Shaped charge

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Sectioned HEAT round with the inner shaped charge visible


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1:Aerodynamic cover 2: Empty room 3: Conical liner 4: Detonator 5: Explosive 6: Piezo-electric sensor


A shaped charge is an explosive charge shaped to focus the effect of the explosive's energy. Various types are used to cut and form metal, initiate nuclear weapons, and penetrate armour. A typical modern lined shaped charge can penetrate armour steel to a depth of 7 or more times the diameter of the charge's cone (cone diameters, CD), though greater depths of 10 CD and above are now feasible.

Overview

Shaped charges are frequently used as warheads in anti-tank missiles (guided and unguided) and also gun-fired projectiles (spun and unspun), rifle grenades, mines, bomblets, torpedoes and various types of air/land/sea-launched guided missiles. They are also used to demolish large obsolete structures by precisely placed and progressively timed cutting charges with the intent of causing an inward collapse that confines the debris to the structure's footprint. Shaped charges find their most numerous application in the petroleum industry, in particular in the completion of oil wells, in which they are used to perforate the metal casing of the well at intervals to admit the influx of oil.

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. The enormous pressure generated by the detonation of the explosive drives the liner contained within the hollow cavity inward to collapse upon its central axis. The resulting collision forms and projects a high-velocity jet of metal forward along the axis. Most of the jet material originates from the innermost layer of the liner, about 10% to 20% of its thickness. The remaining liner material forms a slower-moving slug of material, which is sometimes called a "carrot."

Because of variations along the liner in its collapse velocity, the jet so formed has a varying velocity along its length, decreasing from the front. This variation in velocity stretches the jet and eventually leads to its break-up into particles. In time, the particles tend to lose their alignment, which reduces the depth of penetration at long standoffs.

Also, at the apex of the cone, which forms the very front of the jet, the liner does not have time to be fully accelerated before it forms its part of the jet. This results in its small part of jet being projected at a lower velocity than jet formed later behind it. As a result, the initial parts of the jet coalesce to form a pronounced wider tip portion.

Most of the jet formed moves at hypersonic speed, the tip at 7 to 14 km/s, the jet tail at a lower velocity (1 to 3 km/s), and the slug at a still lower velocity (less than 1 km/s). The exact velocities are dependent on the charge's configuration and confinement, explosive type, materials used, and the explosive-initiation mode. At typical velocities, the penetration process generates such enormous pressures that it may be considered hydrodynamic; to a good approximation, the jet and armor may be treated as incompressible fluids, with their material strengths ignored.

The liner

The shape most commonly used for the liner is a cone, with an internal apex angle of 40 to 90 degrees. Different apex angles yield different distributions of jet mass and velocity. Small apex angles can result in jet bifurcation, or even in the failure of the jet to form at all; this is attributed to the collapse velocity being above a certain threshold, normally slightly higher than the liner material's bulk sound speed. Other widely used shapes include hemispheres, tulips, trumpets, ellipses, and bi-conics; the various shapes yield jets with different velocity and mass distributions.

Liners have been made from many materials, including glass and various metals. The deepest penetrations are achieved with a dense, ductile metal, and a very common choice has been copper. For some modern anti-armor weapons, molybdenum and pseudo-alloys of tungsten filler and copper binder (9:1 thus density is ~18t/m3) have been adopted. Just about every common metallic element has been tried, including aluminium, tungsten, tantalum, depleted uranium, lead, tin, cadmium, cobalt, magnesium, titanium, zinc, zirconium, molybdenum, beryllium, nickel, silver, and even gold and platinum. The selection of the material depends on the target to be penetrated; for example, aluminium has been found advantageous for concrete targets.

For the deepest penetrations, pure metals yield the best results, because they display the greatest ductility, hence postponing the breakup of the stretching jet into particles. In charges for oil-well completion, however, it is essential that a solid slug or "carrot" not be formed, since it would plug the hole just penetrated and interfere with the influx of oil. In the petroleum industry, therefore, liners are generally fabricated by powder metallurgy, often of pseudo-alloys, which if un-sintered, yield jets that are composed mainly of dispersed fine metal particles.

