Computational Analysis of Radiative Heat Loading on Hydrocarbon-Fueled Rockets

Abstract

Several current and future space launch systems feature nozzle clusters which are e.g. mandatory for vertical landing and use liquid hydrocarbon fuels as propellent. The most common choices are Methane and Kerosene (RP-1) combined with liquid oxygen. Methane, in particular, is gaining attention because it is more conducive to engine reusability due to the reduced production of back carbon. RP-1 offers simplified handling and logistics as well as high energy-density. The convective base heating of these launchers at low altitudes is typically very low. This leaves radiation to be the dominant heating mode and changes at intermediate altitudes between 10 an 40km. Here, reverse flow of hot exhaust gases, driven by plume-plume interaction and the associated stagnation zones, impinges on the base region and strongly increases the relative importance of convective heating. However, these plume-plume interactions also cause local pockets of shock-heated exhaust which may in turn also increase the radiative heating. Hence, the prediction of base heating also necessitates an estimate of radiative loads caused by the emission of hot exhaust gases and particles (e.g. soot). The required computational analysis includes the modeling of the local absorption and emission properties of the participating gases and particles and, due to the typically large optical thickness of exhaust plumes, the energy transport of radiation in a self-absorbing medium. In this paper we investigate different relevant methods and simplifications. Due to the enormous complexity of emission and absorption spectra of typical exhaust species with several million spectral lines, line-by-line methods (fully resolved spectra) are generally not feasible for complex plume structures or geometries. To reduce the required spectral resolution, statistical narrow bad (SNB), opacity binning or weighted-sum of gray gases (WSGG) models are usually employed. These methods will be compared and assessed based on a simplified representative plume structure which still allows for accurate line-by-line predictions. The effect of soot particles is included in the analysis and different models (Mie, Rayleigh and geometric scattering) will be compared. Further, a SNB-model will be applied to a realistic plume of a nine-engine cluster of a Falcon 9 like configuration to quantify radiation effects at different flow conditions. One of the questions addressed is the relevance of plume interaction zones for the radiative base heating at medium altitudes. The radiative transport equation will be treated with a Photon-Monte-Carlo model, which enables accurate and computationally efficient predictions of radiative energy transport in participating (absorbing and emitting) media for complex geometries and plume structures.