Radius of Curvature Effects on Throat Thermochemical Erosion in Solid Rocket Motors

Ablative materials are commonly used to protect the nozzle housing and to provide the internal contour to expand the exhaust gases in solid rocket motors. However, thermochemical erosion of nozzle thermal protection material results in a throat area enlargement that reduces the chamber pressure and the thrust as well as the thrust coefficient. The resulting specific impulse loss can be significant, especially for small-scale systems. To understand if throat erosion can be limited by suitable shaping of the nozzle contour, the present work addresses the study of the effect of the throat radius of curvature on the thermochemical erosion behavior of graphite nozzles for a typical metallized solid propellant. The adopted approach relies on a validated full Navier-Stokes flow solver coupled with a thermochemical ablation model based on finite-rate chemistry. A shape change analysis is also conducted to determine how different nozzle contours are modified during the advancement of the erosion process. Results allow the quantification of the overall performance increase that can be obtained by a suitable reduction of the throat radius of curvature.