A three-dimensional-printed solid fuel grain with a star-fractal swirl port was developed to improve the fuel regression rate of hybrid rockets. The printing material used for the grains was an acrylonitrile-butadiene-styrene resin. Nitrous oxide was employed as the oxidizer. Combustion experiments on the hybrid rocket engine with the developed grain showed that the star-fractal swirl port appeared to improve the thrust and specific impulse compared to the classical circular and star-fractal ports when tested using the same oxidizer mass flow rate that is a controllable parameter of hybrid rocket engines. After the experiments, the burned grains were cut and the grain web thicknesses were measured to evaluate the local regression rate. It was found that the fuel surface regressed more rapidly near the concave parts of the star fractal rather than the convex parts. Next, the measured local regression rates were axially averaged. The axially-averaged local regression rates of the star-fractal swirl port were much higher than those of the star-fractal and circular ports. Finally, a numerical approach using the level set method for evaluating time- and space-averaged regression rate was developed. This approach was validated by comparing the space-averaged regression rates with the axially-averaged local rates.
Regenerative cooling using hydrocarbon fuel requires thermal decomposition to increase heat absorption, but a high temperature is required to initiate decomposition and catalysis is expected to enhance decomposition at lower temperature regime. The effects of catalysis on thermal decomposition and coking of normal-dodecane and methyl-cyclo-hexane within a tube heater were experimentally investigated. Platinum (Pt) was spread on the inner surface of a stainless tube, and the inner surface of the tube was heated to target values using an electric heater. Decomposed components were accumulated and analyzed, and heat input was evaluated based on temperature gradient within the stainless tube. The results were compared with those without tube coating to show that the Pt-coating lowered the onset temperature of decomposition by 100 K for both fuels. Additionally, heat absorption per decomposed fuel amount was affected by the Pt-coating and the Pt-coating sizably reduced coking generation.