Shell (projectile)
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A shell is a projectile, which, as opposed to a bullet, is not solid but contains an explosive or other filling, though modern usage includes large projectiles without a filling which are properly termed shot.
These objects of weaponry are generally large rounds fired by artillery, armored fighting vehicles (including tanks), and warships, such as battleships.
Most shells are aerodynamic and hence, tend to have similar shapes to bullets—that is, a cylinder topped by an ogive shaped nose, possibly with a tapering base—but some specialised types are quite different.
History
For the most part explosive shells do not appear to have been in general use before the middle of the 16th century. About that time hollow balls of stone or cast iron were fired from mortars. The balls were nearly filled with gunpowder and the remaining space with a slow-burning composition. This method was fairly ineffective as the charge was not always ignited by the flash from the discharge of the gun, and moreover the amount of composition to burn a stipulated time could not easily be gauged.
The shell was, therefore, fitted with a hollow forged iron or copper plug, filled with slow-burning powder. It was impossible to ignite with certainty this primitive fuze simply by firing the gun; the fuse was consequently first ignited and the gun fired immediately afterwards. This entailed the use of a mortar or a very short piece, so that the fuze could be easily reached from the muzzle without unduly endangering the gunner, in turn implying low muzzle velocities and high trajectories.
In 1823, the first shell guns were invented by the French General Henri-Joseph Paixhans. Paixhans guns were the first guns to combine explosive shells and the flat trajectory of cannons. The guns were adopted by various navies from the 1840s, thereby triggering the demise of wooden ships, and the iron hull revolution in boat building. Cast-iron spherical common shell (because they were used as "common" [usual] targets) were in use up to 1871. For guns they were latterly fitted with a wooden disc called a sabot, attached by a copper rivet, intended to keep the fuze central when loading. They were also supposed to reduce the rebounding tendency of the shell as it travelled along the bore on discharge. Mortar shells were not fitted with sabots.
Cast iron held its own as the most convenient material for projectiles up to the end of the 19th century, steel supplanting it, first for projectiles intended for piercing armour, and afterwards for common shell for high-velocity guns where the shock of discharge has been found too severe for cast iron.
During the First World War, shrapnel shells inflicted terrible casualties on infantry - accounting for nearly 70% of all the casualties of the war. Shells filled with poison gas were used from 1917 onwards. Frequent problems with shells led to many military disasters when shells failed to explode, most notably during the 1916 Battle of the Somme.
Calibre
The calibre of a shell is its diameter. Depending on the historical period and national preferences, this may be specified in millimetres, centimetres, or inches. Care should be taken as the length of gun barrels is frequently quoted in terms of calibre.
Due to problems of manufacture, the lower size limit for shells is a calibre around 20 mm, used in aircraft cannon and on armoured vehicles. Smaller explosive projectiles exist, but they are rare due to the difficulty in manufacture and the small explosive charge that can be carried. The largest shells ever fired were those from the German super-railway guns, Gustav and Dora, which were 800 mm (31.5") in calibre. Very large shells have been replaced by rockets, guided missile, and bombs, and today the largest shells in use are 203 mm (8 inches). Guns of that size are uncommon; 155 mm (6 inches) is the largest calibre in common use.
Gun calibres have standardized around a few common sizes, especially in the larger range, mainly due to the uniformity required for efficient military logistics. Shells of 105, 120, and 155 mm diameter are common for NATO forces' artillery and tank guns. Artillery shells of 122 and 152 mm, and tank gun ammunition of 100, 115, or 125 mm calibre remain in use in Eastern Europe and China. Most common calibres have been in use for many years, since it is no small feat to change over ammunition stockpiles.
The weight of shells varies greatly. A 150 mm (6") shell weighs about 50 kg, a 203 mm (8") shell weighs either 100 kg or 146 kg (concrete demolition variant). Of the largest calibres, used exclusively on battleships, a 280 mm (11") shell weighs about 300 kg, and a 460 mm (18") shell weighs over one and a half tonnes. The two types of projectiles used with the Nazis' Dora mega-gun measured 5 and 8 tonnes, respectively.
