MINIMUM EXPLOSIBLE CONCENTRATION MEASUREMENTS OF HYBRID ROCKET SOLID FUEL DUST

Abstract

Hybrid rockets are considered safe because there is no affinity between the fuel and oxidizer. However, if the solid fuel turns to dust, the dust might cause rapid combustion. Therefore, the dustification of fuel has been evaluated using crushing tests. In an earlier study by the authors, wax was used as a specimen. It was crushed by explosives in a balloon filled with air and oxygen. Then pressure waves near the balloon were measured. Results show that the pressure fluctuation in the oxygen-filled case was greater than that in the air-filled case, but it could not be ascertained whether this result was attributable to the combustion of the wax that had become dust or to the combustion of the wax surface which the explosive contacted. Therefore, the authors measured the lower explosion limit concentration of wax dust under atmospheric conditions and in an atmosphere with a high oxygen concentration to obtain knowledge related to the wax dust mixture flammability. A blow-up type dust explosion test apparatus was used to measure the lower explosion limit. The specimens were produced by grinding and sieving the wax pellets. Powders of four types were produced: 850 μm, 500 μm, 212 μm, and 53 μm. The dust concentration was calculated by dividing the sample mass by the volume of the explosion cylinder. Based on the test results, the lower explosion limits were identified for 212 μm and 53 μm, but not for 850 μm and 500 μm.