Mitsubishi Type 89 Yi-Go Medium Tank (Chi-Ro, I-Go)
Technical Information

Development
The Imperial Japanese Army (IJA) Type 89 Yi-Go medium tank was the second tank built by the Japanese. The designation is also transliterated Chi-Ro or I-Go.

After the development of the first Japanese tank Type 87 Chi-I it was found to be too heavy and too slow and a lighter tank weighing ten tons was sought. In 1925, the Japanese Army Osaka Technical Arsenal began development and rapidly developed this tank due to the designers' experiences with the Type 87 Chi-I. The new tank used steel plate armor developed by Nihonseikosho (Nihonseikosho Company) dubbed "Niseko Steel" an abbreviation of Nihonseikosho.

The design was initiated in 1925 by the Japanese Army Osaka Technical Arsenal. This was Japan's first domestic light tank project. The design was relatively conventional and was influenced by the British Vickers Mark C tank that had been purchased in 1927.

Meanwhile, the development of a diesel engine was ordered from Mitsubishi Jukogyo Company. After efforts over two and a half years, Mitsubishi developed the A6120VD, a reliable and easy to maintain 120 hp air-cooling diesel engine. Starting in 1934, this engine was installed into the Type 89 Medium Tank. This engine was also used in the early production models of the Type 95 Ha Go Light Tank.

In April 1929, the new light tank was finished. Its weight was 9.8 tons and designated the "Type 89 Light Tank." Later, it was re-classified as a "Medium Tank" because the weight increased to over 10 tons after several improvements. Armed with a 57mm gun Type 90 (short barrel) in the turret with two 6.5mm machine guns Type 91 mounted in hull and coaxial.

Wartime Usage
Between 1931–1942, the Type 89 Tank was the Imperial Japanese Army (IJA) main tank used in the Second Sino-Japanese War, Battle of Khalkhin Gol (Nomonhan Incident) and during World War II in the Pacific Theater.

Type 89 Ko Model
The initial production model had a gasoline engine and mounted a machine gun on the right side of the hull. As this design could only attain 15.5 km/hr, and was also limited by the severe winter climate of northern China. A total of 220 units were produced until production ceased in 1934.

Type 89 Otsu Model
The later model "Otsu" had an air cooled diesel 120 psi / 1800 rpm engine and mounted a machine gun on the left side of the hull. The diesel engine had several advantages, notably reduced vulnerability to fire and better fuel economy. This Otsu version was the first diesel-powered tank to be mass-produced in the world.

The improved model had a new gun turret design complete with a cupola for the commander, and with the machine gun relocated to the left side of the hull. The multiple armor plates of the front hull were replaced by a single shallow-sloped frontal armor plate which provided more protection for the driver. However, the major difference between the versions was the Mitsubishi air-cooled 6-cylinder diesel engine, which had several advantages, notably reduced vulnerability to fire and better fuel economy. A diesel engine was also preferred by the Japanese Army because more diesel fuel than gasoline could be produced per barrel of oil. A total of 189 Otsu units were produced.

Production
The Type 89 Yi-Go was the first tank mass produced in Japan between 1931–1942. Production of the Type 89 Ko Model began in 1931–1934. Production of the Type 89 Otsu Model began in 1934–1942.
Type 89 Ko Model: 220
Type 89 Otsu Model: 189

Technical Details
Crew  Four (driver, commander/gunner, loader, hull gunner)
Weight  12 tons (loaded)
Speed  26 km/h
Armor 6-17mm
Engine  Mitsubishi A6120VD 6-cylinder air-cooled petrol diesel
Range  170 km
Armament  57mm gun Type 90 (short barrel) with 100 rounds
Secondary  Two 6.5mm machine guns Type 91 mounted in hull and coaxial with 2,745 rounds