During World War II, liners were made of copper or steel, though other materials were tried or researched. 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) were fitted to some German tanks to give the jet room to disperse and hence reduce its penetrating ability. The plates may also have been used to destabilise small calibre armour piercing (AP) projectiles, and/or strip the penetration cap from larger calibre APC (armour piercing capped) and APCBC (armour piercing capped ballistic capped) projectiles.

The use of skirts today may increase the penetration of some warheads. Due to constraints in the length of the projectile/missile, the built in stand-off on many warheads is not the optimum distance. The skirting effectively increases the distance between the armour and the target, providing the warhead with a more optimum standoff and greater penetration if the optimum stand-off is not drastically exceeded. Skirting should not be confused with bar/slat/chain armour which is used to damage the fuzing system of RPG-7 projectiles. The armour works by deforming the inner and outer ogives and shorting the firing circuit between the rocket's piezoelectric nose probe and rear fuze assembly. If the nose probe strikes the armour, the warhead will function as normal.

The spacing between the shaped charge and its target is critical, as there is an optimum standoff distance to achieve the deepest penetration. At short standoffs, the jet does not have room to stretch out, and at long standoffs, it eventually breaks into particles, which then tend of drift off the line of axis and to tumble, so that the successive particles tend to widen rather than deepen the hole. At very long standoffs, velocity is lost to air drag, degrading penetration further.

The explosive

For optimum penetration, a high explosive having a high detonation velocity and pressure is normally chosen. The most common explosive used in high performance anti-armour warheads is HMX (octogen), though it is never used on its own, as it would be too sensitive. It is normally compounded with a few percent of some type of plastic binder, such as in the plastic bonded explosive (PBX) LX-14, or with another less-sensitive explosive, such as TNT, with which it forms Octol. Other common explosives are RDX-based compositions, again either as PBXs or mixtures with TNT (to form Composition B and the Cyclotols) or wax (Cyclonites). Some explosives incorporate powdered aluminium to increase their blast and detonation temperature, but this addition generally results in decreased performance of the shaped charge. There has been research into using the very-high-performance but sensitive explosive CL-20 in shaped-charge warheads, but, at present, due to its sensitivity, this has been in the form of the PBX composite LX-19 (CL-20 and Estane binder).

Other features

A waveshaper is a body (typically a disc or cylindrical block) of an inert material (typically solid or foamed plastic, but sometimes metal, perhaps hollow) inserted within the explosive for the purpose of changing the path of the detonation wave. The effect is to modify the collapse of the cone and resulting jet formation, with the intent of increasing penetration performance. Waveshapers are often used to save space—a shorter charge can achieve the same performance as a longer one without a waveshaper.

Another useful design feature is sub-calibration, the use of a liner having a smaller diameter (calibre) than the explosive charge. In an ordinary charge, the explosive near the base of the cone is so thin that it is unable to accelerate the adjacent liner to sufficient velocity to form an effective jet. In a sub-calibrated charge, this part of the device is effectively cut off, resulting in a shorter charge with the same performance.

Shaped Charge Variants

There are several different forms of shaped charge.

Linear shaped charges

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Linear shaped charge
A linear shaped charge (LSC) has a liner with V-shaped profile and varying length. The liner is surrounded with explosive, the explosive then encased within a suitable material that serves to protect the explosive and to confine (tamp) it on detonation. The charge is detonated at some point in the explosive above the liner apex. The detonation projects the liner to form a continuous, knife-like (planar) jet. The jet cuts any material in its path, to a depth depending on the size and materials used in the charge. For the cutting of complex geometries, there are also flexible versions of the linear shaped charge, these with a lead or high-density foam sheathing and a ductile/flexible liner material, which also is often lead. LSCs are commonly used in the cutting of rolled steel joists (RSJ) and other structural targets, such as in the controlled demolition of buildings. LSCs are also used to separate the stages of multi-stage rockets.

