Fresh Start, New Beginnings

   After the end of the Second World War, Japan, under the removal order of the GHQ (Supreme Commander for the Allied Powers), had to give up and destroy all military technology and equipment from quickly dissolving Empire. Including with this demand were all Japanese tanks and armoured vehicles kept at home and afar. Documents of old war tank projects were destroyed if they had not been burned during the fire bombings of the United States during the closing stages of the war. To replace the demolished Army of Japan, the United States enacted a National Police Reserve with the purpose of providing national security within the nation in 1950. Two years after the creation of the group, the United States donated M24 Chaffee light tanks to the tank company split between each of the 4 divisions of the Police Reserve.

As the tensions in the Korean Peninsula intensified, Japan came to understand the Korean M24 light tank was insufficient in dealing with the Soviet T-34/85 tank on the battlefield. In 1954 the U.S. and Japan Mutual Defense Assistance Agreement was signed, allowing Japan to ensure its own security via a National Defense Force. This gave Japan the duty to protect its country from foreign threats. When the communist threat was at an alarming high, the US provided 200 units of M4A3E8 (Easy Eight) Sherman tanks. The arrival of the M4 came with good feedback initially. However, the SDF realized the failure the M4 provided in dealing with Japan's defense requirements. The constant need of maintenance and the outdated technology in a new setting proved a hassle for the Self Defense Force. Japan had originally favored the M24 light tank as it was capable of maneuvering the hilly terrain of both Korea and Japan thanks to its light weight. It's armament was obsolete, however.

Japan started to look at the US's modern arsenal of M47 and M48 tanks with their 90mm anti tank guns. However, both tanks were too heavy to adopt and that they would have failed substantially in the given circumstances. This came to resolution in 1955 as the Americans and Japanese agreed to share mutual beneficiaries to aid one another in technological advancements. The Japanese Ground Staff Office then submitted their requirements on a new tank design in January of 1955. Other ideas were present, but in the end the same goal was present. Keeping the weight low, the official project's goals were a weight of 25 tons, with a strong engine output and low ground pressure overall. Coupled with this equipped with a high penetration 90mm anti tank cannon. Provided with good depression, off road capabilities, and managing terrain such as beach heads and rice fields.


Development of the new tank began in June of 1955. The official military request were 2 prototypes. This was then raised to 5 by the years end. The contract for the tank was given to Mitsubishi Heavy Industries and Japan Steel Works. The title of the tank project was ST-A (tank A). Developed alongside this project was Japan's second tank tree route with the tank destroyer themed Type60 Recoilless Rifle. The ST-A underwent changes in its design throughout May as the weight of the vehicle was increased to 30 tons. This change was specified for additional armour to counter anti tank missiles and other anti tank infantry weapons. The new plans for the tank were as follows;

Crew:    4
Full Weight:    Under 30 tons
Length:    about 6.0m
Height:    under 2.8m
Width:    about 2.0m
Ground Clearance:    0.40m
Main Armament:    90mm
Gun Depression:    -15
Rounds:    50
Secondary Armament:    7.62 MGx1
Engine Type:    Diesel air-cooled
Horse Power:    600hp
Horse Power per ton:    20hp/ton
Top Speed:    50km/h
Wading Depth:    over 1.2m
Drive Type:    Rear wheel drive desired
Steering Type:     Hydraulic desired
Ground Pressure:    0.8kg/cm2
Range Finder:     Yes
Infrared vision:     Yes




These placeholder stats were approved for consideration and soon after, the Technical Research Department came together to discuss the idea. This meeting consisted of members of the Ground Staff office, Procurement Head office, and the Defense Agency Bureau Weapons Division. Added to this group was the Japan Weapons Industry Group. Guest to the meeting was Tomio Hara, the former Lieutenant General during the war. Here the decision was to have two tank proposals. The two were classed by weight, 20-25t and 25-30t. The lighter model was originally planned with a 76mm, and kept the original idea of having a light and mobile tank, favored by the Ground Staff Office.

However, the advantage of the heavier design with more armour and a larger cannon tipped the balance. The idea for the 30t project was to give similar maneuverability whilst allowing better protection and firepower.

When the MSA was signed and the US and Japanese lent aid to one another, Japan initially desired a loaned 90mm, used on the M36 tank destroyer. Ultimately this would add weight and make the concept of a lightweight tank hazy. Additionally, the head of the Ground Staff Office was transferred to the Technical Research Department for furthering the heavy tank plan. This effectively scuttled any proposal for a lighter vehicle to come into play.

32 ton mockup by the Technical Research Department

By October, both tank ideas were produced in mockup form. The finalized weights they came up with were 32t and 35t vehicles respectively. Despite requesting a lightweight tank, the features the General Staff Office had increased the final weight to 35 tons. Both designs were presented at the 5th Technical Council of 1955. The names given to these tanks were ST-AI and ST-AII. After the presentation, the JGSDF ordered a prototype of the 32t mockup and further studies. This would eventually lead to the ST-A1 tank, the predecessor prototype to the Type61 MBT. The order given to Mitsubishi was as follows;

「車重35トン、最高速度時速45キロ、90ミリ砲搭載、車高2.5メートルでなるべく低くする」 "A vehicle weighing 35 tons, provided with a top speed of 45 kmh, given 90mm anti tank gun, and a height as low as 2.5 meters."
Formal construction of the ST-A1 and ST-A2 began in 1956. The ST-A1 followed the original order and was designed with a low profile, built using a test steel plating. It was completed in December of 1956.  The ST-A2was larger than the ST-A1 and had a height of 2.5 meters instead of the 1's 2.2. It had an air-cooled diesel engine, torsion bar suspension, torque converter such as a power steering apparatus, and was completed in 1957. It was built using standard SDF steel. However, due to the engine not being produced and still in development the prototype was given the Mitsubishi DL10T V12 liquid-cooled diesel engine (500hp / 2,000rpm) instead.

ST-A1

During trials of the two prototypes, both were deemed inadequate and did not meet expected performance requirements. While both had ultimately failed, the ST-A2 was accepted for further design evaluations. The ST-A1 prototype had low vehicle profile, but this meant that the vehicle could not traverse its turret fully to the rear without elevating the gun. Hence the length of the tank had to be extended to avoid further issues. The suspension wheels became narrower as the track length extended. This increased the ground resistance, which in return caused problems traversing. The prototype had failed the trial.

ST-A2  prototype during trials - December 1956

With the development of the two medium prototypes. The original plan for a lightweight tank was overshadowed and forgotten. However, these new tanks would lead to the concept of a Japanese MBT rather than just a specific goal vehicle as originally sought. These are the routes that led to the creation of the Type61 Main Battle Tank.


In a Part 2 I will cover the further prototype testing of the series and the service implementation of the Type61.

Between a Bolt and a Hard Place

   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.

FineMold's O-I 150t model - The company that purchased the newly shown O-I reports.

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.

Type96 15cm Howtizer - Main armament of O-I

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.

Type1 47mm anti tank gun

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.



This will be a 3 part series covering the O-I superheavy tank. Stay tuned for the final part where I cover what is fake and whats real about the O-I!

Too big to Stop, Too heavy to Start

   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.

Outline of the O-I from record books

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 "Mi-To" shown when the finalized design was completed.

   
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.

Top - Down assembly of the Mi-To.

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.

Driver's periscope located center front hull.

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.

Surviving chain link of the prototype.

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.


This will be a 3 part series covering the O-I superheavy tank. Stay tuned for the next part!