O-I: Japan's Super Heavy Tank

[Mai_Waffentrager 9,657]

O-I

 

If someone came to you and asked the question; "what comes to mind when you hear the term super-heavy tank?", the average answer would be the notorious Maus or E-100 respectively. Big clunking tanks with large slabs of thick steel and armed with monstrous cannons. The idea of this class of vehicle had lingered on since the First World War, often relegated to the domain of prototypes and experimental designs. It would not be until the inter-war period that the concept captured designers' imaginations and drawing-boards as the 'next big thing' to turn the tide in the wars to come. Japan was no exception; in the dawn of the 40's, this super heavy tank would be known to the public as the O-I.

 

 

 

History and development

 

After 1939, the Imperial Japanese Army quickly came to realize that previous forms of mechanized warfare were proved inefficient after their defeat at Khalkhin Gol.

Development of the super-heavy project was spearheaded by Colonel Hideo Iwakuro, the eventual head of the Ministry of War of Japan (陸軍省 Rikugun-shō). Iwakuro opposed Japan’s advances towards the Soviet Union in 1939, and with the Japanese defeat, he decided to initiate a project to construct a heavily armored tank capable of withstanding large-caliber field cannons. Iwakuro assigned Colonel Murata of the 4th Technical Research Group to design and construct the super heavy tank in 1939. Colonel Murata noted Iwakuro’s words as described;

 

「満州の大平原で移動トーチカとして使えるような巨大戦車を作ってほしい。極秘でだ。」
“I want a huge tank built which can be used as a mobile pillbox in the wide open plains of Manchuria. Top secret.”

 

「今の戦車の寸法を2倍に延ばして作れ。」
“Make the dimensions twice that of today’s tanks.”

The 4th Technical Research Group began designing the super-heavy vehicle throughout 1940, attempting to meet Colonel Iwakuro’s vague instructions on the ultimate goal of the project. By March 1941, the research group had finished initial tank design and was ready to begin construction. The following month, a group of pre-selected engineers were chosen to partake in the building of the super-heavy tank. One recorded engineer was Shigeo Otaka, who stated they were sent to the 4th Technical Research Group’s previous headquarters in Tokyo. There, they were guided through a barracks containing multiple small fitting rooms, where they were to conduct meetings and reports on the progress of construction of the super-heavy vehicle. Towards the end of the barracks facility was a fully-enclosed room devoid of windows, with soundproofed walls to prevent external personnel from overhearing discussions related to the project. Each officer present possessed a portion of the project’s blueprint, which, when assembled, projected the full design of the tank, labeled "Mi-To". The name originated from a collection of the Mitsubishi industry and the city, Tokyo; given to the vehicle to uphold secrecy of the tank’s project.

 

 

The chosen engineers voiced their concerns regarding the Mi-To’s design noting that previously, the largest-sized Japanese tank had been the prototype Type95 Heavy in 1934. Issues that had been noted with heavy tank experiments in the years preceding the Mi-To showing Japan’s generally unsuccessful testing on multi-turreted vehicles exceeding the weight of standard armored vehicles. However, with the threat of a second Russo-Japanese conflict becoming more apparent, the project continued despite the engineer’s doubts on the size and mobility of the vehicle.

On April 14th 1941, the engineers began the construction of the Mi-To under secretive means. This entailed privately-made mechanical parts and equipment being shipped to the construction zone. Colonel Murata’s original concept was to complete the super-heavy tank three months after the initiation of Mi-To’s construction. This, ultimately, did not come into fruition; as technical issues on the project began to arise. Due to the limitation on material consumption by the government, the amount of parts that could be secretly shipped-in began to dwindle. By the first month of construction, essential construction resources had been depleted and the issues with the vehicle’s cooling system further caused delays. The construction of the Mi-To was postponed until January 1942, a delay of nine months.
 

 

After the Mi-To’s construction was resumed, the hull was completed on February 8th 1942. The tank had reached near-completion and was being prepared for mobility testing. Mitsubishi built the four turrets for the tank in May of the same year. Initial assembly of the tank’s armament took place soon after the turret’s superstructures were completed. However; the project once again did not have the necessary resources needed for the few remaining parts required for the final assessment. Due to this, the primary turret was removed as it lacked a 35-millimeter-thick roof plate, which had not yet arrived. Thus, the project was put on standby, until further development could continue. The total weight of the vehicle at the time was 96 tons, due to the lack of remaining structural plates and absent 75mm bolted-on armor.

