Shell (projectile)
A shell, in a modern military context, is a projectile whose payload contains an explosive, incendiary, or other chemical filling. Originally it was called a bombshell, but "shell" has come to be unambiguous in a military context. A shell can hold a tracer.
All explosive- and incendiary-filled projectiles, particularly for mortars, were originally called grenades, derived from the French word for pomegranate, so called because of the similarity of shape and that the multi-seeded fruit resembles the powder-filled, fragmentizing bomb. Words cognate with grenade are still used for an artillery or mortar projectile in some European languages.
Shells are usually large-caliber projectiles fired by artillery, armoured fighting vehicles, warships, and autocannons. The shape is usually a cylinder topped by an ogive-tipped nose cone for good aerodynamic performance, and possibly with a tapered boat tail; but some specialized types differ widely.
Background
Gunpowder is a low explosive, meaning it will not create a concussive, brisant explosion unless it is contained, as in a modern-day pipe bomb or pressure cooker bomb. Early grenades were hollow cast-iron balls filled with gunpowder, and "shells" were similar devices designed to be shot from artillery in place of solid cannonballs. Metonymically, the term "shell", from the casing, came to mean the entire munition.In a gunpowder-based shell, the casing was intrinsic to generating the explosion, and thus had to be strong and thick. Its fragments could do considerable damage, but each shell broke into only a few large pieces. Further developments led to shells which would fragment into smaller pieces. The advent of high explosives such as TNT removed the need for a pressure-holding casing, so the casing of later shells only needed to contain the munition, and, if desired, to produce shrapnel. The term "shell," however, was sufficiently established that it remained as the term for such munitions.
Hollow shells filled with gunpowder needed a fuse that was either impact triggered or time delayed.
Percussion fuses with a spherical projectile presented a challenge because there was no way of ensuring that the impact mechanism contacted the target. Therefore, ball shells needed a time fuse that was ignited before or during firing and burned until the shell reached its target.
Early shells
Cast iron shells packed with gunpowder have been used in warfare since at least early 13th century China. Hollow, gunpowder-packed shells made of cast iron used during the Song dynasty are described in the early Ming Dynasty Chinese military manual Huolongjing, written in the mid 14th century. The History of Jin 《金史》 states that in 1232, as the Mongol general Subutai descended on the Jin stronghold of Kaifeng, the defenders had a "thunder crash bomb" which "consisted of gunpowder put into an iron container ... then when the fuse was lit there was a great explosion the noise whereof was like thunder, audible for more than thirty miles, and the vegetation was scorched and blasted by the heat over an area of more than half a mou. When hit, even iron armour was quite pierced through." Archeological examples of these shells from the 13th century Mongol invasions of Japan have been recovered from a shipwreck.Shells were used in combat by the Republic of Venice at Jadra in 1376. Shells with fuses were used at the 1421 siege of St Boniface in Corsica. These were two hollowed hemispheres of stone or bronze held together by an iron hoop. At least since the 16th century grenades made of ceramics or glass were in use in Central Europe. A hoard of several hundred ceramic grenades dated to the 17th century was discovered during building works in front of a bastion of the Bavarian city of Ingolstadt, Germany. Many of the grenades contained their original black-powder loads and igniters. Most probably the grenades were intentionally dumped in the moat of the bastion before the year 1723.
An early problem was that there was no means of precisely measuring the time to detonation reliable fuses did not yet exist, and the burning time of the powder fuse was subject to considerable trial and error. Early powder-burning fuses had to be loaded fuse down to be ignited by firing or a portfire or slow match put down the barrel to light the fuse. Other shells were wrapped in bitumen cloth, which would ignite during the firing and in turn ignite a powder fuse. Nevertheless, shells came into regular use in the 16th century. A 1543 English mortar shell was filled with "wildfire."
File:Boshin War mortar.jpg|thumb|upright=0.65|left|A mortar with a hollowed shell from the Boshin war
By the 18th century, it was known that if loaded toward the muzzle instead,
the fuse could be lit by the flash through the windage between the shell and the barrel. At about this time, shells began to be employed for horizontal fire from howitzers with a small propelling charge and, in 1779, experiments demonstrated that they could be used from guns with heavier charges.
