View source for Engineering:Armour-piercing discarding sabot

{{Short description|Anti-tank projectile}}
[[File:24 mm slpprj m49.png|thumb|300px|Swedish "37/24 mm slpprj m/49" APDS projectile for the Bofors 37 mm anti-tank gun from 1949.<br>37/24 indicates full-calibre with sabot and sub-calibre without sabot — {{convert|37|/|24|mm|in|sigfig=3|abbr=on}}.<br>Left object shows the projectile with sabot, central object shows projectile without sabot and the right object is the projectile's tungsten-core.]]
'''Armour-piercing discarding sabot''' ('''APDS''') is a type of [[Engineering:Rifling|spin-stabilized]] [[Physics:Kinetic energy penetrator|kinetic energy projectile]] for anti-armour warfare. Each projectile consists of a sub-calibre round fitted with a [[Physics:Sabot (firearms)|sabot]]. The combination of a lighter sub-calibre projectile with a full-calibre propellant charge allows for an increase in muzzle velocity compared to full-calibre rounds, giving the round increased armour-penetration performance. To further enhance their armour-penetration capabilities, APDS rounds typically feature a hardened core made from [[Chemistry:Tungsten|tungsten]] or another hard, dense material.

For a given calibre, APDS ammunition can effectively double the armour penetration of a gun when compared to full-calibre rounds such as [[Engineering:Armor-piercing ammunition|AP]], Armour-piercing Capped (APC), and [[Engineering:APCBC|Armour piercing Capped Ballistic Cap]] (APCBC) projectiles.<ref>{{cite web |last1=Shales |first1=Julian |title=WW2 People's War - Armour Piercing Discarding Sabot |url=https://www.bbc.co.uk/history/ww2peopleswar/stories/06/a2187506.shtml |website=BBC |access-date=10 December 2021}}</ref>

APDS-rounds were commonly used in large calibre [[Engineering:Tank gun|tank gun]]s up until the early 1980s, but have since been superseded by [[Engineering:Armour-piercing fin-stabilized discarding sabot|armour-piercing fin-stabilized discarding sabot]] (APFSDS) projectiles, which use fin-stabilization and can be fired from [[Engineering:Smoothbore|smoothbore]] guns.<ref>[http://norfolktankmuseum.co.uk/types-of-ammunition/ an exemplary cut-away cross-section of the internal components of a 105mm APDS projectile]</ref> APDS rounds remain in use for small or medium calibres, such as in saboted light armour penetrator (SLAP) ammunition.<ref>{{Cite web|title=ARMY AMMUNITION DATA SHEETS, SMALL CALIBER AMMUNITION, FSC 1305|url=http://pdf.textfiles.com/manuals/MILITARY/united_states_army_tm_43-0001-27%20-%2029_april_1994.pdf|url-status=live|archive-url=https://web.archive.org/web/20121222120143/http://pdf.textfiles.com:80/manuals/MILITARY/united_states_army_tm_43-0001-27%20-%2029_april_1994.pdf |archive-date=2012-12-22 }}</ref>

== History and development ==
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|Description = British soldiers manning a 76.2 mm [[Engineering:Ordnance QF 17-pounder|Ordnance QF 17-pounder]] [[Engineering:Anti-tank gun|anti-tank gun]] during World War II. Third soldier from the left is holding an APDS round.
}}Armour piercing discarding sabot munitions were developed to increase penetrating performance of anti-tank projectiles by generating higher [[Physics:Terminal velocity|impact velocity]]. A larger projectile would require a completely new weapon system, but increasing velocity faced the limitation that steel armour-piercing (AP) projectiles shattered at velocities above about 850&nbsp;m/s when uncapped. [[Chemistry:Tungsten carbide|Tungsten carbide]], with twice the density of steel, was seen as a solution, due to its greater hardness and ability to withstand shock and pressure, but its greater weight reduced velocity.

