Type 89 "Knee Mortar"

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Type 89 "Knee Mortar" - 50mm. The trigger group is here shown set at minimum firing range.

The Type 89 "Knee Mortar" (jap. 八九式重擲弾筒 Hachikyū-shiki jū-tekidantō "Type 89 Heavy Grenade Launcher") or more formally the Type 89 Grenade Discharger was a rifled infantry light mortar used by Japanese forces during World War II: though referred to as a grenade launcher by the Japanese, it does not really fit the modern definition of such. The weapon's nickname comes from the phrase "Leg Mortar" being associated with it in Japanese, referring to how it was normally carried in a leg bag, but this was mistranslated to "Knee Mortar." This is said to have resulted in some soldiers trying out captured ones by bracing them against their thigh, imagining the curved baseplate was intended to help with this. Actually doing so could result in severe bruising at best and broken bones at worst. The degree to which this occurred is highly exaggerated: any GI who had fired a rifle grenade would understand that firing such a device rested on their thigh was a bad idea.

The mortar was an unconventional design, conceptually influenced by WW1-era light "trench mortars." It resembled a rifle grenade launching cup attached to a short length of "barrel" (which actually contained the firing pin and trigger assembly), attached to a small curved rectangular baseplate. It was rifled and fired using a trigger, both unusual features for a mortar. The mortar was designed for a fixed firing angle of 45 degrees rather than being aimed by elevating the barrel: a spirit level bubble was clamped near the base of the barrel on the right side to ensure the weapon was at the correct firing angle. The range was instead set using an elevating knob on the side of the weapon, which had calibrated scale for both types of round it could fire. Turning the knob moved the trigger and firing pin assembly inside the gun, in turn altering the distance the projectile was allowed to go inside the barrel and so the distance it was under the influence of pressure from the propellant charge before leaving the barrel. Muzzle-loading the rifled mortar was accomplished with a copper driving band around the propellant charge holder of the projectile: this was sub-caliber until the propelling charge detonated: vents in the sides of the propellant base under the copper band then forced it to expand to fit the rifling grooves. The rifling does not seem to have been a factor when not firing the weapon's purpose-built shells. A flexible sheath was provided at the base of the lower part of the weapon, the top of which could be pulled upwards to cover the lower part of the slit the trigger group moved in to prevent dirt or other material from entering the weapon.

In addition to the usual purpose-built impact detonated mortar shells (Type 89 50mm shell, with HE, incendiary and smoke variants), it could also fire the Type 91 Hand Grenade by attaching a propelling charge to a threaded socket in the grenade's base. The Type 91 was intended as a multi-purpose munition which could be thrown, fired from a spigot-type launcher when fitted with a tailfin assembly, or launched from the Knee Mortar or the gas-trap Type 100 Rifle Grenade Launcher. In the latter cases the grenade had an integral inertial arming system to ensure it only armed on firing. The long fuze burn time (7-8 seconds) for use in these other applications made the Type 91 poorly suited to use as an actual grenade as it was incredibly easy for enemy soldiers to pick up and throw back, and it was replaced in service starting in 1937 by the Type 97 hand grenade, which had a 4-5 second delay and no socket for attaching a propelling charge. Many Type 91s still in service were retrofitted with Type 97 fuzes and had their base threading drilled out, the base painted white so they would not accidentally be issued as mortar projectiles, and blobs of welding flux added to the rim at the base so even if they were they would not physically fit in the barrel. Using a grenade would drastically decrease the effectiveness of the weapon: the range scale for firing a Type 91 (on the right with the weapon held in firing position) was only calibrated for ranges from 40 to 190 meters (44-208 yards), while the one for the Type 89 was calibrated out to 650 meters (711 yards).

Since the mortar had no bipod, the shooter had to hold the tube while firing. The normal firing position was with the shooter on one knee, supporting the barrel near the muzzle with their left hand, with their right positioned on the lower part to operate the trigger, but it could also be fired from a prone position. The double-action trigger was operated by pulling it upwards to cock the firing pin and then release the sear, and a lanyard loop was provided to make this action easier as the trigger was quite stiff. The launcher could also be fired horizontally in emergencies by bracing it against a suitable vertical surface such as a tree or wall. It only required a single operator to fire, but was typically issued to a three-man fireteam, the second man acting as loader and the third as spotter. Around 120,000 were made, and they were almost as commonly issued as light machine guns.

Specifications

• Caliber: 50mm (1.97 inch)
• Length: 24 inches (610mm)
• Barrel Length: 10 inches (254mm)
• Weight: 10 lbs 6 oz (4.7 kg)
• Minimum range: 44 yards (40m)
• Effective range: 131 yards (120m)
• Maximum range: 732 yards (670m)
• Rate of fire: Approximately 25 rounds per minute for a three-man crew

The Type 89 "Knee Mortar" and variants can be seen in the following films, television series, video games, and anime used by the following actors:

Film

Title	Character	Notation	Date
Daughters of China (Zhong Hua nu er)	Japanese soldiers		1949
Windtalkers	Japanese soldiers		2002
The Great Raid	Japanese soldiers		2005
Flags of Our Fathers		Seen among dead Japanese soldiers	2006

Television

Title	Character	Notation	Date
Baa Baa Black Sheep	Japanese soldiers		1976-1978
The Pacific	Japanese soldiers	Part 6	2010

Video Games

Game Title	Appears as	Mods	Notation	Release Date
Forgotten Hope				2003
Red Orchestra 2: Rising Storm				2013
World of Guns: Gun Disassembly	"Type 89 Grenade Discharger"			2014
Enlisted				2021

Anime

Title	Character	Note	Date
Mobile Suit Gundam: The 08th MS Team	Anti-Zeon guerrillas	"The Time Limit on Trust"	1995-1999