Laser Induced Spark Ignition of Gaseous and Liquid Rocket Propellants

<p dir="ltr">The desire for an expanded space economy motivates the integration of new technologies into modern space vehicles. The shift to reusable launch configurations puts a greater emphasis on hardware simplicity, reliability, durability, and cost effectiveness. Laser based ignition systems are reliable, light weight, involve no moving parts, provide unlimited relights, and can remove entire fluid systems to reduce vehicle complexity. Successful implementation of this method requires a detailed understanding of interaction between the pre-ignition flow field, plasma deposition, and flame kernel development.</p><p dir="ltr">In this work, the transient ignition process initiated by a laser induced spark is experimentally studied. Two rocket combustors with single element shear-coaxial injectors are tested with gaseous and liquid phase propellants. The nozzle exit pressure of each test article can be reduced to sub-atmospheric conditions with a vacuum ejector system to simulate in-space ignition. The location of energy deposition is positioned throughout each pre-ignition flow field. High-speed imaging and laser diagnostics are used to observe the ignition event.</p><p dir="ltr">Tests performed in a gaseous oxygen/methane combustor at atmospheric nozzle exit conditions are presented. Repeated tests at multiple locations create an understanding of the spatial influence on ignition probability relative to the coaxial jet. Tests initiated within the jet (direct ignition) lead to 100% ignition success. Tests initiated in the recirculation zone (indirect ignition) rely on a hydrodynamic ejection to transport energy to the central reactant jet. These cases result in variable ignition outcome which is found to be correlated to ejecta velocity and deposition energy.</p><p dir="ltr">Additional tests are performed at this condition with high-speed particle image velocimetry, schlieren, and OH* chemiluminescence. Key mechanisms which control the laser ignition process in this combustor are identified. Parameters of important flow features are extracted from images and statistically analyzed using nonlinear regression. The hydrodynamic ejecta are found to have the strongest effect on ignition outcome near the injector face, while turbulence dictates outcome farther downstream. The flame is found to grow through regions of the jet with highest turbulent kinetic energy.</p><p dir="ltr">The pre-ignition flow field of the gaseous oxygen/methane combustor is altered by reducing the nozzle exit pressure to a sub-atmospheric condition. This creates an underexpanded coaxial jet with oblique shocks and expansion fans. The amount of energy deposited within the jet is found to correlate with the local pressure. This directly impacts ignition outcome. Ignition within the recirculation zone is 100% successful due to an optimal mix of reactants at the time of laser introduction. The resulting spatial ignition probability trends are opposite to the atmospheric case.</p><p dir="ltr">The work concludes with the investigation of laser ignition in a gaseous methane liquid oxygen combustor with a sub-atmospheric exit nozzle pressure. The expanding supersonic methane flow confines the liquid oxygen spray, reducing the extent of flashing due to the low chamber pressure. This interaction is optimized to produce gaseous oxygen in the reactant shear layer. Ignition events are studied with high-speed schlieren, chemiluminescence and Mie scattering imaging. Ignition tests within the recirculation zone produce a flame kernel which grows through the low velocity reactants. Tests initiated in the shear layer result in a flame kernel that is advected downstream before propagating upstream toward the injector face. The structure of the spray local to the shear layer deposition is found to correlate with ignition outcome.</p>