Munroe effect

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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 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 rest of the guncotton then the letters on the plate would also be raised above its surface. In 1910, Egon Neumann of Germany discovered that TNT containing a conical indentation would cut through a metal plate which would normally only be dented by that quantity of explosive. 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 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 peaceful engineering applications shaped charges are valued for their versatility and speed. A few hundred pounds of well placed shaped charges can demolish a building faster than several hundred tons of machinery, and in steel manufacturing 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 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. The "Jet-Axe" was roughly 2 feet in diameter. Such a device was demonstrated on British TV in a popular programme called Tomorrow's World with Raymond Baxter.

Compare to the Misznay-Schardin effect.