Numerical investigation on effect of helium on solid–gas hybrid rocket motor with AP/HTPB propellant

A surface pyrolysis and gas-phase combustion of the Ammonium Perchlorate (AP)/Hydroxy Terminated Polybutadiene (HTPB) composite propellant reaction kinetic mechanism with five-step chemical reaction is adopted. The effects of helium injection on the burning rate and combustion of AP/HTPB propellant are analyzed in details, and the characteristics of motor performance are obtained. The numerical simulation results demonstrate that helium injection enhances the combustion chamber pressure, thereby increasing the burning rate of propellant. However, the primary combustion reaction of the AP/HTPB propellant takes place within a thin layer on the burning surface, so the low-temperature helium has minimal impact on the gas-phase combustion. Ultimately, the helium not only elevates the nozzle exit velocity, resulting in specific impulse gain, but also reduces the exhaust plume temperature. With an increase of helium mass flow rate, the area of the velocity increase zone at the nozzle exit continuously decreases, but the average velocity in the motor exit continuously increases. Overall, when the helium flow rate is 2.5 kg/s, the specific impulse can reach 10.5%. Reducing the helium injection hole diameter enhances mixing of helium and combustion gas and expands the velocity increase zone, thereby maximizing the exit velocity gain in average velocity at the nozzle exit. When the injection hole diameter is reduced from 100 mm to 20 mm, the specific impulse gain increases from 3.1% to 10.6%. Furthermore, increasing helium injection temperature greatly boosts the velocity of the mixed gas with the same helium mass fraction ultimately improving specific impulse.