Catalytic Combustion of Rich Methane/Oxygen Mixtures for Micropropulsion Applications

§ NASA Glenn Research Center, Cleveland, OH, 44135 The military and space communities are in need of small scale propulsion and power generation devices for a new class of satellites. These devices need to be built at a relatively low cost and perform a variety of tasks with repeatable performance characteristics. In an effort to meet these goals, this study characterized the combustion of methane/oxygen propellant mixtures in platinum microtubes with inside diameters of 0.4 mm and 0.8 mm. The mixtures tested had equivalence ratios beyond the corresponding rich flammability limits, necessitating the use of a catalytic surface to aid combustion. The experimental studies were conducted at conditions that would favor 1-10 milli-Newtons of thrust as required by the next generation of satellites. Results of this study showed that with the presence of a catalyst, catalytic reactions could support combustion in mixtures even when gas phase chemistry does not play a significant role. The effects of changing equivalence ratio, pressure, mass flow rate, and tube size on the critical temperature leading to catalytic ignition were systematically investigated. Furthermore, the effects of doping the methane/oxygen mixture with hydrogen were explored, demonstrating a significant reduction in the ignition temperature with hydrogen addition. Microtube performance in terms of available thrust, specific impulse, and power required for preheating the microtube were also discussed. With a plug flow model, the experimental conditions were simulated with detailed gas phase chemistry, thermodynamic properties, transport properties, and surface kinetics. The computational results generally supported the experimental findings.