High-explosive anti-tank (HEAT) is the effect of a shaped charge explosive that uses the Munroe effect to penetrate heavy armor. The warhead functions by having an explosive charge collapse a metal liner inside the warhead into a high-velocity shaped charge jet; this is capable of penetrating armor steel to a depth of seven or more times the diameter of the charge (charge diameters, CD). The shaped charge jet armor penetration effect is purely kinetic in nature; the round has no explosive or incendiary effect on the armor.
Unlike standard armor-piercing rounds, a HEAT warhead's penetration performance is unaffected by the projectile's velocity, allowing them to be fired by lower-powered weapons that generate less recoil.
The performance of HEAT weapons has nothing to do with thermal effects, with HEAT being simply an acronym.
History
HEAT warheads were developed during World War II, from extensive research and development into shaped charge warheads. Shaped charge warheads were promoted internationally by the Swiss inventor Henry Mohaupt, who exhibited the weapon before World War II. Before 1939, Mohaupt demonstrated his invention to British and French ordnance authorities. Concurrent development by the German inventors’ group of Cranz, Schardin, and Thomanek led to the first documented use of shaped charges in warfare, during the successful assault on the fortress of Ében Émael on 10 May 1940.
Claims for priority of invention are difficult to resolve due to subsequent historic interpretations, secrecy, espionage, and international commercial interest.Donald R. Kennedy, History of the Shaped Charge Effect: The First 100 Years , D. R. Kennedy & Associates, 1983
The first British HEAT weapon to be developed and issued was a rifle grenade using a cup launcher on the end of the rifle barrel; the Grenade, Rifle No. 68 /AT which was first issued to the British Armed Forces in 1940. This has some claim to have been the first HEAT warhead and launcher in use. The design of the warhead was simple and was capable of penetrating of armor.R F Eather, BSc & N Griffith, OBE MSc – Some Historical Aspects of the Development of Shaped Charges – Ministry of Defence, Royal Armament Research and Development Establishment – 1984 – page 6 – AD-A144 098 The fuze of the grenade was armed by removing a pin in the tail which prevented the firing pin from flying forward. Simple fins gave it stability in the air and, provided the grenade hit the target at the proper angle of 90 degrees, the charge would be effective. Detonation occurred on impact, when a striker in the tail of the grenade overcame the resistance of a creep spring and was thrown forward into a stab detonator.
By mid-1940, Germany introduced the first HEAT round to be fired by a gun, the 7.5 cm Gr.38 Hl/A, (later editions B and C) fired by the KwK.37 L/24 of the Panzer IV tank and the StuG III self-propelled gun . 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, just as did the British, soon turned to integrated warhead-delivery systems: In 1943, the Püppchen, Panzerschreck and Panzerfaust were introduced.
The Panzerfaust and Panzerschreck (tank fist and tank terror, respectively) gave the German infantryman the ability to destroy any tank on the battlefield from 50 to 150 meters with relative ease of use and training (unlike the British PIAT). The Germans made use of large quantities of HEAT ammunition in converted 7.5 cm Pak 97/38 guns from 1942, also fabricating HEAT warheads for the Mistel weapon. These so-called Schwere Hohlladung (heavy shaped charge) warheads were intended for use against heavily armored battleships. Operational versions weighed nearly two tons and were perhaps the largest HEAT warheads ever deployed.Article title A Brief History of Shaped Charges - Defense Technical Information, pg. 9 A five-ton version code-named Beethoven was also developed.
Meanwhile, the British No. 68 AT rifle grenade was proving to be too light to deal significant damage, resulting in it rarely being used in action. Due to these limits, a new infantry anti-tank weapon was needed, and this ultimately came in the form of the "projector, infantry, anti-tank" or PIAT. By 1942, the PIAT had been developed by Major Millis Jefferis. It was a combination of a HEAT warhead with a spigot mortar delivery system. While cumbersome, the weapon allowed British infantry to engage armor at range for the first time. The earlier magnetic hand-mines and grenades required them to approach dangerously near.Ian Hogg, Grenades and Mortars, Weapons Book #37, 1974, Ballantine Books During World War II the British referred to the Monroe effect as the "cavity effect on explosives"."The Bazooka's Grandfather". Popular Science, February 1945, p. 66, 2nd paragraph.
