Type 89 Knee Mortar at James D Julia

The Type 89 grenade discharger, commonly known as the “knee mortar” was a Japanese light infantry weapon introduced in 1929 which blurs the lines between grenade launcher and mortar. Like a mortar, it fires propelled explosive bombs in a high-angle indirect fire role, but it has a rifled barrel and uses a range adjustment mechanism very different from most mortars.

The knee mortar proved to be a very capable and effective weapon in WWII against US forces. It was accurate, effective, and perhaps most importantly, light and very fast to put into action. The closest comparable US weapon was the 60mm light mortar, which had a more effective projectile but was significantly slower to use.

Of course, the “knee mortar” nickname was based on the theoretical belief that one was supposed to rest the curved baseplate of the weapon on a leg while firing, which would actually have resulted in a broken leg. The baseplate was curved to allow it to dig into soft soil and be used against objects like logs and roots.

38 Comments

  1. Another highly informative, well explicated video on an important piece of WWII Pacific War ordnance. Thanks Ian!

    Question: I’ve always suspected, but never had time to actually research, that the U.S. development of the M79 and later M203 40mm grenade launchers owed something to the Japanese grenade dischargers/”knee mortars.” Any evidence?

    It is striking that at Guadalcanal, U.S. Marines were using the old WWI-era French “Tromblom VB” type rifle grenades, whereby a standard ball cartridge shoots through the grenade body, flicking a switch to arm the fuze, while the propellant gases throw the grenade out of the launcher. Meanwhile, the Japanese were employing this weapon, with purpose built shells, or standard hand grenades with a small propellant charge.

    Question 2: Since the Japanese hand grenade relies on striking it against the heel of a boot, or a rifle stock, or a helmet, or a solid object to initiate the time delay, is that something that has to be done prior to launching the hand grenade from the T89? That would be quite a “hot potato” to stuff down the barrel, if so. Also, since the grenade did not have a copper “driving band” like the shell, presumably it was much less accurate?

    • As far as I can see, the Japanese grenade dischargers and U.S. and Soviet grenade launchers occupy a similar tactical niche.

      While I believe that the grenade discharger had threw a heavier weight of explosives, the U.S. and Soviet grenade launchers have a big edge in accuracy. Also, I suspect that the fillings in the U.S. 40mm and Soviet BG-15 grenades are probably more effective (and safer) than the “Shimose” (picric acid) in the Japanese projectiles.

    • The Type 89 projectiles were impact-fused only. Pull the safety pin and it’s ready to go. I would assume that the modified hand grenades certainly were less accurate, but they were also much shorter ranged, which may have made the accuracy mostly irrelevant.

  2. Does that little piece of springy fabric material protect anything underneath it or is it simply to make the weapon more comfortable to carry?

    • There is the experimental non-lethal Beretta LTLX7000. The shotgun have regulated gas valve allowing the gun to vent out some of the gases out of the barrel effectively reducing the muzzle velocity according the need. The need be – achieve constant designated impact velocity at different ranges.
      So it not changing the expansion chamber to achieve different muzzle velocities and as a result different ranges but changing the expansion chamber to achieves different muzzle velocities and as a result constant impact velocity at different ranges.

      Yoel

      • Thanks – I looked into the Beretta LTLX7000 after you posted about it and found this patent:
        http://www.google.com/patents/US6065384

        The Beretta achieves variable pressure by changing the nature of the chamber (amount of venting) instead of by changing the size of the chamber as in the Type 89.

        I find the variable chamber size (the Type 89 approach) fascinating partly for its uniqueness. It does have the restriction that the angle of fire (and thus the angle of impact) is always the same, which of course in the case of the Type 89 is something of an advantage because for its application the operator is not trying to pick a correct angle from many but instead is trying to always achieve the one correct angle.

        It’s so rare to see a truly unique feature in a firearm.

      • “adjustable expansion chamber to adjust projectile range”
        Polish grenade launcher model 36 (Granatnik wz. 36) worked in this manner.
        According to https://en.wikipedia.org/wiki/Granatnik_wz._36
        “the firing angle was fixed at 45 degrees and the range was regulated not by raising or lowering the barrel but by limiting the volume of a gas chamber”

        “achieves variable pressure by changing the nature of the chamber”
        Soviet 50mm mortar RM-38 work on this principle

        • Thanks – interesting.

