Role of Thermal Radiation in HTP-based Small-Scale Hybrid Rocket Engines

Thermal radiation is acknowledged as a contributing mechanism in hybrid rocket engines, yet its quantitative impact across different operating regimes, particularly at small geometric scales, has not been systematically assessed. In this work, the role of radiative heat transfer in a 10 N-class hybrid rocket engine is investigated through a coupled Computational Fluid Dynamics and Gas-Surface Interaction framework, including a participating-medium radiation model that accounts for gas-phase radiation from CO 2 and H 2 O. Numerical simulations are validated against experimental firing tests and used to examine how chamber pressure, propellant combination, and engine size influence the wall heat balance and fuel regression behavior. For hydrogen-peroxide-based configurations operating in the 10 N thrust class, radiation provides a limited contribution to wall heating, remaining below 5% of the total wall heat flux under practical operating conditions. Increasing chamber pressure enhances the radiative heat flux but does not substantially alter its relative weight within the accessible operating envelope. When the same configuration is operated with gaseous oxygen, the radiative fraction increases from 4.5% to 17.5%, reflecting the strong influence of combustion temperature on radiative heat transfer even at small scale. A comparison with a dynamically comparable 1 kN-class firing condition further indicates that larger characteristic dimensions are associated with a rise of the radiative contribution up to 28.1%, consistently with the increased optical thickness of the configuration. Overall, the study establishes a consistent framework to identify when radiative heat transfer can be neglected in small-scale hybrid rocket engines and when it must be explicitly accounted for in this propulsion class. The results contribute to addressing the limited characterization of radiative effects in small-scale hybrid propulsion systems and provide a validated reference for the design and modeling of CubeSat-class hybrid rocket engines.