In the 1930s, to increase terminal velocity without increasing diameter, engineers working for the French Edgar Brandt company developed "saboted" ammunition, in which a heavier sub-calibre core was surrounded by a lightweight "sabot" (the French word for a clog or the hard outer layer of hooves; however Brandt himself used term "propulsive basal member" in his English patents), that was retained for the duration of the flight until impact. They fielded two calibres; the 75&nbsp;mm/57&nbsp;mm for the 75mm Mle 1897/33 anti-tank gun and 37&nbsp;mm/25&nbsp;mm for several 37&nbsp;mm gun types just before the French-German armistice of 1940,<ref>{{cite web|url=http://www.museumoftechnology.org.uk/military.php?cab=shells|title=Shells and Grenades|publisher=The Museum of Technology|access-date=2010-10-23|archive-url=https://web.archive.org/web/20101016002149/http://www.museumoftechnology.org.uk/military.php?cab=shells|archive-date=16 October 2010 |url-status=live}}</ref> before evacuating to the United Kingdom, to join ongoing anti-tank ammunition development efforts there. The British term for this construction was  "Armour-Piercing, Composite Rigid" (APCR)

While the sabot improved short range effectiveness, the additional [[Physics:Drag#Aerodynamics|drag]] caused the performance of the shot to fall off dramatically with increasing range. Between 1941 and 1944, Permutter and Coppock, two designers with the UK Armaments Research Department (ARD), developed a sabot that was discarded immediately after leaving the barrel, so the smaller, heavier, sub-projectile could carry on at the much higher velocity, suffering less drag due to its smaller diameter.

In mid-1944, the resulting APDS (the 'D' standing for "discarding") projectile type was introduced into service for the UK's [[Engineering:Ordnance QF 6-pounder|57mm QF 6-pounder]] [[Engineering:Anti-tank gun|anti-tank gun]] and later in September 1944 for the 76mm [[Engineering:Ordnance QF 17-pounder|QF 17-pounder]] anti-tank gun.<ref>{{cite web|url=http://www.avalanchepress.com/17-Pounder.php|title=The 17-Pounder|author=Jason Rahman|publisher=Avalanche Press|date=February 2008|access-date=2010-10-23|archive-url=https://web.archive.org/web/20101109112825/http://www.avalanchepress.com/17-Pounder.php|archive-date=9 November 2010 |url-status=live}}</ref> {{efn|Muzzle velocity of the 17 pdr when firing APDS was around {{convert|3,900 |ft/s|abbr=on}}. Weight of the sub-calibre [[Chemistry:Tungsten carbide|tungsten carbide]] projectiles was approximately 7 lb (3.1 kg), considerably less than the normal AP round's weight of 17 lb (7.7 kg).{{citation needed|date=March 2022}} }}

== Construction ==
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|caption1=APDS-[[Physics:Projectile|projectile]] with [[Chemistry:Tungsten|tungsten]]-core for a {{wipe|United Kingdom}} WW2 76,2 mm [[Engineering:Ordnance QF 17-pounder|Ordnance QF 17-pounder]] [[Engineering:Anti-tank gun|anti-tank gun]]
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=== Sabot construction ===
The sabot of a large calibre APDS consists of a light high strength alloy full diameter pot and base unit, which is screwed together. The front part of the pot has three-four petals (sabots) which are covered with a centring band (often a [[Chemistry:Nylon|nylon]] derivative). The rear half has a rubber [[Physics:Obturating ring|obturator]] and driving band (again nylon) held in place by the screw-in base unit. The base unit, if a tracer element is attached to the sub-projectile, has a hole located at the centre. Before firing, the sub-projectile and sabot are locked together.

Due to the high setback forces ([[Physics:G-force|g-force]]s), friction between the pot and sub-projectile allows spin to be transferred, thus stabilising the sub-projectile. Small/medium calibre APDS use a lightweight high strength alloy base pot and three or more plastic petals. To transfer the spin to the core in small/medium calibre weapons, the core tends to have a notch at its base. Under bore acceleration, which can be higher than 100,000&nbsp;g, the uneven base is forced into the softer pot material, locking the sub-projectile to the pot and imparting spin.

Not all small/medium calibre APDS rely on this technique, another method for spin coupling is by using the forward plastic petals. The petals are of a slightly larger diameter than the [[Engineering:Rifling|lands]] in the rifled bore. This forces the petals tightly against the core, increasing the friction between them and allowing the spin to be transferred.

=== Projectile construction ===
The sub-calibre projectile consists of a high density core with a penetrating cap, enclosed within a high strength sheath (steel) with a lightweight alloy (aluminium-magnesium alloy) ballistic cap. For modern small/medium calibre APDS projectiles, the core is not sheathed and the ballistic and penetrating caps are combined. A tracer element may be added to the APDS sub-projectile, for large calibre weapons this is part of the outer sheath, for small/medium calibre weapons it is contained within a hollow cavity in, or attached to, the base of the core.

