Combustion of boron and fluorocarbon solid fuels in a hybrid rocket

A composition comprised of 80% polytetrafluoroethylene and 20% boron (by weight) is considered as a potential high-density solid fuel mixture for mixed hybrid rocket propulsive applications. Constant pressure strand burner experiments were conducted over a pressure range from approximately 1.46 to 10.6 MPa (198 to 1,538 psia) under nearly constant pressure in nitrogen environment to determine the low-pressure self-deflagration limit and measure burning rates as a function of pressure in an optically accessible chamber. A burning rate correlation r_b[cm/s] = 0.042(P[MPa])^0.531 was determined for the given formulation. A low-pressure self deflagration limit of approximately 2.2 MPa (319 psia) was obtained. Pressurized counterflow burner experiments conducted using pure oxygen revealed formation of surface char which prevented measurement of solid regression rates below 2MPa indicating an additional resistance for heat and mass transfer. Static-fired rocket motor experiments were conducted to determine the pressure and flow dependencies of the system by variation of oxidizer flow rates and nozzle throat areas, and to evaluate propulsive performance parameters. Characteristic exhaust velocity efficiency (C*efficiency), which provides a measure of combustion efficiency, ranged from approximately 86 to 96% depending on motor operating conditions. While classical hybrids do not have a strong dependence of fuel regression rate on pressure, a pressure dependence was observed in this system below the self deflagration limit due to the pressure dependence of the decomposition and fluorination kinetics of the solid fuel mixture. Below the self-deflagration limit, the motor operated at a constant pressure, typical of a classical hybrid, while 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. C*efficiencies were not noticeably affected by oxidizer dilution with nitrogen over the range considered.