Explosively Formed Penetrator

The Explosively Formed Penetrator (EFP) is also known as the Self-Forging Fragment (SFF), Explosively Formed Projectile (EFP), SElf-FOrging Projectile (SEFOP), Plate Charge, and Misznay-Schardin (MS) Charge. An EFP uses the action of the explosive's detonation wave (and to a lesser extent the propulsive effect of its detonation products) to project and deform a plate or dish of ductile metal (such as copper, iron, or tantalum) into a compact high-velocity projectile, commonly called the slug. This slug is projected towards the target at about two kilometres per second. The chief advantage of the EFP over a conventional (e.g., conical) shaped charge is its effectiveness at very great standoffs, equal to hundreds of times the charge's diameter (perhaps a hundred meters for a practical device).

The EFP is relatively unaffected by first-generation reactive armour and can travel up to perhaps 1000 charge diameters (CDs) before its velocity becomes ineffective at penetrating armour due to aerodynamic drag, or successfully hitting the target becomes a problem. The impact of a ball or slug EFP normally causes a large-diameter but relatively shallow hole, of, at most, a couple of CDs. If the EFP perforates the armour, extensive behind armour effects (BAE, also called behind armour damage, BAD) will occur. The BAE is mainly caused by the high temperature and velocity armour and slug fragments being injected into the interior space and the overpressure (blast) caused by this debris. More modern EFP warhead versions, through the use of advanced initiation modes, can also produce long-rods (stretched slugs), multi-slugs and finned rod/slug projectiles. The long-rods are able to penetrate a much greater depth of armour, at some loss to BAE, multi-slugs are better at defeating light and/or area targets and the finned projectiles have greatly enhanced accuracy. The use of this warhead type is mainly restricted to lightly armoured areas of main battle tanks (MBT), the top, belly and rear armoured areas for example. Its use in the attack of other less heavily protected armoured fighting vehicles (AFV) and in the breaching of material targets (buildings, bunkers, bridge supports, etc), it is well suited. The newer rod projectiles may be effective against the more heavily armoured areas of MBTs. Weapons using the EFP principle have already been used in combat; the "smart" submunitions in the CBU-97 cluster bomb used by the US Air Force and Navy in the 2003 Iraq war employed this principle, and the US Army is reportedly experimenting with precision-guided artillery shells under Project SADARM (Seek And Destroy ARMor). There are also various other projectile (BONUS, DM 642) and rocket submunitions (Motiv-3M, DM 642) and mines (MIFF, TMRP-6) that use EFP principle.

Tandem warhead

Some modern anti-tank rockets and missiles use a tandem warhead shaped charge, consisting of two separate shaped charges, one in front of the other, typically with some distance between them. Usually, the front charge is somewhat smaller than the rear one, as it is intended primarily to disrupt explosive reactive armor.

References

Fundamentals of Shaped Charges, W.P. Walters, J.A. Zukas, John Wiley & Sons Inc., June 1989, ISBN 0-471-62172-2.

Tactical Missile Warheads, Joseph Carleone (ed.), Progress in Astronautics and Aeronautics Series (V-155), Published by AIAA, 1993, ISBN 1-56347-067-5.

See also

External links

explosive material is a material that either is chemically or otherwise energetically unstable or produces a sudden expansion of the material usually accompanied by the production of heat and large changes in pressure (and typically also a flash and/or loud noise) upon initiation;
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Armour or armor (see spelling differences) is protective clothing intended to defend its wearer from intentional harm in combat and military engagements, typically associated with soldiers.
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explosive material is a material that either is chemically or otherwise energetically unstable or produces a sudden expansion of the material usually accompanied by the production of heat and large changes in pressure (and typically also a flash and/or loud noise) upon initiation;
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detonator is a device used to trigger an explosive device.

Detonators can be chemically, mechanically, or electrically initiated, the latter two being the most common.

