Pyrolytic Graphite and Boron Nitride as Low-Erosion Nozzle Materials for Long-Duration Hybrid Rocket Testing

摘要

Results from proof-of-concept tests of a composite-material nozzle, designed for low-erosion in hybrid rockets are presented. In this design, anisotropic thermal conduction properties of two synthetically-derived materials, pyrolytic graphite and boron nitride are leveraged to produce unique low-erosion properties. Both anisotropic materials exhibit a significant difference in thermal conductivity when one compares the values perpendicular (edge-plane) and parallel (basil-plane) to the planes of deposition. In this configuration, materials are arranged to give the nozzle throat fabricated from pyrolytic-graphite a high thermal conductivity along a plane lying radially to the direction of flow, with low-conductivity plane along the axial-flow direction. The conduction axes for the surrounding boron nitride insulator are arranged orthogonally to the throat, and allow for a high level of insulation in the radial direction with the highest level of conduction along the longitudinal axis. This configuration allows heat from the motor plume to be conducted away from the nozzle throat and into the high heat capacity boron-nitride insulating layer, resulting in significantly reduced erosion rates. Test results from several long duration burns, compared against a pure-graphite nozzle of similar geometry, show a 5-fold decrease in erosion rates under similar burn conditions. Comparisons to analytical heating models are presented.