Editing High-explosive squash head (section)

== 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>