Thermo-mechanical and ballistic investigation of HTPB/paraffin wax blended solid fuels for hybrid rocket applications

This study investigates hydroxyl-terminated polybutadiene (HTPB), paraffin wax, and HTPB-paraffin blended solid fuels for hybrid rocket applications using gaseous oxygen as the oxidizer. The work addresses the central trade-off in paraffin-based hybrid propulsion: paraffin provides high regression rates through melt-layer droplet entrainment, whereas HTPB improves grain toughness, elasticity, and handling safety. Thermo-mechanical characterization was performed using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and tensile testing to assess decomposition behavior, melting transitions, tensile strength, Young's modulus, and elongation at break. Ballistic tests in a laboratory-scale hybrid rocket motor were used to evaluate regression rate, chamber pressure, thrust response, and combustion stability. The results show that pure paraffin gives the highest regression rate but is mechanically brittle, while increasing HTPB content improves structural integrity and suppresses unstable wax sloughing at the cost of lower regression rate. The blended formulations therefore provide a practical compromise between ballistic performance and fuel-grain robustness, supporting their use in compact, controllable, and safer hybrid propulsion systems, including CubeSat and small-satellite applications.