Influence of Chamber Pressure, Oxidizer Port Velocity, and Port Diameter on Combustion-Mode Transition in a Liquid Oxygen - Solid Fuel Duct

This study investigates the combustion-mode transition from stabilized combustion to flame spread in a liquid oxygen–solid fuel duct, representing an axial-injection end-burning hybrid-rocket configuration. A total of 61 combustion tests were conducted while independently varying chamber pressure (0.5–3.3 MPa), oxidizer port velocity (1.3–5.7 m/s), and port diameter (2.0–4.0 mm). High-speed visualization enabled clear identification of two distinct combustion modes based on flame behavior and propagation rate. The resulting combustion-mode maps demonstrate that flame spread is promoted at higher chamber pressures, lower oxidizer port velocities, and larger port diameters. These trends are consistent with prior findings under gaseous-oxidizer conditions. A conceptual transition model is introduced based on observed flame behavior, suggesting that operating-condition-dependent flame anchoring near the port exit governs the onset of flame spread. The established maps offer practical guidance for ensuring stable end-burning operation in liquid oxygen-based hybrid rockets.