Measurement and analysis of an unstable model rocket combustor

Experimental data from a longitudinally unstable uni-element model rocket combustor are analyzed using two levels of modeling. Linear acoustic model results showed that perfect acoustic coupling between the injector oxidizer tube and the combustion chamber is not necessary to produce unstable combustion. At a chamber length of 50.8 cm (20-in) perfect coupling was predicted at the chamber second longitudinal mode frequency, but tests were unstable at the first longitudinal mode. Linear growth rates were iteratively determined by fitting the output from a four-mode, longitudinal stability model including nonlinear gas dynamics to the temporal and spectral characteristics of measured pressure oscillations from a single unstable test. Results showed that both the first and second longitudinal modes must be linearly unstable. Acoustically induced vortex shedding was postulated as the driving instability mechanism in the experiment. Two modified experiments were designed: the first had a decreased injector face width, an assumed controlling geometry for the mechanism, and the other was outfitted with an array of point light emission measurements. Test data showed that the injector face width had a drastic influence on stability. At a chamber length of 25.4 cm (10-in) and injector face width of 0.76 cm (0.30-in) the magnitude of pressure oscillations increased to 40% of mean chamber pressure from 2% at a face width of 1.1 cm (0.43-in). Highly unstable tests showed distinct phase relationships between light emission and pressure and their spatial variation indicated the propagation of a light source. The location of combustion driving varied with chamber length. The oscillation decrement, a useful parameter for estimating stability margin from dynamic pressure, is described along with its application to full-scale test data. Four injector configurations shown to have noticeably different oscillation decrements were tested to verify the uni-element scaling methodology. The oxidizer tube length and axial inlet design, subsonic jet and choked, were the main design variables. Analysis of test data showed that the choked inlet does not behave as an ideal closed acoustic boundary. The most stable injector configuration had a subsonic jet inlet and a 19.1 cm (7.5-in) oxidizer tube length in agreement with full-scale data