Maeda Ku-6 (So-Ra)

Empire of Japan (1943)
Experimental Glider Tank – 1 Mock-up Built

While tanks can provide excellent offensive firepower, they can not always be easily transported to where they are needed. In the case of Japan during WW2, this was possible to achieve by using ships to transport them to where they were needed. During the war, the concept of a flying tank was becoming an interesting concept for the Japanese military hierarchy. Transporting tanks via air could potentially offer benefits to the airborne troops, who were often left without proper firepower support. This would lead to the creation of the Maeda Ku-6 tank glider.

The Concept of Airborne Operations

The idea of placing airborne troops behind enemy lines offers many tactical advantages. These can attack weak points and enemy supply lines. This in turn would force the opposing side to redistribute its own forces to deal with this problem. On the other hand, airborne forces often lack proper artillery or armor support, making them somewhat an easy target for well-equipped enemies. Some nation armies responded to this by employing light field artillery or even recoilless guns. Transporting armored vehicles proved a more daring task to implement. Tanks, for example, could not be easily carried inside a transport plane or even parachuted due to their weight. The American and British responded by developing lightly armored and armed light tanks, such as the M22 Locust or the Light Tank Mk VII Tetrarch. The Soviet Union, on the other hand, designed an auxiliary glider contraption that would be used to transport a tank, the Antonov A-40. This principle would also be tested by the Japanese Army during the war, which led to the creation of the Maeda Ku-6 project.

Airborne Japan

The Japanese began developing simple glider designs for civilian use in 1937. Following the successful use of gliders by the Germans during their conquest of the West in May 1940, the Imperial Japanese Army (IJA) began developing new gliders in June 1940. In response to this, the Imperial Japanese Navy (IJN) began its own project soon after. In Japanese terminology, these were designated Kakku (English: to glide).

Both the IJA and IJN had and used parachute units. It is important to note that these were relatively small units that were rarely employed in their intended role. For these reasons, their equipment was more or less the same as that of ordinary infantry formations. They saw the most active service during the fight for the Dutch East Indies in 1942. These were mainly used to capture various vital strategic points, such as airfields or weakly defended positions deep into the enemy’s rear lines. Following the end of this campaign, the Japanese did not use paratrooper units in their primary role. These paratroops formations were instead mainly used as ordinary infantry units.

Japanese paratrooper IJN units had two notable deployments: in the successful Battle of Manado (11th-12th January 1942) on Celebes Island (also known as Sulawesi), and in the Battle of Timor (19th February 1942-10th February 1943), where IJN paratroopers suffered heavy casualties. Their IJA counterparts were used more as a commando unit and were only ever airdropped during the conquest of Sumatra in February 1942.

In 1943 attempts were made in order to increase their firepower. It is unclear how much impact the experiences from the airborne operations of February 1942 had. It was proposed to use specially designed glider tanks that could be flown to their designated target and thus provide necessary firepower to generally weakly armed parachute formations. In addition, this vehicle could be airlifted to any other theater of war without a need for them to be transported by ships which were by this time becoming dangerous due to US navy and submarine activities.

The Maeda Ku-6

The project was initiated by the Army Head Aviation Office in collaboration with the Fourth Army Research Center. The first drawings of this new design were soon ready and were allocated to the Maeda research center for the construction of a working prototype. In the early stage of development, the new tank was to be transported by an especially designed glider. But as Maeda was unable to create a glider that could transport a light tank another solution was needed. Maeda engineers suggested another approach to this problem. As no glider could be developed to carry a tank, maybe the tank itself could be modified to be used as a glider.

While Maeda was responsible for the glider development, the design of the tank was given to Mitsubishi Heavy Industries. It is unclear if it was a completely new tank design or if Mitsubishi reused some of the existing vehicles that were in service. According to the Japanese Army and Navy Aircraft Complete Guide, the Type 98 light tank was used for the project. This tank was intended as a replacement for the Type 95 Ha-Go but this was never achieved as it was built too late and in too few numbers.

