Ballistic Performance of Paraffin-Based Solid Fuels Enhanced by Catalytic Polymer Degradation

Hybrid propellants are of great interest in space propulsion, but their use has been limited due to the low burning rate of their solid fuel. The use of paraffin was a breakthrough, but these grains exhibit excessive brittleness. Recently, it was proposed that the application of paraffin particles agglutinated by a polymer matrix eliminates these issues, but the high temperatures of the binder degradation still impose a limiting factor to the propulsive performance. In the present work, this challenge is overcome by employing a metallic complex as catalyst that selectively lowers the temperature of the polymer degradation, increasing the compatibility between binder and paraffin. For hydroxyl-terminated polybutadiene (HTPB) degradation, Cu, Ni, Co, and Cr acetylacetonate complexes demonstrated catalytic effects in high temperatures, whereas copper acetylacetonate stood out in lower temperatures. Test fires allowed determining the regression rate by using the weight loss method and comparing with the data available in the literature, in which paraffin and HTPB grains set the highest and the lowest limits, respectively. The addition of paraffin particles to HTPB showed some enhancement, while the inclusion of the copper catalyst substantially improved the regression rate, suggesting this fuel grain as a promising alternative for hybrid rocket motors.