Propulsion tests on ultra-high-temperature ceramic matrix composites for reusable rocket nozzles

Ultra-high-temperature ceramic matrix composites (UHTCMCs) based on a ZrB₂/SiC matrix have been investigated for the fabrication of reusable nozzles for propulsion. Three de Laval nozzle prototypes, obtained by sintering with either hot pressing (HP) or spark plasma sintering (SPS), were tested 2–3 times in a hybrid rocket motor for proving reusability. Sections were extracted after oxidation tests to study the microstructural changes and oxidative and thermomechanical stresses induced by the repeated tests. Compared to a reference graphite nozzle, no measurable erosion was observed for the UHTCMC-based nozzles. The oxidation mechanism consisted in the formation of a ZrO₂ intermediate layer, with a liquid silicon oxide (SiO₂) layer on the surface that was displaced by the action of the gas flux towards the divergent part of the nozzle, protecting it from further oxidation. Both specimens obtained by HP and SPS displayed similar performance, with very slight differences, which were attributed to small changes in porosity. These tests demonstrated the capability of complex-shaped prototypes made of the developed UHTCMCs to survive repeated exposure to environments representative of a realistic space propulsion application, for overall operating time up to 30 s, without any failure nor measurable erosion, making a promising step towards the development of reusable rocket components.