Old-style British classification by weight
Historically, shells were often described in pounds in the UK, e.g., as "two-pounder ammunition" shortened to "2-pdr". Usually this refers to the actual weight of a high explosive (HE) shell, but confusingly, this was not always the case. Some were named after the weights of obsolete shell types of the same calibre, or even obsolete shell types that were considered to have been functionally equivalent. Also, shells fired from the same gun, but of different weight, took their name from the gun. Thus, conversion from "pounds" to an actual barrel diameter requires consulting a historical reference.
Types
There are many different types of shells. The principal ones include:
High Explosive (HE)
The most common shell type is high explosive, commonly referred to simply as HE. HE shells have a strong steel case, a bursting charge, and a fuze. When the fuze initiates the shell, the bursting charge shatters the case and scatters hot, sharp fragments of steel at high velocity. Most of the damage is caused by being struck by these fragments, rather than directly by the blast. Depending on the type of fuze used the HE shell can be set to burst on the ground, in the air above the ground, or after penetrating a short distance into the ground (either to transmit more ground shock to covered positions, or to reduce the spread of fragments).
Armour-Piercing (AP)
In naval warfare and older anti-tank shells, the shell had to withstand the shock of punching through armour plate. Shells designed for this purpose had a greatly strengthened case with a specially hardened and shaped nose, and a much smaller bursting charge or even no bursting charge for smaller calibres. A further refinement of the design improved penetration by adding a softer metal cap to the penetrating nose giving APC (Armour piercing - capped). The softer cap took away some of the initial shock that would otherwise shatter the round. However the best profile for the cap was not the best for flight. To restore aerodynamics a further hollow cap was added to give APCBC (APC + Ballistic Cap). Explosive AP shells were sometimes distinguished by appending the suffix "-HE" or "/HE". Solid shot AP projectiles were so uncommon, that for unnecessary repetition the suffix "-HE" is usually not used; all projectiles can be assumed to have even small explosive charge. Plain AP shell is now very rarely seen except in naval usage, and is uncommon even there. See also: Armor-piercing shot and shell
Armour-Piercing, Discarding Sabot (APDS)
Image:Obus 501556 fh000022.jpg APDS was developed by the United Kingdom and put into British service in March 1944 with their 6 pdr and 17 pdr anti-tank guns. For increased penetrating power a high velocity round was required, this in turn required a stronger material (such as tungsten) to withstand the greater shock of impact. Such a shot was too heavy at full bore to be accelerated to a sufficient muzzle velocity, so a lightweight outer carrier, the Sabot, (a French word for a wooden shoe) which filled the barrel was fitted around the smaller-diameter shot. This gives the projectile a higher acceleration in the gun's barrel, due to the larger surface area for the gases to impinge upon relative to its weight. Once outside the barrel, the sabot is stripped off by a combination of centripetal force and aerodynamic force, giving the shot low drag in flight. For a given caliber, this type of ammunition can effectively double the anti-tank performance of a gun over those using "simple" shot.
A kinetic energy penetrator that is a cross between APDS and APFS (armour-piercing, fin-stabilized) is APFSDS (armour-piercing, fin stabilized, discarding sabot). In this the projectile is made long and thin to increase its sectional density and thus penetration. However once a projectile is more than about ten times longer than it is wide, spin stabilisation becomes ineffective, so the projectile is instead stabilised by fins attached at its base, and is fired from an unrifled barrel. An APFSDS projectile looks like a big metal arrow. APFSDS projectiles are often made from tungsten alloys, but the denser depleted uranium offers greater penetration.
APDS, APFS, and APFSDS rounds are solid "shot" and contain no explosive charge and are not therefore "shells".
Armour-Piercing, Composite Rigid (APCR)
Armour-Piercing, Composite Rigid is a British term, the US term for the design was High Velocity Armour Piercing (HVAP) and German, AP40. The APCR projectile is a core of a high-density hard material such as tungsten carbide surrounded by a full bore shell of a lighter material (e.g. an aluminium alloy). The projectile shape in general resembles that of the standard APCBC shot (though the German Pzgr 40 and some Soviet designs resembled a stubby arrow) but the projectile weight is less - up to half that of a standard AP shot of the same calibre. This means that it can be fired to a higher velocity. The kinetic energy of the shot is concentrated in the core and hence a smaller impact area improving the penetration of the target armour. To prevent shatter on impact, a shock buffering cap was placed between the core and the outer ballistic shell as with APC rounds. However because the shot is lighter but still the same overall size it has poorer ballistic qualities and loses velocity and accuracy at longer ranges. The APCR was superseded by the design of the APDS which dispensed with the outer light alloy shell once the shot had left the barrel. The Germans were using an APCR round, the Pzgr 40 "arrowhead" shot, for their 5 cm Pak 38 tank guns in 1942 and it was also developed for their 75 and 88 mm tank guns. As the Hartkernmunition it was used for German aircraft mounted anti-tank guns. Shortages of the key component, tungsten, led to the Germans dropping the use of the type during World War 2 because it was more useful in machine tools than fired at Soviet tanks.
