Munroe Effect

The Munroe effect refers to the partial focusing of blast energy caused by a hollow or void cut into a piece of explosive, a property which is exploited by a shaped charge.

Explosive energy is released directly away from (normal to) the surface of an explosive, so shaping the explosive will concentrate the explosive energy in the void. If the void is properly shaped (usually conically), a high-velocity jet of plasma will form.

It is named after Charles E. Munroe, who discovered it in 1888. Whilst working at the Naval Torpedo Station at Newport, Rhode Island in the United States, he noticed that when a block of guncotton with the manufacturer's name stamped into it was detonated next to a metal plate, the lettering was cut into the plate. If letters were raised in relief above the surface of the guncotton then the letters on the plate would also be raised above its surface. In 1910, Egon Neumann of Germany discovered that a block of TNT which would normally dent a steel plate would cut a hole right through it if the explosive contained a conical indentation. However, the military usefulness of this effect was not appreciated until the Second World War.

In modern military applications, a Munroe-effect shaped-charge warhead can be expected to penetrate solid steel armor of thickness equal to 150–250% of the warhead diameter, though it will tend to be somewhat less effective against modern composite armors and reactive armor, which was developed specifically as a counter to shaped-charge weapons.

In non-military applications, shaped charges are valued for their versatility and speed. Shaped charges are used in the explosive demolition of buildings: a few hundred pounds of well-placed shaped charges can raze a building faster than several hundred tons of machinery. In steelmaking, small shaped charges are often used to pierce taps that have become plugged with slag.

A device called a "Jet-Axe" was also used sometime around the 1960s or 1970s by fire brigades in the United Kingdom to cut holes through reinforced doors and walls to help gain access to fight fires or rescue people. The "Jet-Axe" was the shape of a flattened doughnut with a hole in the middle and contained a ring of a shaped charge. The device, which was roughly 2 feet (60 cm) in diameter, would be hung on a door or wall with the shaped charge facing the door or wall and when detonated it would cut a circular hole through it.