Steven J. Schneider

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.

Catalyzed Ignition of Using Methane/Hydrogen Fuel in a Microtube for Microthruster Applications

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 0.40-mm-diam platinum microtube is investigated experimentally. All tests are conducted in a vacuum chamber with an ambient pressure of 0.0136 atm to simulate high-altitude conditions. Experimental results show that the critical temperature needed to catalytically lightoff 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.

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

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.

Catalyzed Ignition of Bipropellants in Microtubes

This paper addresses the need to understand the physics and chemistry involved in propellant combustion processes in micro-scale combustors for propulsion systems on micro-spacecraft. These spacecraft are planned to have a mass less than 50 kilograms with attitude control estimated to be in the 10 milli-Newton thrust class. These combustors are anticipated to be manufactured using Micro Electrical Mechanical Systems (MEMS) technology and are expected to have diameters approaching the quenching diameter of the propellants. Combustors of this size are expected to benefit significantly from surface catalysis processes. Miniature flame tube apparatus is chosen for this study because microtubes can be easily fabricated from known catalyst materials and their simplicity in geometry can be used in fundamental simulations to more carefully characterize the measured heat transfer and pressure losses for validation purposes. Experimentally, we investigate the role of catalytically active surfaces within 0.4 and 0.8 mm internal diameter micro-tubes, with special emphases on ignition and extinction processes in fuel rich gaseous hydrogen and gaseous oxygen. Flame thickness and reaction zone thickness calculations predict that the diameters of our test apparatus are below the quenching diameter of the propellants in sub-atmospheric tests. Temperature and pressure rises in resistively heated platinum and palladium micro-tubes are used as an indication of exothermic reactions. Specific data on mass flow versus preheat temperature required to achieve ignition are presented.

Development and Testing of a Methane/Oxygen Catalytic Microtube Ignition System for Rocket Propulsion

This study sought to develop a catalytic ignition advanced torch system with a unique catalyst microtube design that could serve as a low energy alternative or redundant system for the ignition of methane and oxygen rockets. Development and testing of iterations of hardware was carried out to create a system that could operate at altitude and produce a torch. A unique design was created that initiated ignition via the catalyst and then propagated into external staged ignition. This system was able to meet the goals of operating across a range of atmospheric and altitude conditions with power inputs on the order of 20 to 30 watts with chamber pressures and mass flow rates typical of comparable ignition systems for a 100 Ibf engine.

Catalyzed Combustion of Bipropellants for Micro-Spacecraft Propulsion

Abstract This paper addresses the need to understand the physics and chemistry involved in propellant combustion processes in micro-scale combustors for propulsion systems on micro-spacecraft. These spacecraft are planned to have a mass less than 50 kilograms with attitude control estimated to be in the 10 milli-Newton thrust class. These combustors are anticipated to be manufactured using Micro Electrical Mechanical Systems (MEMS) technology and are expected to have diameters approaching the quenching diameter of the propellants. Combustors of this size are expected to benefit significantly from surface catalysis processes. Miniature flame tube apparatus is chosen for this study because microtubes can be easily fabricated from known catalyst materials and their simplicity in geometry can be used in fundamental simulations for validation purposes. Experimentally, we investigated the role of catalytically active surfaces within 0.4 and 0.8 mm internal diameter microtubes, with special emphases on ignition processes in fuel rich gaseous hydrogen and gaseous oxygen. Flame thickness and reaction zone thickness calculations predict that the diameters of our test apparatus are below the quenching diameter of the propellants in sub-atmospheric tests. Temperature and pressure rise in resistively heated platinum and palladium microtubes was used as an indication of exothermic reactions. Specific data on mass flow versus preheat temperature required to achieve ignition are presented. With a plug flow model, the experimental conditions were simulated with detailed gas-phase chemistry, thermodynamic properties, and surface kinetics. Computational results generally support the experimental findings, but suggest an experimental mapping of the exit temperature and composition is needed.

A Computational Study of the Ignition of Premixed Methane and Oxygen via a Hot Stream

With new priorities today that had not existed in the past, the development of new technology has caused the reevaluation of fuels that have been traditionally used. One such resource is methane which may see new implementations in engine technology. A great deal of research into non-premixed methane-air systems has been carried out, but premixed ignition has not been as studied. This study computationally focuses on the ignition of methane and oxygen mixtures in a counterflow configuration by a heated stream. Most combustion systems have non-uniform and/or non-homogenous flow, species, and temperature fields. This configuration allows for the study of both chemical kinetics as well as convective-diffusive transport effects. The effects of strain rate, pressure, and equivalence ratio on ignition are examined. A catalytic simulation code is used to generate ranges of combustion product composition and temperature when using a catalytic reactor for the hot stream. The vitiation of these into the hot stream is conducted to determine their influence on ignition. A combined case simulating the potential of a catalytic igniter is also run to confirm the feasibility of such a technology.