Most modern APDS projectiles use high strength shock resistant tungsten alloys. The main constituent is tungsten, alloyed or sintered with/to [[Chemistry:Cobalt|cobalt]], [[Chemistry:Copper|copper]], [[Chemistry:Iron|iron]] or [[Chemistry:Nickel|nickel]]. Very few APDS use [[Chemistry:Depleted uranium|depleted uranium]] (DU) [[Chemistry:Titanium|titanium]] alloy for the penetrator material{{Citation needed|date=October 2019}}, though the retired 20&nbsp;mm MK149-2 [[Engineering:Phalanx CIWS|Phalanx CIWS]] round did use DU.

== Function ==
=== Discarding of sabot ===
When a large calibre APDS is fired and while still within the bore, the setback forces shear the forward petals, partly unlocking the sub-projectile from the sabot, but still holding it rigidly within the pot. Gas pressure is used to delay the unlocking of the pins holding the rear part of the sub-projectile by gyroscopic forces. Once outside the barrel, the pins, centring band and forward petals are released or discarded by projectile spin, the aerodynamic drag removes the pot/base unit. 
<gallery mode="packed" heights="50" style="text-align:left">
File:Sabot Diagram Example.png|Diagram showing the operation of the discarding sabot on APDS-projectiles. This example uses a "spindle-sabot".
</gallery>

[[File:Sabot separating.gif|thumb|A modern APFSDS-T projectile shortly after muzzle exit, just as the sabot petals are separating from the penetrator.]]
As an APDS sub-projectile does not require driving bands and the core is supported at the base and [[Ogive|ogive]] region, a far more aerodynamic projectile shape can be chosen. This, in combination with the sub-projectile's higher [[Physics:Sectional density|sectional density]], gives the resulting sub-projectile vastly reduced aerodynamic drag in comparison to full-calibre tungsten-core projectiles, such as armour-piercing, composite rigid (APCR). Both the higher initial velocity and the reduced drag result in higher velocity at impact. This also lowers flight time and improves accuracy. Accuracy can suffer if there are unwanted sabot/sub-projectile interactions during discard.

=== Impact example ===
The sequence upon impact of the APDS projectile, for example the [[Engineering:Royal Ordnance L11A5|120&nbsp;mm L11]], as used on the Chieftain tank, fired L15 APDS <ref>{{cite web|url=http://www.janes.com/articles/Janes-Ammunition-Handbook/120-mm-RO-Defence-tank-gun-ammunition-United-Kingdom.html|title=120 mm RO Defence tank gun ammunition (United Kingdom), Tank and anti-tank guns|publisher=Jane's|access-date=2010-10-23|date=5 Jan 2010}}</ref> (muzzle velocity 1370&nbsp;m/s), goes as follows: the lightweight ballistic cap is crushed, the penetrating cap then strikes the armour, distributing the shock across the whole surface of the core's nose, reducing the initial shock experienced by the core. The steel sheath surrounding the core peels away, and the core goes on to penetrate the armour. The penetration of the L15 APDS is approximately 355&nbsp;mm of rolled homogeneous armour at 1000&nbsp;m.

== FAPDS ==
During penetration, a frangible projectile's core, a high density alloy, fragments into many high-velocity pieces. The resulting projectiles are called Frangible Armour Piercing Discarding Sabot (FAPDS) for APDS types, or FRAP (Frangible Armour Piercing) for full-calibre projectiles. As of 2010, Diehl Defence called its FAPDS a "Penetrator with Enhanced Lateral Effect" (PELE). It was developed with Rheinmetall.<ref name="Marketing Material">{{cite web |url=http://www.diehl.com/fileadmin/diehl-defence/user_upload/flyer/PELE_21.10.10.pdf |publisher=Diehl |title=Penetrator with Enhanced Lateral Effect (PELE) |url-status=dead |access-date=2013-06-26 |archive-url=https://web.archive.org/web/20130612055933/http://www.diehl.com/fileadmin/diehl-defence/user_upload/flyer/PELE_21.10.10.pdf |archive-date=2013-06-12 }}</ref>

The effect of a frangible projectile on a lightly armoured target is much the same as a high explosive incendiary round, but with a cloud of dense, high-velocity fragments penetrating deeper into the target's interior. Upon striking heavy armour the effect of FAPDS is more akin to a standard APDS, albeit with higher fragmentation of the core, and hence lethality if the armour is perforated.{{citation needed|date=March 2023}}

==Notes==
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==References==

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[[Category:Projectiles]]

{{Sourceattribution|Armour-piercing discarding sabot}}