          The wiki article says “The Granatnik wz.36 was a Polish grenade launcher designed in originally in 1927 as “wz. 30″ and later modified in 1936. It entered service in 1936 becoming the standard grenade launcher of the Polish Army…”

          so the wz.30 was designed at about the same time as the Type 89. The wz uses a separate expansion chamber (kind of like a giant gas tube, mounted on the actual cannon tube)

          I wonder if this is an example of coincidental parallel invention or if there was “cross talk” that influenced both designs.

  3. Very cool video, Ian!

    This reminded me of my own army service in the IDF; 1986-1994. My “packal” (platoon weapon) was the 52mm mortar, not all that dissimilar to this weapon.

    My mortar lacked the bubble level but had a thick white line running down the middle and a trigger in the form of a turnable knob which allowed for a second or third firing pin strike in case the primer didn’t ignite intially.

    I have no idea if this weapon is still in the IDF inventory or has been replaced/retired.

    • Certainly the British 50mm platoon mortar was very, very long-lived in its service life… Didn’t Belgium postwar produce a a simple mortar where the carry strap has markings on where to put your boot heel in order to fire the desired range?

      IDF used to use lots of rifle grenades until fairly recently, when the U.S. type 40mm was adopted, yes?

      • It’s a 52mm mortar, not 50mm. The M-16/M-203 combo was in use when I was in, as were locally-produced rifle grenades.

        Haven’t really kept abreast of IDF weaponry, so I couldn’t say what they are using now…………

  4. I’d always naively assumed you ranged these by holding it at the appropriate angle, with a plumb or bubble “display”. Obviously the actual system is better, but I can’t imagine being able to hold it at exactly 45 degrees without flinching on the trigger pull.
    That copper driving band is genius.

  5. Ian, Great review! I thought I knew quite a bit about these until now. My Dad (a china Marine) stated these were quite effective and intimidating weapons. The other reason these were so fast to employ is you did not have to use/adjust the number of increments (smaller charges) used to proper a typical mortar round. Very ingenious!

  6. Someone makes an electronic sight for an M224 60mm mortar. The sight calculates range based on inclination of the mortar and displays the calculated range to the firer. The firer sits spread-eagled on the ground with the mortar between his legs, with no base plate or bipod. I guess it’s kind of a new spin on an old idea.

  7. Some interesting facts (well, I think they’re sort of interesting) about the Type 89, and a couple of responses.

    Dave Carlson;

    The origins of the M79 /M203 family owed a good bit to the Type 89 tactically. As well as to the German Kampfpistole, or “battle pistol”, the grenade-launching development of the original single-shot 27mm signal pistol, and its follow-on, the Sturmpistole;

    http://www.dieselpunks.org/profiles/blogs/light-and-death-grenade-launching-pistols

    The SP can be distinguished by its folding stock, which is actually the same as the “emergency” stock issued with the MG13 flexible 7.9 MG used in Luftwaffe bombers. (Two MG13s in each plane were issued with this stock, plus a bipod, apiece so they could be used as LMGs in event the crew were forced down in unfriendly territory.)

    The actual mechanism of the M79/M203, known as the “High-Low Pressure” system, was based on that of the German 8cm Panzerabwehrwerfer 600, a lightweight anti-tank gun developed in mid-1944;

    https://en.wikipedia.org/wiki/8_cm_PAW_600

    Like the PAW 600, the GLs use the HLP system to reduce overall weight and “kick”, accepting that there is an in-built range penalty.

    (It’s the reason you can’t fire MK19 auto-grenade launcher rounds from the ’79 or ‘203; they generate more pressure to operate the AGL’s blowback mechanism, and it’s too much for the individual GLs. The least serious thing that will happen is a cracked barrel and the “blooper guy” getting a dislocated shoulder. Consequences go downhill from there.)

    As for the Type 89 technically, it belongs to the family of “spigot mortars”, with a mechanism working very much like those of the British PIAT and Blacker Bombard.

    The Japanese also had a 5cm bipod-mounted rifled mortar that operated much like the Type 89, and apparently could use the same round. The difference was that its spigot/firing pin was fixed, and range was governed by changing the elevation angle as with other more “orthodox” mortars.

    The other 5cm mortar, the Type 98, was very different. It was smoothbored, fired like the other two, but its “round” was like that of the WW1 German “Krupp Trench Howitzer” or the British copy thereof, the Vickers 40mm. The round had a bore-diameter “stalk” surmounted by a thin-walled steel “can” containing 5 pounds of “shimose”- the official IJA term for picric acid. It was used mainly for sorting out bunkers.

