Regenerative Cooling of Graphite Nozzles for Throat Erosion Suppression

This study concerns the first successful demonstration of a regenerative cooled graphite nozzle for hybrid rockets and its combustion characteristics. Calculations show that both liquid oxygen and nitrous oxide are effective coolants for hybrid rockets. Liquid oxygen is selected for initial experiments to avoid concerns related to the effects of thermal decomposition of nitrous oxide. Four experiments are conducted with varying flow rates of liquid oxygen using a laboratory-scale hybrid rocket equipped with a metallic nozzle retainer containing 3D-printed cooling channels to enable regenerative cooling of graphite nozzle inserts. Nozzle erosion does not occur in any of the experiments, which are based on conditions in previous research without regenerative cooling in which substantial nozzle erosion (10-25% increases in throat diameter) occurred. Relationships between liquid oxygen flow rate and nozzle temperature, as well as between liquid oxygen flow rate and liquid oxygen temperature after being used for cooling, are obtained and correlations are confirmed.