Armour-Piercing, Composite Non-Rigid (APCNR)
Armour-Piercing, Composite Non-Rigid (APCNR), the British term, is based on the same projectile design as the APCR - a high density core within a shell of soft iron or other alloy. The difference is in the gun that fires it which has a tapered barrel, either a taper in a fixed barrel (Gerlich design in German use) or a final added section as in the British "squeeze -bore" (Littlejohn adaptor). On firing the round is full bore but the outer shell deforms as passes through the taper. Flanges or studs are swaged down in the tapered section, so that as it leaves the muzzle the projectile has a smaller overall cross section. This gives it better flight characteristics with a higher sectional density and the projectile retains velocity better at longer ranges than an equivalent weight shell. As with the APCR, at impact, the kinetic energy of the round is concentrated at the core. The Germans deployed their 28/20mm PzB 41, their initial taper barrelled design, as a light anti-tank weapon early in the war, but although HE projectiles were designed and put into service, the limiting of the shell design to the muzzle bore reduced their mass to only 85 grammes and hence effectiveness. The British used the Littlejohn squeeze-bore adaptor which could be attached or removed as necessary, to extended the usefulness of their QF 2 pdr gun in armoured cars and light tanks which could not take a larger gun. Although it meant that a full range of shells and shot could be used changing the adaptor in the heat of battle was highly impractical. The APCNR was superseded by the APDS design which was compatible with straight bore barrels.
High Explosive, Anti-Tank (HEAT)
HEAT shells are a type of shaped charge used to defeat armoured vehicles. They are extremely efficient at defeating plain steel armour but are becoming less useful with the growing prevalence of composite and reactive armour. The power of the shell is independent of the velocity of the shell and is as effective at 1000 metres as at 100 metres. A HEAT charge is most effective when detonated at a certain, optimal, distance in front of the target and HEAT shells are usually distinguished by a long, thin nose probe sticking out in front of the rest of the shell, e.g., PIAT bomb.
Discarding Sabot Shell (DSS)
In principle the same as the APDS shot but applied to High Explosive Shells. It is a means to deliver a shell to a greater range. The design of the sub-projectile carried inside the sabot can be optimised for aerodymanic properties since and the sabot can be built for best performance within the barrel of the gun. The principle was developed by a Frenchman, Edgar Brandt, in the 1930s. With the occupation of France, the Germans took the idea for application to anti-aircraft guns - a DSS projectile could be fired at a higher muzzle velocity and reach the target altitude quicker reducing errors in aiming.
High Explosive, Squash Head (HESH) or High Explosive, Plastic (HEP)
HESH is another anti-tank shell based on the use of explosive. Developed by the British inventor Sir Charles Dennistoun Burney in WW2 for use against fortifications. A thin case contains a charge of a plastic explosive. On impact the explosive flattens against the face of the armour. The fuze then detonates. Energy is transferred through the armour plate. When the compressive shock reflects off the air/metal interface on the inner face of the armour, it is transformed into a tension wave which spalls a "scab" of metal off into the tank damaging the equipment and crew without actually penetrating the armour.
HESH is completely defeated by spaced armour (provided that the plates are individually able to withstand the explosion), but remains popular because not all vehicles are equipped with spaced armour, and it is also the most efficient weapon for demolishing brick and concrete.
Proof shot
A proof shot is not used in combat but to test the gun barrel before use. The proof shot, which is heavier than a normal shot or shell to be fired, and an oversize propelling charge are used to subject the barrel to greater than normal stress. The proof shot is inert (no explosive or functioning filling) and is often a solid unit, although water, sand or iron powder filled versions may be used for testing the gun mounting. Although the proof shot resembles a functioning shell (of whatever sort) behind so that it behaves as a real shell in the barrel, they are not aerodymanic since its job is over once it has left the muzzle of the gun. This is advantageous because the shot travels a much shorter distance and if stopped by an earth bank or similar the impact is less.
