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Re-Visiting the Mass-Flux Model for Explosive Reactive Armor and the Effect of Plate Thickness



Explosive Reactive Armor (ERA) is usually composed of an explosive layer in between two steel plates. The plate that is being hit first moves towards the jet and is called the Backward Moving Plate (BMP). The second plate moves with the jet direction and is therefore named the Forward Moving Plate (FMP). In 1984, the first theoretical paper explaining the mechanisms by which ERA defeats shaped-charge jets was presented [1]. The paper revealed several physical mechanisms for the first time, including the hydrodynamic mass-flux model. Since each plate has a different velocity relative to the incoming jet, the interaction time interval and velocity between each jet element and plate, is different. As soon as the jet tip hits the cassette, the explosive is detonated, causing the metallic plates to move aside and disrupt the jet. The plates' velocity is usually one order of magnitude lower than that of the tip of the jet, and since the jet is usually thin, the craters created in the plates during the sideways interaction are also small, causing long, thin craters in the plates which were named “strips”. It was therefore hypothesized that the interaction between the main part of the jet and the plates could be regarded as an interaction between two streams: the jet and the strip. The massflux ratio of the two will determine the ERA effectiveness. A detailed consideration of the mass-flux distribution of the jet throughout the entire interaction process was missing. The present work will describe this distribution, and address the benefits for the ERA effectiveness that are formed by the plate thickness. Moving from jet tip to tail (excluding the tip itself, the appendix, and the slug), the jet velocity decreases while its diameter increases. The distribution of the massflux of the jet emerging from a 60º point initiation charge [6] computed by the MSCAN code (for a mass-flux arriving at a fixed target)[8] is presented in Fig. 1. This mass-flux consideration overlooks two main problems, which are solved differently.


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