A shaped charge, commonly also hollow charge if shaped with a cavity, is an explosive charge shaped to focus the effect of the explosive's energy. Different types of shaped charges are used for various purposes such as cutting and forming metal, initiating nuclear weapons, penetrating armor, or perforating wells in the oil and gas industry.
The effectiveness of a shaped charge is significantly influenced by manufacturing technology. From the military and civilian use of shaped charges, it is known that a typical modern shaped charge, having a conical cavity equipped with a metal liner, and produced using conventional technologies and traditional lining materials, can penetrate a hardened steel barrier to a depth of no more than 6 charge diameters (CD). However, by increasing the precision of shaped charge manufacturing, meeting all quality requirements for the lining material and the explosive charge, using advanced methods of filling with high-explosives, and using initiation systems capable of precisely controlling the detonation front, it is possible to increase this limit to 8–10 charge diameters. Finally, for perfectly manufactured shaped charges, it is expected that penetration depths of steel barriers equal to 12 calibers or charge diameters can be achieved.
Contrary to misconception, possibly resulting from the acronym HEAT (high-explosive anti-tank), the shaped charge does not depend in any way on heating or melting for its effectiveness; that is, the jet from a shaped charge does not melt its way through armor, as its effect is purely kinetic in nature. However, the process does create significant heat and often has a significant secondary incendiary effect after penetration.
Function
The shock wave from an explosive is perpendicular to the surface of the explosive. The inside of a cone focuses and concentrates the shock wave to points along the axis of the cone. As the explosion progresses from the point of detonation, the concentrated shock wave progresses along the axis of the cone, gathering energy along the way.
The addition of a liner increases the effect of the explosion by providing a heavy mass that is ejected as a high velocity jet from the cone.
Munroe effect The Munroe or Neumann effect is the focusing of blast energy by a hollow or void cut on a surface of an explosive. The earliest mention of hollow charges was in 1792. Franz Xaver von Baader (1765–1841) was a German mining engineer at that time; in a mining journal, he advocated a conical space at the forward end of a blasting charge to increase the explosive's effect and thereby save powder. About 1805, the idea was adopted, for a time, in Norway and in the mines of the Harz mountains of Germany, although the only available explosive at the time was gunpowder, which is not a high explosive and hence incapable of producing the shock wave that the shaped-charge effect requires.
The first true hollow charge effect was achieved in 1883, by (1845–1905), chief of the nitrocellulose factory of Wolff & Co. in Walsrode, Germany.Kennedy (1990), pp. 5, 66.See:
By 1886, Gustav Bloem of Düsseldorf, Germany, had filed for hemispherical cavity metal detonators to concentrate the effect of the explosion in an axial direction.
The Munroe effect is named after Charles E. Munroe, who discovered it in 1888. As a civilian chemist working at the U.S. Naval Torpedo Station at Newport, Rhode Island, he noticed that when a block of explosive guncotton with the manufacturer's name stamped into it was detonated next to a metal plate, the lettering was cut into the plate. Conversely, if letters were raised in relief above the surface of the explosive, then the letters on the plate would also be raised above its surface.See:
In 1894, Munroe constructed his first crude shaped charge:
Although Munroe's experiment with the shaped charge was widely publicized in 1900 in Popular Science Monthly, the importance of the tin can "liner" of the hollow charge remained unrecognized for another 44 years.Kennedy (1990), p. 6. Part of that 1900 article was reprinted in the February 1945 issue of Popular Science, describing how shaped-charge warheads worked. It was this article that at last revealed to the general public how the United States Army bazooka actually worked against armored vehicles during WWII.
In 1910, Egon Neumann of Germany discovered that a block of TNT, which would normally dent a steel plate, punched a hole through it if the explosive had a conical indentation. The military usefulness of Munroe's and Neumann's work was unappreciated for a long time. Between the world wars, academics in several countries Myron Yakovlevich Sukharevskii (Мирон Яковлевич Сухаревский) in the Soviet Union,M. Sukharevskii:
William H. Payman and Donald Whitley Woodhead in Britain, See also: and Robert Williams Wood in the U.S. recognized that projectiles could form during explosions.
In 1932 Franz Rudolf Thomanek, a student of physics at Vienna's Technische Hochschule, conceived an anti-tank round that was based on the hollow charge effect. When the Austrian government showed no interest in pursuing the idea, Thomanek moved to Berlin's Technische Hochschule, where he continued his studies under the ballistics expert Carl Julius Cranz.For a biography of Carl Julius Cranz (1858–1945), see:
German Wikipedia: Carl Cranz There in 1935, he and Hellmuth von Huttern developed a prototype anti-tank round. Although the weapon's performance proved disappointing, Thomanek continued his developmental work, collaborating with Hubert Schardin at the Waffeninstitut der Luftwaffe (Air Force Weapons Institute) in Braunschweig.
