Experimental investigation of regenerative cooling performance in hybrid rocket engines

An experimental investigation regarding the reliability and feasibility of a regenerative cooling system in hybrid rocket engines is presented. The novelty of the work is the introduction of a regeneratively cooled carbon-based nozzle throat using liquid oxidizer, for thermal management of the detrimental heat fluxes developed in the nozzle. The benefits and drawbacks of using regenerative cooling based on the observations of the current experimental campaign are critically discussed. Cryogenic oxygen in subcritical conditions is used as a coolant and oxidizer, for combustion with a fuel grain of high-density polyethylene stored within the combustion chamber. Pressure and temperature measurements are taken from multiple locations in the oxygen feed line and within the graphite nozzle throat. Eight firing tests were performed under various oxidizer mass flow rates and thus various thrusts. It was confirmed that cooling the graphite throat prevented the occurrence of nozzle erosion in all eight firing tests. The heat transfer rate and the physical state of the coolant are evaluated indirectly. Results show that increasing coolant gasification correlates with decreasing mass flow rate. The cooling performance improved more with higher vapour fractions than larger mass flow rates, however, dry-out occurred in tests with the highest gasification level. In summary, is it shown that while the current regenerative cooling system will supress nozzle erosion and limit nozzle temperatures in long duration firing, it reduces the predictability of flow rate and thus thrust during operations.