Chemical Erosion of Refractory-Metal Nozzle Inserts in Solid-Propellant Rocket Motors

An integrated theoretical/numerical framework is established and validated to study the chemical erosion of refractory-metal (tungsten, rhenium, and molybdenum) nozzle inserts in solid-rocket-motor environments, with a primary focus on tungsten. The formulation takes into account multicomponent thermofluid dynamics in the gas phase, heterogeneous reactions at the surface, energy transport in the solid phase, and nozzle material properties. Typical combustion species of nonmetallized ammonium-perchlorate/hydroxyl-terminated-polybutadiene propellants at practical motor operating conditions are considered. The erosion rates calculated by employing three different sets of chemical kinetics data available in the literature for the tungsten-steam reaction have been compared. The effect of considering either of two different tungsten oxides, WO2 or WO3, as the final product of surface reactions is also investigated. The predicted erosion rates compare well with experimental data. The oxidizing species of H2O proved more detrimental than CO2 in dictating the tungsten nozzle erosion. The material recessionrateiscontrolledbyheterogeneouschemicalkineticsbecausethediffusionlimitisnotreached.Theerosion rate increases with increasing chamber pressure, mainly due to higher convective heat transfer and enhanced heterogeneous surface reactions. The tungsten nozzle erodes much more slowly than graphite, but at a rate comparable with that of rhenium. The molybdenum nozzle exhibits the least erosion for flame temperatures lower than 2860 K. Its low melting temperature (2896 K), however, restricts applications for propellants with high flame temperatures.