Boron and Polytetrafluoroethylene as a Fuel Composition for Hybrid Rocket Applications

A composition consisting of 80% polytetrafluoroethylene and 20% boron (by weight) was considered as a potential high-density solid fuel mixture for mixed hybrid rocket propulsive applications. Constant-pressure strand burner experiments for the given formulation indicated a low-pressure self-deflagration limit of approximately 2.2 MPa (319 psia), and a burning rate correlation was determined. Pressurized counterflow burner experiments conducted using pure oxygen revealed formation of surface char, which prevented measurement of solid fuel regression rates below 2 MPa, indicating an additional resistance for heat and mass transfer. Static-fired rocket motor experiments, conducted to determine the pressure and flow dependencies of the system, exhibited characteristic exhaust velocity efficiencies ranging from approximately 86 to 96%. Whereas classical hybrids do not have a strong dependence of fuel regression rate on pressure, a pressure dependence was observed in this system below the low-pressure self-deflagration limit due to the pressure dependence of the decomposition and fluorination kinetics of the solid fuel mixture. Below the low-pressure self-deflagration limit, the motor operated at a constant pressure, typical of a classical hybrid, whereas above the limit, a progressive burn was observed, characteristic of a composite propellant. Systematic oxidizer dilution with nitrogen revealed a decrease in pressurization rate with decreasing oxygen content, and an ignition limit was achieved for this system when the oxygen mass fraction was reduced from 0.65 to 0.6. Characteristic exhaust velocity efficiencies were not noticeably affected by oxidizer dilution with nitrogen over the range considered.