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A mortar is a muzzle-loading indirect fire weapon that fires shells at low velocities, short ranges, and high-arcing ballistic trajectories. It typically has a barrel length less than 15 times its caliber.

Contents

Function

81 mm high explosive, white phosphorus and illumination mortar rounds

A mortar is relatively simple and easy to operate. A modern mortar consists of a tube into which gunners drop a shell. A firing pin at the base of the tube detonates the propellant and fires the shell. The tube is generally set at between 45 and 85 degrees angle to the ground, with the higher angle giving shorter firing distances.

These attributes contrast with the mortar's larger siblings; howitzers and field guns, which fire at higher velocities, longer ranges, flatter arcs, and sometimes using direct fire.

From the 18th to the early 20th century very heavy, relatively immobile siege mortars were used, of up to one metre calibre, often made of cast iron and with outside barrel diameter many times that of the bore diameter.

French mortar diagram, 18th century.

A mortar can also be a launcher for fireworks, a hand-held or vehicle-mounted projector for smoke shells or flares, or a large grenade launcher.

Light and medium mortars are portable, and usually used by infantry units. The chief advantage a mortar section has over an artillery battery is the flexibility of small numbers, mobility and the ability to engage targets in the defilade with plunging fires. It is able to fire from the protection of a trench or defilade. In these aspects the mortar is an excellent infantry support weapon, as it can be transported over any terrain and is not burdened by the logistical support needed for artillery.

Heavy mortars are typically between 120- and 300-mm caliber[1]. These weapons are usually towed or vehicle-mounted, sometimes breech-loaded, and normally employed by infantry units attached to battalion through division level. Even at this size, mortars are simpler and less expensive than comparable howitzers or field guns.

A mortar can be carried by one or more men (larger mortars can usually be broken down into components), or transported in a vehicle. An infantry mortar can usually also be mounted and fired from a mortar-carrier; a purpose-built or modified armoured vehicle with a large roof hatch.

A heavy mortar can be mounted on a towed carriage, or permanently vehicle-mounted as a self-propelled mortar. Twin-barrelled self-loading mortars — such as the Patria AMOS PT1 — are the latest evolution of these heavy mortars and are mounted on platforms such as armored personnel carriers, tank chassis, and coastal patrol boats.[2]

Design

Most modern mortar systems consist of three main components: a barrel, a base plate, and a bipod.

Modern mortars normally range in caliber from 60 mm (2.36 in) to 120 mm (4.72 in). However, mortars both larger and smaller than these specifications have been produced. An example of the smaller scale is the British 51 mm Light Mortar which is carried by an individual and consists of only a tube and a base plate. Conversely, a large example is the Soviet 2S4 M1975 Tyulpan (tulip flower) 240 mm self-propelled mortar.

Smaller mortars (up to 82 mm) are commonly used and transported by infantry based mortar sections as a substitute for, or in addition to, artillery.

Ammunition for mortars generally come in two main varieties: fin-stabilised and spin-stabilised. The former have short fins on their posterior portion which control the path of the shell in flight. Spin-stabilized mortar shells rotate as they travel along and leave the mortar tube, which stabilizes them in much the same way as a rifle bullet. Both types of rounds can be either illumination (infra-red or visible illumination), smoke, or high explosive.

Spin-stabilised rounds require a rifled barrel. Since mortars generally are muzzle loaded the mortar shell has a pre-engraved band, called an obturator, that engages with the rifling of the barrel. They are more accurate, but slower to load and the trajectory is affected by drift: any spinning projectile is subject to the Magnus effect which causes the trajectory to drift perpendicular to the spin axis; this is also what makes spinning balls follow a curved trajectory in sports such as baseball, cricket, football, and tennis.

Mortars are made in a range of calibers. The French 81 mm mortar became standard for many countries. The Soviets took advantage of this by standardising on 82 mm mortars, allowing Soviet mortars to use mortar ammunition of other countries found on the battlefield, albeit with less accuracy, while their own would be too large for their opponents. This advantage was used during the Vietnam War and at other times.[citation needed]

Mortars suffer from instability when used on snow and soft ground, because the recoil pushes then into the ground or snow unevenly. A solution to this problem is the Raschen Bag.

