Microwave Electrothermal Thrusters for Improved Satellites and Spaceflight Published May 21, 2009 By Maria Callier Air Force Office of Scientific Research ARLINGTON, Va. -- AFRL-funded research of microwave electrothermal thruster (MET) technology may lead to higher-performing miniaturized electric propulsion systems for satellites and other spacecraft. The Pennsylvania State Universitybased team found that using a thruster powered by a microwave generator creates a localized plasma (without electrodes) that heats a propellant gas--a reaction which, in turn, causes nozzle expansion. This technology is important to weather radar, aircraft radios, and a variety of communications applications. Further, a fully tested, spaceflight-ready mini-MET system would mean improved satellite maneuverability, endurance, and fabrication. With just 100 W of power, the 2 in. long, 1.25 in. diameter MET generated the highest-ever thrust for a low-power electrothermal thruster. Requiring just one-third the propellant used by other systems, the technology could potentially reduce the onboard power requirements of satellites, as well as extend their productive life cycle. In addition, the miniature MET is safer than chemical-based systems, because the thruster operates only when the magnetron produces a microwave. When the magnetron is turned off, the plasma needed for heating the fuel is inoperable as well. Focused on storable propellants such as hydrogen, nitrogen, ammonia, helium, hydrazine, and water, the team's work most recently turned towards optimization of thruster operations at 8 GHz and 350 W of microwave power. With increasing performance, however, comes ever more extreme conditions: at this level of operation, for example, chamber temperatures effectively double when an ammonia propellant is involved. Consequently, a search is now under way to identify other nozzle materials--either metal or ceramic--that exhibit the necessary characteristics: high melting temperature, sufficient structural strength, and low thermal conductivity. The lab-sponsored MET has thus far undergone ground test. In order to realize its ultimate goal, the research team plans to design and fabricate a spaceflight-ready MET for testing in the harsh extremes of space.