Categories
Cold War Soviet Prototypes

K-91 (Rear-Mounted Turret)

Soviet Union (1949)
Heavy Tank – Blueprints Only

After the Councils of Ministers of the USSR had terminated all heavy tank projects of 50 tonnes and more in 1949, many Soviet design bureaus and factories saw their opportunities to design new vehicles. One of these was OKB IC SV (Design Bureau of the Engineering Committee of the Armed Forces) led by Anatoly Fedorovich Kravtsev. They designed three vehicles, two heavy tanks and one SPG, under the name K-91.

Development

The Councils of Ministers of the USSR stated on 18th February, 1949 in the decree No.701-277§ that all design, development and production of heavy tanks heavier than 50 tonnes shall end. Kravtsev’s team at OKB IC SV set to work almost immediately to design some lighter heavy tanks that would replace both heavy and medium tank classes, whilst also being the platform for Self-Propelled Guns (SPGs). The lead engineer was I.T. Levinov and the designer was Matyukhin.

The bureau designed three vehicles, named K-91. The first was a heavy tank with a large, front-mounted turret that encompassed all 4 crewmembers. The second was a rear-mounted turret heavy tank that featured an odd-shaped hull and an autoloader. There was also an SPG.

Design

The second variant of the K-91 heavy tank design was less elaborately depicted than the first (there is no knowledge on the chronological order in which the tanks were designed, and are numbered as such simply to differentiate them). From the 4 available drawings, the tank had the turret mounted in the rear of the hull and the engine and transmission were mounted all the way in the front. This offered a lot more protection to the crew, as the mechanical components acted as a form of armor against fragments following penetration. The crew, consisting of 3 men, were all seated in the turret. Most curiously, the hull had a very odd shape. From above, it was almost shaped like an egg, while from the side, it was flat. This awkward hull design, combined with the stowage of ammunition in the turret, resulted in a lot of empty space in the hull.

Side view of the K-91 rear-mounted proposal. From this angle, the hull seems very flat and unremarkable.
Source: Technic and Weapons No. 9, 2013, M.V. Pavlov, I.V. Pavlov Domestic armored vehicles of 1945-1965, page 35

Crew

As aforementioned, the tank had a crew of 3; commander, gunner, and driver. They were all seated in the turret. The gunner sat on the left side of the gun and only had his main gun sight as vision, having to rely entirely on the commander’s calls. The commander sat right behind him, surrounded by the ammunition, and had only one periscope to view out of. He had a protrusion from the otherwise semi-spheric turret, akin to a commander cupola. This is where the periscope was mounted and, most likely, this was his entry and exit hatch. The gunner most likely also used this hatch. The commander was close to the gun breech, potentially meaning that he could manually load the gun if the automated system failed.

Lastly, the driver was seated on the right side of the gun. Since he was seated in a rotating turret, he was given a pivoting device that would allow him to remain facing the front of the hull regardless of the rotation of the turret. This, however, makes it unclear if the turret was able to perform a full 360° rotation. Right behind him was the plethora of ammunition. Since the driver was placed quite high in the turret, he also had a protrusion from the rounded cast turret.

From this angle, the bizarre, almost UFO-like hull shape is clear. Despite it having no vertical angling, the Z-axis angling gave it very good LOS protection.
Source: Technic and Weapons No. 9, 2013, M.V. Pavlov, I.V. Pavlov Domestic armored vehicles of 1945-1965, page 35

Armament

The main armament on the K-91 heavy tanks was to be a 100 mm D-46T. At the point when these tanks were designed, this was a brand new gun. The project started on 28th May, 1948 and it was intended as a replacement to the D-10T gun used on the T-54. Two prototypes were built at Factory No.9 in 1949. Development was canceled shortly after though, but it was important in the development of the D-54.

The ammunition had a muzzle velocity of 1,000 m/s and weighed between 16 to 17 kg. The ammunition was not two-part, despite being very large. The gun had +12° of elevation and -3° of depression.

