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Japanese "knee" mortars.

Let me start by saying that the World-War-II-era, Japanese Type 89 launcher is no more a "knee" mortar than the German World War II-era MP40 submachine gun was a "Schmeisser." Hugo Schmeisser had no more to do with the design of the Erma-Werke-developed MP40 than deployment of the Type 89 mortar Off a soldier's knee could be accomplished without shattering every bone in. his leg.

Where the expression "knee" mortar came from, no one knows for sure. Some have 'stated that the term came about because the Type 89 was carried close to the leg. This is quite impossible, as it was actually carried over the shoulder.

Most likely this "G.I. Joe" designation was derived from the small, curved baseplate, which looks like it would fit on the thigh or knee. The baseplate was, in fact, curved to fit on a medium-sized tree trunk as commonly encountered in the jungle terrain of the Pacific islands or jammed into the soft earth. In addition, the baseplate of its predecessor, the Type 10 grenade launcher, was also curved, in this case so that all the bits and pieces could be disassembled and stored compactly inside the cylindrical launcher tube.


That settled--sort of, before we discuss World-War-II Japanese "knee" mortars, we should first determine exactly what is a mortar? Like artillery, machine guns and modern automatic grenade launchers; mortars are "area target" weapons. This is in juxtaposition to small arms such as handguns, submachine guns and rifles, which are dearly "point target" weapon systems.

Conventional mortars usually, but not always, consist of a smoothbore tube resting on a baseplate. Most often they are supported by a bipod, along with elevation and traversing mechanisms to adjust their high trajectory fire. However, extremely lightweight, so-called "patrol" mortars usually rest only upon a monopod under, and attached to, the launcher tube. Muzzle loaded, they usually fire fin-stabilized subsonic bombs. Zones of fire are established by varying the propellant charge weight, and several other means as we shall see. The recoil impulse of their bombs is passed directly to the ground through the baseplate.

Whenever possible, mortars are best deployed from a defilade position. A mortar is in defilade when the tube and its crew are hidden from enemy ground observation by a landmass such as the crest of a prominent hill. Positioned on the reverse side of the hill, the mortars should be placed as close to the hill's vertical slope as practical. This effectively protects them from counter-battery fire, which will then almost always overshoot their position.

Traditionally, mortars have been divided into three categories: light, medium and heavy. Light mortars include tubes up to 60mm (2.36 inches) in caliber. These types usually never weigh more than about 40 pounds. Their bombs weigh 1 to 5 pounds. Maximum ranges vary from 500 to 2,000 yards.

Medium mortars include calibers from more than 60mm to 100mm. They can weigh from 75 up to 150 pounds. Their bombs weigh in the range from 7 to 15 pounds. Maximum ranges go from 2,000 to 6,000 yards.

Heavy mortars include all those with calibers in excess of 100mm. They usually weigh more than 200 pounds. Their bomb weight is above 15 pounds. They have a maximum range of up to 10,000 yards.

A mortar's firing mechanism is at the base of the tube, either in the form of a fixed firing pin on which the bomb is dropped by gravity, or commonly in the case of a light patrol mortar it may be a spring-loaded striker. Many mortars permit retraction of the firing pin for safety reasons This is a desirable feature when applying a misfire drill that involves dropping the bomb out the muzzle into the hands of a crewmember, as it could slide back onto the striker with disastrous results.

Today, both the Japanese Type 10 and Type 89 launchers would be called light weight "patrol mortars." Both have 50mm bores. The first to be fielded by the Japanese Imperial army was the Type 10 grenade discharger, which is quite a rarity in any Japanese World-War-II-era military small arms collection.

It's a true "grenade"' launcher, as it was designed specifically to propel the Type 91 fragmentation grenade, to which was added an adapter that principally consisted of a finned tail boom assembly. Of smoothbore design, it was intended to bridge the gap between hand grenades and mortars. Small and compact, it weighs only 5.25 pounds, with an overall length of 20 inches. The launcher tube is 9.5 inches in length, which is the length also of the entire unit when broken down with all of its components stowed inside the tube and carried in a leather harness over the shoulder. The purported range was approximately 175 yards, but I think that would be really stretching its maximum potential.


