The ablation performance of phenolic and carbon matrix composites under the free jet conditions of hybrid propellant rocket

Ablative composite materials are commonly preferred in rocket nozzle regions where high temperature durability and shock resistance are required. In hybrid rocket motor applications, these materials are exposed directly to the hot gas products formed by the combustion reaction of fuel and oxidizer. Accordingly, ablation testing under the free-jet of lab-scale hybrid rocket motor would be flight-like simulation for the application of these materials in hybrid propellant rockets. As observed in the literature; Polymer Matrix Composites (PMCs), reinforced with silica and carbon fibers, can withstand short-term exposure to hot gases of combustion, whereas Ceramic Matrix Composites (CMCs), as Carbon/Carbon (C/C), exhibit exceptional durability without significant recession in similar conditions. In this work, chopped and 2D fabric reinforced phenolic matrix PMCs, and C/Cs were tested under the hot exhaust of paraffin fuel and gaseous oxidizer-based lab-scale hybrid rocket for 7 sec. In order to observe the oxygen-to-fuel ratio (O/F) effect on the ablation performances, each sample was tested in both oxygen-rich and fuel-rich firing conditions. Surface temperatures of samples were measured by two-colour IR pyrometer, also ablation rates were determined from the mass loss of samples. Test results show that phenolic matrix and C/C composites are significantly ablated by fiber-matrix debonding. Heat treatment of C/C samples in 1500 °C increased the ablation resistance by enhancing fiber-matrix bonding against hot gas exposure. For both phenolic matrix and C/C composite samples, chopped fiber reinforced samples propose better ablation resistance than 2D reinforcement. The variety of samples provides clear understanding about the ablation mechanisms of composite materials, specifically in the hybrid rocket applications.