The use of exploding shells from field artillery became relatively commonplace from early in the 19th century. Until the mid 19th century, shells remained as simple exploding spheres that used gunpowder, set off by a slow burning fuse. They were usually made of cast iron, but bronze, lead, brass and even glass shell casings were experimented with. The word bomb encompassed them at the time, as heard in the lyrics of The Star-Spangled Banner, although today that sense of bomb is obsolete. Typically, the thickness of the metal body was about a sixth of their diameter, and they were about two-thirds the weight of solid shot of the same caliber.
To ensure that shells were loaded with their fuses toward the muzzle, they were attached to wooden bottoms called sabots. In 1819, a committee of British artillery officers recognized that they were essential stores and in 1830 Britain standardized sabot thickness as a half-inch. The sabot was also intended to reduce jamming during loading. Despite the use of exploding shells, the use of smoothbore cannons firing spherical projectiles of shot remained the dominant artillery method until the 1850s.
Modern shell
The mid–19th century saw a revolution in artillery, with the introduction of the first practical rifled breech loading weapons. The new methods resulted in the reshaping of the spherical shell into its modern recognizable cylindro-conoidal form. This shape greatly improved the in-flight stability of the projectile and meant that the primitive time fuzes could be replaced with the percussion fuze situated in the nose of the shell. The new shape also meant that further, armour-piercing designs could be used.During the 20th century, shells became increasingly streamlined. In World War I, ogives were typically two circular radius head – the curve was a segment of a circle having a radius of twice the shell caliber. After that war, ogive shapes became more complex and elongated. From the 1960s, higher quality steels were introduced by some countries for their HE shells, this enabled thinner shell walls with less weight of metal and hence a greater weight of explosive. Ogives were further elongated to improve their ballistic performance.
Rifled breech loaders
Advances in metallurgy in the industrial era allowed for the construction of rifled breech-loading guns that could fire at a much greater muzzle velocity. After the British artillery was shown up in the Crimean War as having barely changed since the Napoleonic Wars, the industrialist William Armstrong was awarded a contract by the government to design a new piece of artillery. Production started in 1855 at the Elswick Ordnance Company and the Royal Arsenal at Woolwich.The piece was rifled, which allowed for a much more accurate and powerful action. Although rifling had been tried on small arms since the 15th century, the necessary machinery to accurately rifle artillery only became available in the mid-19th century. Martin von Wahrendorff and Joseph Whitworth independently produced rifled cannons in the 1840s, but it was Armstrong's gun that was first to see widespread use during the Crimean War. The cast iron shell of the Armstrong gun was similar in shape to a Minié ball and had a thin lead coating which made it fractionally larger than the gun's bore and which engaged with the gun's rifling grooves to impart spin to the shell. This spin, together with the elimination of windage as a result of the tight fit, enabled the gun to achieve greater range and accuracy than existing smooth-bore muzzle-loaders with a smaller powder charge.
The gun was also a breech-loader. Although attempts at breech-loading mechanisms had been made since medieval times, the essential engineering problem was that the mechanism could not withstand the explosive charge. It was only with the advances in metallurgy and precision engineering capabilities during the Industrial Revolution that Armstrong was able to construct a viable solution. Another innovative feature was what Armstrong called its "grip", which was essentially a squeeze bore; the 6 inches of the bore at the muzzle end was of slightly smaller diameter, which centered the shell before it left the barrel and at the same time slightly swaged down its lead coating, reducing its diameter and slightly improving its ballistic qualities.
Rifled guns were also developed elsewhere – by Major Giovanni Cavalli and Baron Martin von Wahrendorff in Sweden, Krupp in Germany and the Wiard gun in the United States. However, rifled barrels required some means of engaging the shell with the rifling. Lead coated shells were used with the Armstrong gun, but were not satisfactory so studded projectiles were adopted. However, these did not seal the gap between shell and barrel. Wads at the shell base were also tried without success.
In 1878, the British adopted a copper "gas-check" at the base of their studded projectiles and in 1879 tried a rotating gas check to replace the studs, leading to the 1881 automatic gas-check. This was soon followed by the Vavaseur copper driving band as part of the projectile. The driving band rotated the projectile, centered it in the bore and prevented gas escaping forwards. A driving band has to be soft but tough enough to prevent stripping by rotational and engraving stresses. Copper is generally most suitable but cupronickel or gilding metal were also used.