Electric armor against shaped charges: Analysis of jet distortion with respect to jet dynamics and current flow

Shaped charges are a warhead technology often applied to rocket propelled grenades and represent a dangerous threat for armored vehicles in combat as well as in peace-keeping operations. Their armor piercing performance rests upon an explosively induced collapse of a metallic liner to a stretching jet with very high particle velocities. A copper jet produced by a shaped charge can be distorted by high electric currents injected into the jet by means of spaced electrode plates connected to a high voltage capacitor. In tests carried out at Fraunhofer EMI, a shaped charge with a well characterized jet was used for the experiments in order to examine the current flow through the jet and its effect on the jet evolution. The measured current flow is related to the jet dynamics and the distortion pattern observed by multiple flash X-ray images. As expected, the current flow starts when the jet tip reaches the back electrode plate. No significant change of the current flow is observed at the characteristic jet break-up time. The current flow continues after the tail of the copper jet has left the electrodes and resembles a damped sinusoidal. A distortion of the jet is observed where the jet particles are not aligned along the jet axis. Instead the particles are stretched orthogonally to the jet axis with increased separation along the jet axis. The tip part of the jet is hardly affected. The jet distortion is analyzed with respect to jet dynamics and current flow which allows formulating criteria for the design of electric armor systems. The current injection effective for jet distortion is limited to a time slot of a magnitude of 60 mu s for the 44-mm caliber-shaped charge used in the experiments. To a first approximation, the current flow can be modeled by an electric arc. An electric circuit model can describe the current flow behavior with respect to the electric impedance and allows designing an electrical circuit adequate for the time slot. By the analogy of a wire explosion the necessary current magnitude for an effective jet disruption with respect to the interaction time slot can be estimated to begin at 300 kA. For the tip portion of the shaped charge jet, the time of effective current injection is very short. When the current starts to set in, the jet tip is already passing the back electrode plate. For this reason, an effective distortion of the jet tip represents a challenge that has to be mastered.