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The shaped charge effect, variously known as the hollow charge effect, the cavity effect, or the Munroe Effect, dates to the 1880s Naval Torpedo Station, Newport, Rhode Island. At that time the Navy was conducting experiments using guncotton blocks (a solid block of nitrocellulose impregnated with nitroglycerin) as a potential warhead for torpedoes. These blocks were cast and had identification letters/numbers cast into the block. Dr. Charles Munroe discovered that when a block of guncotton was detonated with its lettered surface against a steel surface, a mirror image of the letters were impressed into the steel. Similar effects were observed about the same time in Germany and Norway.

After a period of neglect, a pair of Swiss inventors undertook serious development of the Munroe Effect to penetrate armor plate. They tried to disguise their achievement during sales attempts with European and American armaments firms by claiming they discovered a new explosive. Ordnance experts eventually discovered that a shaped charge created the demonstrated armor penetration, with serious development of infantry antitank weapons using this technology in the United States, Germany, Soviet Union and Britain. The latter three nations began fielding shaped charge anti-tank hand grenades in 1940. After extensive research during the 1930s at the Ballistics Research Laboratory, Aberdeen Proving Ground, MD, shaped charge infantry antitank weapons began being fielded in the U. S. forces. Notable were a rifle-propelled antitank grenade and the renown Bazooka. The Bazooka was a totally improvised development - being created from the rifle grenade, a rocket motor and a tube constructed of fire extinguisher casings.

In both the hand-launched and rifle- or rocket-propelled weapons, the shaped charge warhead is a mass of a highly brisant explosive formed around a hollow metal liner, usually copper or aluminum. Brisance is the speed of a detonation wave produced by an explosive. Highly brisant explosives, like nitroglycerin, have a shattering effect on solid materials, while low brisance explosives, like conventional blasting powder, tend to displace solid objects. The warhead cavity is usually a conical, hemispherical, or parabolic shape. The warhead has an external container and fuze train. The warhead can be either base-fuzed or nose-fuzed with an ignition train leading to an explosive initiation system in the base of the warhead.

When the warhead strikes a target, the explosion initiates from the rear of the explosive charge. The detonation wave sweeps forward and, as it encounters the cavity liner, it collapses the metal cone. As the detonation continues, the metal liner is squeezed into a narrow jet of molten, high temperature microscopic particles and plasma. The jet takes an elongated form because at the time of the explosion, the detonation imparts different velocities to some particles of the metal liner. Part of the reason for this is the detonation vector (speed and direction) through the explosive; part is the due to the shape of the cavity and differences in liner thickness and material. Those particles and atoms in the plasma that are moving faster begin to extend away from the slower moving particles, elongating the jet as micro-seconds of time pass. The tip of the jet approaches speeds of 8,500 to 10,000 meters/second, the trailing end of the jet moves at about 1,500 meters/second, while the denser particles form a "slug" that travels along the same direction as the jet but at much lower velocities, e.g. 600 to 1,000 meters/second. This slug contains about 80% of the liner material while the high speed jet contains 20%.

As the jet strikes the target, such as steel or armor plate, the point of the jet exerts pressures in the range of hundreds of kilobars - far above the yield strength of the metal. The target metal immediately liquefies and flows out of the path of the jet - a phenomenon termed hydrodynamic penetration (the same phenomenon that enables high pressure jets of air or water to cut through solid objects in modern manufacturing processes). Harder materials, like armor plate, resist the radial flow of the material away from the jet more strongly than do comparatively softer materials, like steel - this is why the same warhead exploding on steel will leave a larger hole and greater penetration than for armor plate.

Penetration generally depends upon five factors:

the length of the shaped charge jet,
the density of the shaped charge jet,
the focus of the shaped charge jet (e.g. pointed vs. difused),
the density of the target material, and
the hardness of the target material.

Many shaped charge weapons employ a mechanism to initiate the detonation some distance from the target surface. This is called standoff, and this distance is termed standoff distance. This distance allows the jet to lengthen and therefore increase the depth of penetration. There is an optimum standoff distance, where the cone reaches an ideal length, after which it begins to particulate - break up into separate elements - just as water forced from a nozzle retains begins to break up from a solid stream with increasing distance from the nozzle. Empirical data shows the distance where particluation begins is typically 6 to 8 times the maximum diameter of the cone.

The standoff distance, and hence the effectiveness of the shaped charge, also depends upon the terminal velocity of the warhead, the speed at which it strikes the target. A high velocity will collapse the standoff before the charge can be fully detonated and the jet fully formed - this greatly reduces penetration.

The focus, or precision, of the jet describes the straightness and pointedness of the jet. If the detonation occurs when the warhead is oscillating or moving laterally, then the cross section of the jet will be larger, and the jet itself will take on a bending shape. This reduces the depth of penetration, and the reason why explosives with very high detonation speeds are used. High detonation speed means the amount of movement of the warhead, and hence the jet, is infinitesimal during the detonation interval. This is also why shaped charge rounds are generally not effective when fired from rifled weapons - consider the AMX-30 main gun that fires a specially designed shaped charge round that inhibits rotation. Empirical data shows that jet degradation begins at about 10 revolutions per second.

Damage to the target is, however, not due to the perforation of the armor plate, but the effects of the jet escaping inside the target, the effects of the jet on the internal surface of the armor plate, and the effects of that portion of the slug that follows the jet into the target interior:

The point of the jet perforating the armor plate creates a shock wave at the speed of the jet. This shock wave is relatively small owing to only a small amount of jet material penetrated (less than 20% of the warhead cone volume carrying explosive energy). While this pressure wave dissipates within a short distance, its effects would be relatively larger in a confined space like a tank turret. This shock wave may stun or injure humans, and create a sympathetic detonation in ammunition.
Spalling occurs as the interior surface of the armor plate breaks away with the approach of the tip of the jet. Part of this spalling is due to to the stress imparted by the approach of the jet exceeding the tensile strength of the armor material, and part of it is due to fluid, or hydrodynamic effects. For hydrodynamic effects, molten armor solidifies to form metal droplets, which at speed have the effect of firing a small gauge shotgun shell inside the target. For true spall, the particles are larger and irregularly shapes, approximating the effect of a small grenade or shell detonating inside the target. Spall is the principal cause of casualties inside the target.
While about half of the slug will splash and spatter off the surface of the armor plate around the perforation point, that portion of the slug that penetrates through the perforation creates a spray of relatively high speed, molten metal particles. These particles, along with those formed by the jet, splash the inside of the vehicle. These may ignite flammables, burn through fabric, plastic, hoses, and electrical wire insulation, and may initiate ammunition detonation.

A small shaped charge, approximately 5cm diameter and 9cm long can penetrate 25 to 30 cm of armor steel. [In 1967 the author witnessed a demonstration where a shaped charge the size of a large "bankers" fountain pen completely perforated 8 one-inch steel slabs with a 5-8 mm diameter hole.] Larger shaped charges, like those used in contemporary antitank missiles penetrate 50 cm of steel.