Effect of surface roughness and radiation on graphite nozzle erosion in solid rocket motors

A numerical analysis is performed to study the effect of surface roughness and radiation on graphite nozzle erosion in solid rocket motors loaded with non-metalized ammonium perchlorate/hydroxyl-terminated polybutadiene composite propellants. An integrated theoretical/numerical formulation established previously to predict the chemical erosion of nozzle material has been extended to investigate the effects of surface roughness and radiation. The framework takes into account propellant chemistry, detailed thermofluid dynamics for a multi-component flow, energy transport in the solid phase, heterogeneous reactions at the nozzle surface, and nozzle material properties. Typical combustion species of non-metalized composite propellant at practical motor operating conditions are considered at the nozzle inlet. The two-layer turbulence model has been modified to account for surface roughness. The contribution from radiative heat transfer is included in the overall energy balance at the gas-solid interface. Results show that the heat-transfer rate to the nozzle material is enhanced for a rough surface, leading to a higher erosion rate than that for a smooth surface. For non-metalized propellant, the erosion rate has been found to decrease slightly due to radiation, and change in the rate strongly depends on the gas-phase emissivity.