ImportMaster at 19:05, 20 April 2022

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{{multiple image
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| header = High explosive squash head {{big|HESH}}
| image1 = 120MM HESH Round.jpg
| caption1 = Cross section of a 120 mm HESH round from the [[Arjun (tank)|Arjun MBT]]
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'''High explosive squash head''' ('''HESH''') in British terminology, or '''high explosive plastic/plasticized''' ('''HEP''') in American terminology,<ref>{{Cite web|title=Explosive_weapon_effects_web.pdf|url=https://www.gichd.org/fileadmin/GICHD-resources/rec-documents/Explosive_weapon_effects_web.pdf|url-status=live|access-date=2021-10-10|website=gichd.org|page=22}}</ref> is a type of [[explosive]] [[projectile]] which utilizes a plastic explosive that conforms to the surface of a target before detonating, to improve the transfer of explosive energy to the target. Squash head projectiles are similar to high explosive projectiles and are well suited to many of the same targets. However, while HESH projectiles are not armour-piercing, they are capable of defeating armored targets by causing [[spall]] which can injure or kill a vehicle's occupants.<ref name=":0" />

== Design ==
{{external media
|video1={{YouTube|0CfQ7hDZss8|HESH round working principle animation}}
|video2={{Youtube|Uhz3w8-PSl8|HESH round comparison}}
}}
=== Function ===
HESH rounds are thin metal shells filled with inert material (like [[Tar|coal-tar pitch]]<ref name=":1" />), [[plastic explosive]] and a delayed-action base [[fuze]]. Upon impacting, the inert material followed by plastic explosive is "squashed" against the surface of the target and spreads out to form a disc or "pat" of explosive. The inert material helps to prevent premature detonation of the plastic explosive and to sustain the impact pressure and temperature.<ref name=":1" /><ref name=":0" />

The base fuze then detonates the explosive milliseconds later, creating a [[shock wave]] that, owing to its large surface area and direct contact with the target, is transmitted through the material. In the case of the metal armour of a tank, the compression shock wave is conducted through the armour to the point where it reaches the metal-air interface (the hollow crew compartment), where some of the energy is reflected as a tension wave, a phenomenon called ''impulsive loading''. At the point where the compression and tension waves intersect, a high-stress zone is created in the metal, causing pieces of steel to be projected off the interior wall at high velocity.<ref name=":0" />
<gallery widths="250" heights="100">
File:Municia delostrelecka granat 08m (cropped 2).png|Shell design: charge, fuze, shell
File:HESH function.png|Function of HESH-munition
</gallery>

=== Effect ===
This fragmentation by blast wave is known as ''scabbing'' or ''spalling'', with the fragments themselves known as ''scabs'' or ''[[spall]]''.<ref name=":2" /><ref name=":0" />

Depending upon the armour thickness, a heavy chunk of target material (4 to 10 kg for a standard 120 mm round<ref name=":2" />) can separate out from the other end of the target with supersonic velocities. This spall is sufficient to permanently damage the essentials of tank, igniting the ammunition or fuel storage and severely damaging the crew to achieve a "total kill" of the target. In general, the higher the armour thickness, the higher the scab weight will be.<ref name=":0" />

The fragmentation achieved by impulsive loading of armour block by an HESH round is more lethal compared to similar [[High-explosive incendiary/armor-piercing ammunition|high explosive]] rounds.<ref name=":2">{{Cite web|last=Design aspects of 120MM HESH round|first=For [[Arjun (tank)|Arjun MBT]]. DRDO, Government of India|title=120MM HESH round|url=https://www.drdo.gov.in/sites/default/files/inline-files/59-2CIR-16_120mm_High_Explosive_Squash_Head_Mark-I.docx|url-status=live|website=[[DRDO]]}}</ref><ref name=":0" />
<gallery widths="250" heights="100">
File:HESH round ,impact from front and back side of an armour block and.jpg|Front and back side of armour block impacted by HESH rounds
File:Damage caused by HESH fort nelson.jpg|Example of light [[spall]]ing damage caused by HESH rounds
File:HESH round spall damage.png|Example of heavy spalling damage caused by HESH rounds
</gallery>

=== Use ===
HESH rounds are mostly fired from guns with [[rifled]] barrels rather than [[smoothbore]] ones. Rifling decreases the penetrating power of HEAT warheads because the centrifugal force of the spinning projectile tends to disperse the [[shaped-charge]] jet, but this same effect can assist a HESH shell by increasing the surface area of contact, the higher the spinning the larger the spat contact patch will be. Unlike [[High-explosive anti-tank warhead|high-explosive anti-tank]] (HEAT) rounds, which are shaped-charge ammunition, HESH shells are not specifically designed to perforate the armour of main battle tanks. HESH shells rely instead on the transmission of the shock wave through the solid steel armour.<ref name=":0">{{Cite journal|last=Deshpande|first=P.U.|last2=Prabhu|first2=V.D.|last3=Prabhakaran|first3=K.V.|date=2003-10-01|title=Impulsive Loading of Armour by High Explosive Squash Head Munition|url=http://publications.drdo.gov.in/ojs/index.php/dsj/article/view/2282|journal=Defence Science Journal|volume=53|issue=4|pages=357–365|doi=10.14429/dsj.53.2282}}</ref><ref>{{Cite web|title=115 mm, 120 mm & 125 mm Tank Guns|url=http://characterisationexplosiveweapons.org/studies/annex-d-115-120-125-mm-tank-guns/|access-date=2021-10-04|website=GICHD|language=en-US}}</ref>