 

The date on which the construction of the tank resumed is unknown, although active testing of the tank was scheduled for late 1943. The tank was unveiled to the Imperial Japanese Army’s highest command in 1943, and received a name change to O-I. This followed Japanese naming convention (O translating to Heavy, I for First, making it "First Heavy") that was standard. In his place was Lieutenant Colonel Nakano, Murata's assistant and colleague. Tomio Hara, head of the Sagamia Army Arsenal, was also present. Following the demonstration, senior officials within the IJA requested that field trials begin in August of the same year. The tank was disassembled at 2:00 AM one night in June of 1943 and sent to the Sagami Army Arsenal in Sagamihara, 51 kilometers from Tokyo. The vehicle arrived at the depot in June, and was reassembled and tested on the 1st of August.

 

 

On the day of the trials, the O-I performed satisfactorily until the second hour of the tests. While maneuvering on off-road terrain, the tank sank into the ground by up to a meter; attempts at traversing the hull to extricate the vehicle proved fruitless, resulting in further sinking due to the vehicle’s suspension coils compressing. The tank was eventually towed out, and further testing was continued on concrete. However, the earlier damage to the suspension resulted in vehicle’s movement damaging the concrete, which in turn, further damaged the suspension bogies to the point that further testing could not continue. The trials were postponed, and later canceled the following day.

Nevertheless, the trials conducted at the testing field were considered to be a success, and the vehicle was deemed ready for use in spite of its flaws. The engineers began disassembly of the tank on the 3rd of August due to resources being limited and the inability to maintain the tank in the field. Disassembly of the tank was completed on August 8th. Two days later, the engineers noted in a log that they were to inspect the parts and conduct research to fix the issues the O-I would face.

 

 

The fate of the O-I after its field-trials which took place on the 1st of August is unclear. Russian reports claim the Japanese were in possession of a wooden O-I mock-up mounting a Daimler-Benz DB 601A engine in 1945, however other sources point to the scrapping of the remaining parts of the same year. The remains of the O-I reside at the Wakajishi Shrine, with a track link of the prototype still present.

 

 

Design

 

  In modern interpretations, Japan is often seen as designing tanks held together by numerous bolts and rivets. A concept considered elegant in the inter-war period, but outdated during the war as new and more reliable methods of cast and welds become mass preferred upon in competing nations. The most iconic tanks grouped with Japan are vehicles designed and manufactured as early as 1934, nearly 7 years prior to the introduction of the infamous American M4 Sherman and Russian T-34. By the time the Second World War initiated, Japan had since started to deviate from their reliance on bolted tanks.

However, when you see the O-I up and about, the first thing you may notice are the bolts blanketing the tank's entirety. Don't be confused, the bolts on this superheavy vehicle are only a camouflage for the secrets lying underneath the steel. In first glance, the tank's design comes off as simple, flawed, and simply over excessive. It's design, albeit massive and complex, was given a simple and straightforward goal. That being as an armoured bunker in Manchuria, not as a combat tank to engage alone in the field.

 

 

The O-I was conceived out of the necessity to produce a mobile bunker to contest the Soviet Union in the then-expected Second Russo-Japanese conflict. The flaw with the routine bunker or pillbox is that you cannot maneuver and relocate them with the frontline constantly being pushed. Japan would need a sustainable fortress that could push with the infantry and advance further into the USSR without the need to construct more immobile bunkers with resources already scarce.

Japan relied on the North Expeditionary Doctrine when dealing with the threat of the USSR. After the defeat at Khalkhin Gol, the Government practically outcasted the Imperial Japanese Army for embarrassing Japan while its Navy met unrivaled. However, to counter their prior loss they had planned to once again prepare for another conflict that had seemed inevitable with the initiation of the German invasion of Poland and declining of relations. The tank was designed to withstand the guns of the Soviet Union's arsenal, while all the same countering with use of a 15cm howtizer against enemy positions and advancing armour.

 

O-I's main armament was chosen to be the newly produced Type96 Howitzer. A 4,140 kilogram cannon built and pressed into service in 1937, the cannon saw extensive use against the National Revolutionary Army in China and during the border conflicts with the Soviet Union. The cannon was picked to accommodate the need of targeting enemy fortified positions to cover the Infantry's pushes. By design, this is not an anti tank armament, it does not have wide options of anti tank shells with high penetration. The main shell of the howitzer is the Type95 APHE shell, recorded with 540 m/s and an average penetration of 125mm at a range of 230 meters. The cannon saw useage of both the Type92 HE and High penetrating HE shells respectively.

