K-91 (Front-Mounted Turret)

Soviet Union (1949)
Heavy Tank – Blueprints Only

With the end of the Second World War (Great Patriotic War in Russia) and the uncertainties of a new war, many nations sought to design weapons as revolutionary and powerful as possible. This often involved thinking outside the box, resulting in some odd and interesting designs. One of these projects was the K-91, born at a time when the Soviet Union had a hypersaturated market in regards to general tank development and especially heavy tanks.

Development

On 18th February, 1949, the Council of Ministers of the USSR published statement No.701-277§, which effectively canceled all development of heavy tanks weighing 50 tonnes and above, putting an end to tanks such as the IS-7. Instead, work was shifted to designing lighter heavy tanks. Thus SKB-2 and Factory No.100 of Chelyabinsk were assigned work in designing a new heavy tank, which would eventually become the T-10.

With the cancellation of most heavy tanks programs, the Design Bureau of the Engineering Committee of the Armed Forces (OKB IC SV), led by Anatoly Fedorovich Kravtsev, saw the opportunity to design a unique set of vehicles. By this point, Kravtsev’s design bureau had experience in designing light tanks and APCs, but never had them mass produced, such as the K-75. Kravtev’s team envisioned something special. This was not to be any regular heavy tank, rather, they would look back at wartime vehicles which tried to combine and replace both medium and heavy tanks, while still being a solid platform for self-propelled guns, a concept that became mainstream later on.

In charge of the program was lead engineer I.T. Levinov while the designer was Matyukhin. They designed three vehicles: two heavy tanks, one with a front mounted turret, one with a rear mounted turret, and a tank destroyer/self-propelled gun. In this article, the first variant will be discussed.

Design

The first variant, besides being the most sensible, was also the one that was considered the most, with a total of 5 drawings. It featured a massive turret that housed all 4 crewmen – including the driver. The hull was extremely low, thanks to the movement of the driver to the turret and usage of a boxer engine. This was done not only to decrease the area and silhouette of the vehicle, but also to make it lighter and harder to hit. The bizarre aura of the vehicle continues, with a very strange set of roadwheels, with torsion bar suspension and a large frontal sprocket. To create more room inside the tank, the sides of the hull follow the shape of the track, which required the addition of round skids to prevent the track from hitting the hull violently. The armament would be a modest 100 mm gun with a coaxial DShK and one more on the roof for AA protection. In terms of protection, however, the tank stood out, with around 200 mm of raw thickness on the upper frontal plate and turret.

Crew

The crew consisted of 4 men, a commander, a gunner, a driver, and a loader. They were all seated in the turret. The gunner sat on the left side of the gun, in the front of the turret. He had no periscopes, but had to rely on his gunsight for vision. Considering the shape of the turret, which featured two unevenly sized bulges protruding from the otherwise rounded turret, the gunner had to share the entry and escape hatch with the commander. The commander sat right behind the gunner, and had only one periscope for vision. This meant that he had a hard time surveilling the battlefield and searching for targets for the gunner. His reliance on the intercom and radio with other tanks must have been increased.

On the front right side of the gun sat the driver, who had a pivoting driving system. This allowed the turret to turn freely, while the driver would still be in the same position. It is unclear if the turret would have been able to complete a full 360° turn with this system. The driver did have two pericopes for vision (one could be the loader’s, it is hard to tell from the drawings). The loader sat behind the driver, slightly more towards the center, with full access to the gun breach. He had the not-so-easy task of maneuvering the large 100 mm rounds through the very low roof of the turret. The ammunition was placed all around the rear of the turret and inside the turret ring. As proof that even the designers found the turret roof to be too low, they had to make a cutout and slight bulge in the armor so that the head of the loader would actually fit. To put it into perspective, the average tanker was between 160 to 170 cm in height.

Armament

The main gun was to be a 100 mm D-46T. This was a brand new gun developed by OKB No.9 as a replacement to the D-10T. The project was greenlit on 21st May, 1948 and two were produced in Factory No.9 in 1949. It was, however, most likely canceled and used for the development of the D-54. The shells weighed between 16 and 17 kg and would have a muzzle velocity of 1,000 m/s. The gun had +20° of elevation and -3° of depression.

