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Original Date: 04/26/1999
Revision Date: 01/18/2007
Information : Solar Thermal Propulsion
In the distant future, low cost propulsion will be needed for interplanetary travel and unmanned exploration. NASA foresees Solar Thermal Propulsion as a way to boost future payloads from a low earth orbit to a geosynchronous earth or higher orbit. These payloads would also undergo round trips of 30 days to transit between orbits. Solar Thermal Propulsion is an excellent choice because it requires only one propellant gas and combines moderate thrust with moderate propellant efficiency. For more distant travel, a solar thermal engine using this propulsion would act like a simple, efficient tugboat in space.
In the operation of a solar thermal engine, the absorber functions as a heat exchanger. Sunlight is concentrated via a lens or mirror, and then focused into the absorber cavity. This cavity is comprised of three, vacuum plasma, spray- formed coaxial shells with two, double helical flow passages through which the propellant gas flows. As the gas flows through the helical channels, it absorbs energy, expands, and then exits the nozzle. Through this process, solar energy is converted to kinetic energy-thrust. The test units built at Marshall Space Flight Center (MSFC) are designed to produce two to two-and-one-half Newtons of thrust using hydrogen as the propellant. The intended service temperature of the ground test absorber cavity is 2450° C, with an internal gas pressure of 170 kPa, using hydrogen as the working fluid.
MSFC’s Solar Thermal Facility has built the heliostat mirror, concentrator, quartz-windowed vacuum test chamber, absorber/thruster, and gaseous hydrogen plumbing. The mirror measures 20 feet by 24 feet, and is positioned 200 feet from the concentrator. During a test, the mirror follows the sun via tracking software and redirects the solar energy to an 18-foot diameter concentrator. Using 144 hexagonal reflective sections, the mirror focuses incident solar radiation through the test chamber’s front-fused silica window and into the opening of the absorber/thruster.
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