Catalytic Ignition of Methane/Hydrogen/Oxygen Mixtures for Microthruster Applications

Abstract

Catalyzed combustion of propellants in a microtube serves as a model of a microthruster that has potential applications for micropropulsion for small satellites/spacecraft. The effect of hydrogen addition on fuel-rich methane/oxygen ignition within a 0.4 mm diameter platinum microtube is investigated experimentally and numerically. All tests are conducted in a vacuum chamber with an ambient pressure of 0.0136 atm to simulate the high-altitude conditions. Experimental results show that the critical temperature needed to catalytically light-off fuel-rich methane/oxygen mixtures is reduced by the addition of small amounts of hydrogen to the mixture. Two-stage ignition phenomena are observed for low levels of hydrogen addition (2-7% by volume), with the first and second ignition conditions corresponding to the reactions of hydrogen and methane, respectively. The effects of changing flow rate (residence time), equivalence ratio, and amount of hydrogen addition on the critical ignition temperature are investigated. The ability of the catalyst to sustain chemical reactions once the input power is turned off is also explored, and for most cases self-sustainability is realized. Various microtube performance parameters are estimated for all experiments, which include thrust, specific impulse, and power required to ignite reactions within the microtube. Steady-state and transient numerical models with detailed gas-phase and surface chemistry are used to provide insight into the different ignition phenomena observed experimentally.