The Type 10, as well as the later Type 89, are both fired by means of a firing pin thrust upward by a geared lever on the outside of the launcher body. This is significantly different than larger mortars that, as previously stated, most often have a fixed firing pin at the base of the tube and are fired by simply dropping the mortar bomb down the tube.

This works with relatively heavier mortars as they are well stabilized by their baseplate and usually adjustable bipod. As patrol mortars are stabilized only by means of the operator holding onto the tube itself while aiming the launcher and this is quite unsteady, it's best that the operator himself should determine the exact moment of ignition rather than waiting for the bomb to drop down to the bottom of the tube and ignite by impacting against a fixed firing pin. The exact instant of ignition there is often a-surprise.

The range of the propelled grenade is controlled by a gauge .at the base of the Type 10 launcher. Rotating a knurled ring, with range indicator marks from "0" to "20" (zero to 200 meters), opens or closes,: to a greater or lesser extent, a gas port at the back side of the tube away from the operator, which regulates the gas pressure developed within the launching tube.

The shorter the range to the target, the larger the opening of the port, and thus the more gas that is released out into the atmosphere. This system of regulating the distance that the grenade was propelled' was used on the Soviet Model 1938 50mm mortar. Aiming is accomplished by sighting along a vertical channel in the tube from the muzzle to the tube's base, painted red.

Accuracy of the Type 10 grenade launcher was only mediocre. After the Type 89 "knee" mortar was introduced, the Type 10 grenade discharger was usually relegated to discharging pyrotechnic signal flares and smoke shells.



The rifled-tube Type 89 (1929) 50mm mortar represented a significant advance in Japanese small mortar technology. It was also the beginning of high/low pressure grenade concept and although it was not fully understood by the U.S. Ordnance Department at the time, the notion was eventually used in the development of the 40mm grenade used in the M79 launcher in Vietnam and eventually in the M203 grenade launcher still in service with the U.S. military.

Overall length of the Type 89 is 24 inches, with a 10-inch launcher tube. It could launch a high explosive (HE) bomb, the Type 91 hand grenade, in the manner described above, and smoke, practice, incendiary and pyrotechnic signal flares. The maximum effective range of the HE bomb was purportedly 711 yards. A copper driving band at the base of the HE bomb expands against the launcher tube's rifling as it's propelled up the tube and prevents "blow by" of the propellant gases, as well as stabilizing the shell as it exits the tube.

The baseplate of the Type 89 is curved like the Type 10's, but larger. The trigger mechanism is also similar to that of the Type 10. But the range adjusting assembly attached to bottom of the launcher tube is unique and represented the high/low pressure grenade concept in its incipient stage, although its designers did not understand it as such.

When the range adjusting assembly's knurled knob is turned, it not only moves the firing pin housing up or down inside the launcher tube, it--more importantly--increases or decreases the volume of space between the base of the bomb and the firing pin housing. As the range dial is rotated to a longer target range, the volume between the base of the bomb and the firing housing decreases and the gases escaping out the eight bleed holes at the base of the bomb are compressed within a smaller volume, thus increasing the pressure that is propelling the bomb out of the launcher tube and, of course, thereby increasing its velocity and the distance it travels. Very clever indeed, but do 1 need to remind you that they still lost the war?

The high explosive 50mm Type 89 mortar bomb weighs 1.76 pounds with an overall fused length of 5.69 inches. The explosive filling is 5.29 ounces of TNT. The propellant is nitrocellulose diphenylamine flaked powder. The fuse is a spring-loaded, instantaneous, impact-type. The blast radius is 4 feet with an effective burst radius of 11 yards and a fragmentation radius of-60 yards. The time of flight at the maximum range is 13 seconds.

The body of this mortar bomb is composed of three parts. The base houses the propellant container and the percussion primer. The main section holding the explosive filler is threaded at both ends: The fuse assembly forms the third part.

