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Mortars

Photograph of a
          mortar crew in action

National Archives. Via Wikipedia Commons

Strictly speaking, any heavy weapon that fires a ballistic projectile only at an angle of elevation greater than 45 degrees is classified as a mortar. However, with the exception of a few large caliber coastal defense mortars at Corregidor and elsewhere, the mortars of the Pacific War were all infantry weapons consisting of a  hollow tube, open at one end and with a firing pin in the other end, supported by a base plate and a bipod. These fired a teardrop-shaped explosive shell with a propellant cartridge in its tail, which was dropped into the muzzle of the mortar to be set off by the firing pin. Most were smoothbore, and stability in flight was achieved by putting fins on the tail of the mortar shell.

Mortars were fired from positions very close to the front line, giving the infantry a measure of rapid on-call fire, and they could drop their shells on the far side of ridges, houses, or other obstacles to direct fire. Their nonspinning, low-velocity shells were relatively quiet in flight, giving little warning. These characteristics also made mortars difficult targets for counterbattery fire. Mortars were relatively easy to manufacture and  transport. On the other hand, mortars had a relatively short range and only moderate accuracy.

The infantry mortars used in the Second World War ranged in caliber from 50mm to 120mm. The lighter mortars were organic to the line infantry companies, while the heavier mortars were usually allocated to heavy weapons companies. Most were originally equipped with leveling bubbles and elaborate sights, but these tended to disappear as the war progressed and the various armies involved realized that an experienced mortar man could do just as well without.

Allied operational researchers concluded that a medium mortar had the firepower of three machine guns.

Japanese Mortars


Photograph of
          Japanese Type 89 mortar

U.S. Army. Via ibiblio.org

The Japanese were fond of mortars and issued some very good designs in large numbers. The most common was the Type 89 50mm "Knee Mortar," so named because its Japanese nickname, "Leg Mortar," was mistranslated by Allied intelligence units. This lightweight mortar (10 lb or 4.5 kg) was typically carried strapped to the leg of the mortar man, but it certainly was not intended to be fired from the knee; Allied soldiers who tried this invariably suffered broken thighs. The mortar fired a projectile weighing 1.44 lb (0.65 kg) to 700 yards (640 meters). It was the only mortar used in the Pacific War with a rifled barrel, and the range was set by screwing the firing pin a variable distance into the barrel. This made for a simple, robust design. The mortar was issued in large numbers to infantry platoons, was supplied with an illuminating round in addition to the explosive round, and in a pinch it could fire a Type 91 hand grenade to which a propellant cartridge was attached. Its chief disadvantage in jungle fighting was the risk that the shell would be set off by overhead branches immediately after leaving the barrel, doing more damage to its crew than to the enemy.


Photograph of
          Japanese Type 98 mortar

U.S. Army. Via ibiblio.org

The Japanese also made limited use of a rather primitive 50mm mortar, the Type 98, which fired a box containing 7 lbs (6.4 kg) of picric acid explosive with a stick at one end. Elevation was set at 40 degrees (so technically this was not a mortar at all!) and range was determined by how far the stick was inserted into the barrel. The projectile was set off by a delay fuse which was ignited by pull cords. There was also a lightweight round and a bangalore torpedo round for the weapon.

Photograph of
          Japanese Type 94 mortar

U.S. Army. Via ibiblio.org

Japanese heavy mortars included the Type 94, a 90mm mortar that fired a 11.5lb (5.2kg) projectile to 4050 yards (3700 meters). It was a very heavy weapon at 340 lbs and it broke down into four components of 75-104 lbs (34-47 kg) each. It had an elaborate recoil absorption system that kept the base plate from being driven into the soil when it was fired. In addition to the normal high explosive round, an incendiary projectile was supplied which scattered 40 pellets of a mixture of white phosphorus and carbon disulfide over a wide area. This was apparently developed for use in China. Unfortunately for the Japanese, this highly effective weapon was not manufactured in large numbers.

Photograph of
          Japanese Type 99 mortar

U.S. Army. Via ibiblio.org

Instead, the standard heavy mortar was the Type 99 81mm mortar, which threw a 6.93lb (3.14kg) projectile out to 2200 yards (2000 meters). A heavier projectile with reduced range was also planned but apparently did not see service. White phosphorus and various signalling rounds were also available. This weapon did have one peculiarity. Like most heavy mortars, it could be fired simply by dropping the projectile into the barrel, where the primer was set off by the weight of the projectile striking the firing pin. However, the firing pin could also be retracted so that the weapon could be loaded without firing. The projectile was then fired by striking a cross-shaft with a heavy wooden mallet.

Photograph of
          Japanese Type 97 mortar

U.S. Army. Via ibiblio.org

The Type 97 mortar also saw use at Guadalcanal and elsewhere. It was similar to the Type 99 and very similar to the American 81mm mortar, with a barrel considerably longer than that of the Type 99. This gave it greater range at the cost of a heavier weapon (145 lbs or 66 kg versus 52 lbs or 24 kg for the Type 99.) The Type 97 could throw a 6.93lb (3.14kg) projectile out to 3100 yards (2800 meters). It could be broken down into three sections for transport and had a rather complicated collimated site.

