High Explosive Anti-Tank (HEAT) Armor Piercing Shaped Charge

From the days of sword and shield to the present, the history of warfare has been a competition between assault weapons and armor. As more powerful and sophisticated projectiles have been created, armor systems have begun to employ reinforced plastics, ceramics and even strategically placed empty space in addition to or in lieu of conventional metallic armor.

Anti-armor projectiles can be classified in three general categories: those employing solid penetrators, usually driven by mass/velocity consideration (HEP--high energy penetrator); those employing shaped charges to generate a high velocity rod formed by the explosive collapse of a metallic cone for penetration (HEAT--high explosive anti-tank) and those employing a different shaped explosive charge to promote spalling on the inside of the armor (HESH--high explosive squash head). A subcategory of the solid penetrators also exists whereby incendiary materials are placed in combination with the penetrator to burn during penetration and after breaching the armor. These weapons are generally dependent upon achieving their penetrating and explosive effects more or less instantaneously with impact with the target.

HEAT - high explosive anti-tank - armor piercing shells comprise a special type of anti-tank ammunition which is provided with a hollow charge warhead. In principle, a hollow charge comprises an outer casing, a metal cone and an explosive. When the explosive detonates, the metal cone is squeezed together and a metal jet is formed which, with great force, penetrates even very thick and hard armor. Due to its good effect in armored targets, the hollow charges have long constituted a serious threat to armored vehicles.

The High Explosive Anti-Tank (HEAT) rounds take a cone-shaped shaped charge warhead to targets. This shaped charge warhead, with its inherent blast and fragmentation capability, also provides additional weapon defeat capability. A copper shaped charge liner and wave shaper are contained within the warhead.

A sophisticated heavy two-stage shaped-charge warhead is capable of piercing armor of equivalent to 900mm thickness. A triple-shaped charge warhead offers 50mm more penetration. The RPG-7 grenade, with a shaped-charge warhead, has very good armor penetration (330 mm), capable of defeating most types of armored vehicles. Even a small 440 gram shaped-charge explosive is extremely destructive, and can penetrates more than 14 inches (35.6 cm) of armor. The M77 submunition's antimateriel capability is provided through a shaped charge with a built-in standoff, which can penetrate up to four inches of armor. The smaller artillery-delivered M46 submissions have a shaped charge warhead that penetrates 2.75 inches of homogeneous armor.

High explosive anti-tank (HEAT) rounds have been in the American arsenal since World War II. The penetration mechanism within the HEAT warhead is the shaped-charge. A shaped-charge consists of a thin-walled metallic cone (the liner) with high explosive molded around the outside of the cone. When the warhead reaches a preset stand-off distance from the target, the explosive is detonated. A shock front passes over the liner causing the liner to progressively deform and collapse upon itself.

Under the extreme pressures of the explosion, the solid metallic liner converts to a semi-liquid or amorphous state. When opposing sides of the converging liner meet during collapse, a portion of the metal liner is "squirted" forward with high velocity (approx. 9 km/sec). This material comprising a train of discrete masses at high temperature moving at supersonic velocity is called the jet, and constitutes the penetrating element of the warhead. In the prior art, the flight axis of the jet is generally forward along the axis of symmetry of the warhead. Where guidance, target seeking or sensing components are located in a warhead ogive forward of the shaped charge, the amorphous jet must penetrate such components enroute to the target material which degrades its performance against the target.

The phenomena of shaped-charge or HEAT rounds is particularly important in deflecting enemy combat vehicles such as tanks. Such vehicles are defended primarily by two mechanisms; namely, (a) composition or internal structure of armor plates and sheets mounted on the vehicle, and (b) angle of obliquity presented to incoming anti-tank rounds by such plates or sheets.

Overcoming sloped armor system surfaces in enemy vehicles has been a major challenge to the U.S. Army over the years. Various techniques have been employed previously to accomplish this while more shallow obliquity angles are achieved to deflect incoming warheads. Having the projectile strikes the target while travelling a path of descending arc is one technique which lessens the effects of shallow obliquity of protective armor around the target. Recently, some projectiles have been constructed so that the penetration mechanism is built into the warheads that its penetration force is angularly displaced from the warhead axis of symmetry. Thus upon detonation, the penetrating mechanism of the warhead (a shaped-charge jet) leaves the projectile at a predetermined and built in displacement angle relative to the warhead axis or trajectory.

Each of the previous techniques used to overcome sloped armor defended targets carries with it some drawbacks. Having the projectile strike the target while travelling a descending arcuate path requires that the speed of the projectile be low enough to respond to aerodynamic forces as necessary to cause a continuously changing flight path. Having the projectile travel at low speeds introduces certain additional problems, one of which is that aiming a low speed projectile is considerably more difficult than aiming a high speed projectile. Incorporating the warhead obliquely into the projectile may eliminate the need to fire the projectile at a slow rate of speed. With this construction, the projectile can strike a sloped target surface ineffectively head-on, but the penetrating jet train travels along a path other than the flight axis of the warhead, thus overcoming successfully the target surface obliquity which defeats the projectile as it hits the target.

However, this construction produces certain additional problems. Constructing the warhead at an angle in the projectile lessens the allowable size of the warhead for any given projectile diameter. An example of this phenomenon is as follows: if one were to build a shaped charge into the projectile at an angle of ninety degrees (so that the penetrating element would travel perpendicular to the center axis of the projectile), the allowable length of the shaped charge could not exceed the projectile diameter. For built in obliquities or angles between zero (conventional) and ninety degrees (perpendicular), the allowable length of the shaped charge is decreased from a maximum effectiveness length criterion to the projectile diameter, thus comprising its lethality or penetration effectiveness.