By 1937, Schardin believed that hollow-charge effects were due to the interactions of shock waves. It was during the testing of this idea that, on February 4, 1938, Thomanek conceived the shaped-charge explosive (or Hohlladungs-Auskleidungseffekt (hollow-charge liner effect)).Kennedy (1990), p. 9. (It was Gustav Adolf Thomer who in 1938 first visualized, by flash radiography, the metallic jet produced by a shaped-charge explosion.See:
) Meanwhile, Henry Hans Mohaupt, a chemical engineer in Switzerland, had independently developed a shaped-charge munition in 1935, which was demonstrated to the Swiss, French, British, and U.S. militaries.See: H. Mohaupt, "Chapter 11: Shaped charges and warheads", in: F. B. Pollad and J. A. Arnold, ed.s, Aerospace Ordnance Handbook (Englewood Cliffs, New Jersey: Prentice-Hall, 1966). Kennedy (1990), pp. 10–11. William P. Walters (September 1990) "The Shaped Charge Concept. Part 2. The History of Shaped Charges", Technical Report BRL-TR-3158, U.S. Army Laboratory Command, Ballistic Research Laboratory (Aberdeen Proving Ground, Maryland), p. 7. Available on-line at: Defense Technical Information Center
During World War II, shaped-charge munitions were developed by Germany (Panzerschreck, Panzerfaust, Panzerwurfmine, Mistel), Britain (No. 68 AT grenade, PIAT, Beehive cratering charge), the Soviet Union (RPG-43, RPG-6), the U.S. (M9 rifle grenade, bazooka),Donald R. Kennedy, "History of the Shaped Charge Effect: The First 100 Years ", D.R. Kennedy and Associates, Inc., Mountain View, California, 1983 Italy (Effetto Pronto Speciale shells for various artillery pieces) and Japan (Type 3 grenade, Lunge mine). The development of shaped charges revolutionized anti-tank warfare. Tanks faced a serious vulnerability from a weapon that could be carried by an infantryman or aircraft.
One of the earliest uses of shaped charges was by German glider-borne troops against the Belgian Fort Eben-Emael in 1940. These demolition charges – developed by Dr. Wuelfken of the German Ordnance Office – were unlined explosive charges and did not produce a metal jet like the modern HEAT warheads. Due to the lack of metal liner they shook the turrets but they did not destroy them, and other airborne troops were forced to climb on the turrets and smash the gun barrels.
Applications
Modern military
The common term in military terminology for shaped-charge warheads is high-explosive anti-tank (HEAT) warhead. HEAT warheads are frequently used in anti-tank guided missiles, unguided rockets, gun-fired projectiles (both spun (spin stabilized) and unspun), rifle grenades, land mines, bomblets, torpedoes, and various other weapons.
Protection During World War II, the precision of the charge's construction and its detonation mode were both inferior to modern warheads. This lower precision caused the jet to curve and to break up at an earlier time and hence at a shorter distance. The resulting dispersion decreased the penetration depth for a given cone diameter and also shortened the optimum standoff distance. Since the charges were less effective at larger standoffs, side and turret skirts (known as Schürzen) fitted to some German tanks to protect against ordinary anti-tank rifles were found to give the jet room to disperse and hence also reduce HEAT penetration.
The use of add-on spaced armor skirts on armored vehicles may have the opposite effect and increase the penetration of shaped-charge warheads in some cases. Due to constraints in the length of the projectile/missile, the built-in stand-off on many warheads is less than the optimum distance. In such cases, the skirting effectively increases the distance between the armor and the target, and the warhead detonates closer to its optimum standoff. Skirting should not be confused with cage armor which is primarily used to damage the fusing system of RPG-7 projectiles, but can also cause a HEAT projectile to pitch up or down on impact, lengthening the penetration path for the shaped charge's penetration stream. If the nose probe strikes one of the cage armor slats, the warhead will function as normal.
Non-military In non-military applications shaped charges are used in explosive demolition of buildings and structures, in particular for cutting through metal piles, columns and beams and for boring holes. In steelmaking, small shaped charges are often used to pierce taps that have become plugged with slag. They are also used in quarrying, breaking up ice, breaking log jams, felling trees, and drilling post holes.
Shaped charges are used most extensively in the petroleum and natural gas industries, in particular in the completion of oil and gas wells, in which they are detonated to perforate the metal casing of the well at intervals to admit the influx of oil and gas. Another use in the industry is to put out oil and gas fires by depriving the fire of oxygen.
A shaped charge was used on the Hayabusa2 mission on asteroid 162173 Ryugu. The spacecraft dropped the explosive device onto the asteroid and detonated it with the spacecraft behind cover. The detonation dug a crater about 10 meters wide, to provide access to a pristine sample of the asteroid.
Technical characteristics
A typical device consists of a solid cylinder of explosive with a metal-lined conical hollow in one end and a central detonator, array of detonators, or detonation wave guide at the other end. 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 hollow is properly shaped, usually conically, the enormous pressure generated by the detonation of the explosive drives the liner in the hollow cavity inward to collapse upon its central axis.
The resulting collision forms and projects a high-velocity jet of metal particles forward along the axis. Most of the jet material originates from the innermost part of the liner, a layer of about 10% to 20% of the thickness. The rest of the liner forms a slower-moving slug of material, which, because of its appearance, is sometimes called a "carrot".