Scaling of the propulsive capability of aluminized gelled nitromethane

Abstract

Small mass fractions (< 20%) of micron-scale aluminium particles added to a high explosive can react quickly and with sufficient exothermicity to improve metal-acceleration ability (AA) relative to an equal volume of only the base explosive. In order for the aluminium to increase AA, exothermicity must more than offset losses in gas-production and from heating and accelerating the solid particles in the flow. Furthermore, particles must react promptly to deliver this energy prior to loss in driving pressure from product expansion or acoustic decoupling from the driven material. For these reasons, many aluminized formulations exhibit slight or no increase in AA. Furthermore, AA is typically studied using the cylinder test, which specifies a fixed, heavy copper wall. In the present study the authors have used symmetric sandwiches of flyer plates of widely different thicknesses to examine how charge scaling and plate acceleration time-scales influence the enhancement in AA for different sizes of aluminium particles. Nitromethane gelled with 4% Poly(methyl methacrylate) by mass was used for the explosive. 3M K1 microballoons were added at a mass fraction of 0.5% to sensitize the mixture. Aluminium with a mean particle size of 3.5 µm or 108 µm was added at 15% total mixture mass fraction. At 15% mass fraction, an enhancement in AA was observed for both particle sizes and flyer configurations. Results indicated an onset of reaction close to the sonic plane of the detonation wave.