In operation the Type 89 "knee" mortar is held at a 45[degrees] angle and the bomb inserted into the launcher tube base first after the safety pin has been pulled away from the fuse assembly. The sight is that of the Type 10 grenade launcher, a grooved line painted red extending from the muzzle down a short distance on the tube. To fire, pull the trigger handle, to which was usually attached a piece of cord. And remember, don't put it on your knee.

These devices and their munitions are now relatively rare. While I recently saw a live, NFA-registered Type 89 "knee" mortar for sale, they are most often encountered in a deactivated state as per BATFE requirements with a large hole machined out of the base end of the launcher tube. In that condition they are not considered to be destructive devices or any other type of regulated device and can be transferred from one individual to another without paperwork.

Deactivated Type 89 "knee" mortars sell for $1,500 to $2,000. Type 10 grenade launchers are far less common and will fetch up to $3,000. Inert Type 89 mortar bombs run from $300 up to $1,500, depending upon condition and type. These are fascinating and historical pieces of militaria and certainly complement any Japanese World-War-II-era small arms collection.


A Brief History of Mortars

The first documented use of a mortar was by Sultan Mehmed during the siege of Constantinople, which fell in 1453 A.D. These early types were deployed as siege weapons against cities and fortresses and also purportedly against ships close to shore.

Mortars at this stage of their development consisted of very little more than a metal pot attached to a wooden baseplate. Referred to as a "bombard," it was not well received by the troops, as all too often the device would explode upon discharge, killing the crew. As artillery continued to evolve, the bombard fell from favor and all but disappeared from service.

During the Russo-Japanese War of 1904-05, mechanisms were employed to project grenades to distances farther than they could be pitched by hand. By 1908 the Germans had developed a mortar that fired only at angles less than 45[degrees]. In 1912, Krupp introduced a trench howitzer that featured an explosive head attached to a rod. It became known by the troops as the "Toffee Apple Mortar." However, it took the advent of trench warfare during World War I to force advanced development of true mortars.

The term "bomb" for mortar projectiles was derived from the mortars of the 17th century, which fired hollow projectiles containing an explosive filler called "bombs." Historically, mortar bombs have been of two general configurations: teardrop-shaped and tapering at the rear to a tail boom with stabilizing fins, and cylindrical with a parallel-walled configuration in front of the tapering rear end. Cylindrical bombs carry more payload, but are heavier and of shorter range.



The mortar's form today remains essentially that of the weapon designed by Wilfred Stokes at the early part of World War I in 1915. It had a 3-inch diameter steel tube (76mm) resting on a steel baseplate with a traversing and elevation mechanism. It fired a cylindrical bomb without stabilizing fins and with a 12-gauge shorshell at the rear filled with Ballistite.

First made in 1887, Ballistite consisted of 40% nitroglycerine and 60% nitrocellulose. Also known as Nobel powder, it was the first of the modern smokeless powders and quite similar to British Cordite in performance. The fusing assembly on the Stokes bomb resembled that of the famous British hand grenade, the Mills Bomb. A 4-inch (102mm) version of the Stokes mortar was adopted by the United States upon its entry into the war in 1917.


During the 1920s, a French ordnance engineer, Edgar Brandt, began to make significant improvements in the Stokes design. Brandt's company was eventually absorbed by Hotchkiss and became known as Hotchkiss-Brandt. By the 1930s he had perfected modifications and the United States adopted both the 60mm and 81mm Brandt mortars. The German 80mm Granatwerfer 34 was basically a Stokes/Brandt type. Thus, by World War II the belligerents on both sides of the line were lobbing bombs on each other using mortars of basically the same pattern.

Graphics by Peter G. Kokalis and Crittenden Schmitt Archives
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Title Annotation:Mostly Machine GUNS
Author:Kokalis, Peter G.
Publication:Shotgun News
Geographic Code:9JAPA
Date:Jan 10, 2009
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