The secondary armament consisted of two 12.7 mm DShK heavy machine guns. One was mounted coaxially, on the right side of the gun. This does raise questions of who could have loaded it and cleared jams. The driver was the only one that could realistically reach it, but that involved the driver not driving the tank. The loader would have been required to almost lay over the gun to reach the machine gun. The roof-mounted DHsK was mounted on a rotating pintle, similar to the IS-3 and IS-4 heavy tanks. The loader or commander could have fired it.

Propulsion

To decrease the height of the hull as much as possible, a boxer engine was used. Boxer engines have the cylinders arranged horizontally, facing away from each other. This allows for much lower engines, but considerably wider, compared to straight-heads or V-shaped ones. Additionally, boxer engines offer finer and more responsive performances, but are more expensive to build. The engine was most likely a V-64 12-cylinder diesel, outputting circa 700 to 800 horsepower. On this variant, the engine was placed in the rear, behind the turret, while the gearbox and transmission were in the front, where the drive wheels were also located. To transmit the power, a large shaft ran through the entire length of the vehicle, in between the torsion bars and turret ring floor. There were two fuel tanks, one underneath each cheek of the turret.

Suspension

The suspension of the K-91 was very unusual. It had 9 roadwheels per side, attached with suspension arms to torsion bars. The first three arms were facing the opposite from the last 4. The first and last 2 wheels were sprung by just one torsion bar and attached via a pivoting bogie. The idler was the same as the road wheels, while the sprocket was very large to allow for good crossing of obstacles. The suspension seems to have had very little space in which the wheels could move, meaning either that it had to be quite hard or that the wheels would easily hit the bump stops and transfer the rest of the shock to the hull.

Armor

As expected, the K-91 was very well protected, with around 200 mm of armor on the upper frontal plate, angled at 45°. The lower frontal plate was around 150 mm angled at 50°. The frontal cheeks were vertical but angled outward from a frontal viewpoint. They were 150 mm thick, and so was the side armor, which was completely flat. Rear armor seems to have been two angled 75 or 100 mm plates.

The turret was extremely complex in its design. It appears to have been cast, with several uneven bulges for the crew. It was 200 mm thick at the base and got exponentially thinner as the angling got higher. The bulges remained 200 mm thick, as they were less angled.

It is hard to tell if the vehicle remained under the 50-tonne threshold, but considering its small profile and smaller gun (compared to most Soviet heavy tanks), it could have reached 45+ tonnes.

Other variants

Kravtsev’s bureau designed two more vehicles as the K-91; a self-propelled gun version based on this variant and a heavy autoloading tank with a rear mounted turret.

The video game company Wargaming has made a fake hybrid of this variant and the rear-mounted variant, taking the turret and putting it in the back, for their video game World of Tanks.

Conclusion

None of the three K-91 vehicles designed at OKB IC SV got far due to the apparent lack of improvement over contemporary heavy and medium tanks. The vehicles were quite complex and expensive from a design viewpoint but were fundamentally crude and rudimentary. The designs were terminated in late 1949. Kravtev’s bureau went back to designing APCs and light tanks and developed the K-78, K-90 and K-61, among others.

K-91 (front turret) specifications
Dimensions (L-W-H)	10.230 (6.300 w/o barrel) – 3.340 – 2.150
Total Weight	circa 49 tonnes
Crew	4- commander, gunner, driver, loader
Propulsion	V-64 boxer 12-cylinder diesel, est. 700-800 hp
Speed	N/A
Armament	1x 100 mm D-46T 1x co-axial 12.7mm DShK 1x AA 12.7 mm DShK
Armor	200 mm around turret 200 mm UFP 150 mm LFP, cheeks, side
Total Production	blueprints only

Sources

Technic and Weapons No. 9, 2013, M.V. Pavlov, I.V. Pavlov Domestic armored vehicles of 1945-1965
Yuri Pasholok on the Soviet STG – Status Report (ritastatusreport.live)
https://military.wikireading.ru/56371