Non-Gurney scaling of explosives heavily loaded with dense inert additives

For most high explosives, the ability to accelerate material to some terminal velocity scales with the ratio of material-mass to charge-mass (M/C) according to the Gurney model. In planar geometry, the Gurney model accurately estimates the terminal velocity of the driven material until M/C is reduced to 0.2 or lower. Below this value, gasdynamic departures from the assumptions of the model result in under-prediction of the material terminal velocity. In the present study, a modified Gurney model was used to predict the scaling of flyer velocity with M/C for explosives heavily diluted with either low density (3M K1 glass microballoons—GMBs), or high density (steel beads) diluent. The modified model accounted for explosive energy being transferred to accelerate the diluent. The model was compared with a series of flyer experiments propelled using either Poly(methyl methacrylate)-gelled nitromethane (96% NM/4% PMMA) diluted with 10% GMBs by mass or diethylenetriamine-sensitized liquid nitromethane saturating a packed bed of steel particles. Both grazing and normally incident detonations of the test mixtures were used to propel the flyers. The Gurney model accurately predicted the terminal velocity for the gelled NM/GMB mixture although it did not account for the contribution from the detonation shock nor the initiating slapper plate. The model failed to predict flyer velocity for the NM/steel mixture. The propulsive efficiency of this mixture increased with M/C relative to the baseline explosive so that the model under-predicted velocity for small M/C but over-predicted for large M/C.