Mortar

A mortar is a device that fires projectiles at low velocities and short ranges. The mortar has traditionally been used as a weapon to propel explosive mortar bombs in high-arcing ballistic trajectories. The weapon is typically muzzle-loading with a short, often smooth-bore barrel, generally less than 15 times its caliber. Mortar bombs are often referred to, incorrectly, as "mortars".[1]

Mortars are also used for a non-weapon purpose, to deploy ballistic parachutes. Such applications have included the deployment of drogue parachutes in preparation for water landing of the Mercury, Gemini, and Apollo space capsules.

Origins
The Dardanelles Gun, cast in 1464 and based on the Orban bombard that was used for the Ottoman besiegers of Constantinople in 1453 (British Royal Armouries collection).

Mortars have been used for hundreds of years, originally in siege warfare. Many historians consider the first mortars to have been used at the 1453 siege of Constantinople by Fatih Sultan Mehmed. A European account of the 1456 siege of Belgrade by Giovanni da Tagliacozzo said that the Ottoman Turks used seven mortars that fired "stone shots one Italian mile high".[2] The time of flight of these was apparently long enough that casualties could be avoided by posting observers to give warning of their trajectories.[3]

French mortar diagram from the 18th century.

Early mortars, such as the Pumhart von Steyr, were also 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[4]). These mortars fired an exploding shell, which had a fuse lit by the hot gases when fired. This innovation was quickly taken up, necessitating a new form of naval ship, the bomb vessel. Mortars played a significant role in the Venetian conquest of Morea and in the course of this campaign an ammunition store in the Parthenon was blown up (see diagram).

Engraving depicting the Venetian siege of the Acropolis of Athens, September 1687. The trajectory of the shell that hit the Parthenon, causing its explosion, is marked.

An early use of these more mobile mortars as field (rather than siege) weapons was by British forces in the suppression of the 1719 Jacobite rising at the Battle of Glen Shiel. High angle trajectory mortars held a great advantage over standard field guns in the rough terrain of the West Highlands of Scotland.

US Army 13-inch mortar "Dictator" was a rail-mounted gun of the American Civil War.

Coehorn-type mortars of approximately 180 pounds (82 kg) weight were used by both sides during the American Civil War. At the Siege of Vicksburg, General US Grant reported making such mortars "by taking logs of the toughest wood that could be found, boring them out for six or twelve-pound shells and binding them with stong iron bands. These answered as coehorns, and shells were successfully thrown from them into the trenches of the enemy."[5]

The mortar had fallen out of general use by the Napoleonic era and interest in the weapon was only revived at the beginning of the 20th century. During the Russo-Japanese War, Lieutenant-General Leonid Gobyato of the Imperial Russian Army applied the principles of indirect fire from closed firing positions in the field and, with the collaboration of General Roman Kondratenko, he designed the first mortar that fired navy shells.

The German Army studied the Siege of Port Arthur, where heavy artillery had been unable to destroy defensive structures like barbed wire and bunkers. The solution they developed was a short-barrelled rifled muzzle-loading mortar called the Minenwerfer, and was built in three sizes. Recognizing the advantages of the Minenwerfer in trench warfare, production was stepped up and, by 1918, the numbers had increased dramatically to 1,234 heavy, 2,361 medium and 12,329 light Minenwerfer.

The Eprouvette was a mortar used to test the strength of gunpowder. It fell out of use by the middle of the 19th century.

Modern portable mortar
The Stokes trench mortar was introduced into service in 1915, as the first portable mortar.

It was not until the Stokes trench mortar devised by Sir Wilfred Stokes in 1915 during the First World War, that the modern mortar transportable by one person was born. In the conditions of trench warfare, there was a great need for a versatile and easily portable weapon that could be manned by troops undercover in the trenches. Stokes's design was initially rejected in June 1915 because it was unable to use existing stocks of British mortar ammunition, and it took the intervention of David Lloyd George (at that time Minister of Munitions) and Lieutenant-Colonel J. C. Matheson of the Trench Warfare Supply Department (who reported to Lloyd George) to expedite manufacture of the Stokes mortar. The weapon proved to be extremely useful in the muddy trenches of the Western Front, as 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.[6]

The Stokes mortar was a simple weapon, consisting of a smoothbore metal tube fixed to a base plate (to absorb recoil) with a lightweight bipod mount. When a mortar bomb was dropped into the tube, an impact sensitive primer in the base of the bomb would make contact with a firing pin at the base of the tube, and detonate, firing the bomb towards the target. It could fire as many as 25 bombs per minute and had a maximum range of 800 yards firing the original cylindrical unstabilised projectile.[7]

Portuguese Expeditionary Corps soldiers loading a Stokes Mortar, in the Western Front.

