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A HEAT round. The copper-lined conical shaped area can be clearly seen in this cutaway.
PARS 3 LR with HEAT warhead of the German Army.

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

Contents

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 cape or probe in front of the warhead.[notes 1]
  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 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, HEAT rounds can 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%[1]

HEAT rounds are less effective if they are spinning, the normal method for giving a shell accuracy. The centrifugal force disperses the jet, so the round needs to be fired from smoothbore weapons, or else modified for use with rifled guns. A further problem is that if the warhead is contained inside the barrel, then its diameter is restricted to the caliber of the gun. Cannon-fired HEAT rounds are not as accurate and have a shorter effective range than armor piercing rounds that depend on kinetic energy to penetrate armor. These characteristics are a result of the shape of cannon-fired HEAT rounds, a shape that is optimized for delivery of the round's high velocity metal jet, but less than optimal for long flight and accuracy. The lessening of accuracy increases dramatically with range. A stationary Soviet T-62 tank, firing 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, the T-62 could only expect a first-round hit probability of 25% when firing its HEAT round.[2] These characteristics of HEAT rounds are disadvantageous only on open battlefields 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.

In non-gun applications, when HEAT is used as the warhead for guided missiles, glide bombs, rifle grenades, or spigot mortars, warhead size is not a limiting factor, as these are not contained within the firing weapon's barrel. In these cases HEAT rounds often use seemingly oversized warheads on smaller bodies. Classic examples include the German Panzerfaust and Soviet RPG7, while most modern examples are based on missile bodies. In recent years, with the ending of the cold war, attention has turned to lighter armour, and especially weapons that are useful against bunkers and similar targets. In these cases, weapons originally intended to deal with light armour using HEAT are once again finding use in the field, the Swedish Carl Gustav) has recently re-entered production for use by U.S. forces.

Contrary to a widespread misconception, 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. This confusion has arisen from the acronym HEAT, as well as early descriptions of how the weapons worked, including the first official manuals for the M72 LAW issued to the US Army and USMC at the start of the Vietnam War.

History

Soviet 125 mm HEAT BK-14

The development of HEAT weapons was spurred by some Swiss inventors who exhibited a "new" weapon before the Second World War. Observers from several countries realised that the principle was not new but an application of the shaped charge.

The first HEAT warhead was a rifle grenade, the British No. 68 AT grenade. It was followed by more effective combinations of warhead and delivery systems in the US "Bazooka", and the British PIAT spigot mortar. In mid-1940 Germany introduced the first HEAT round to be fired by a gun, the 7.5 cm Gr. 38 fired by the 7.5 cm Kw.K. of the Panzer IV tank and the Stug-III self propelled gun. In mid-1941 the first HEAT rifle-grenade (issued to paratroopers) and the improved 7.5 cm Gr. 38 Hl/A design followed. In 1942 Germany started the production of HEAT rifle-grenades for regular army units and introduced another improved design, the Gr. 38 Hl/B. In 1943 finally the Püppchen, Panzerfaust and Panzerschreck were introduced together with the design of the Gr. 38 HL/C. Other uses of HEAT, by the Germans, was for the neutralisation of the armoured gun turrets of the Belgian Fort Eben-Emael by commando troops, and later as warhead on the "Mistel" weapons. The latter was perhaps the largest HEAT warhead ever used, and was intended for use against heavily armored warships, for example battleships.

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 outdated (and basically useless) 37 mm anti-tank guns to produce a medium-range low-velocity weapon. A more convincing system was created by making a much larger tripod-mounted version of the Panzerschreck, producing the 7.5 cm Leichtgeschütz 40, what is today known as a recoilless rifle. The recoilless rifle had the range to stay easily hidden on the battlefield, was light enough to be portable by a small team, but had the performance needed to defeat any tank. The main drawbacks, was a short range and a large back blast that gave the weapon's position away when fired.

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).

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 antitank guided missiles (ATGM) tipped with HEAT warheads since the early 1960's. 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.[3]

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.[4] Today HEAT rounds are primarily used in shoulder-launched and in jeep- and helicopter-based missile systems. Tanks mostly use the more effective APFSDS rounds.

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 penetration power is greatly reduced. 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

A Russian 3BK29 HEAT round

Many HEAT-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 rather more expensive to produce.

Some anti-armor weapons incorporate a variant on the shaped charge concept that, depending on the source, can be called a Self Forging Fragment (SFF), Explosively Formed Penetrator (EFP), 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 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 (Main Battle Tanks), the top, belly and rear armored areas for example. It is well suited for use in the attack of other less heavily armored AFVs (armored fighting vehicles) 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.

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High explosive dual purpose

M430A1 HEDP.

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.

See also

Notes

  1. ^ Both the US TOW and the French-German MILAN wire guided antitank missiles almost doubled their maximum penetration by the addition of a standoff probe.

References

  1. ^ Jane's Ammunition Handbook 1994, pp. 140-141, addresses the reported ~700 mm penetration of the Swedish 106 3A-HEAT-T and Austrian RAT 700 HEAT projectiles for the 106 mm M40A1 recoilless rifle.
  2. ^ Jane's Armour and Artillery 1981-82, p. 55.
  3. ^ Michael P Kelley, Where We Were In Vietnam.
  4. ^ Aviation Week Report.

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