compact absorber, which is in turn heated to > 2200 K. This heat can then be used in two major ways. By flowing hydrogen or another working fluid through the absorber, high efficiency thrust can be generated with 800 sec or more specific impulse (Isp), almost twice that of conventional cryogenic stages and comparable with typical solid-core nuclear thermal stages. Within a decade, advances in materials and fabrication processes hold the promise of the Isp ranging up to 1,100 sec. In addition, attached thermionic or alkali metal thermoelectric converter (AMTEC) power converters can be used to generate 20 to 100 kilowatts (kW) of electricity. The SOTV Space Experiment (SOTV-SE), planned to be flown in 2003, will demonstrate both hydrogen propulsion and thermionic power generation, including advanced lightweight deployable concentrators suitable for large-scale applications. Evolutionary geosynchronous-transfer orbit/ geosynchronous-Earth orbit (GTO/GEO) payload lift capability improvements of 50% or more to the Delta IV launch vehicles could be implemented as part of the Delta IV P4I plan shortly thereafter. Beyond that, SOTV technology should allow long-term storage of stages in orbits up to GEO with tremendous maneuvering capability, potentially 4 to 5 km/sec or more. Servicing of low-Earth orbit (LEO) and GEO assets and reusable (ROTVs) are other possible applications. Offering a combination of high Isp and high thrust/weight together with thrust levels from 10 to 100 lb or more, a large-scale SOTV offers a low-cost alternative to nuclear thermal or solar electric stages for cargo missions to the Moon and Mars. This paper describes the SOTV-SE as well as these and other potential applications of this integrated power/propulsion technology.