Numerical Analysis of Nozzle Material Thermochemical Erosion in Hybrid Rocket Engines

Ablative materials are commonly used to protect the nozzle metallic housing and to provide the internal contour to expand the exhaust gases in both solid and hybrid rockets. Because of interaction with hot gas, these materials are chemically eroded during rocket firing, with a resulting nominal performance reduction. The objective of the present work is to study the erosion behavior of graphite nozzles in hybrid engines at different operating conditions and compare results with those obtained for solid motors. A main distinctive feature of hybrid engine operating conditions is, in fact, a greater concentration of oxygen-containing combustion products than solid motors. The adopted approach relies on a validated full Navier-Stokes flow solver coupled with a therm chemical ablation model that takes into account heterogeneous chemical reactions at the nozzle surface, rate of diffusion of the species through the boundary layer, ablation species injection in the boundary layer, heat conduction inside the nozzle material, and variable multispecies thermophysical properties. The parametric analysis performed in this study allows one to assess the impact of various parameters that affect the nozzle erosion rate, taking into account various combinations of fuels and oxidizers operating at different conditions.