Effect of metal and metalloid solid-fuel additives on performance and nozzle ablation in a hydroxy-terminated polybutadiene based hybrid rocket motor

Abstract

The performance of hybrid rocket motor can be significantly boosted by metal and metalloid solid-fuel additives. However, the ablation rate of nozzle is dramatically increased by the additives at the same time. In this study, the effect of aluminum, boron and aluminum hydride additives on the performance and nozzle ablation in a lab-scale hybrid rocket motor is investigated. The propellant combination of 95% hydrogen peroxide and HTPB-based fuels is adopted. Nine tests are operated in various oxidizer mass flux to analyze the combustion performance. The regression rate for each test is measured by the fuel mass reducing. The ablation rate of nozzle is measured by three-jaw caliper and Computed Tomography scans. Electron microscope scanning and energy spectrum analysis are carried out on nozzle surface to investigate the microscopic morphology and composition of the nozzle surface after test. Results show that the difference between the specific impulse values of the two grains with the same aluminum element content is negligible. The specific impulse of aluminum-containing grains is generally higher than that of boron-containing grains. The regression rate of fuel adding 58% aluminum is higher than the fuel adding 20% aluminum and 38% aluminum hydride by 21.8%, suggesting that aluminum has a superior regression rate improvement than that of aluminum hydride. Among these three grains, the combustion efficiency and specific impulse of the fuel adding boron is the lowest, which indicates that the combustion performance of boron as an additive is inferior to aluminum and aluminum hydride. The addition of aluminum and aluminum hydride increases the ablation rate of nozzle and aggravates the deposition of alumina particles on the inner wall of the nozzle, whereas the addition of boron nearly has no effect on nozzle ablation.