Explosive Ordnance Devices or EOD (hand grenades, naval mines etc.
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Detonation is a process of supersonic combustion in which a shock wave is propagated forward due to energy release in a reaction zone behind it. It is the more powerful of the two general classes of combustion, the other one being deflagration.
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hypersonic speeds are speeds that are highly supersonic. In the 1970s, the term generally came to refer to speeds of Mach 5 (5 times the speed of sound) and above. The hypersonic regime is a subset of the supersonic regime.
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Hydrodynamics, also known as liquid-dynamics in limited academic circles, (literally, "water motion") is fluid dynamics applied to liquids, such as water, alcohol, oil, and blood. However, this distinction from fluid dynamics as a whole is not always fully observed.
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Cone (from the Greek κώνος, Latin conu) is a basic geometrical shape. It may also refer to:
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2, 1
(mildly basic oxide)
Electronegativity 1.90 (Pauling scale)
Ionization energies
(more) 1st: 745.5 kJmol−1
2nd: 1957.9 kJmol−1
3rd: 3666 kJmol−1

Atomic radius 135 pm
Atomic radius (calc.
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6
(strongly acidic oxide)
Electronegativity 2.16 (scale Pauling)
Ionization energies
(more) 1st: 684.3 kJmol−1
2nd: 1560 kJmol−1
3rd: 2618 kJmol−1
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6, 5, 4, 3, 2, 1, 0, −1
(mildly acidic oxide)
Electronegativity 2.36 (scale Pauling)
Ionization energies 1st: 770 kJ/mol
2nd: 1700 kJ/mol
Atomic radius 135 pm
Atomic radius (calc.
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reaper-binder, or binder, was a farm implement that improved upon the reaper. The binder was invented in 1872 by Charles Withington. In addition to cutting the small-grain crop, it would also tie the stems into small bundles, or sheaves.
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6, 5, 4, 3, 2, 1, 0, −1
(mildly acidic oxide)
Electronegativity 2.36 (scale Pauling)
Ionization energies 1st: 770 kJ/mol
2nd: 1700 kJ/mol
Atomic radius 135 pm
Atomic radius (calc.
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5, 4, 3 (mildly acidic oxide)
Electronegativity 1.5 (scale Pauling)
Ionization energies 1st: 761 kJ/mol
2nd: 1500 kJ/mol
Atomic radius 145 pm
Atomic radius (calc.
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Depleted Uranium (DU) is uranium remaining after removal of the isotope uranium-235. It is primarily composed of the isotope uranium-238. In the past it was called by the names Q-metal, depletalloy, and D-38, but these have fallen into disuse.
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2
(Amphoteric oxide)
Electronegativity 2.33 (scale Pauling)
Ionization energies
(more) 1st: 715.6 kJmol−1
2nd: 1450.5 kJmol−1
3rd: 3081.
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TIN may refer to:
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2, 3
(amphoteric oxide)
Electronegativity 1.88 (Pauling scale)
Ionization energies
(more) 1st: 760.4 kJmol−1
2nd: 1648 kJmol−1
3rd: 3232 kJmol−1

Atomic radius 135 pm
Atomic radius (calc.
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Magnesium has the symbol Mg, the atomic number 12, and an atomic mass of 24.31. Magnesium is the ninth most abundant element in the universe by mass. It constitutes about 2% of the Earth's crust by mass, and it is the third most abundant element dissolved in seawater.
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Titanium (IPA: /tʌɪˈteɪniəm/) is a chemical element; in the periodic table it has the symbol Ti and atomic number 22.
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Zinc (IPA: /ˈzɪŋk/, from German: Zink) is a chemical element in the periodic table that has the symbol Zn and atomic number 30.
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Zirconium (IPA: /zəˈkəʊniəm, ˌzɛːˈkəʊniəm, zɜːɹ'kəʊniəm) is a chemical element that has the symbol Zr and has the atomic number 40.
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6
(strongly acidic oxide)
Electronegativity 2.16 (scale Pauling)
Ionization energies
(more) 1st: 684.3 kJmol−1
2nd: 1560 kJmol−1
3rd: 2618 kJmol−1
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Beryllium (IPA: /bəˈrɪliəm/) is the chemical element that has the symbol Be and atomic number 4.
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