High explosive anti-tank warhead
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High explosive anti-tank (HEAT) rounds are made of an explosive shaped charge that uses the Neumann effect (a development of the Munroe effect) to create a very high-velocity jet of metal in a state of superplasticity that can punch through solid armor.
The jet 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. Spacing is critical, as the jet disintegrates and disperses after a relatively short distance, usually well under 2 metres. The jet material is formed by a cone of metal foil lining, usually copper, though tin foil was common 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 width, although modern versions claim numbers as high as 700%[citation needed].
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 warhead design 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. Increasing the caliber to allow a greater diameter makes the gun heavier. Recoilless rifles using lighter barrels and mounts firing HEAT rounds (e.g. the British WOMBAT or Swedish Carl Gustav) have proven to be effective.
Where HEAT is used as the warhead for guided missiles, rifle grenades and spigot mortars, warhead size is not a limiting factor, as these are not contained within the firing weapon's barrel.
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 is merely an unfortunate side-effect of the name HEAT.
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[edit] History
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. Germany introduced in summer of 1940 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 summer 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.
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.
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.
[edit] Armor developments in response to HEAT rounds
Increased size and changes to the armor of main battle tanks have reduced the usefulness of HEAT to a degree, by making the needed warhead size large enough to be no longer man portable. 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.
[edit] Variations
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. Its 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), it is well suited. 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 tank (the Abrams' predecessor), carried 63 rounds for its 105 mm M68 gun.
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