Name of the Project

According to E. M. Dyer (Japanese Secret Projects Experimental Aircraft of the IJA and IJN 1939-1945), the new light tank was designated as So-Ra (Sora-Sha), which could be translated as the “sky” or “air” tank. The whole project would be designated Kuro-Sha, with the Ku and Ro (meaning the number ‘6’) taken from the Ku-6 glider designation. Lastly, the Sha stands for “tank”. An older source, J. E. Mrazek (Fighting Gliders of World War II), mentions that the tank design originated in late 1939. According to Mrazek, the tank was initially designated ‘special Tank project 3’. It received the Sora-Sha designation before being changed to Kuro-Sha (English: Black Vehicle).

Technical Specification

Given its experimental nature and the loss of the original documentation, not much is known about the So-Ra light tank.

Hull 

Due to its intended use, the So-Ra had to be as light as possible. Its total combat weight was estimated to be around 2.9 tonnes. The overall hull design was likely to have consisted of the rear engine compartment and the front crew compartment.

Suspension

Based on the available picture of the wooden mock-up of the So-Ra, it was to have an unspecified five-road suspension. What kind of suspension would be used is unclear. If the previously mentioned fact was sorely based on a Type 98 light tank then it would be using a  bell-crank suspension which was quite common on Japanese tanks. Its later improved version, the Type 2 Ke-To, used a much-simplified bell crank suspension. To complicate the matter further, author D. Nešić (Naoružanje Drugog Svetsko Rata-Japan) mentions that the tank glider project would use a torsion bar suspension. It would likely have had a front-drive wheel, rear idler, and two return rollers.

Engine

The So-Ra was to be powered by an air-cooled four-cylinder 50 hp @ 2,400 rpm gasoline engine. Fuel capacity was to be around 150 liters. Thanks to its small weight, the maximum speed with this engine was estimated to be around 42 km/h.

Superstructure and Turret

On top of the hull, a superstructure was placed. Its central part appears to have had a simple box-shaped structure to allow the crew to fit inside the short and small vehicle. The front driver compartment was provided with three large vision ports. This was necessary, as the driver was also the glider pilot, so giving him the best possible view of the surroundings was important. On top of the driver compartment, an escape hatch was likely located. The centrally-positioned turret also appears to have a simple box-shaped design.

Armor

The armor thickness of this tank is unknown, but given its role, it would have been very light. E. M. Dyer mentions that it would have likely not exceeded more than a maximum of 12 mm.

Armament 

Regarding armament, three different configurations were suggested. The first would have been a 37 mm gun. Which precise type is unclear, but given its availability, it was possibly the Type 94 tank gun, as some sources have suggested (E. M. Dyer Japanese Secret Projects Experimental Aircraft of the IJA and IJN 1939-1945). This tank gun was actually a modified 37 mm Type 94 infantry anti-tank gun. It had a muzzle velocity of 575 m/sec, it could penetrate 35 mm of armor at 300 m with Armor Piercing (AP) rounds. The gun could also fire High-Explosive (HE) rounds, although the effect of 37 mm HE was rather minor. A semi-automatic sliding breechblock fed the ammunition. Loading the gun would have been extremely easy to do one-handed, as the cartridges were rather small, at around 13 cm long and 4 cm in diameter.

If this vehicle was truly based on Type 98 or the improved Type 2 Ke-To, the 37 mm Type 1 may have been used. This is basically the same gun but with a longer barrel which offered a better velocity of 797 m/sec. The armor penetration was also increased to 44 mm at ranges of 500 m. Note that these numbers may vary between different sources.

The second variant consisted of a single 7.7 mm or larger caliber machine gun. This would have possibly been the Type 97 7.7 mm heavy ‘tank’ machine gun. This was an air-cooled machine gun, fed from a top-loading magazine, similar to the British Bren gun. This machine gun was actually a Japanese version of the Czech ZB vz 26 machine gun. It was equipped with a stock that was angled off to the right, allowing a gunner to line their eye up with the sight.

