Laser ignition of low vulnerability propellants based on RDX

Low vulnerability gun propellants are energetic materials specifically designed to resist unintended initiation stimuli and aim at replacing the usual nitrocellulose (NC) based ones. The present work focuses on the ignition and combustion characteristics of three propellants that belong to the family of Low Vulnerability Ammunition (LOVA). The propellants consist of hexogen (RDX) and NC. Three different ratios of NC were studied in order to have a better understanding of the NC role in the propellant ignition. The weight percentages of RDX/NC are of 95-5, 90-10 and 85-15. The energetic materials studied in this work are produced at the French-German Research Institute of Saint-Louis (ISL). The laser ignition of these materials is obtained thanks to a laser diode. The experiments are performed in a 55 cm3 cylindrical closed reactor. The initial pressures investigated are ranging between 25 and 70 bars. Two different atmospheres are studied, nitrogen and argon. Depending on the initial pressure, the surrounding gas and the laser power, several combustion characteristics are measured. Parameters such as ignition delay, maximal overpressure and propagation rate are compared for the three studied propellants. It is shown that the ignition delays increase when the weight rate of NC is decreasing. Moreover, ignition energies are also investigated thanks to the modified Langlie method. This adapted dichotomous method allows to reach ignition probabilities with at most 30 shots. Energies giving 50 % of ignition probabilities (E50) are given in the paper. By changing the pulse time of the laser, the ignition probabilities are calculated for each energetic material. Thereby, these values are compared between each propellant for a given initial pressure, gas nature and laser power. It is found that the rate of NC has a significant impact on the ignition probability. Indeed, the lowest E50 is obtained for the propellant made of 85 % of RDX and 15 % of NC, and the highest E50 for the 95-5 RDX/NC ratio.