A modified version of the mortar, which fired a modern fin-stabilised streamlined projectile and had a booster charge for longer range, was developed after World War I;[8] this was in effect a new weapon. By World War II, it could fire as many as 30 bombs per minute, and had a range of over 2500 yards with some shell types.[9]

The French developed an improved version of the Stokes mortar as the Brandt Mle 27, further refined as the Brandt Mle 31; this design was widely copied with and without license.[10][11][12] About 700 Stokes mortars were acquired by Poland between 1923 and 1926. These weapons were the prototypes for all subsequent light mortar developments around the world.

Mortars today, while substantially similar in design to the Stokes mortar, are greatly improved versions; these offer 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.

Largest mortars
An 1832 "Monster Mortar" invented by Henri-Joseph Paixhans.

For the largest mortars, see list of the largest cannon by caliber.

The largest mortars ever developed were the Belgian "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; 914.4 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" saw action (at the Battle of Antwerp in 1832).[13]

The largest mortars to see active use in modern warfare were the German 60 cm (24 inch) Karl-Gerät.

Improvised mortars
Improvised, or "home-made", mortars have been used by insurgent groups, usually to attack fortified military installations or to terrorize civilians. The Provisional Irish Republican Army used some of the best-known examples during the 1970s, 1980s and 1990s. The largest types came to be known as "barracks busters" and were usually constructed from heavy steel piping mounted on a steel frame. The largest had a calibre of 320 millimetres (13 in) and fired home-made rounds carrying from 80 to 100 kilograms (180 to 220 lb) of explosive.

IRA improvised mortar tube.

As each tube fired only one round, mortars were usually deployed as a battery of four or six welded onto a steel frame. This was often concealed inside a van, such as a Ford Transit. The vehicle would be parked, pointing roughly at the target. A timer fired the propellant charges after a delay – this allowed the mortar gunner time to escape. After firing, a timer-operated incendiary device could set the vehicle on fire in order to destroy any forensic evidence it contained.

Well-known incidents using these weapons include the 1985 Newry mortar attack, when nine members of the Royal Ulster Constabulary were killed, and the Downing Street mortar attack in 1991. In the latter, the IRA launched three bombs at 10 Downing Street, the British Prime Minister's residence, during a cabinet meeting there. The only bomb that detonated landed in the building's back garden, and shattered the rear windows. Prime Minister John Major had to move to Admiralty House while repairs were effected.[14]

Function
LLR 81mm mortar of the 2e REI.

The mortar is a relatively simple and easy weapon to operate. A modern mortar consists of a tube into which assistant gunners drop a purpose-designed bomb. The tube is generally set at an angle of between 45 and 85 degrees to the ground, with the higher angle giving shorter firing distances. The bomb has a small baseline charge and no cartridge case; for extra range, propellant rings are attached to the bomb's fins. When it reaches the base of the tube, the bomb hits a fixed firing pin, which detonates the baseline charge and fires the projectile. Some mortars have a moving firing pin, operated by a lanyard; others may be fired by a trigger.

These attributes contrast with the mortar's larger siblings, howitzers and field guns, that fire shells at higher velocities, longer ranges, flatter arcs, sometimes using direct fire. These weapons are also breech-loaded, while most mortars are muzzle-loaded.

From the 17th 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 an outside barrel diameter many times that of the bore diameter. An early example was Roaring Meg used during The English Civil War. Smaller and more portable designs were introduced during the First World War, primarily for trench warfare, which took place at relatively close ranges. Mortars continue to be in used by militaries to the present day.

81 mm high explosive, white phosphorus and illumination mortar rounds.

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 cover with plunging fire. Mortars are able to fire from the protection of a trench or other type of cover. In these aspects, the mortar is an excellent infantry 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 calibre.[15] 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 to operate and less expensive than comparable howitzers or field guns.

A mortar can be carried by one or more people (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. Heavy mortars 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 armoured personnel carriers, tank chassis, and coastal patrol boats.[16] A mortar can also be a launcher for fireworks, a hand-held[citation needed] or vehicle-mounted projector for smoke shells or flares, or a large grenade launcher.[citation needed]

Design
Picture of the base plate and bipod attached to the barrel

Most modern mortar systems consist of three main components: a barrel, a base plate, and a bipod. Modern mortars normally range in calibre 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 comes in two main varieties: fin-stabilized and spin-stabilized. Examples of the former have short fins on their posterior portion, which control the path of the bomb in flight. Spin-stabilized mortar bombs 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. There are also training rounds and precision-guided rounds, such as the Strix mortar round.

