Hybrid Combustion Studies on Regression Rate Enhancement and Transient Ballistic Response

The ballistic response of hydroxyl-terminated polybutadiene (HTPB)- and paraffin-based fuel formulations burning under quasi-steady and transient operating conditions is analyzed by different diagnostics developed at the Space Propulsion Laboratory (SPLab) of Politecnico di Milano. The analysis of different fuel formulations under quasi-steady operating conditions focused on the regression rate (r f ) enhancement. HTPB loaded with coated nano-sized aluminum showed promising performance in terms of oxidizer mass flux sensitivity reduction and r f increases in the order of 40 % over the baseline. The solid wax fuels exhibited higher r f increases (∼200 % for microcrystalline waxes) but poor mechanical properties. Due to this the possible mechanical properties enhancement by blending the solid waxes with thermoplastic polymers was investigated. The developed strategy appears promising, possibly enabling significant r f increases over polymeric fuels thanks to the entrainment. The ballistic response of HTPB to forced transient conditions was investigated considering throttling down events. Under the tested conditions, the solid fuel exhibited a non-monotonic behavior for linear oxidizer mass flow decrease. The observed, faint r f oscillations are probably due to thermal delay of the condensed phase.