Paraffin-Based Fuels and Energetic Additives for Hybrid Rocket Propulsion

Energetic additives for hybrid rocket propulsion, investigated at the Space Propulsion Laboratory (SPLab) of Politecnico di Milano, are discussed in this paper in order to estimate the potential increase of hybrid rocket performance. Traditional HTPB-based and paraffin-based fuels are considered, with special emphasis to the second ones. For fuel formulations based on hydroxyl-terminated polybutadiene (HTPB), the attention is focused on innovative, mechanically activated Al-polytetrafluoroethylene (PTFE) composites, produced and characterized at SPLab. The ballistics of fuel is investigated in a micro-burner by a time-resolved technique using gaseous oxygen. Al-PTFE composites show promising results in terms of enhanced ignition reactivity and rf. Paraffin-based solid fuels filled with lithium aluminum hydride (LiAlH4, LAH) are then investigated. Two different formulations containing 5% and 10% of a styrene-based thermoplastic elastomer (Polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene, grafted with maleic anhydride) for the strengthening of paraffin waxes, with LAH 5% mass fractions are considered. The paraffin-based blends filled with LiAlH4 are found to be stable when exposed to air. An extensive thermal, rheological and mechanical characterization is performed and discussed in the paper. Firing tests, in gaseous oxygen, are performed in a dedicated lab-scale hybrid motor. The local and instantaneous regression rate, measured using a fiber optic technique, shows a peculiar behaviour.