The proof testing takes the form of a test firing with the proofshot, remotely operated for safety, then an examination for damage. If the barrel passes the exmaination then "proof mark"s are added to the barrel.
Cluster shells
Like cluster bombs, an artillery shell may be used to scatter smaller submunitions, including anti-personnel grenades, anti-tank top-attack munitions, and landmines. These are generally far more lethal against both armor and infantry than simple high explosive shells, since the multiple munitions create a larger kill zone and increase the chance of achieving the direct hit necessary to kill armor. Most modern armies make significant use of cluster munitions in their artillery batteries.
Artillery-scattered mines allow for the quick deployment of minefields into the path of the enemy without placing engineering units at risk, though artillery delivery may lead to an irregular and unpredictable minefield with more duds than if mines were individually emplaced. Signatories of the Ottawa Treaty have renounced the use of artillery-scattered mines.
Chemical
Chemical shells contain just a small explosive charge to burst the shell, and a larger quantity of a chemical weapon such as a poison gas. Signatories of the Chemical Weapons Convention have renounced such shells.
Non-lethal shells
Not all shells are designed to kill or destroy. The following three types are designed to achieve particular non-lethal effects on the battlefield. They are not completely harmless, however; smoke and illumination shells can accidentally start fires, while all three types can cause minor damage (or potentially kill) if property or a person is unlucky enough to be struck by the discarded carrier.
Smoke
The smoke shell is designed to create a smokescreen. The main types are bursting (usually filled with white phosphorus, WP) and base ejection (a shell which scatters smoke grenades).
Illumination
Another non-lethal shell type is illumination. An illumination shell has a fuze which ejects the "candle" (a pyrotechnic flare emitting white, coloured, or infrared light) at a calculated altitude, where it slowly drifts down beneath a heat resistant parachute. These are also known as starshell.
Carrier
The carrier shell is simply a hollow carrier equipped with a fuze which ejects the contents at a calculated time. They are often filled with propaganda leaflets (see external links), but can be filled with anything that meets the weight restrictions and is able to withstand the shock of firing. Famously, on Christmas Day 1899 during the siege of Ladysmith, the Boers fired into Ladysmith a carrier shell without fuze, which contained a Christmas pudding, two Union Jacks and the message "compliments of the season". The shell is still kept in the museum at Ladysmith.
Fireworks
Aerial firework bursts are created by shells. In the United States, consumer firework shells may not exceed 1.75 inches in diameter.
Unexploded shells
The fuze of a shell has to keep the shell safe from accidental functioning during storage, due to (possibly) rough handling, fire, etc, it also has to survive the violent launch through the barrel, then reliably function at the correct time. To do this it has a number of arming mechanisms, which are successively enabled under the influence of the firing sequence.
Sometimes, one or more of these arming mechanisms fails, and if the fuze is installed on a HE shell, it fails to detonate on impact. More worrying and potentially far more hazardous are fully armed shells on which the fuze fails to initiate the HE filling. This may be due to shallow, low velocity or soft impact conditions. Whatever the reason for failure, such a shell is called a blind or unexploded ordnance (UXO). The older term, "dud", is discouraged because it implies that the shell cannot detonate. Blind shells often litter old battlefields and depending on the impact velocity may be buried some distance into the earth, all remain potentially hazardous. For example, antitank ammunition with a piezoelectric fuze can be detonated by relatively light impact to the piezoelectric element, and others, depending on the type of fuze used can be detonated by even a small movement. The battlefields of the First World War still claim casualties today from leftover munitions. Thankfully modern electrical and mechanical fuzes are highly reliable, if they do not arm correctly they keep the initiation train out of line, or if electrical in nature, discharge any stored electrical energy.
If a blind shell is discovered, it should be avoided, other people warned of its presence, and it should be reported to the local police or armed forces for safe destruction.
External links
- WW2 propaganda leaflets: A website about airdropped, shelled or rocket fired propaganda leaflets. Example artillery shells for spreading propaganda.de:Granate