The tank was not only given the 15cm howitzer, however. Located on the front hull, two turrets with a Experimental 47mm's were present. Today we know the cannon as the main gun of the Chi-Ha Kai. This cannon was introduced in 1940 and became the nation's primary gun for anti tank measures. With penetration ranging from 80 to 114.3mm depending on the individual shell type.  The weapon’s barrels were reinforced with steel to secure them to the tank, due to the standard gun not adequately fitting into the turret.

 

The O-I was designed with 150mm of total armour thickness in both the front and rear of the vehicle. However, the production of the tank proved difficult to manufacture a 150mm plate, so to counter this crossroad, Mitsubishi split the plate into two separate slabs of 75mm armour thickness. The second 75mm plate would be bolted and sealed onto the existed plate on the vehicle to provide the expected over all thickness of the tank. The side armor on the hull superstructure was 70 millimeters thick. the base having standard thickness of 35mm, supported by an additional 35mm plate bolted on. There were eight wheel-supporting beams located on both sides of the suspension area which added an additional 40 millimeters of armor to specific locations on the side of the O-I. On the lwoer half of the side, a measurement of 110mm of armour thickness is present. 40 ladder pieces were placed around the tank to provide crew with the ability to climb onto of the vehicle with ease.

 

.

The tank had a length of 10.1 meters, width of 4.8 meters, and a height of 3.6 meters. The dimensions of the vehicle closely matched those of the Panzer VIII Maus. These proportions were massive and required the equally large amount of crew to operate it. The crew consisted of 11 manned positions. These were; 1 Driver, 1 Co Driver, 3 Main turret gunners, 1 Commander, 2 secondary turret operators, 1 rear turret operator, 1 Radio signaler, and 1 Engineer to maintain the tank. The tank was both designed and built with two inner armor plates to divide the interior into three sections; walls with two doors each and an ultimate thickness of 20mm. This allowed the crew and modules to remain relatively safe while the structure was kept safe with supporting stands. These supports allowed the interior armor plates to stay stable and also prevented collapse.

 Inside the O-I were two Kawasaki V-12 engines, both located in the rear, parallel lengthwise, to give room for the rear turret operator and transmission. The output of the engine is 550hp, both combined gave the tank an over all of 1100hp. The tank had a 6 gear system and weighed 1020kg. Speed of the tank ranged in 40kmh on flat roads in the 96 ton prototype. Paper speed with 150t weight was 30. The transmission copied that of the Type97 Chi-Ha’s, but used larger parts and gears making the total weight heavier. The vehicle had a coil spring system, with eight 2 wheeled boggies, totaling 16 individual wheels. Truly, a design of high proportions, with little feasibility for the weight of the expectations put on the tank.

 

 

Edited with revised article. You can find it on my blog here; http://sensha-manual.blogspot.com

Edited December 23, 2016 by Mai_Waffentrager

[Hopit 4,416]

I take it then there isn't any info about the amount of pen the 15cm HEAT had?

But I'm sure the tank would have made an impressive sight, if they had managed to get it operational. :good:

 

Well, this certainly was an interesting read, thanks for your effort in making this info-bomb shell! 

Edited January 30, 2016 by Hopit

[Aceinthehole257 11,769]

Probably kept that thing as a secret for 75 years out of embarassment.

[TheJoker1432 1,974]

thats a lot of data wow

[BHmaster 285]

Great job on all your effort and research!  :salute:

[Mai_Waffentrager 9,657]

And for the recorded diagrams from the engineers,

 

[Armoredbeercan 2,808]

so they went from mobile tin cans to immobile bunker tanks? 

[CmdNomad 1,392]

Awesome, good work! :good:

[BoBaH_u3_5A 4,118]

Thanks waffen

[andreacip2002 129]

Fantastic!!! Realy fantastic: Is a impossible tank

[Nomad_Gaming 6,221]

they actually built it, which is surprising. How the vehicle would have held up in combat would have been another issue entirely. Against the KMT, however, it would have served its purpose as intended until someone called in an airstrike on it; if deployed in 1944 it would have been all but invulnerable to the early Shermans and Hellcats that the Chinese Nationalist Army was equipped with at the time. 

[Professor_Lohere 9,004]

It's really ugly and it would be  useless too. We don't need this in the game. :3

Edited January 30, 2016 by Lohere

[bellezza03 6,004]

This thing is hard to balance. Just because it have a crappy gun. Too high and will be useless, too low, OP. God headcache for gaijin XD
Anyway the 47 mm APHE is no way to be filled with 250 grams of explosive and have that penetration. That is something wrong there.