    As for the story about the Type 89 causing broken thighbones, it must have happened at least a few times. According to Ian Hogg in Grenades and Mortars (Ballantine, 1974, p.123), “By late 1944 all Allied descriptive material on the grenade launcher carried, in large print, the warning ‘THIS WEAPON MUST NOT BE FIRED FROM THE THIGH’.”

    So at least somebody must have ended up in hospital because of trying it.

    The type 89 did cause U.S. troops to try firing the 60mm Stokes-Brandt minus the bipod, with resultant poor accuracy. But in the process, it was to become the inspiration for the post-war family of 5cm and 6cm “commando” mortars that have a handle, a spade, a bubble-level sight, and no bipod at all.

    So in the end, the little “Knee Mortar” did pretty well for itself.

    cheers

    eon

    • Yes, eon, that stupid translation mistake did land a few marines in hospital. The Type 89 was called “leg mortar” in Japanese because it was strapped to the leg while the soldier was on the march and unstrapped when the soldier was called to engage the enemy. The American translator got it wrong, hence the “knee mortar” story.

    • Ah, thanks. I was aware of the 80mm PAW kanone, but entirely unaware that the Sturmpistole had any effect on tactical doctrine or development. Now perhaps I could actually get on with some research of my own.

      By way of thanks, allow me to add two unusual weapons from parts of the world I’m more familiar with. First, in El Salvador during the 1980s Civil War, a single shot grenade launcher turned up, sans markings. It was apparently used with Soviet AGS-30 grenades, but I’ve been assured that like the difference between the Mk.19 and the M79/203, this simply cannot be done. It is at lease conceivable that these might have used U.S. 40mm munitions and the description was sorely mistaken.

      Also, same place, in instances where a mortar could not be contemplated by the left-wing guerrillas, the expedient was adopted of rigging an 81mm mortar shell to an RPG-7 rocket propelled grenade motor and using the RPG launcher to lob the round a short distance (it was heavier than the original warhead). Apparently the blast radius is very highly dangerous to the firer, but the trade off was something like mortar support without the weight and encumbrance of the mortar and its base plate.

      Did the IJA use picric acid in hand grenades too? My limited understanding of the far east has it that the Chinese stick grenade was filled with picric acid explosive.

      • The three most common grenades were the Type 97, the Type 99 aka “Kiska”, and the older Type 91 which was more commonly used as a rifle-launched grenade from a discharger cup similar to the WW1 British SMLE discharger. It is also apparently the one fired by the Type 89 GL.

        The 91 and 97 have serrated exteriors like most other “frag” grenades of the time, supposedly to ensure an even “ring” of fragments of the same size. The pre-WW1 Hale grenades patented by the Cotton Powder Co. in Britain were apparently the first to have this feature; it rarely works as intended.

        The 99 “Kiska” (so-called by U.S. forces because that was the first place it was encountered) had a smooth surface and a thinner-walled case, and may have been intended as a pure blast grenade. Or maybe it was just cheaper and faster to make it that way.

        The 91 and 97 were each filled with 65 gm. of TNT. The later 99 was filed with 58 gm. of picric acid. That may have been an “economy” measure. (Picric acid is easier to make than TNT when you’re dealing with limited supplies of aniline, etc.)

        The 97 and 99 had 4 to 5 second pyrotechnic delay fuse trains. The 91 had a 7 to 8 second pyro delay, reflecting its use in the GL.

        Note that these numbers can’t be taken too literally. Like a lot of Soviet-era Russian grenades, the pyro delays were a bit unpredictable in delay time. The best course with any of the Japanese grenades was to pull the safety cotter pin, smack the head against your boot heel or something similarly solid to hit the percussion primer and start the delay, and then throw the sucker and run like Joe Namath in the opposite direction.

        One of my profs, a Marine vet of the PTO from Guadalcanal to Iwo Jima, stated that the most-often observed delay on the 97 and 99 was three seconds, and the 91 usually blew after five, which meant that at its maximum range of ~200 meters from the Type 98 it was usually an airburst about 20 feet up, which was very nasty if you were on the receiving end.

        The 91 was almost always seen with the GL propelling “module” attached, as here;

        https://upload.wikimedia.org/wikipedia/commons/e/ee/Japanese_Type_91_50_mm_grenade.gif

        I imagine that the propellant package of the Type 89’s dedicated round probably was about the same.

