Effects of Oxidizer Species on the Transition Between the Regression Mode and the Flame-Spreading Mode

Abstract

Axial-injection end-burning hybrid rockets (AIEB-HR) operate in a combustion mode called the regression mode, which enables efficient and high-power propulsion. However, under certain conditions, a transition to flame-spreading mode (RtS transition) occurs, leading to operational failure. This study experimentally investigates the RtS transition conditions using three oxidizers: oxygen, nitrous oxide, and 33 vol% oxygen diluted with nitrogen. A total of 183 firing tests were conducted over a pressure range of 0.5–2.0 MPa and a wide range of oxidizer mass fluxes to identify the RtS transition conditions. Unlike previously proposed metrics such as friction velocity and Damköhler number, the stand-off distance showed universal scaling across different oxidizers. It is revealed that a reduction in the stand-off distance and perturbations can trigger the RtS transition, and it successfully explains experimentally observed behaviors. These findings demonstrate that stand-off distance provides a robust and physically meaningful criterion for predicting the RtS transition in AIEB-HRs.