Combustion of PTFE-Boron compositions for propulsion applications

An experimental study was conducted to evaluate the potential of solid fuels based on PTFE and boron mixtures for hybrid rocket motor applications. Specifically, a processing technique based on sintering was studied to determine the viability of these fuels. Sintering of the fuels provided reasonable mechanical properties to allow for exploration of these fuels without the addition of performancerobbing ingredients. Linear regression rates of sintered and unsintered fuels were collected in a diffusion flame setting with gaseous oxygen as the oxidizing component demonstrating that the sintering process had no effect. This family of fuels has shown that they will not combust at atmospheric pressure unless pure oxygen is present. However, sintered fuels with boron loadings greater than or equal to 25% by weight do self-propagate at atmospheric pressure once ignited in the presence of oxygen, whereas unsintered fuels do not self-propagate unless they have boron loadings greater than or equal to 30% by weight. At pressures up to 12 MPa, fuels containing 10% by weight boron would not self-propagate in a nitrogen atmosphere, whereas fuels containing 20% boron would self-propagate at pressures greater than about 5.7 MPa. Preliminary lab-scale rocket motor firings demonstrate the viability of a hybrid rocket based on PTFE and boron mixtures. In addition, they demonstrate that the regression rates of these fuels show dependencies on pressure and possibly oxidizer flow rate as well. Thermochemical analysis suggests that these fuels offer a significant performance benefit in terms of density impulse, while also presenting a significant technological challenge due to excessively high flame temperatures for some mixtures.