During the war, the French communicated Mohaupt's technology to the U.S. Ordnance Department, and he was invited to the US, where he worked as a consultant on the bazooka project.
The need for a large bore made HEAT rounds relatively ineffective in existing small-caliber anti-tank guns of the era. Germany worked around this with the Stielgranate 41, introducing a round that was placed over the end on the outside of otherwise obsolete 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 ability of the round, which is defined by explosive power, HEAT rounds were particularly useful in long-range combat where slower terminal velocity was 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. The HEAT round was particularly useful to them because it allowed the low-velocity large-bore guns used on their many assault guns to also become useful anti-tank weapons.
Likewise, the Germans, Italians, and Japanese had in service many obsolescent infantry guns, short-barreled, low-velocity artillery pieces capable of 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. High-explosive anti-tank rounds for these old infantry guns made them semi-useful anti-tank guns, particularly the German guns (the Japanese 70 mm Type 92 battalion gun and Italian 65 mm mountain gun also had HEAT rounds available for them by 1944 but they were not very effective).
High-explosive anti-tank rounds caused a revolution in anti-tank warfare when they were first introduced in the later stages of World War II. One infantryman could effectively destroy any existing tank with a handheld weapon, thereby dramatically altering the nature of mobile operations. During World War II, weapons using HEAT warheads were termed hollow charge or shape charge warheads.
Post World War II
The general public remained in the dark about shape charge warheads, even believing that it was a new secret explosive, until early 1945 when the US Army cooperated with the US monthly publication Popular Science on a large and detailed article on the subject titled "It makes steel flow like mud". It was this article that revealed to the American public how the fabled bazooka actually worked against tanks and that the velocity of the rocket was irrelevant.
After the war, HEAT rounds became almost universal as the primary anti-tank weapon. Models 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 at long-ranges also. Anti-tank missiles altered the nature of tank warfare from the 1960s to the 1990s; due to the tremendous penetration of HEAT munitions, many post-WWII main battle tanks, such as the Leopard 1 and AMX-30, were deliberately designed to carry modest armour in favour of reduced weight and better mobility. Despite subsequent developments in vehicle armour, HEAT munitions remain effective to this day.
Design
Penetration performance and effects
The jet moves at hypersonic speeds in solid material and therefore erodes exclusively in the local area where it interacts with armor material. The correct detonation point of the warhead and spacing is critical for optimal penetration, for two reasons:
If the HEAT warhead is detonated too near a target's surface, there is not enough time for the jet to fully form. 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. The jet stretches, breaks apart and disperses during travel, reducing effectiveness. Depending on the quality of the HEAT warhead, this happens around 6 to 8 times the charge diameter for low quality and around 12 to 20 times the charge diameter for high quality warheads. For example the 150 mm diameter warhead of the newer BGM-71 TOW (TOW 2) reaches a peak penetration of ~1000 mm RHA at 6.5x diameter (~1 m) and is down to ~500 mm RHA penetration at 19x diameter (~2.8 m).
An important factor in the penetration performance 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 diameters, and these numbers were typical of early weapons used during World War II. Since then, 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%.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.
As for any antiarmor weapon, a HEAT round achieves its effectiveness through three primary mechanisms. Most obviously, when it perforates the armor, the jet's residual can cause great damage to any interior components it strikes. And as the jet interacts with the armor, even if it does not perforate into the interior, it typically causes a cloud of irregular fragments of armor material to spall from the inside surface. This cloud of behind-armor debris too will typically damage anything that the fragments strike. Another damage mechanism is the mechanical shock that results from the jet's impact and penetration. Shock is particularly important for such sensitive components as electronics.
Stabilization and accuracy
Spinning imparts centrifugal force onto a warhead's jet, dispersing it and reducing effectiveness. This became a challenge for weapon designers: for a long time, spinning a shell was the most standard method to obtain good accuracy, as with any rifled gun. Most hollow charge projectiles are fin-stabilized and not spin-stabilized.
In recent years, it has become possible to use shaped charges in spin-stabilized 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 forming the liner in such a way that it has a crystalline structure which imparts spin to the jet.