Shaped charge

A shaped charge is an explosive shaped to focus the released energy. They are used to cut and form metal, ignite nuclear weapons, and penetrate Armour.

A modern shaped charge can commonly penetrate about 7 times the diameter of the charge. The explosion creates a small coin-size hole in the armour, so the destructive effects mainly comes from shrapnel spalled from the inside of the plating.

Shaped charges are frequently used in anti-tank missiles and mortar shells.

Design

There are several basically different forms of shaped charge.

An important nonmilitary use of shaped charges is to cut large pieces of metal in confined areas, especially in rescues.

Metal-cutting charges are flat triangles of plastic explosive (usually plastique). A blasting cap is placed in the point of the triangle. The triangle is laid down with the wide end wrapped around the material to cut. When the blast reaches the end of the explosive, it is reflected into the metal, cutting it.

A scissors charge is two of the above triangular charges, placed on opposite sides of the metal, on opposite sides of the cutting line. They must be sized and timed so the blast wave of each arrives at the same time at the cutting line. The opposing blast waves shear the metal.

Differential cones transfer momentum from the explosive to another material, usually a metal such as copper, with a speed of sound different from the speed of the blast in the explosive. The slope of the cone is chosen so that the speed of the blast at the surface of the cone is equal to the speed of sound of the metal on the surface of the cone. The result is usually a dense drop or slug of fluid moving at hypersonic speeds. The slug penetrates armour.

Differential cones are easy to manufacture. Usually the penetrator cone is swaged to a casing, and the explosive is cast inside the resulting shape.

Explosive lenses are machined (usually) from an alloy of a rigid explosive with a compatible plastic. The surface of the lense is calculated to cause the blast front to go where desired. This is possible because the blast ignition in the explosive almost always moves at a different speed than the shockwave in the explosive reaction products.

Some explosive lenses (notably those in nuclear weapons) use two different materials with different speeds of ignition.

The ignition wave is presumed to start as a circular expanding wave-front around the blasting cap. IN precision lenses, the shockwave is started from an electrically exploded wire called an initiator.