Studies on Combustion Instability for Liquid Propellant Rocket Engines

To build a framework of a prediction tool for injection-coupled combustion instability with coaxial-type injectors, Hutt and Rocker’s methodology including Crocco’s n-τ model was applied. This linear stability analysis considers a coupling among LOX/fuel flow-path acoustics, chamber responses, and Crocco’s combustion characteristics. To validate the tool, LOX/methane subscale firing tests, which were performed and reported by NASA, were analyzed. The injection-coupled oscillating combustion, which was occurred at 5 kHz, was selected as unstable case. A stable combustion case was also selected for comparison purposes. Injection and combustion characteristics, which amplify the oscillation, were found to be dominated by the LOX-post acoustic characteristics. Chamber responses, which decay the oscillation, were estimated with two approaches: (a) onedimensional acoustic analysis with Natanzon’s methodology with a short-nozzle approximation, and (b) threedimensional acoustic analysis with a commercial software, ACTRAN. The stability was evaluated with AFC (amplitude-frequency characteristics) diagram, in which the injection and combustion characteristics and chamber responses are compared with regard to amplitudes. As a result, significant differences were not seen in the AFC diagrams between the unstable and stable cases (both analyses showed “unstable”). Further investigations for the chamber responses are needed to evaluate the potential instabilities of the system, correctly. In addition, we need to introduce a phase relationship into the tool to understand the underlying physical phenomena.