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Distinctive features of mortars

Mortars and their ammunition are generally smaller and lighter than other artillery. They are suitable for use at short range, but not at long range. In particular, the mortar can drop shells on close-by targets, even behind obstacles, due to its "lobbing" trajectory. This also makes it possible to launch attacks from positions lower than the target of the attack; for example, conventional long-range artillery could not shell a target 1 km away and 30 metres (100 ft) higher, but shelling the target by mortar would be easy.

Mortars are also very effective when used from concealed positions, such as the natural escarpments on hillsides or from woods, especially if observers are being employed in strategic positions to direct fire.

Fin-stabilized mortar bombs do not have to withstand the rotational forces placed upon them by rifling, and can carry a higher payload in a thinner skin than rifled artillery ammunition. Due to the difference in available volume a smooth-bore mortar of a given diameter will have a greater explosive yield than a similarly sized artillery shell. For example a 120 mm mortar bomb has about the same explosive capability as a 155 mm artillery shell.

Georgian-era portable Trench Mortar.

Spigot mortar

Spigot mortars are a particular type of mortar consisting of a mostly solid rod or spigot, and a hollow tube in the projectile into which the spigot fits, inverting the normal tube mortar arrangement. At the top of the tube in the projectile is a cavity containing propellant such as cordite. There is usually a trigger mechanism built into the base of the spigot, with a long firing pin running up the length of the spigot activating a primer inside the projectile and firing the propellant charge.

The advantage of a spigot mortar is that the firing unit (baseplate and spigot) is smaller and lighter than a conventional mortar of equivalent payload and range. It is also somewhat simpler to manufacture.

The disadvantage is that the mortar projectile requires additional material to contain the propellant gases during firing. While most mortar shells have a streamlined shape towards the back that naturally fits a spigot mortar application well, using that space for the spigot mortar tube takes volume and mass away from the explosive warhead payload and fragmentation mass of the projectile. If a soldier is carrying only a few projectiles, the projectile weight disadvantage is not significant. However, the weight of a large quantity of the heavier and more complex spigot projectiles offsets the weight saved due to the spigot mortar being lighter than a conventional mortar.

A near silent mortar can be made using the spigot principle. Each round has a close-fitting movable plug in the tube that fits over the spigot. When the round is fired, the projectile is pushed off the spigot, but before the plug clears the spigot it is caught by a constriction at the base of the tube. This traps the gases from the propelling charge and hence the sound of the firing. Post World War II the silent Belgium Fly-K spigot mortar was accepted into French service as the TN-8111.

Spigot mortars are generally out of favor in modern usage, replaced by small conventional mortars.

Mallet's Mortar with 36 inch shells which would have contained 480lb (217kg) of gunpowder.
A 1377 made Chinese Bombard
Mortar of the Knights of Saint John of Jerusalem, Rhodes, 1480-1500, fired 260 kg cannon balls.
An 1832 "Monster Mortar" invented by Henri-Joseph Paixhans.
19th century mortar, facing the sea, in the walls of Acre.

Military applications of spigot mortars include

  • Anti-tank launchers
    • The Blacker Bombard and PIAT anti-tank launcher used by Britain in World War II utilised a spigot mortar type of launcher.
  • Anti-submarine launchers
    • The Hedgehog launcher, used from the deck of a ship, used 24 spigot mortars which fired a circular pattern of anti-submarine projectiles into the sea ahead of the ship. A sinking projectile detonated if it struck a submarine, and the pattern was such that any submarine partly in the landing zone of the projectiles would be struck one or more times.

Nonmilitary applications include use of small-caliber spigot mortars to launch lightweight, low-velocity foam dummy targets used for training retriever dogs for bird hunting. Extremely simple launchers use a separate small primer cap as the sole propellant (similar or identical to the cartridges used in industrial nail guns).