Mallet's Mortar with 36 inch bombs which would have contained 480lb (217kg) of gunpowder.

Operators may fire spin-stabilized rounds from either a smoothbore or a rifled barrel. Rifled-mortars are more accurate, but slower to load. Since mortars are generally muzzle-loaded, mortar bombs for rifled barrels usually have a pre-engraved band, called an obturator, that engages with the rifling of the barrel. Exceptions to this were the U.S. M2 4.2 inch mortar and M30 mortar, whose ammunition had a sub-caliber expandable ring that enlarged when fired. This allows the projectile to slide down the barrel freely, but grip the rifling when fired. The system resembles the Minié ball for muzzle-loading rifles.

Mortars are made in a range of calibres. The French 81 mm mortar became standard for many countries, while the Soviet bloc standardized on the 82mm mortar.

Mortars suffer from instability when used on snow or soft ground, because the recoil pushes them into the ground or snow unevenly. A Raschen Bag addresses this problem.

Distinctive features of mortars
American soldiers firing an Israeli-made M120 mortar

Modern mortars and their ammunition are generally much smaller and lighter than artillery, such as guns and howitzers, which allows light and medium (typically, 60mm and 81/82mm) mortars to be considered light weapons; i.e. capable of transport by personnel without vehicle assistance. They are short-range weapons, and often more effective than artillery for many purposes within their shorter range. In particular, due its high, parabolic trajectory with a near vertical descent, the mortar can land bombs on nearby targets, including those behind obstacles or in fortifications, such as light vehicles behind hills or structures, or infantry in trenches or spider holes. This also makes it possible to launch attacks from positions lower than the target of the attack. (E.g., long-range artillery could not shell a target 1 km away and 30 metres (100 ft) higher; a target easily accessible to a mortar.)

Mortars are also highly effective when used from concealed positions, such as the natural escarpments on hillsides or from woods, especially if forward observers are being employed in strategic positions to direct fire; an arrangement where the mortar is in relatively close proximity both to its FO and its target, allowing for fire to be quickly and accurately delivered to lethal effect.

Fin-stabilised mortar bombs do not have to withstand the rotational forces placed upon them by rifling or greater pressures, and can therefore 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. Also, fin-stabilised munitions fired from a smooth-bore, which do not rely on the spin imparted by a rifled-bore for greater accuracy, do not have the drawback of veering in the direction of the spin.

Spigot mortar
British Home Guard soldiers with a 'Blacker Bombard' spigot mortar (1943)

Spigot mortars, a particular type of mortar, consist mainly of a solid rod or spigot, onto which a hollow tube in the projectile fits—inverting the normal tube-mortar arrangement. At the top of the tube in the projectile, a cavity contains 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 tube mortar of equivalent payload and range. It is also somewhat simpler to manufacture. Further, most spigot mortars have no barrel in the conventional sense, which means ammunition of almost any weight and diameter can be fired from the same mortar.

The disadvantage is that while most mortar bombs have a streamlined shape towards the back that fits a spigot mortar application well, using that space for the spigot mortar tube takes volume and mass away from the payload 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 operate using the spigot principle. Each round has a close-fitting sliding 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. After World War II the Belgium Fly-K silent 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. Military applications of spigot mortars include launcher, used from the deck of a ship, used 24 spigot mortars which fired a diamond 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.
 * The 290 mm petard mortar used on the Churchill AVRE by Britain in World War II.[17]
 * The 320 mm Type 98 mortar used by Japan in World War II to some psychological effect in the battles of Iwo Jima and Okinawa
 * 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

Non-military applications include the use of small-calibre 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).

Gun-mortars
Gun-mortars are hybrid weapons, the archetype being the Brandt 60 mm LR Gun-mortar, which combine features of modern infantry mortars together with those of modern cannon. Such weapons are smoothbore, firing fin stabilised rounds, using relatively small propellant charges in comparison to projectile weight, they have short barrels in comparison to guns and are much more lightly built than guns of a similar calibre-all characteristics of infantry mortars. However, they also have gun like features, such as vehicle mountings (or mounting on a gun carriage) together with sighting systems that allows direct fire, breech loading and recuperation; some such as the 2B9 Vasilek are autoloading. This produces a weapon capable of engaging area targets with indirect high angle fire, and also specific targets such as vehicles and bunkers with direct fire. Such hybrids are a lot heavier and more complicated than infantry mortars, superior to rocket propelled grenades in the anti armour and bunker busting role, but have a reduced range compared to modern gun-howitzers and inferior anti-tank capability compared to modern anti tank guided weapons. However, they do have a niche in, for example, providing a multi-role anti-personnel, anti-armour capability in light mobile formations.