Effects of Characteristic Chamber Length on c Efficiency in CAMUI-Type Hybrid Rockets Using Hydrogen Peroxide*

This study experimentally investigated the effect of characteristic chamber length, L*, on combustion efficiency and stability in CAMUI-type hybrid rockets using 70 wt% and 80 wt% hydrogen peroxide under non-catalytic spray-injection conditions. Combustion tests were conducted by systematically varying L* through changes in the nozzle throat diameter while maintaining the combustor volume constant. For both oxidizer concentrations, the characteristic exhaust velocity efficiency, ηc*, increased with increasing L*. The 70 wt% cases required a larger L* than the 80 wt% cases to achieve comparable efficiency, and flame blowoff occurred in the low-L* region. The normalized RMS pressure fluctuation was also larger for the 70 wt% cases, particularly in the low-L* region, indicating lower combustion stability. These results indicate that reducing the hydrogen peroxide concentration increases the L* required to maintain stable and efficient combustion. As a key outcome of this study, stable and efficient combustion of 70 wt% hydrogen peroxide was demonstrated without catalytic assistance when a sufficiently large L* was provided. These results demonstrate the capability of the CAMUI-type combustor to extend stable operation toward lower oxidizer concentrations and experimentally clarify the concentration-dependent L* requirement as a practical design guideline for catalyst-free hydrogen peroxide hybrid rockets.