        You can find further information on these plus the other Japanese grenades here;

        http://inert-ord.net/jap02h/grenades/index.html

        Note the 40mm HEAT round with the pre-engraved driving band at the bottom of its “stick”. The German rifled cup discharger used on the Mauser Kar98 used the same arrangement, and the Sturmpistole had its own HEAT grenades with 27mm “sticks” that worked the same way. One had a warhead about the size of the one on the U.S. M9 rifle grenade- ouch!

        cheers

        eon

        • Picric acid is pretty unstable to use as the secondary explosive (as opposed to a booster which I think it used to be commonly used as) so I am a bit surprised about that.

          You can see an example of recommending Picric Acid as a booster in an improvised device in the Improvised Munitions Handbook (TM 35-210) https://archive.org/details/milmanual-tm-31-210-improvised-munitions-handbook or http://www.amazon.com/Improvised-Munitions-Pentagon-U-S-Military/dp/0975900900 where it is mentioned several times (for example with Mercury Fulminate as primary)

          • Picric acid is a remarkably twitchy primary HE, that’s for sure. But it was used as a main filling in everything from grenades and mortar rounds on up to heavy artillery rounds by all sides in WW1, simply because it was easy and cheap to make compared to, say, TNT.

            As Ian Hogg pointed out in The Guns 1914-18, everybody had their own name for it. Shimose in Japan, Lyddite in the UK, Granatfullung 88 in Germany, but it was all just picric acid.

            One reason the Germans were the first to use phosgene in artillery shells was that their standard HE high-capacity shell was built a lot like a “shrapnel” round, with a screwed-on nose giving access to the interior like a pickle jar. This was because it was designed to be filled with pre-cast blocks of, yep, picric acid. They just substituted two half-cylinder glass “bottles” of phosgene in liquid form, with a central “tube” moulded in to accept a narrow cylinder of waxed cardboard containing the bursting charge, a lot like a stick of dynamite. Except the tube was full of picric acid, just enough to “pop” the casing and shatter the glass bottles, releasing the gas.

            The French had a tougher time, because their shells were designed to have the HE poured in molten through the nose or base fuse hole and cool and harden inside. They finally figured out how to blow glass in through the fuse hole, like the liner in a Thermos bottle, pour the liquid phosgene (or later mustard gas) in, and slip the burster tube (waxed tinplate in their case) in through the same hole, with an asbestos gasket around the screwed-in detonator to form a chemical proof seal. And their burster charge was picric acid, too.

            This was all necessary because both phosgene and mustard are remarkably corrosive buggers, attacking the cast iron or wrought iron or steel in a shell body with the alacrity of a school of piranha that have just spotted a llama going for a swim. What’s worse, leakage could also form esters with the HE filling that just might cause gas evolution inside the shell, resulting in a corroded shell going “pop!” in an ammo dump. Not exactly what they had in mind. Picric acid was actually a bit less vulnerable to this chemical reaction than TNT.

            BTW, the IMBB is available for free online any one of a number of places. It is, after all, an official U.S. Government publication. 😉

            cheers

            eon

        • As you know much better than I, there were innumerable instances of marines and soldiers pitching Japanese grenades back during battles of the Pacific War, and as a result, very many cases of men diving on grenades to spare their comrades in arms.

        • If you are looking for 30mm VOG-17M or 30mm VOG-30 weapons system lighter than AGS-17 see ТКБ-0249 «Арбалет» (“Crossbow”) grenade launcher:
          http://weaponland.ru/load/granatomet_tkb_0249_arbalet/61-1-0-273
          It is designed to be fired from bipod, notice the tube over stock which house recoil-absorber. Technical data:
          Bore: 30mm
          Overall length: 900mm
          Mass: 10.0kg
          Muzzle velocity: 185m/s
          Range (maximal): 1700m
          Sights scaled to: 1000m
          Magazine: 5 or 10 grenades

          • And Baryshev (Барышев) grenade launcher. Baryshev designed whole system of weapons:
            http://www.bratishka.ru/archiv/2002/9/2002_9_6.php
            including:
            АБ-7,62 (avtomat firing 7.62×39 cartridge)
            АВБ (automatic rifle firing 7.62×54 cartridge, can fire in full-auto mode)
            КПБ (big-bore machine gun firing 12.7×108 cartridge, it has interesting feature – magazine was loaded with clip, this allow practical RoF 50 rpm)
            АРГБ – 30mm automatic grenade launcher:
            This weapon use Baryshev delayed blow-back principle of operation, it is equipped with optical sight and angle-meter.
            Technical data:
            Bore: 30mm
            Mass: 15,3kg
            Length: 950mm
            Length (stock collapsed): 700mm
            Length (barrel): 300mm
            Muzzle velocity: 185m/s
            RoF (cyclic): 350rpm
            Magazine capacity: 5 (photos in linked article show this grenade without magazine)

  8. you mentioned that they were used in similar numbers as LMG’s, do you know how they were distributed as in how many per squad or platoon ?