HESH ammunition has good general purpose use, being effective against most targets, though the round is generally used at relatively low velocities because high velocity excessively disperses the pat of explosive. While only effective against tanks without [[spaced armour]] or spall liners, the round is still favoured for combat [[demolition]] purposes. The flattened high-velocity explosive pat is capable of destroying concrete constructions much faster than a HEAT round (which is designed for armour penetration), and without the dangerous fragmentation of a traditional [[Explosive material|high-explosive]] (HE) fragmentation round.<ref name=":1">{{Cite web|last=Design and functional aspects of HESH round|first=[[DRDO]], Government of India|title=HESH round design specifications|url=https://publications.drdo.gov.in/ojs/index.php/dsj/article/view/6834|url-status=live|website=Defence Science Journal}}</ref>

== History ==
HESH was developed by [[Dennistoun Burney]] in the 1940s for the UK's war effort, originally as an anti-[[bunker|fortification]] "wallbuster" munition for use against concrete. He also led British developments in [[recoilless rifle]]s as a means to deliver the shell. An early application of the HESH principle was the [[Armoured Vehicle Royal Engineers|AVRE]]'s [[Royal Ordnance L9|165 mm demolition gun]].<ref>{{Cite book|last=Edgerton|first=David|url=https://www.worldcat.org/oclc/761379325|title=Britain's war machine : weapons, resources and experts in the Second World War,Chapter-8 Boffins|date=2012|publisher=Penguin|isbn=978-0-14-102610-7|location=London|oclc=761379325}}</ref>

HESH was found to be surprisingly effective against metallic armour and concrete structures as well. It was widely used as a primary round in most of the large calibre rifled guns.

== Users ==
[[File:105mm HEP typeB (cropped 2).JPG|thumb|Japanese 105 mm Type B HESH shell for the [[Royal Ordnance L7]] rifled gun in use on the [[Type 74]] tank]]
[[File:US Navy 111214-N-BA263-276 Explosive ordnance disposal technicians assigned to Commander, Task Group (CTG) 56.1 build a 1,500-pound munitions dispo.jpg|thumb|[[US Navy]] technicians building a munitions disposal for HESH (HEP) shells]]
HESH rounds were fielded primarily by the [[British Army]] as the main explosive round of its [[main battle tank]]s during the [[Cold War]]. It was also used by other military forces, particularly those that acquired the early post-World War II British 105 mm [[Royal Ordnance L7A1]] tank gun, including Germany, India, Israel and Sweden.

Since the 1970s, HESH ammunition has increasingly fallen out of favour as armour designs have trended towards layered composites of hard metal and heat-resistant materials. This type of armour is a poor conductor of shock waves. Anti-spalling devices (''spall liners''), made of materials such as [[Kevlar]], are commonly fitted to the interior surface of modern armoured vehicles to minimise spalling effects.<ref>{{Cite web|date=2021-07-30|title=Modern Anti-Tank Ammunition|url=https://www.ontrmuseum.ca/tankmuseum/blog-post/modern-anti-tank-ammunition/|access-date=2021-10-02|website=The Ontario Regiment RCAC Museum|language=en-US}}</ref> Another reason for the declining use of HESH rounds is the preference of most armies for smoothbore guns due to the use of high power [[Kinetic energy penetrator|armour-piercing fin-stabilized discarding sabot]] which significantly decrease the rifled barrel life.

British [[Challenger 1]] and [[Challenger 2]] tanks, India's [[Arjun (tank)|Arjun tank]] (which has the same rifled 120 mm gun as the UK's MBTs) use HESH rounds as their primary ammunition. Amongst other ammunition types, the [[Stryker Mobile Gun System]] variant is to be equipped with a 105 mm HESH round for demolition and bunker-busting purposes. Argentina's [[TAM (tank)|TAM medium tanks]], Canada's [[Leopard C1]] and [[Leopard C2]] main battle tanks (all of which mount the same 105 mm gun as the Centurion), the Australian [[Leopard AS1]] main battle tank, and the Chinese [[VT-4 Main Battle Tank|VT-4 main battle tank]] (which mounts a 125 mm smoothbore gun).

HESH rounds are also carried by [[Combat engineering vehicle|armoured engineer vehicles]]; they are typically intended for use against [[fortification]]s rather than [[armoured fighting vehicle]]s. A 165 mm HESH round is used by the [[United States Army]] for the main gun of the [[M728 combat engineer vehicle]], an [[M60 Patton|M60 tank]] equipped with a [[bulldozer]] blade. Similarly, the UK's [[Centurion AVRE]] was equipped with a short 165 mm gun solely for a 29 kg HESH shell.

== See also ==
* [[Munroe effect]]
* [[High-explosive incendiary]]

== References ==
{{reflist}}

[[Category:Artillery ammunition]]
[[Category:Anti-tank rounds]]
[[Category:Anti-fortification weapons]]
[[Category:English inventions]]
[[Category:History of the tank]]

[[pl:Amunicja przeciwpancerna#przeciwpancerny pocisk odkształcalny]]

High-explosive squash head - Revision history

ImportMaster at 19:05, 20 April 2022