LOX Injector Geometry Changes to Reduce the Risk of Injection-Coupled Combustion Instabilities in a Cryogenic Rocket Thrust Chamber

Abstract

High-frequency combustion instabilities present a severe risk in the development of liquid propellant rocket engines. Since the detailed coupling mechanisms leading to the instabilities are still not fully understood today, the problem is usually addressed by adding damping elements such as injector baffles or resonators to the combustion chamber. However, their design is mostly based on experience and the damping characteristics are very difficult to predict. Therefore it is necessary tune the resonators and verify sufficient damping with expensive and complex full-scale engine tests. At the Institute of Space Propulsion of DLR the research combustion chamber BKD which shows self-excited combustion instabilities is used to gain a better understanding of the underlying coupling mechanism. As was found out in previous publications, in BKD the combustion instabilities are driven by injection-coupling. A phenomenon which has been reported to also lead to combustion instabilities in cryogenic full-scale engines using shear coaxial injection elements, as the LE-X or J-2S. An improved method to damp injection-coupled instabilities, by directly damping the injector acoustics instead of the chamber pressure oscillations has been developed and tested at representative operating conditions. The principle of the damping method is described in this paper. The damping elements have been installed in an additive manufactured part of the injector head and had no measurable influence on the performance. Test results are presented which imply that the flame dynamics were reduced by damping the injector resonance frequencies.