Chemical erosion of refractory metal nozzle inserts in solid-propellant rocket motors

An integrated theoretical/numerical framework is established to study the chemical erosion of refractory metal (tungsten, rhenium, molybdenum) nozzle inserts in solid rocket-motor environments. The numerical framework takes into account propellant chemistry, detailed thermofluid dynamics, heterogeneous reactions at the nozzle surface, and nozzle geometry and material properties. The gas-phase flame dynamics is based on the complete conservation equations for multi-component reacting system. The energy equation is solved for the process in the nozzle material with appropriate species and energy boundary conditions at the gas-solid interface. Typical combustion species of non-metallized AP/HTPB at practical motor operating conditions are considered at the nozzle inlet. Heterogeneous reactions involving refractory metals and the oxidizing species of H₂O and CO₂ are considered at the interface. Three different sets of chemical kinetics data available in the literature for tungsten and steam reaction at high temperatures have been studied. The erosion rate increases with increasing chamber pressure, mainly due to higher convective heat transfer and enhanced heterogeneous surface reactions. The predicted erosion rates compare well with experimental data. Tungsten nozzle erosion is much lower than graphite nozzle, but higher than rhenium nozzle. For lower flame temperatures (2860 K), the least erosion was exhibited by the molybdenum nozzle. On account of its low melting temperature, the use of molybdenum is restricted to non-metallized propellant environment.