History

Mortars have existed for hundreds of years, first seeing use in siege warfare. However, the early incarnations of these weapons such as the Pumhart von Steyr were large and heavy, and could not be easily transported. Simply made, these weapons were no more than iron bowls reminiscent of the kitchen and apothecary mortars from where they drew their name.

An early transportable mortar was invented by Baron Menno van Coehoorn (Siege of Grave, 1673[3]). Coehorn-type mortars of approximately 180 pounds (82 kg) weight were used by both sides during the American Civil War.

During the Russo-Japanese War, Leonid Gobyato for the first time applied deflection from closed firing positions in the field and with General Roman Kondratenko designed the first mortar that fired navy shells. However, it was not until the Stokes trench mortar devised by Sir Wilfred Stokes in 1915, that the modern mortar transportable by one person was born. The Germans also developed a series of trench mortars or Minenwerfer in calibers from 7.58 cm to 25 cm during World War I, though these were rifled.

Extremely useful in the muddy trenches of the Western Front, mortars were praised because of the shell's high angle of flight; a mortar round could be aimed to fall directly into trenches where artillery shells, due to their low angle of flight, could not possibly go. Modern mortars have improved upon these designs, offering a weapon that is light, adaptable, easy to operate, and yet possesses enough accuracy and firepower to provide the infantry with quality close fire support against soft and hard targets more quickly than any other means.

During the battle of Iwo Jima, the Imperial Japanese Army used twelve 320 mm mortars against the American forces.

Largest mortars

The largest mortars ever developed were the French "Monster Mortar" (36 French inches; 975 mm; developed by Henri-Joseph Paixhans in 1832), Mallet's Mortar (36 inches; 910 mm; designed by Robert Mallet and tested by the Woolwich Arsenal, London, in 1857) and the "Little David" (36 inches; 910 mm; developed in the United States for use in World War II). All three mortars had a caliber of 36 inches, but only the "Monster Mortar" was used in action (at the Battle of Antwerp in 1832).[4]

Improvised mortars

Improvised, or "homemade", mortars have been used by insurgent groups, usually to attack fortified military installations. Some of the best-known examples were those used by the Provisional Irish Republican Army during the 1970s, 1980s and 1990s. These were known as "barracks busters" and were usually constructed from heavy steel piping of 3-4 inches in diameter mounted on a steel frame. This could be constructed easily inside a van such as the MK 1 Ford Transit. Bombs were also home-made and had simple propellant fuses.

The mortars were usually deployed as a battery of four or six welded onto the same steel frame. The idea was that the improvised propellant fuses could be set once the mortar carrier was aimed roughly at the target and the mortars would automatically fire after a short delay. This allowed the mortar gunner to escape even before the mortar is fired. After firing, the vehicle may have been set on fire by a timer-operated incendiary device, to destroy any forensic evidence it contained. In 1985 the IRA killed nine members of the Royal Ulster Constabulary when it carried out a mortar attack on Newry RUC barracks.

A famous use of this weapon was an IRA assassination attempt on 7 February 1991. The IRA mortared 10 Downing Street as a Cabinet meeting was in session. However the bomb landed in the back garden of the British Prime Minister's residence and only shattered the rear windows. Prime Minister John Major was forced to move to Admiralty House while repairs were effected.[5]

Images

See also

Citations and notes

  1. ^ Vesa Toivonen, 2003, From Tampella to Patria, 70 Years of Finnish Heavy Weapons Production, Tampere, ISBN 952-5026-26-4
  2. ^ Toivonen, 2003
  3. ^ BARON VAN MENNO COEHOO... - Online Information article about BARON VAN MENNO COEHOO
  4. ^ "Largest Mortar". Guinness World Records. http://www.guinnessworldrecords.com/index.asp?id=46267. Retrieved 2006-04-04. 
  5. ^ http://www.keesings.com/search?kssp_a_id=38019n03uki&kssp_selected_tab=article Mortar attack on 10 Downing Street

Recommended reading

  • FM 3-22.90 Mortar Gunnery (US Army)
  • FM 3-22.91 Mortars (US Army)

External links


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