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.

Limited by the energy level of solid propellant, the specific impulse of solid rocket motor is relatively low. The helium-injected solid-gas hybrid rocket motor can effectively improve the specific impulse of solid rocket motor by injecting helium with low molecular weight and strong expansion capacity into the rocket motor. To improve the performance of the helium-injected solid-gas hybrid rocket motor, the effect of micro-hole arrays on the performance of the helium-injected solid-gas hybrid rocket motor is investigated. Results show that the specific impulse of the motor presents a parabolic trend with the variation of helium injection ratio (mass flow ratio of helium to combustion gas), and the peak value is reached when the helium injection ratio is 1∶4. The motor can obtain a maximum specific impulse gain of 5.77% with the micro-hole diameter of 2 mm. The mixed gas with low helium mass fraction provides positive gain to the specific impulse of the motor, while the mixed gas with high helium mass fraction causes negative gain due to the low total temperature and weak expansion capacity. Therefore, the specific impulse of the motor depends on the combination of the mixed gas with high and low helium mass fraction. Furthermore, motor thrust increases with the increase of helium injection ratio. By changing the helium injection ratio from 0 to 2∶1, a motor can achieve a maximum thrust regulation range of 100%–313%. The injection of helium greatly reduces the nozzle outlet temperature by up to 1 600 K. Therefore, the helium-injected solid-gas hybrid motor can effectively reduce the plume temperature of the rocket motor, suppress the infrared radiation characteristics, and achieve a better infrared stealth effect.