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Shaped Charge

Category: Term of the day

Shaped Charge

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.

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 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.

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).

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