Devices that generate both high energy and high temperature are required to ignite reliably the propellant mixtures in combustion chambers like those present in rockets and other combustion systems. This catalytic microtube rocket igniter generates these conditions with a small, catalysis-based torch. While traditional spark plug systems can require anywhere from 50 W to multiple kW of power in different applications, this system has demonstrated ignition at less than 25 W. Reactants are fed to the igniter from the same tanks that feed the reactants to the rest of the rocket or combustion system. While this specific igniter was originally designed for liquid methane and liquid oxygen rockets, it can be easily operated with gaseous propellants or modified for hydrogen use in commercial combustion devices.
For the present cryogenic propellant rocket case, the main propellant tanks — liquid oxygen and liquid methane, respectively — are regulated and split into different systems for the individual stages of the rocket and igniter. As the catalyst requires a gas phase for reaction, either the stored boil-off of the tanks can be used directly or one stream each of fuel and oxidizer can go through a heat exchanger/vaporizer that turns the liquid propellants into a gaseous form. For commercial applications, where the reactants are stored as gases, the system is simplified. The resulting gas-phase streams of fuel and oxidizer are then further divided for the individual components of the igniter.
One stream each of the fuel and oxidizer is introduced to a mixing bottle/apparatus where they are mixed to a fuel-rich composition with an O/F mass-based mixture ratio of under 1.0. This premixed flow then feeds into the catalytic microtube device. The total flow is on the order of 0.01 g/s. The microtube device is composed of a pair of sub-millimeter diameter platinum tubes connected only at the outlet so that the two outlet flows are parallel to each other. The tubes are each approximately 10 cm long and are heated via direct electric resistive heating. This heating brings the gasses to their minimum required ignition temperature, which is lower than the auto-thermal
ignition temperature, and causes the onset of both surface and gas phase ignition producing hot temperatures and a highly reacting flame.
The combustion products from the catalytic tubes, which are below the melting point of platinum, are injected into the center of another combustion stage, called the primary augmenter. The reactants for this combustion stage come from the same source but the flows of non-premixed methane and oxygen gas are split off to a secondary mixing apparatus and can be mixed in a near-stoichiometric to highly lean mixture ratio. The primary augmenter is a component that has channels venting this mixed gas to impinge on each other in the center of the augmenter, perpendicular to the flow from the catalyst. The total cross-sectional area of these channels is on a similar order as that of the catalyst. The augmenter has internal channels that act as a manifold to distribute equally the gas to the inward-venting channels. This stage creates a stable flame kernel as its flows, which are on the order of 0.01 g/s, are ignited by the combustion products of the catalyst. This stage is designed to produce combustion products in the flame kernel that exceed the autothermal ignition temperature of oxygen and methane.
While these combined components will mix and produce a near stoichiometric flame with a temperature high enough to ignite the reactants in most combustion devices, the overall mass flow rate and energy is still relatively low. For the extreme conditions of igniting a cryogenic propellant chemical rocket, this total may not be enough to maintain a flame in the adverse environment. To enable this operation, another gas phase stage called the secondary augmenter is added in series with the first two components. As more heat release is required, the mass flow rate is increased by an order of magnitude to more than 0.1 g/s for this stage. The flows are kept separate, however, until injected where they impinge and mix within this secondary augmenter. Again, the flows are distributed via a manifold system then injected through ports that are sized more than an order of magnitude larger than the total port area of the first two components. The mixture is kept fuel rich so that the temperature is regulated below the melting point of the components. With the ignition of this stage, a large stable torch is produced to ignite the cryogens.
The hardware is designed so that the total size of the device was similar to that of a traditional spark plug. Likewise, the outlet of the igniter mimics that of a spark plug in order to have it act as a direct replacement in combustion devices. In tests it functioned as such, lighting chambers with propellant flows an order of magnitude larger. Operation was demonstrated with back pressures as low as 0.01 atmospheres up to approximately 10 atmospheres and in theory, these bounds could be wider. Ignition was demonstrated with reactant temperatures near chilled-in cryogenic conditions. This igniter serves as a low-energy alternative to spark ignition and can operate as an ignition source for a variety of commercial combustion devices.
This work was done by Steven J. Schneider and Matthew C. Deans of Glenn Research Center.
Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steven Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-18565-1.