While the frontal mounted turret variant featured a human loader, the rear-mounted turret K-91 had an autoloader. However, no details are provided and the drawings do not offer any details on the mechanism. Autoloaders were still in a relatively infant stage, especially at such large calibers. The Soviets had some experience with autoloaders, as they had experimented with such devices on tanks even during the war, and the final variant of the IS-7 featured an autoloading mechanism for the massive 130 mm main gun. However, the system on the K-91 was different from that on the IS-7, which featured two-part ammunition stored in a rack, and were pushed in by a conveyor belt. The gun required to be brought back to neutral position after each shot. The K-91 system featured ammunition stowed all across the turret being pushed in towards the center. There, a conveyor belt or arm would ram the shells in. Since the ammunition was one single piece, reloading would have taken less time.

The tank had one secondary armament, a 12.7 mm DShK heavy machine gun, mounted coaxially.

The autoloading mechanism on the IS-7.
Source: Pinterest

Propulsion

When moving the turret to the back, the engine was moved to the front, right behind the transmission (the drive sprockets were in the front). The specific engine used was, allegedly, a V-64 12-cylinder boxer diesel. Boxer engines have pistons mounted horizontally, facing away from each other. This makes them a lot lower (it allowed the hull to only be 1,170 mm tall), but wider than other piston configurations, such as in-line or V-shaped. In this case, the width did not matter due to the large amounts of empty space next to and behind the engine. The position of the fuel tanks is unknown. They could either have been in front of the turret cheeks, which was the furthest possible spot from the engine, or between the basket turret floor and tank floor, between the torsion bars.

Rear cutout view of the K-91 with a rear turret. The wide boxer engine can be seen, with the gun and massive diagonally mounted 100 mm shells.
Source: Technic and Weapons No. 9, 2013, M.V. Pavlov, I.V. Pavlov Domestic armored vehicles of 1945-1965, page 35

Armor

In terms of armor protection, the tank had plenty. The thickest was around 260 mm. However, this spot was rather small, as it was only the ‘tip’ of the hull. This thick piece of armor was bent into shape and then connected to the rest of the hull armor via a puzzle connection weld. The rest of the hull was 200 mm all throughout. These seem to be entire sheets of armor that were bent into shape. The frontal lower plate seems to have been 100 mm thick, although angled at -40°. The rear armor plates were even thinner, probably around 75 mm. The belly armor was 10 mm and top 20 mm.

The turret was a classic Soviet-style semi-sphere, with the addition of the two domes for the driver and commander. The turret seems to have been 200 mm thick all around, with only various mantlet and roof areas being thinner.

Side cutout view of the vehicle. Here, the armor layout is well visible.
Source: Technic and Weapons No. 9, 2013, M.V. Pavlov, I.V. Pavlov Domestic armored vehicles of 1945-1965, page 35

Suspension

Although it is different compared to most Soviet heavy tanks of the time, the suspension was fairly standard. It featured a small frontal mounted sprocket, 9 roadwheels sprung to 7 torsion bars and a rear idler. The first two and last two wheels were connected to each other and were attached via a bogie.

Other K-91s

As previously stated, Kravtsev’s team proposed three vehicles. Besides this one, there was a frontal mounted turret heavy tank and an SPG. The video game company Wargaming took this variant and the frontal mounted turret version and created an unhistorical hybrid.

Conclusion

Despite having some more technologically advanced features over the other K-91 variant, such as an autoloader, the design never got very far and all K-91 projects were canceled later in 1949, most likely as they did not bring any realistic improvement over existing and future combat vehicles.

K-91 rear-mounted turret specifications

Dimensions (L-W-H) 9,020 m (7,650 m w/o barrel) – 3,350 m – 2,140 m
Total Weight, Battle Ready circa 49 tonnes
Crew 3- commander, gunner, driver
Propulsion V-64 boxer 12-cylinder diesel, est.700-800 hp
Speed N/A km/h
Armament 1x 100 mm D-46T autoloaded
1x co-axial 12.7mm DShK
Armor 200 mm around turret
260 mm front tip
100 mm lower frontal plate
200 mm hull side and cheeks
75 mm rear hull
Total Production none – paper 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

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