Numerical investigation of helium injection effects on plume thermal characteristics of solid–gas hybrid rocket motors

• A coupled thermodynamic–CFD framework was developed for helium-injected hybrid rockets. • Helium injection markedly lowers the plume core temperature under sea-level conditions. • The cooling effect remains effective across the entire 0–60 km flight altitude range. • A new “Radiation Potential” metric quantifies plume infrared emission capability. • Dual suppression mechanisms—thermal cooling and radiating species dilution—jointly reduce infrared radiation. This study presents a numerical study on the plume temperature field of a novel helium-injected solid-gas hybrid rocket motor. First, a thermodynamic model based on Gibbs free energy minimization and one-dimensional isentropic flow was established to determine the initial conditions of the combustion chamber, which served as boundary inputs for a two-dimensional axisymmetric Reynolds-Averaged Navier-Stokes computational model. The numerical framework integrates the Shear Stress Transport k-ω turbulence model and the Eddy Dissipation Concept combustion model and has been validated against experimental data. This study analyzes the effects of helium injection ratio and flight altitude (0–60 km) on the plume's thermodynamic and radiative characteristics. The results show that helium injection significantly reduces the plume temperature: under sea-level static conditions, a helium-to-gas mass flow ratio of 2:1 lowers the plume core temperature by 73%. This cooling efficacy is maintained at high altitudes, with a 1:4 injection ratio achieving maximum temperature reductions of 18.4–22.0% in the 10 to 60 km altitude range. This paper introduces a "Radiation Potential" metric, which couples local temperature with the concentration of radiating species (H₂O, CO₂). This metric reveals a dual suppression mechanism: at a 2:1 injection ratio, the maximum RP is reduced by 99.87% (nearly three orders of magnitude) due to the synergistic effect of direct cooling and the dilution of infrared-active gases. This research demonstrates that helium injection is a highly effective active suppression technology, providing key insights for the low-observability design of advanced propulsion systems. This is particularly crucial for advanced tactical and strategic missiles to enhance their survivability and penetration capabilities against the increasing prevalence of infrared-guided threat systems.