Experimental Investigation into the Effect of Solid-Fuel Additives on Hybrid Rocket Performance

An experimental performance evaluation, conducted with static firings of a laboratory-scaled hybrid rocket burning gaseous oxygen and several fuel combinations, aimed at raising the fuel regression rate is addressed in this paper. Pure hydroxyl-terminated polybutadiene has been tested as a baseline fuel and compared to hydroxyl-terminated polybutadiene loaded with nano- or micrometer-sized aluminum, nanosized Viton®-coated aluminum, magnesium hydride, magnesium, nanosized iron, and iron with magnesium. After a first exploratory test series in which both radial- and axial-injection arrangements have been analyzed, an axial nozzle has been selected to feed oxygen into single-port fuel grains; in that configuration, the port diameter effect upon the regression rate has been further investigated, revealing faster regression with larger port diameters at a given mass flux. The outcomes in terms of regression rate, combustion efficiency, and motor stability are examined. A thorough comparison with the available literature has been drawn, demonstrating that a clear regression-rate improvement trend has not been established yet. Current test results show a moderate average regression-rate increase (on the order of 10%) over the pure fuel, though considerably high characteristic-velocity and impulse efficiencies (on average, around 95 and 92%, respectively) have been achieved. Stable combustion has been observed in all the tested conditions with all the fuel combinations considered, excluding hydroxyl-terminated polybutadiene containing uncoated nanoaluminum, with which high-amplitude, very low-frequency pressure oscillations have occurred. A tentative explanation of this behavior is proposed, resorting to large metal particle periodic accumulation on the nozzle surface.