High explosive anti-tank
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High explosive anti-tank, more commonly known as 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%.
HEAT rounds are less effective if they are spinning; the normal method for giving a shell accuracy. The centrifugal forces disperses the jet, as a result the warhead design needs to be 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 rifle weapon designs use lighter barrels and carriages and the combination of a light recoiless rifle gun and HEAT round e.g. WOMBAT (105 mm) or Carl Gustav (66mm) have proven effectiveness. Where HEAT is used as the warhead for guided missiles, rifle grenades and spigot mortars, warhead size is not a limiting factor.
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History
The development of HEAT weapons was spurred by some Swiss inventors who exhibited a "new" weapon before the war. Observers from several countries realised that the principle was not new but an application of the shaped charge. Presumably the Swiss made no sales.
The first HEAT warhead was a rifle grenade, the British No 68 grenade. It was followed by more effective combinations of warhead and delivery systems in the US "Bazooka", and the British PIAT spigot mortar. The Germans were quick to realize the potential of the HEAT design, introducing a larger copy of the Bazooka as the Panzerschreck, and a feared single-use system known as Panzerfaust. These systems were all short-range, meaning that combined arms could reduce their effectiveness on the battlefield. At the same time grenades using HEAT appeared the German arsenal.
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) 37mm 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 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 150mm guns (the Japanese 70mm and Italian 65mm infantry guns also had HEAT rounds available for them by 1944 but they weren't 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 kill 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.
Armor developements 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 which is 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 homogenous armor steel, the then preferred material in the construction of armored fighting vehicles.
Variations
Many HEAT-missiles today have 2 separate warheads to be more effective against reactive armor; the first, smaller warhead ignites the reactive layer, while the second, larger warhead penetrates the normal armor below. This approach requires highly sophisticated electronics to set off the two warheads milliseconds apart, and is therefore rather expensive.
Some newer anti-armor weapons incorporate a relatively new variant on HEAT that different sources call SEFOP (SElf FOrging Projectile), SFF (Self Forging Fragment) EFP (Explosively Formed Projectile) that shoots a solid or semisolid slug of metal at 6 to 8 kilometers per second rather than a jet of molten metal. The metal slug is unaffected by reactive armor and can travel up to 100 meters, or possibly more in some cases, before it disintegrates due to air friction. 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 recent Iraq war use this principle, and the US Army is reportedly experimenting with precision guided artillery shells using this principle under Project SADARM (Seek And Destroy ARMor). It seems a natural enough principle to adapt to wire-guided and laser-guided antitank missiles and short-range antitank rockets, as well (and rumor has it that a SEFOP round for the Russian RPG-7 antitank rocket is being manufactured for export by Oerlikon in Switzerland as we speak).
With the effectiveness of gun-fired HEAT rounds being lessened by the increasingly sophisticated armor, a new class of HEAT rounds known as high explosive dual-purpose, or HEDP, has become popular. These are essentially HEAT rounds effective against older tanks and other armored vehicles, with enough explosive to have a reasonable burst effect that can be used in place of conventional explosive rounds in anti-infantry roles. Typically its antipersonnel lethality is improved by incorporating coils of notched wire wrapped around the high explosive charge within the shell, or adding ball bearings to it, to increase the amount of shrapnel produced. 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, because the 120mm shells used by the main guns are so massive, bulky, and heavy that the tank can carry only 40 of them -- the M60A3 tank, which the M1 Abrams replaced in service, carried 63 rounds for its 105mm gun.