Numerical Analysis of Bell Rocket Nozzle Performance at Variable Ambient Pressure Conditions

This study investigates the aerodynamic performance and combustion plume structure of a baseline Rao bell rocket nozzle across launch ascent at ambient pressure conditions by Computational Fluid Dynamics (CFD) and Rocket Propulsion Analysis (RPA) methods. The baseline configuration was first analysed to investigate the variations in thrust coefficient, specific impulse, combustion plume structure, and flow separation from a sea level of ambient pressure 1 atm up to a low altitude of ambient pressure 500 Pa. Two modified geometry variants were derived to mitigate potential issues incurred at high altitudes. For example, at high altitude, the expansion ratio (ER) of the bell nozzle increased to 50 compared to 25 at low altitude. Further investigations continued with the two modified nozzle designs at various ambient altitude conditions to evaluate the nozzle geometry’s impact on its performance in order to establish a correlation of bell nozzle geometric design and ambient pressure conditions. It was found that the bell nozzle with a shortened divergent length improves aerodynamic performance at low altitude, while at high altitude, the bell nozzle with an increased divergent length produces similar performance as the Baseline Bell nozzle (BB) at sea level, which is mainly due to its ability to delay under-expansion (i.e., exhaust gas pressure < ambient pressure). The study demonstrates the performance trade-offs of a bell nozzle during a launch ascent in relation to nozzle geometries and ambient conditions. The aerodynamic performance of a bell nozzle can be influenced by altering its geometry to mitigate potential issues experienced at extreme conditions of high post-exit over-expansion with internal shocks and flow separations.