Explosively formed penetrator

An explosively formed penetrator (EFP), also known as an explosively formed projectile, a self-forging warhead, or a self-forging fragment, is a special type of shaped charge designed to penetrate armor effectively at standoff distances. As the name suggests, the effect of the explosive charge is to deform a metal plate into a slug or rod shape and accelerate it toward a target. They were first developed during WWII.

A conventional shaped charge generally has a conical metal liner that projects a hypervelocity jet of metal able to penetrate steel armour to great depths; in travel over some distance the jet breaks up along its length into particles that drift out of alignment, greatly diminishing its effectiveness at a distance.

An EFP, on the other hand, has a liner face in the shape of a shallow dish. The force of the blast moulds the liner into any of a number of shapes, depending on the shape of the plate and how the explosive is detonated. Some sophisticated EFP warheads have multiple detonators that can be fired in different arrangements causing different types of waveform in the explosive, resulting in either a long-rod penetrator, an aerodynamic slug projectile, or multiple high-velocity fragments. A less sophisticated approach for changing the formation of an EFP is the use of wire-mesh in front of the liner: with the mesh in place the liner fragments into multiple penetrators.

In addition to single-penetrator EFPs (also called single EFPs or SEFPs), there are EFP warheads whose liners are designed to produce more than one penetrator; these are known as multiple EFPs, or MEFPs. The liner of an MEFP generally comprises a number of dimples that intersect each other at sharp angles. Upon detonation the liner fragments along these intersections to form up to dozens of small, generally spheroidal projectiles, producing an effect similar to that of a shotgun. The pattern of impacts on target can be finely controlled based on the design of the liner and the manner in which the explosive charge is detonated. A nuclear-driven MEFP was apparently proposed by a member of the JASON group in 1966 for terminal ballistic missile defense. A related device was the proposed nuclear pulse propulsion unit for Project Orion.

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