Comparative analysis of flow field and thrust behaviour of plug nozzles in continuum vs. rarefied conditions

Abstract

Plug nozzle of a single-stage-to-orbit launch vehicle moves through continuum to rarefied regime, as it approaches high altitudes. Despite multiple studies on plug nozzle flows in the rarefied regime, there is significantly less work that compares continuum and rarefied flows through plug nozzles. Even the works that conducted such comparative analysis did not consider the effect of varying nozzle pressure ratios (NPR). The present work compares flow field and thrust performance of a plug nozzle in continuum and rarefied regimes over a range of NPR. In the continuum domain, the exhaust jet displays typical characteristics such as shocks, expansion fans and flow separation. However, the influence of viscosity becomes more noticeable in the rarefied regime. Furthermore, some consequences of flow expanding outside the boundaries of the nozzle walls are noted. The nozzle thrust coefficients exhibit a comparable pattern in both the continuum and rarefied regimes, but noticeable disparities in magnitude are seen. The significant disparity in the thrust coefficients can be attributed to two factors: low mass flow rates and elevated frictional losses. In an effort to eliminate the impact of low mass flow rates, the computed specific impulse values provide insight into the effect of frictional losses on thrust performance of a plug nozzle operating in a rarefied domain.