Editing High-explosive squash head

{{Short description|Ammunition type}}
{{Multiple image
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| header = High explosive squash head<br/>(HESH)
| image1 = 120MM HESH Round.jpg
| caption1 = Cross section of a 120 mm HESH round from the [[Arjun (tank)|Arjun]] [[main battle tank]]
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A '''high-explosive squash head''' ('''HESH'''), in British terminology, or a '''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|access-date=2021-10-10|website=gichd.org|page=22|archive-date=2023-03-15|archive-url=https://web.archive.org/web/20230315005137/https://www.gichd.org/fileadmin/GICHD-resources/rec-documents/Explosive_weapon_effects_web.pdf|url-status=dead}}</ref> is a type of [[explosive]] [[projectile]] with plastic explosive that conforms to the surface of a target before detonating, which improves 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 can defeat armored targets by causing [[spall]], which can injure or kill a vehicle's occupants or detonate some types of ammunition.<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 [[plastic explosive]] and a delayed-action [[fuze]] at the base of the shell.<ref name=":1"/> On impact, 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 prevent premature detonation of the plastic explosive and sustains the impact pressure and temperature.<ref name=":1"/><ref name=":0"/>

Milliseconds later, the base fuze detonates the explosive, creating a [[shock wave]] that, owing to its large surface area and direct contact with the target, is transmitted through the material. In 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: <span style="background:#0AF20C">charge</span>, <span style="background:#F40A0C">fuze</span>, <span style="background:#0CF6F4">shell</span>
File:HESH function.png|Function of HESH-munition
</gallery>

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

Depending upon the armour thickness, a heavy piece of target material ({{convert|4|to|10|kg|abbr=on}} for a {{convert|120|mm|abbr=on}} round used in [[Arjun MBT]]<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 a 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 a HESH round is more lethal than similar [[High-explosive incendiary/armor-piercing ammunition|high explosive]] rounds.<ref name=":2">{{Cite web|author=<!-- Unstated -->|title=120 mm High Explosive Squash Head Mark-I|url=https://www.drdo.gov.in/sites/default/files/inline-files/59-2CIR-16_120mm_High_Explosive_Squash_Head_Mark-I.docx|publisher=[[Defence Research and Development Organisation]] (DRDO), Government of India}} Design aspects of 120 mm HESH round for [[Arjun (tank)|Arjun MBT]]</ref><ref name=":0"/>{{clarify|date=January 2023}}
<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|Light spalling damage caused by HESH rounds
File:HESH round spall damage.png|Heavy spalling damage caused by HESH rounds
</gallery>

=== Use ===
HESH rounds are mostly fired from guns with [[rifling|rifled]], rather than [[smoothbore]], barrels. Rifling causes a projectile to spin, thereby allowing longer projectiles to be used, while at the same time improving accuracy. This also may make a HESH shell more effective on impact by increasing the surface area of contact for the explosive: the faster the spin, the larger the resultant contact patch. HESH shells are not specifically designed to perforate the armour of vehicles, unlike [[high-explosive anti-tank]] (HEAT) rounds, with their [[shaped charge]] jets. HESH shells rely instead on transmitting a shock wave through the solid steel armor.<ref name=":0">{{Cite journal|last1=Deshpande|first1=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|access-date=2021-10-02|archive-date=2015-07-03|archive-url=https://web.archive.org/web/20150703025739/http://publications.drdo.gov.in/ojs/index.php/dsj/article/view/2282|url-status=dead|url-access=subscription}}</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 (generally under {{convert|800|m/s|abbr=on}}) 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 can destroy concrete constructions much faster than a HEAT round (which is designed to penetrate armour), and without the dangerous fragmentation of a traditional [[Explosive#High|high explosive]] (HE) fragmentation round.<ref name=":1">{{Cite journal|last=Roy|first=S. N., Armament Research & Development Establishment, Kirkee|date=29 March 2014|title=Some design and functional aspects of squash head shell|url=https://publications.drdo.gov.in/ojs/index.php/dsj/article/view/6834|journal=Defence Science Journal|volume=12|issue=1|pages=55–58|doi=10.14429/dsj.12.6834|doi-broken-date=1 November 2024 }}</ref>

== History ==
HESH was developed by [[Dennistoun Burney]] in the 1940s for the British 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 post WWII was the  [[Royal Ordnance L9|L9 165 mm demolition gun]] fitted to [[Armoured Vehicle Royal Engineers|AVRE]] combat engineer vehicles.<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 |at=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 as well as concrete structures. It was widely used as a primary round in most 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|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 mainly 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, especially those that acquired the early post-World War II British {{convert|105|mm|abbr=on}} [[Royal Ordnance L7A1]] tank gun, including Germany, India, Israel, and Sweden.

Since the 1980s, HESH ammunition has increasingly lost favour as armour designs have trended towards layered composites of hard metal and heat-resistant materials. This type of armour conducts shock waves poorly. 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 using smoothbore guns due to the usage of powerful [[armour-piercing fin-stabilized discarding sabot]], which would significantly decrease the rifled gun's barrel life.

British [[Challenger 1]] and [[Challenger 2]] tanks, and India's [[Arjun (tank)|Arjun tank]] (which has the same rifled {{convert|120|mm|abbr=on}} 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 {{convert|105|mm|abbr=on}} 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 {{convert|105|mm|abbr=on}} 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 {{convert|125|mm|abbr=on}} smoothbore gun) all use HESH rounds.

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 {{convert|165|mm|abbr=on}} 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 {{convert|165|mm|abbr=on}} gun solely for a {{convert|29|kg|abbr=on}} HESH shell.

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

== 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]]
[[Category:Tank ammunition]]