[llXeusll 902]

Wondering if this will seal club as hard as it does in World of Tanks.

[Mai_Waffentrager 9,657]

This thing is hard to balance. Just because it have a crappy gun. Too high and will be useless, too low, OP. God headcache for gaijin XD
Anyway the 47 mm APHE is no way to be filled with 250 grams of explosive and have that penetration. That is something wrong there.

 

17 grams, 250 grams would be near impossible as you said.

 

It will take time to balance. I naturally suggest a BR a little higher than the KV2. The tank is large and will be vunerable to both air and to the sides of the vehicle where it can be penetrated mostly every time. The prototype is a lot easier since it has halved armour and only two 47mms. 

[Bringo_Gaboso 3,819]

If deployed in 1944 it would have been all but invulnerable to the early Shermans and Hellcats that the Chinese Nationalist Army was equipped with at the time. 

with 75mm of unsloped side armor it can be penned by a hellcat 1km away. 

Edited January 30, 2016 by Bringo_Gaboso

[bellezza03 6,004]

Everything depends on BR. And it will be a difficul one. Against IS-2 M26 and KT this tank have no chance, despite being of the same period, but against IS-1/Tiger and jumbo, it will be a new black price. The problem is that the main gun is really bad. Seriusly an angled Tiger will stop that round. Bounche fest lol

with 70mm of unsloped side armor it can be penned by a hellcat 1km away.

And also one word from the front: HVAP

[Mai_Waffentrager 9,657]

Everything depends on BR. And it will be a difficul one. Against IS-2 M26 and KT this tank have no chance, despite being of the same period, but against IS-1/Tiger and jumbo, it will be a new black price. The problem is that the main gun is really bad. Seriusly an angled Tiger will stop that round. Bounche fest lol
And also one word from the front: HVAP

 

I'll make an effort to try and locate better shells for the gun, since this is an artillery cannon there could be some stronger explosive rounds. 

[Bringo_Gaboso 3,819]

 

Due to this, the primary turret was removed as it lacked a 35-millimeter-thick roof plate, which had not yet arrived.

maybe the first version should be open topped with the turret to reflect this. then the one with the add on armor and a turret top plate 

Edited January 30, 2016 by Bringo_Gaboso

[Mai_Waffentrager 9,657]

maybe the first version should be open topped with the turret to reflect this 

 

I havent been able to translate the specifics of this event, but I'm assuming the roof plate contained frames needed to use the turret installment. If it was just a roof plate it doesn't make sense to test without it. 

[Kemosabe_WT 281]

But the O-I was not desighet to be armed with a Type 92 105mm gun with 60 rounds? Where the Type 96 150mm came from?

 

http://japan.greyfalcon.us/O.htm

http://www.militaryfactory.com/armor/detail.asp?armor_id=528

http://forum.warthunder.com/index.php?/topic/90232-o-i/

 

Also in this image you see 105mm gun (someone said the tank in image is O-I)

Edited January 30, 2016 by Kemosabe_WT

[Mai_Waffentrager 9,657]

But the O-I was not desighet to be armed with a Type 92 105mm gun with 60 rounds? Where the Type 96 150mm came from?

 

http://japan.greyfalcon.us/O.htm

http://www.militaryfactory.com/armor/detail.asp?armor_id=528

http://forum.warthunder.com/index.php?/topic/90232-o-i/

 

Also in this image you see 105mm gun (someone said the tank in image is O-I)

 

 

Because thats what the blueprints and report confirm. We used the Type92 for decades because it was the only logical gun presumed at the time. The 10cm cannon was never used or considered, just the 15cm. 

[Kemosabe_WT 281]

Because thats what the blueprints and report confirm. We used the Type92 for decades because it was the only logical gun presumed at the time. The 10cm cannon was never used or considered, just the 15cm. 

Ok, and can we see the report/blueprint or are copyright protected? Image are too small to read :)

[Mai_Waffentrager 9,657]

Inside diagram of the O-I. Will try getting a larger res when I'm given permission.

 

 

Edited January 31, 2016 by Mai_Waffentrager

[moshe770 393]

Very nice and interesting, you did good work here :salute:

this HE shell with 6.15kg of TNT(?) will be the strongest shell in the game! still, use this shell not be easy due to the velocity (540m/s).

how long is the reload time of the main cannon?