End-effects regime study in full-scale and laboratory scale setups
Full-scale launch data from a cluster of three rocket engines employing a Thrust Optimized Parabolic contour nozzle and hot gas was compared with a laboratory-scale representation. The laboratory setups comprised of both three-nozzle and four-nozzle cluster aerodynamically scaled for use with cold gas. The evolving free-shock separated and restricted separated shock flow states seen in the full-scale was reproduced in the laboratory as well as an end-effects regime prior to flowing full. Acoustic pressure waveforms recorded on the base of both vehicles and behind the rocket clusters are analyzed using various statistical metrics as well as time-frequency analysis, along with the influence staggered starts have on these waveforms. Wall pressure data captured near the lip of these nozzles were compared with the acoustics and analyzed for evidence of nozzle interaction. During the end-effects regime, the nozzles produced high intensity loads and steepened waveforms given by raises the overall sound pressure level, and in both the skewness and kurtosis values of the acoustic pressure time derivative. The finding reveals a 3 dB reduction in end-effect regime loads when a stagger was introduce. However, the effects of stagger had neglible influence on the skewness and kurtosis of the acoustic pressure time derivative as they rose to the same levels, thereby demonstrating the intermittence and impulsiveness of the acoustic waveforms that form during rocket engine startup.