Further Development of Low-Erosion Nozzle Materials for Long-Duration Hybrid Rocket Burns

Results from proof-of-concept tests of a composite nozzle, designed for low-erosion in hybrid rockets are presented. Three generations of nozzles were designed, built and tested. The low-erosion configurations rely on pyrolytic graphite throat inserts, surrounded by high heat-capacity absorbing layers. In addition to testing of two types of pyrolytic-graphite as throat inserts, two insulating materials, hexagonal boron nitride and reinforced carbon fiber composite, were also investigated. The first generation was also encased in a thin steel shell. The pyrolytic graphite inserts, with high radial thermal conductivity pull heat away from the throat and into a high heat-capacity insulating layer. The pyrolytic graphite inserts protect the insulation material from the highly-oxidizing plume flow; resulting in significantly reduced throat erosion rates. Measured erosion rates are correlated with numerical predictions. Potential issues associated with thermally-induced stress build-up, were investigated using an axisymmetric thermo-structural model to analyze the transient temperature profile and resultant thermal stresses in each of the low-erosion nozzle generations. The 1st and 2nd generation nozzles with the boron nitride insulator are predicted to fail before a 15-second burn duration. In contrast, the 3rd generation design will survive intact for burns exceeding 30-seconds. This analytical result agrees well with test observations.