The Type 89 Grenade Discharger, commonly known as the "Knee Mortar," was a unique and innovative weapon developed by the Imperial Japanese Army during the late 1920s. It played a significant role in Japanese infantry tactics during World War II, demonstrating a combination of simplicity, effectiveness, and portability that set it apart fromconventional mortars. The Type 89 earned the “knee mortar” nickname during the Allied campaigns of the South Pacific. The weapons curved baseplate fits nicely around the upper leg, and when in a kneeling position, nicely points the weapon in a 45-degree angle. If the weapon were to be fired from this position, it would probably lead to a broken femur or other severe injury.
Development and Design
Japanese grenade dischargers were conceived as a response to the limitations of conventional mortars in terms of portability and ease of use. The Type 89 was an improvement over the earlier Type 10 grenade discharger. The Type 10 fired the Type 91 hand grenade from a smoothbore barrel. Although the Type 10 was innovative and provided an infantry squad with immense indirect firepower, it was greatly hampered by its smooth bore barrel and inadequate ammunition. Since the Type 91 grenade had no way of creating a gas seal inside the barrel once fired, it was limited to an effective range of just under 200 meters, and since the barrel was smoothbore, there was no stabilization of the projectile and accuracy suffered. To improve the original design, development began on a new grenade discharger in the late 1920’s. This new and improved grenade discharger would feature a rifled barrel, and new ammunition. Finally in 1929 the Type 89 was accepted into Japanese military service.
The Type 89 was characterized by its compact and lightweight design, weighing only around 10 pounds (4.5kg). This made it easily portable by a single soldier, who could carry it in a sling across their back when not in use. The weapon could be brought to bear on an enemy within mere seconds. This allowed for a rapid and extremely accurate deployment of indirect fire, particularly in the dense and often challenging terrains of the Pacific islands. Unlike a traditional mortar, the Type 89 is fired by a trigger located on the bottom portion of the mortar. This trigger is a lever with a leather lanyard attached. As the lanyard is pulled, the lever cocks the firing pin to the rear as its pulled backward. Once the firing pin has reached its full rearward travel it is released and strikes a primer located in the base of the shell, igniting the propelling charge.
Technical Specifications
The Type 89 was chambered for the new Type 89 High-Explosive 50mm grenade. The grenade contained 1-pound, 12-ounces (0.8 kg) of picric acid explosive and was propelled by a powder charge screwed onto the base of the shell. The Type 89 shell featured a copper band around the base of the propelling charge. As the propelling charge detonated, gases expanded and escaped through holes drilled into the perimeter of the base behind the copper band. This gas then forces the copper band to expand outward and seal against the bore. This ensured that gas could not escape around the projectile, thus increasing the weapons range and now the shell could grip the rifled bore and stabilize the projectile, greatly increasing its accuracy. Even with the new ammunition, the Type 89 could still fire the Type 91 grenades, just with less range and accuracy than the new Type 89 projectiles. Not only could the Type 89 grenade discharger fire high explosive shells and the Type 91 grenade, smoke and incendiary shells were also developed. The Japanese also designed and employed signal flares for the Type 89 that further enhanced its combat effectiveness and employment on the battlefield.
The Type 89 had an effective range of almost 800 meters, with an adjustable range setting on the lower portion of the mortar to accommodate different distances. Unlike traditional mortars, the knee mortar is always fired from the 45-degree position and the propelling charges are always the same. Type 89’s were fitted with a bubble level clamped around the base of the barrel to indicate 45-degrees to the gunner. Aiming was provided by a line scribed into the length of the barrel and usually filled with white or red paint.
Range is adjusted by turning a knurled drum just under the barrel. The drum is connected to a threaded rod inside the tube of the mortar that moves in or out inside the barrel. If the maximum range was desired, you would turn the drum to until the rod was at the bottom of the barrel. This would allow the Type 89 shell to sit at the very bottom of the tube, thus increasing the amount of propellant gas acting upon the shell during firing. If the opposite effect was desired, and the user wanted the shortest range possible, the drum would be turned until the rod was fully inside the barrel. When a shell is loaded in this configuration, the fuse of the Type 89 shell actually sticks out the end of the barrel. Since the mortar was always fired from the 45-degree position, this provided it with a high trajectory, enabling it to hit targets behind obstacles or in defilade positions.
Tactical Employment
The Type 89 played a crucial role in the Japanese infantry's tactics, particularly in the Pacific theater of World War II, where the dense jungles and rugged landscapes often made traditional mortars impractical. Its light weight and portability allowed it to be easily carried by infantry squads, and its rapid deployment made it a valuable asset in ambushes and defensive positions. Moreover, the Type 89 provided the Japanese forces with a distinct advantage in close-quarter combat situations. Its high-angle fire and the ability to fire indirectly allowed it to hit targets behind cover, making it an effective tool for dislodging entrenched enemy positions.
Typically, a Japanese infantry platoon would be issued three Type 89 grenade dischargers with a three-man team assigned to each weapon. A grenade discharger team generally consisted of the gunner, who carried the weapon and eight rounds of ammunition, and two assistant gunners who carried extra ammunition and provided security to the gunner while firing. When used in combat, the gunner would rapidly deploy the weapon, sight and range the target. The second assistant would prepare and load ammunition, while the third assistant provided security for the team and passed ammunition.
Some allied sources from the period indicate that a trained and experienced grenade discharger team could sustain a rate of fire of up to 25 rounds per minute with deadly accuracy. This eye-watering volume of high-explosive firepower made Japanese ambushes and attacks extremely deadly and gave Japanese platoon commanders a flexible and effective means of indirect fire support.
Limitations and Legacy
While the Type 89 was a formidable weapon in certain situations, it had its limitations. Its effective range was relatively short when compared to larger mortars, and it was less accurate at longer ranges. Additionally, while the explosive charge of the Type 89 shell provided an adequate amount of concussive force, the shell’s steel hull was quite thin and didn’t provide as much fragmentation as was really needed to fully exploit the weapons deadliness.
Despite these limitations, the Type 89 knee mortar remains an iconic and distinctive piece of military history. Its innovative design and tactical utility in specific situations made it a crucial part of Japanese infantry tactics during World War II. It is remembered not only for its effectiveness but also for the ingenuity of its design, showcasing the adaptability and resourcefulness of the Imperial Japanese Army.
Today, the legacy of the knee mortar lives on with the US Army and US Marine Corps with the M224 Lightweight Company Mortar System (LWCMS). The M224 fires a 60mm fin-stabilized projectile with various options including high-explosive, smoke, and illumination rounds. The M224 can be fitted with a baseplate and bipod and utilized in a traditional mortar role, or it can be used in a stand-alone role with just the tube. The M224 uses a trigger system and is aimed in a similar manner to the Type 89 knee mortar. The M224 has been used in Iraq but saw extensive service in Afghanistan for the same reasons the Japanese employed the knee mortar in the pacific; its light weight and portability and being able to provide a large amount of indirect fire in moment’s notice.
In conclusion, the Type 89 Grenade Discharger, or "Knee Mortar," stands as a testament to Japanese ingenuity in military technology during World War II. Its unique design, portability, and versatility allowed it to play a significant role in infantry tactics, particularly in challenging terrains where conventional mortars were less practical. Despite its limitations, the Type 89 remains an iconic weapon that reflects the innovative spirit of its time.
Thought these had TNT filler.