Numerical investigation on thrust gain mechanism of helium injected solid–gas hybrid rocket motor

The specific impulse of solid rocket motors is much lower than that of liquid rocket motors due to limitations in solid propellant formulations. Inspired by the nuclear thermal rockets, the concept of helium injected solid–gas hybrid rocket motors (SGHRMs) is innovatively proposed and its thrust performance is numerically investigated in the present study. The injected helium is regarded as a working medium with strong expansion capacity, and the high-temperature combustion gas of the solid propellant is used as a heat source to heat the helium. Then, the mixed gas including the combustion gas and helium flows through the nozzle producing high boost thrust. Results show that the maximum specific impulse gain is up to 4.92%, and by adjusting the helium injection ratio from 0 to 2:1, the thrust regulation range of 100%–303% is achieved. When the helium is injected from the motor head, mixed gas with various helium fraction exhibits stratified flow characteristics. Hence, the mechanism of specific impulse gain can be elucidated by one-dimensional internal ballistics analysis. That is, mixed gas with a low helium mass fraction can significantly stimulate velocity gain with a slight reduction in total temperature, thereby increasing the specific impulse. However, mixed gas with a high helium mass fraction significantly reduces the total temperature, leading to a decreased expansion ability and a corresponding drop in the specific impulse. Finally, competition between mixed gases with various helium fractions determines the specific impulse gain level of SGHRMs.