Performance comparison of HTPB-based solid fuels containing nano-sized energetic powder in a cylindrical hybrid rocket motor

An experimental investigation was conducted to determine the relative propulsive and combustion behavior of various HTPB-based solid-fuel formulations containing nano-sized energetic particles. At the present time, ten solid fuel formulations have been studied. These solid-fuel grains contained nano-sized energetic particles such as Alex® aluminum, Technanogy aluminum, boron, boron carbide, coated and uncoated aluminum flakes, and WARP-1 aluminum. The nano-sized particles were cast in an HTPB solid-fuel grain and burned in the Long-Grain Center- Perforated (LGCP) hybrid rocket motor using pure oxygen as the oxidizer injected at the head-end of the motor. The oxidizer mass flux was varied from 140 to 850 kg/m²-s at average chamber pressures ranging from 2.3 to 4.6 MPa (320 to 650 psig). The LGCP hybrid rocket facility was used as a screening device in order to determine which formulation and/or nano-sized particles would be most effective for the development of future generation solid fuels or solid propellants. At a specified average mass flux rate, the addition of 13% energetic powders showed an increase of up to 63% in mass burning rate compared to the pure HTPB fuel (such as Alex®-containing solid fuels). The C*_eff for the Alex®-containing formulation is around 88 to 92%, which is higher than other fuel formulations. Viton-A coated aluminum flakes supplied by Naval Air Warfare Center (NAWC)-China Lake showed a mass burning rate increase of nearly 43% compared to HTPB and 2x the increase compared to the uncoated Al flakes (Silberline). It is believed that the fluorine and fluorine-containing compounds produced from the dissociation of Viton-A contributes to the rapid ignition and combustion of the aluminum flakes. Coated aluminum flakes are extremely inexpensive and show great promise. Boron-based solid fuels showed significant increase in mass burning rate as well (~45%). This increase could be associated with the higher amount of energy feedback from the combustion zone to the regressing fuel surface. The calculated instantaneous regression rate profile was integrated and compared to the measured experimental regression rate and matches to within 8% at all conditions.