Effect of Characteristic Chamber Length on c* Efficiency in CAMUI Hybrid Rockets Using Hydrogen Peroxide

Hybrid rocket engines using hydrogen peroxide (H2O2) are promising for small-satellite kick-motor applications due to their safety and storability, but their combustion characteristics are strongly affected by oxidizer concentration. This study investigates the influence of characteristic chamber length L*, on combustion efficiency and stability in CAMUI-type hybrid rockets using 80 wt% and 70 wt% H2O2 under non-catalytic spray injection. For the 80 wt% H2O2, stable combustion was achieved for nozzle throat diameters up to 4.25 mm, allowing systematic variation of L* in the stable regime, whereas larger throats resulted in blowout. The c* efficiency increased with L* and reached a plateau at approximately 4–5 m, indicating a practically sufficient residence time for complete droplet vaporization and mixing. In contrast, all tests with 70 wt% H2O2 exhibited strong low-frequency combustion instability, preventing meaningful evaluation of c* efficiency. Analysis of pressure fluctuations showed that reduced flame temperature and slower droplet evaporation weakened the recirculation-driven stagnation-point combustion essential for stable CAMUI operation. These findings clarify the concentration-dependent combustion behavior of H2O2-hybrid systems and provide design guidelines for future compact kick-motor development.