Solid-fuel regression rate behavior of vortex hybrid rocket engines

A series of static engine firings were conducted to investigate the solid-fuel regression rate behavior and operating characteristics of vortex hybrid rocket engines. The vortex hybrid engine configuration is characterized by a coaxial, coswirling, counterflowing vortex combustion field in a cylindrical fuel port. To generate this flowfield, oxidizer is injected through a swirl injector located between the aft end of the fuel grain and the inlet to the converging portion of the exit nozzle. Test firings with thrusts up to 960 N were conducted with gaseous oxygen and hydroxyl-terminated polybutadiene solid fuel. Average fuel regression rates up to seven times larger than those in similar classical hybrids were measured. Empirical correlations were developed to describe accurately the experimental regression rates over more than an order of magnitude variation in mass flux. In addition to local mass flux and oxygen injection velocity, geometric engine variables, such as engine contraction ratio and length-to-diameter ratio, had a significant influence on the measured regression rates. Nondimensional regression rate and heat transfer correlations were also developed. Throttling and restart capability were demonstrated.