High-explosive anti-tank warhead

High-explosive anti-tank (HEAT) warheads are made of an explosive shaped charge that uses the Munroe effect to create a very high-velocity partial stream of metal in a state of superplasticity that can punch through solid armor.

Contrary to a widespread misconception perpetuated by the acronym, HEAT rounds do not depend in any way on thermal phenomena for their effectiveness. In particular, the shaped charge jets do not "melt their way" through armor, only reaching 500-600°C, and instead rely on extreme pressure to force a hole through the armor.

Operation
The stream moves at hypersonic speeds (up to 25 times the speed of sound) in solid material and therefore erodes exclusively in the contact area of jet and armor material. The correct detonation point of the warhead and spacing is critical for optimum penetration, for two reasons:


 * 1) If the HEAT warhead is detonated too close to the target's surface there is not enough time for the particle stream to fully develop.  That is why most modern HEAT warheads have what is called a "standoff", in the form of an extended nose cap or probe in front of the warhead.
 * 2) The distance is critical because the stream disintegrates and disperses after a relatively short distance, usually well under 2 metres. The stream material is formed by a cone of metal foil lining, usually copper, though ductile iron and tin foil was commonly used during the Second World War.

The key to the effectiveness of a HEAT round is the diameter of the warhead. As the penetration continues through the armor, the width of the hole decreases leading to a characteristic "fist to finger" penetration, where the size of the eventual "finger" is based on the size of the original "fist". In general, very early HEAT rounds could expect to penetrate armor of 150% to 250% of their diameter, and these numbers were typical of early weapons used during World War II. Since the Second World War, the penetration of HEAT rounds relative to projectile diameters has steadily increased as a result of improved liner material and metal jet performance. Some modern examples claim numbers as high as 700%.

HEAT projectile stabilization and accuracy
HEAT warheads become much less effective if they are rapidly spinning, which became a challenge for weapon designers – for a long time, spinning the shell was the most standard method for obtaining good accuracy, as with any rifled gun. However, the centrifugal force of a spinning shell disperses the charge jet. Consequently, most hollow charge projectiles are fin-stabilized and not spin-stabilized. The round could be fired from smoothbore barrel, losing some accuracy.

In recent years it has become possible to use shaped charges in spin-stabilised projectiles by imparting an opposite spin on the jet so that the two spins cancel out and result in a non-spinning jet. This is done either using fluted copper liners, which have raised ridges or by manufacturing the liner in such a way that it has a crystalline structure which itself imparts a spin on the jet.

Besides spin-stabilisation, another problem with any barreled weapon (that is, a gun) is that large-diameter shell has worse accuracy than small-diameter shell of the same weight. The lessening of accuracy increases dramatically with range. Paradoxically, this leads to situation when a kinetic armor-piercing projectile is more usable at long ranges than a HEAT projectile, despite the latter having a higher armor penetration. To illustrate this: a stationary Soviet T-62 tank, firing from (smoothbore) cannon at a range of 1000 meters against a target moving 19 km/h, was rated to have a first-round hit probability of 70% when firing a kinetic (APFSDS) projectile. Under the same conditions, it could expect 25% when firing HEAT round. This affects combat on open battlefield with long lines of sight; the same T-62 could expect a 70% first-round hit probability using HEAT rounds on target at 500 meters.

A further problem is that, if the warhead is contained inside the barrel, its diameter becomes overly restricted by the caliber of that barrel. In non-gun applications, when HEAT warhead is delivered with missiles, rockets, bombs, grenades, or spigot mortars, the warhead size is no longer a limiting factor. In these cases HEAT warhead often seems oversized in relation to the round's body. Classic examples of this include the German Panzerfaust and Soviet RPG-7.

History
Shaped charge warheads were promoted internationally by the Swiss inventor Henry Mohaupt, who exhibited the weapon before the Second World War. Prior to 1939 Mohaupt had demonstrated his invention to British and French ordnance authorities. During the war the French communicated the technology to the U.S. Ordnance Department, who recognized the significance of Mohaupt's research and invited him to the U.S.A. He arrived there in October 1940 to work as a consultant on the Bazooka project, one of the first applications of the shaped charge. Independent secret development by the German group of Cranz, Schardin, and Thomanek led to the first documented use of shaped charges in warfare, in a successful assault on the fortress of Eben Emael on 10 May 1940. Claims for priority of invention are difficult to resolve due to subsequent historic interpretations, secrecy, espionage, and international commercial interest. It is not even certain the German and Swiss groups were truly independent.