 

Lastly, a flamethrower was considered a potential armament.  Given the small size of the Ku-6 vehicle, the ignition fuel load for it would have been quite limited.

The ammunition load of either gun or machine gun version is unknown. However, given the cramped space inside the small glider tank, and the need to save weight, this would likely be quite limited.

Crew

The So-Ra would have been operated by two crew members. The driver was positioned in the front of the vehicle. He was also responsible for piloting the whole glider. Behind him, in the turret, the commander, who was also responsible for operating the main armament, was positioned. This small crew would greatly affect the tank’s overall performance. Given the limitation in size and weight, adding more crew members was not possible.

Wing Configuration

The Ku-6 was designed as a tank transporting glider. Due to the loss of the original documents, not much is known about its overall design. Over the years, historians have devised two different designs of how this contraption may have looked based on available information.

According to the first proposals, the tank itself was designed to act as an improvised glider fuselage. The wings and the tail assembly would be attached to it. The tank crews would be provided with wired control wires inside the vehicle. In front of the tank, a towing cable would be added.

The second version is completely different. Above the tank, a larger wing with a twin tail boom was added. These two components would be connected by struts. In both cases, once the tank hit the ground, the wing assemblies could be easily removed, which meant that the tank could immediately go into action with relative ease.

As the tracks would cause huge drag, specially designed sleds would be attached to them during take-off to facilitate an easier take-off. With the whole wing assembly, the Ku-6 had a length of between 12.8 to 15 m (depending on the source) with a width of 22 m. The maximum speed that could be achieved during the flight was 174 km/h.

It is important to note that these are both speculations on behalf of historians, and the actual method of constructing the glider is currently unknown.

Testing and Project’s Fate

Due to the slow pace of work, the first operational glider prototype was completed in 1945. The tank itself was not ready by this time. As a temporary solution, a wooden mock-up of it with extra weight was intended to be used instead. The prototype was taken to the sky by a Mitsubishi Ki-21 aircraft. Almost from the start, the Ku-6 proved to have poor overall flight performance. The pilot had a poor view. Lastly, as it was specially designed to carry the So-Ra, its transport capacity for other vehicles was limited. The IJA officials quickly became disinterested in the Ku-6, focusing instead on the Ku-7, which looked more promising. Another aspect that we must take into account was the generally poor state of the Japanese Army in 1945. By this point, it was so battered and depleted that realistically undertaking an airborne operation was an impossible task. In the end, the Ku-6 would be terminated and the fate of the single prototype is unknown, but it was either scrapped or lost during Allied bombing raids.

Conclusion

The Ku-6 seems like an interesting concept that could have offered a number of benefits to the Japanese in the early years of the Pacific theater. By 1943, when the project was initiated, the war situation for Japan had rapidly deteriorated, with the Allies pressing on all sides. In reality, the Ku-6 proved to be too flawed in design. It was difficult to control and the pilot had poor visibility. Given that it was a glider, it would make an easy target for enemy fighters which, by its construction time, had almost complete air supremacy.

Maeda Ku-6 specifications
Tank Dimensions	4.07 x 1.44 x 1.88 m
Total weight	2.9 tonnes
Crew	2 (driver and commander)
Propulsion	Four-cylinder 50 hp@2,400 rpm gasoline engine
Armament	37 mm gun or 7.7 machine gun or a flamethrower
Armor	Possibly up to 12 mm
Top speed	42 km/h

 

Sources

• D. Nešić (2008), Naoružanje Drugog Svetsko Rata-Japan, Beograd
• E. M. Dyer (2009) Japanese Secret Projects Experimental Aircraft of the IJA and IJN 1939-1945, Midland
• J. E. Mrazek (1977) Fighting Gliders of World War II, ST Martin Press
• S. J. Zaloga (2007) Japanese tanks 1939-45, New Vanguard
• Tomio Hara’s Japanese Tanks 1978
• Japanese Army and Navy Aircraft Complete Guide
• L. Ness (2015) Rikugun Guide To Japanese Ground Forces 1937-1945, Helion and Company