Speaker
Description
High-temperature thermal radiation is a long-standing modeling challenge shared by very different classes of high-enthalpy flows, from atmospheric re-entry to chemical rocket propulsion. In rocket engines, the combustion products of solid, hybrid and liquid propellants (H$_2$O, CO$_2$, CO, soot for hydrocarbon-fuelled systems, and Al$_2$O$_3$ particles for aluminized propellants) behave as strongly non-grey participating media, and radiative heat transfer can locally contribute a significant, and in some regimes dominant, share of the total wall heat flux, affecting nozzle and combustion-chamber thermal protection design, fuel-grain regression in hybrid engines, and the aerothermal environment of reusable boosters during supersonic retro-propulsion (SRP). Over the past several years, the T(H)RUST group at Sapienza University of Rome has developed and progressively validated a common numerical framework, based on the Discrete Transfer Method (DTM), to address this problem consistently across propulsion architectures and flow regimes, rather than treating each application in isolation (1). This work aims to show how this problem can be tackled through a unified numerical framework spanning propulsion architectures and flow regimes, and how radiative heat transfer can be an extremely relevant contribution to the overall thermal loads generated by the high-enthalpy flows produced by rocket engines.
The core of our framework is an in-house DTM solver, recently released as open-source software (2), which solves the radiative transfer equation for participating medium and diffusely-reflecting walls and can be coupled, one-way or two-way, to CFD solvers. Depending on the required accuracy and affordable computational cost, gas radiative properties are modeled at different levels of fidelity: line-by-line calculations as a benchmark, statistical narrow-band (SNB) model as a practical spectral reference, gray-gas models based on mean Planck absorption coefficients, and the Weighted-Sum-of-Gray-Gases (WSGG) approach; custom WSGG formulations, calibrated up to 300 bar and 4000 K to cover liquid-rocket-engine (LRE) conditions, were also developed. Soot radiation is modeled through Rayleigh-approximation models with temperature-dependent complex refractive index, coupled, where relevant, to soot-formation models based on mixture-fraction or C$_2$H$_2$-precursor chemistry. Properties of alumina particles are computed according to Mie theory. This framework has been applied to solid-rocket-motor nozzle ablation (3), hybrid-rocket fuel regression (4,5), LRE thrust-chamber wall heat flux (6,7), and the aerothermal environment of a reusable launcher during retro-propulsion manouvers (8). As a unifying example, the same methodology is applied to estimate the radiative wall heat flux of a representative hydrocarbon-fuelled LRE across two conditions within its life cycle: steady in-chamber/nozzle operation, and the plume/recirculation environment generated during an supersonic retro-propulsion (SRP) descent, using consistent spectral and soot modeling assumptions in both cases.
For the representative hydrocarbon-fuelled LRE considered, radiative heat transfer is found to be a relevant fraction of the total wall heat flux in both operating conditions. Inside the combustion chamber, gas-phase (H$_2$O/CO$_2$) radiation typically accounts for 5-10% of the total heat flux, locally rising up to 30% in the cylindrical section and near the injection plate. Analysis of the open literature revealed a lack of reduced-order spectral models calibrated for the high pressure and temperatures typical of LRE combustion chambers, which motivated the development of new WSGG formulations validated up to 300 bar and 4000 K. Addition of soot further increases the radiative share on the total heat load.
During SRP, the same combustion products, once expelled and mixed with atmospheric air, contribute to the heating of the rocket base plate and side wall, with the radiative share of the total heat flux rising up to about 70-75% under high-thrust conditions, exceeding the convective contribution.
Radiative heat transfer modeling in high enthalpy flows and especially in propulsive systems remains a genuinely complex problem, involving strongly non-grey, high-temperature, high-pressure participating media whose accurate description generally requires computationally expensive spectral calculations. However, industrial practice, and in particular engine development, relies on extensive parametric analyses across operating conditions and design variants, for which such high-fidelity tools are rarely affordable, creating a need for reliable models with a computational cost compatible with these design phases. The unified framework and body of work presented here show that radiative heat loads are not a negligible detail in the thermal assessment of rocket propulsion systems, from combustion chambers to vehicle recovery, but also that dedicated reduced-order and global models, when properly developed and validated against spectral references over the relevant temperature and pressure range, can reproduce this contribution with good accuracy at a small fraction of the cost of full spectral calculations, making systematic, physically consistent radiative assessments practically feasible for engineering design.
(1) Francesco Nasuti et al., "Progresses in Applied Research on Liquid Rocket Propulsion by T(H)RUST Research Team at Sapienza University of Rome", 75th International Astronautical Congress (IAC), 2024.
(2) Marco Fabiani, ``GROOT.'' Zenodo, May 21, 2026, https://doi.org/10.5281/zenodo.20327076.
(3) Daniele Bianchi et al., "Numerical Modeling and Experimental Validation of Thermochemical Ablation in Solid Rocket Motor Nozzles", 11th EUCASS, 2025. https://doi.org/10.13009/EUCASS2025-739.
(4) Marco Fabiani et al., "Numerical Analysis of Gas and Soot Radiation in Hybrid Rockets with Pyrolyzing Fuels", 35th International Symposium on Space Technology and Science, 2025.
(5) Marco Fabiani et al., "Radiative Wall Heat Transfer Evaluation in Hybrid Rocket Thrust Chambers",AIAA SciTech 2024 Forum. https://doi.org/10.2514/6.2024-1608.
(6) Marco Fabiani et al., "Spectral and Global Radiative Heat Transfer Models for Liquid Propellant Rocket Engines", Journal of Propulsion and Power, Vol.41, No.5, 2025, pp.650-664. https://doi.org/10.2514/1.B39892.
(7) Marco Fabiani et al., "Reduced-order Models for Radiative Heat Loads Estimation in Liquid Rocket Engines", 76th International Astronautical Congress, Sydney, Australia, 2025.
(8) Marco Grossi et al., "Numerical Investigation of Thermal Loads during Supersonic Retropropulsion of a Reusable Launch Vehicle", 27th AIAA International Space Planes and Hypersonic Systems and Technologies Conference, Naples, Italy, 2026, https://doi.org/10.2514/6.2026-5034.
Summary
This work summarizes the efforts of T(H)RUST:a Sapienza university research Team in the field of thermal radiation, presenting a unified radiative heat transfer framework applied to combustion products of chemical rocket engines (solid, hybrid and liquid rockets). Both internal flows (combustion chambers) and external flows (e.g. re-entry maneuvers) are considered. Using a representative hydrocarbon-fuelled liquid rocket as a case study, radiation is shown to be non-negligible both in-chamber and during supersonic retro-propulsion. The results demonstrate that dedicated reduced-order models (e.g. purpose-built WSGG formulations) can capture these effects with spectral-model accuracy at a fraction of the computational cost, supporting practical, parametric radiative assessments in engine design.