By mid-1940 Germany introduced the first HEAT round to be fired by a gun, the 7.5 cm fired by the Kw.K.37 L/24 of the Panzer IV tank and the Stug III self-propelled gun (7.5 cm Gr.38 Hl/A, later editions B and C). In mid-1941 Germany started the production of HEAT rifle-grenades, first issued to paratroopers and by 1942 to the regular army units (Gewehr-Panzergranate 40, 46 and 61), but as with the British soon turned to the integrated warhead-delivery systems: In 1943 the Püppchen, Panzerschreck and Panzerfaust were introduced. The Panzerfaust and Panzerschreck or 'tank terror' gave the German infantryman the ability to destroy any tank on the battlefield from 50-150m with relative ease of use and training (unlike the UK PIAT). The Germans also made use of large quantities of HEAT ammunition in converted 7.5 cm Pak 97/38 guns from 1941, later fabricating a HEAT warhead for the Mistel weapon. The latter weighed nearly two tons and was perhaps the largest HEAT warhead ever deployed. It was intended for use against heavily armoured warships, like battleships.

During World War II the British referred to the Monroe effect as the cavity effect on explosives. The first British HEAT weapon to be developed and issue was a rifle grenade using a 2 1/2 inch cup launcher on the end of the barrel; the British No. 68 AT grenade issued to the British army in 1940. By 1943 the combination of HEAT warhead and spigot mortar delivery in the cumbersome PIAT weapon at last allowed British infantry to engage armour at ranges not bordering on suicide, about 115 yards; as was sometimes the case with magnetic hand-mines and grenades. The Germans came up with an effective counter to hollow-charged or HEAT weapons such as the PIAT or Bazooka through the use of armoured skirts or mesh skirts on their tanks, which caused the HEAT shell to detonate prematurely.

The need for a large bore made HEAT rounds relatively ineffective in existing small-caliber anti-tank guns of the era. The Germans were able to capitalize on this, however, introducing a round that was placed over the end on the outside of their otherwise obsolete 37 mm anti-tank guns to produce a medium-range low-velocity weapon.

Adaptations to existing tank guns were somewhat more difficult, although all major forces had done so by the end of the war. Since velocity has little effect on the armor-piercing capability of the round, which is defined by explosive power, HEAT rounds were particularly useful in long-range combat where the slower terminal velocities were not an issue. The Germans were again the ones to produce the most capable gun-fired HEAT rounds, using a driving band on bearings to allow it to fly unspun from their existing rifled tank guns. HEAT was particularly useful to them because it allowed the low-velocity large-bore guns used on their numerous assault guns to become useful anti-tank weapons as well. Likewise, the Germans, Italians, and Japanese had many obsolescent "infantry guns" in service (short-barreled, low-velocity artillery pieces capable of both direct and indirect fire and intended for infantry support, similar in tactical role to mortars; generally an infantry battalion had a battery of four or six). HEAT rounds for these old infantry guns made them semi-useful anti-tank guns, particularly the German 150 mm guns (the Japanese 70 mm and Italian 65 mm infantry guns also had HEAT rounds available for them by 1944 but they were not very effective).

The general public remained in the dark about shape charge warheads (i.e. they even believed that it was a new super secret explosive) until early 1945 when the US Army co-operated with the US monthly publication Popular Science on a large and detailed article on the subject. Most Americans were shocked to learn that even their enemies in World War II had what the US Army referred to as shape charge warhead weapons. During World War II, weapons using HEAT warheads were known as having a hollow charge or shape charge warhead.

HEAT rounds caused a revolution in anti-tank warfare when they were first introduced in the later stages of World War II. A single infantryman could effectively destroy any existing tank with a handheld weapon, thereby dramatically altering the nature of mobile operations. After the war HEAT became almost universal as the primary anti-tank weapon. HEAT rounds of varying effectiveness were produced for almost all weapons from infantry weapons like rifle grenades and the M203 grenade launcher, to larger dedicated anti-tank systems like the Carl Gustav recoilless rifle. When combined with the wire-guided missile, infantry weapons were able to operate in the long-range role as well. Anti-tank missiles altered the nature of tank warfare throughout the 1960s and into the 80s, and remain an effective system today.

Helicopter armament
Helicopters have also carried anti-tank guided missiles (ATGM) tipped with HEAT warheads since the early 1960s. The first example of this was the use of the Nord SS.11 ATGM on the Aérospatiale Alouette II helicopter by the French armed forces. Subsequently, such antitank-capable helicopters were widely adopted by other nations.

On April 13, 1972, Chief Warrant Officer Barry McIntyre, Major Larry McKay, First Lieutenant Steve Shields, and Captain Bill Causey became the first helicopter crews to destroy enemy armour in combat during the Vietnam War. A flight of two Cobra helicopters from Battery F, 79th Artillery, 1st Cavalry Division, U.S. Army, were armed with the newly developed 2.75" HEAT rockets, which were yet untested in combat. The specially modified Huey which was shipped in an emergency destroyed three T-54 tanks that were about to overrun a U.S. command post. McIntyre and McKay engaged first, destroying the lead tank.