  9. Reminds me of the french army’s “LGI” F1 (“Lance Grenade Individuel”, which means “individual grenade launcher”).

    The overall shape and usage is the same but the LGI uses a captive piston which ensure no heating, no sound, no blast and no light on firing.

    It can be used by a man or attached to a tree and used as a trip-wire trap.

    • That is a seriously gnarly and vicious weapon! First time I heard of it. The Japanese knee mortar lives on, I guess. The English language wiki site I found described a French Armée de terre squad as being divided into a 300m element with FAMAS rifles and AT-4 anti-tank rockets, and a 600m group with a Minimi, rifles, and that grenade launcher thing…

      I know that the French armed forces also use the HK AG36 40mm grenade launcher too. Are there also still rifle grenades as well?

  10. I don’t have the book handy for the actual quantities, but according to “Hell To Pay” by DM Giangreco, the Japanese had a very large stock of these hidden away in caves and other spots for defense of the Home Islands from the expected invasion by the Allied Forces. IIRC, they had 10’s of thousands of the mortars, and many millions of warheads.

    What the Japanese had squirreled away for invasion defense was astounding, and Allied officers who handled disposition of those stocks were aghast at the thought that they would have had to face it, if the two A-bombs hadn’t brought things to an end. The intelligence groups concluded that the proposed two invasions would have, in all probability, failed to conquer the islands as had been expected. Would have made our previous island hopping experiences seem like a walk in the park by comparison.

    An eye opening book.

    • The text that Will is referring to is on page 166 with an endnote on page 336. Both are below:

      “The apparent lack of concern among Japanese commanders in the summer of 1945, when it became clear that the Americans would not strike Kyushu immediately after their Okinawa victory, was derived from their understanding that an invasion of southern Kyushu could now not take place until the fall, and an assault on the Tokyo area itself would not likely begin for a full nine or ten months. In the meantime, rifles and automatic weapons of adequate quality could be manufactured very easily in small, decentralized armories As an example, production was already flying on the simple Type 89 grenade launcher — really a personal mortar — in the summer of 1945 because it had become apparent that the weapon, nicknamed “knee mortar,” had been responsible for a high percentage of the American casualties on Iwo Jima and Okinawa. Some 28,428 were available at wars’ end, and production of their ballistic grenades had far surpassed individual weapons needs as 51 million, or 1,794 per barrel, were turned over to the Americans.43”

      “43. Ibid. [Reports of General MacArthur, vol. 1, suppl., p.], 136. Could this figure be a typo? Even if the an extra zero was inadvertently added and the true number of rounds was “just” 5.1 million, this quantity of mortar bombs per tub would still have to be judged as more than ample for the upcoming operation.”

  11. I remember how succinctly one U.S. Marine veteran of Guadalcanal and subsequent Pacific island-hopping campaigns summed up the 6.5mm Arisaka sniper rifle and the Type 89 “knee mortar” — “Accurate sons-of-bitches”.

  12. There are available online many of the after-action reports / interviews from US ground forces that had been engaged with the Japanese. They were written during the war, most quite soon after the actions took place.

    Our soldiers and marines had a lot of respect for this weapon. I recall one quote exactly from one interview: “Why don’t we have a weapon like this?” (in reference to the Type 89).

    I believe that the M79 and addition of a dedicated grenadier to each rifle squad was a direct result of combat experience against the Type 89.

  13. I showed one of these to my neighbor a Para Marine from Bougainville to Iwo Jima . He said every one they saw had a bullet hole in the barrel to insure that it wouldn’t be used against them later. He thought the Japanese had practiced with them for months before they landed and knew the ranges to every point on the islands.

  14. So, is this World War I German granatenwerfer the ancestor of the Japanese grenade discharger/ “knee” mortar?

    http://www.passioncompassion1418.com/decouvertes/ImagesDecouvertes/Grenades/proj3.jpg

    Looks like it has a spike and small base-plate, and a spigot or “mandrel” with a simple bolt and tension spring/striker assembly. The trigger or lanyard was pulled, and the striker detonated a cartridge embedded in the base of the winged shell. Apparently these were rather quiet, a bit like the contemporary French weapon mentioned up post that only produced 52 decibels when fired. Was the Japanese knee mortar similarly low signature?

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