American Mortars


Photograph of
          FIlipino soldiers training on an 81mm mortar

U.S. Army. Via ibiblio.org

The most common U.S. mortar was the M2 60mm, which could fire a 2.94lb (1.33kg) projectile to a range of 1985 yards (1815 meters). It was somewhat complex and heavy (42lb or 19kg) compared with the light mortars of other armies. However, it could be broken down into three 12-16 lb (5-7 kg) components for easier transport.  An illuminating round was supplied that suspended a 100,000 candlepower star unit from a parachute, which burned for about 25 seconds. Issued to every infantry platoon, the 60mm mortar was also issued to antitank units to illuminate targets at night.

The round was sufficiently light that there were rare instances of mortar crews in close combat throwing their shells like grenades. One such incident occurred on the night of 12 April 1945 on Okinawa, where Staff Sergeant Beauford Anderson received the Medal of Honor for holding off a Japanese night attack by throwing mortar shells into a draw from which the Japanese were forming up for an attack on his position.

60mm mortar rounds originally used an aluminum fuse requiring a pound of metal and costing about a dollar. Early in the war, General Electric invented a plastic fuse that required no aluminum and cost only $0.25. Over 29 million were produced during the war.

Photograph of
          FIlipino soldiers training on an 81mm mortar

U.S. Army. Via ibiblio.org

The Americans also fielded the M1 81mm mortar, which could throw a 6.87lb (3.1kg) projectile to 3290 yards (3010 meters) or a 10.62lb (4.82kg) projectile to 2558 yards (2340 meters). A smoke projectile was also supplied. The heavy projectile had supercaliber fins, which fully deployed when the projectile left the barrel. These theoretically gave excellent stability, but in practice they were mechanically unreliable and were replaced by a more conventional fin assembly early in the Pacific War.  The mortar could be broken down into three components, each weighing 44 pounds (20 kg), which was still rather a heavy load for an infantryman. As a result, the mortar was often carried in a halftrack (Mortar Carrier M4 or M21) and there were experiments with mounting the mortar in a tank chassis.

Photograph of 4.2" mortar in action at Okinawa

U.S. Army. Via ibiblio.org

The Americans also fielded a 4.2" mortar intended for firing chemical shells, but this was considered artillery rather than an infantry weapon. It fired a 26 pound (12 kg) shell out to 4400 yards (4000 m) at a rate of 5 rounds per minute. Since chemical agents were never employed during the war, the 4.2" was used primarily to lay smoke screens or fire conventional explosive shells. The mortars were initially assigned to independent battalions of 48 mortars and 1010 men, but in September 1943 the manpower was reduced to 622, barely enough to properly man the mortars.

A number were given to the Navy which used them on LCI(M) gunboats to provide smoke screens for amphibious operations.

British Mortars


Photograph of
          Australian troops firing a 51mm mortar

Australian War Memorial. Via Wikimedia Commons

The most common British mortar was 2" (51mm) in caliber and fired a 2.25lb (1 kg) projectile out to 500 yards (460 meters). It was increasingly simplified as the war progressed, with the fancy sight being among the first items to go. It was supplied with a variety of special projectiles, including smoke, illuminating, white phosphorus, and signal smoke shells. There were even specialized nonexplosive projectiles designed to carry a harpoon (for snagging trip lines of booby traps) or to cast an explosive net over a suspected minefield for clearing. The 2" mortar was issued to every infantry platoon and to antitank units to illuminate targets at night. It weighed less than 11 pounds (5 kg) in its simplest version.

Photograph of
          British soldiers firing a 76mm mortar

Imperial War Museum. Via Wikimedia Commons

The British also issued a 3" (76mm) mortar that could throw a 10lb (4.5kg) projectile 1600 yards (1460 meters). An improved version increased the range to 2800 yards (2560 meters). The mortar could be broken down into three 37-45 lb (17-20 kg) components for transport. The Australian Army developed a lightweight jungle version with a shortened barrel and a special fast-burning propellant cartridge, but it was produced only in small numbers.

Kuomintang

The Kuomintang were terribly deficient in heavy weapons and particularly in artillery. Mortars provided a cheap substitute for heavier guns, and the Chinese seem to have developed considerably proficiency in their use. However, the numbers were low compared with any Western army, at an estimated 1 mortar per 400 men, versus about 1 mortar per 100 men in a 1943 U.S. division. The U.S. supplied some 60mm mortars as Lend-Lease, but the Chinese also apparently produced their own mortars based on 81mm German models.


References

Bailey (2004)

Bergerud (1996)

"CBI Roundup" (1942-10-8; accessed 2011-5-4)

Hastings (2007)

Hogg (1977)

Kleber and Birdsell (1966; accessed 2015-3-26)

Klein (2013)

Leckie (1995)

Smith (2000)



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