Armor developments
Improvements to the armor of main battle tanks have reduced the usefulness of HEAT warheads by making man portable HEAT missiles heavier, although many of the world's armies continue to carry man portable HEAT rocket launchers for use against vehicles and bunkers. In unusual cases, shoulder-launched HEAT rockets are believed to have shot down U.S. helicopters in Iraq.

The reason for the ineffectiveness of HEAT munitions against modern main battle tanks can be attributed in part to the use of new types of armor. The jet created by the explosion of the HEAT round must have a certain distance from the target and must not be deflected. Reactive armor attempts to defeat this with an outward directed explosion under the impact point, causing the jet to deform and so greatly reducing penetration power. Alternatively, composite armor featuring ceramics erode the liner jet more quickly than rolled homogeneous armor steel, the then-preferred material in the construction of armored fighting vehicles.

Spaced armor and slat armor are also designed to defend against HEAT rounds, protecting the vehicle by causing a premature detonation of the explosive at a relatively safe distance away from the main armor of the vehicle.

Variations
Many HEAT-armed missiles today have two (or more) separate warheads (known as a tandem charge) to be more effective against reactive or multilayered armor; the first, smaller warhead initiates the reactive armor, while the second (or other), larger warhead penetrates the armor below. This approach requires highly sophisticated fuzing electronics to set off the two warheads the correct time apart, and also special barriers between the warheads to stop unwanted interactions; this makes them more expensive to produce.

The latest HEAT warheads, such as 3BK-31, feature triple charge, where the first one penetrates the spaced armor, the second - reactive or first layers of armor, and the third one finishes the penetration. The total penetration value may reach up to 800 mm.

Some anti-armor weapons incorporate a variant on the shaped charge concept that, depending on the source, can be called an Explosively Formed Penetrator (EFP), Self Forging Fragment (SFF), SElf FOrging Projectile (SEFOP), plate charge, or Misznay Schardin (MS) charge. This warhead type uses the interaction of the detonation wave(s), and to a lesser extent the propulsive effect of the detonation products, to deform a dish/plate of metal (iron, tantalum, etc.) into a slug-shaped projectile of low length to diameter ratio (L to D) and project this towards the target at around two kilometres per second.

The SFF is relatively unaffected by first generation reactive armor, it can also travel up to, and above 1000 cone diameters (CDs) before its velocity becomes ineffective at penetrating armor due to aerodynamic drag, or hitting the target becomes a problem. The impact of a SFF normally causes a large diameter, but relatively shallow hole (in comparison to a shaped charge) of, at best, a few CDs. If the SFF perforates the armor, extensive behind armor damage (BAD, also called behind armor effect (BAE)) occurs. The BAD is mainly caused by the high temperature and velocity armor and slug fragments being injected into the interior space and also overpressure (blast) caused by the impact.

More modern SFF warhead versions, through the use of advanced initiation modes, can also produce rods (stretched slugs), multi-slugs and finned projectiles, and this in addition to the standard short L to D ratio projectile. The stretched slugs are able to penetrate a much greater depth of armor, at some loss to BAD, 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 armored areas of MBTs—the top, belly and rear armored areas for example. It is well suited for use in the attack of other less heavily armored armored fighting vehicles (AFVs) and in the breaching of material targets (buildings, bunkers, bridge supports, etc.). The newer rod projectiles may be effective against the more heavily armored areas of MBTs.

Weapons using the SEFOP principle have already been used in combat; the smart submunitions in the CBU-97 cluster bomb used by the US Air Force and US Navy in the 2003 Iraq war used 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 SFF principle.

With the effectiveness of gun-fired single charge HEAT rounds being lessened, or even negated by the increasingly sophisticated armoring techniques, a class of HEAT rounds known as high-explosive anti-tank multi-purpose, or HEAT-MP, has become more popular. These are essentially HEAT rounds which are effective against older tanks and other armored vehicles, but have improved fragmentation, blast and fuzing. This gives the projectiles an overall reasonable light armor and anti-personnel/materiel effect so that they can be used in place of conventional high-explosive rounds against infantry and other battlefield targets. This reduces the total number of rounds that need to be carried for different roles, which is particularly important for modern tanks like the M1 Abrams, due to the sheer size of 120 mm rounds used. The M1A1/M1A2 tank can carry only 40 rounds for its 120 mm M256 gun—the M60A3 Patton tank (the Abrams' predecessor), carried 63 rounds for its 105 mm M68 gun. This effect is reduced by the higher first round hit rate of the Abrams with its improved fire control system compared to the M60. The frequent fuel replenishments required for the Abrams' fuel-hungry turbine also make simultaneous ordnance replenishment a marginal burden.

High-explosive dual purpose
Another variation on HEAT warheads is surrounding them with a conventional fragmentation casing, to allow the warhead to be more effectively used for blast and fragmentation attacks on unarmored targets. In some cases this is merely a side effect of the armor-piercing design, in other cases a dual role is specifically designed in. Some warheads have been known as HEDP—high-explosive dual purpose.