Tuesday 22 August 2023

Failed Modernization of the KV-1S

Evolutionary development is a common sight in tank development. The fact that engineers do not seek to make work for themselves should not be mistaken for laziness. This is often a necessary requirement set by the customer, especially when it is necessary to keep up the rate of production. For this reason, revolutionary projects were often vetoed from above. This happened with the KV-13 and T-34M in 1942 when both were put away until better days.

Object 238 on trials, August 1943.

Stalin signed GKO decree #3289ss titled "On the issue of increasing the firepower of tanks and SPGs" on May 5th, 1943. According to the decree, two prototypes of IS and KV-1S tanks with 85 mm guns were due on July 1st, 1943. Work went in four separate directions. Two different chassis were combined with two different weapons. The Central Artillery Design Bureau decided to make the fewest necessary changes the existing tank. This led to the creation of the Object 238, a simple but unsuccessful variant of rearming the KV-1S.

Evolution of experiments from 1942

It is often claimed that the Soviet military was caught with its pants down when the Tiger tank arrived on the scene. This was not the case. The appearance of a German heavy tank was an unpleasant surprise, but it's incorrect to say that there was no way to fight it. The issue was that this work was not prioritized because there was no point in it, but there was a steady stream of prospective projects. This includes development of 85 mm tank guns.

85 mm ZIS-25 gun, precursor to the S-31.

V.G. Grabin's design bureau at factory #92 was the first to develop an 85 mm tank gun. It was meant for the T-220 heavy tank. The bulky gun needed a new large turret and the army quickly lost interest in it. Factory #92's design bureau returned to the topic of 85 mm guns in October of 1942. The ZIS-25 gun was created under the direction of I.M. Lependin. Lipendin was one of the creators of the ZIS-5 gun prior to this, and so 80% of the new gun's parts came from here. This simplified development and production. Factory #92 missed two things. The first was that creating a gun for the KV-1 in October of 1942 was pointless since the KV-1S had replaced it in production more than two months prior. The gun mount and turret of the KV-1S were different. Second, the GAU found a number of issues with the design, including difficulty in loading.

The ZIS-25 gun moved to the TsAKB in early 1943.

The 85 mm tank gun was put on a high shelf, but not for long. Grabin's Central Artillery Design Bureau picked up the topic shortly after. This began with a transfer of the ZIS-25 project in January of 1943. According to correspondence, Grabin was aware of the issues with the ZIS-25 and prepared to alter the KV's turret for it. To do this, he wanted a KV-1S and one ZIS-5 gun without a barrel or breech. The GBTU didn't hurry to help him. On March 14th, 1943, they replied that the TsAKB already had a KV-2. A KV-1S turret could be allocated for it and the hybrid tank would be used to test the gun. 

A universal turret for the KV-13 and KV-1S equipped with an 85 mm S-31 gun.

Tactical-technical requirements for the design and development of the 85 mm tank gun were signed on March 26th, 1943. The gun would be shorter than the 85 mm 52-K AA gun and ZIS-25 tank gun, since a muzzle velocity of 600 m/s was considered sufficient. The gun could penetrate 90 mm of armour at 500 meters. These requirements vanished soon after. The cause of this was the capture of two Tiger tanks. Even before #121 was shot up, it was clear that a more powerful 85 mm gun was needed. On April 12th, 1943, the GAU Artillery Committee developed preliminary requirements for a tank destroyer armed with an 85 mm gun. The required penetration was 120 mm, the estimated thickness of the Tiger's front armour. This was done before the penetration trials, but the trials only confirmed that such a gun was necessary.

The turret made up for the ZIS-25's drawbacks, but the turret itself was a drawback. The NKTP did not want to build it.

GKO decree #3289ss issued on May 5th, 1943, only approved the existing requirements. This time, there would be a tender. Factory #9 began two work on the D-5 gun that could take 85 mm and 122 mm barrels. The TsAKB continued with their 85 mm gun under the designation S-31. The gun was designed very quickly since they had a head start. A draft and an explanatory memo were finished on May 14th. Work on the gun was headed by the chief of the 17th Department of the TsAKB P.F. Muravyev and chief of the 3rd Department Ye.V. Sinilshikov. The design was reminiscent of the ZIS-25, but not without alterations.

Conclusions on the TsAKB's turret, June 7th, 1943. This decision meant that the turret would not enter production.

The biggest change was that there was a new turret. The KV-1S and KV-13's turrets were not a good fit for the TsAKB's gun. The turret ring was preserved, but it was somewhat enlarged. A turret basket and ready rack were added. The turret was made universal, compatible with the KV-1S and KV-13. Therein lay the problem. The turret was developed without input from the factories that would have had to build it, which led to a conflict. In addition, the KV-13/IS-1 (#233) was rejected in May of 1943. The fact that factory #9 worked closely with ChKZ during their design added fuel to the fire. Their D-5T-85 gun was created for the new Object 237 tank (then called IS-3). 

An analogous turret was built anyway in the fall of 1943, by which point it was no longer necessary.

The S-31 project was reviewed on May 22nd, 1943. This was a promising design. Out of 449 parts, 350 came from the ZIS-5. On the other hand, the GAU was also skeptical of the TsAKB's turret. The mass of the tank grew by 2 tons. The turret basket made the turret crew's work easier, but also complicated access to the ammunition racks in the hull. Instead of the required 30 degrees of gun elevation, the S-31 was only capable of 25 degrees. The maximum recoil length was 520 mm instead of the required 350. The turret was finally dead and buried on June 7th. Formally, ChKZ was ordered to produce the turret, but they were reluctant to comply. A turret was finally built and put to trial in late October of 1943, by which point the KV-1S and S-31 were already history. Meanwhile, the TsAKB was feverishly looking for a solution to this unexpected problem.

Unlucky quick fix

It seems that a backup plan was developed by the TsAKB right after the review at the Artillery Committee. The Committee's skepticism caused Grabin to seek an alternative. This time, work was conducted in tight cooperation with factory #100's OKB design bureau. The installation of the S-31 gun into the turret of the KV-1S began to take shape by June of 1943.

Draft of the Object 238, June 1943.

This tank is sometimes called KV-85G, but there was no such index. It was likely invented at the NIBT Proving Grounds or elsewhere much later. This tank was only referred to as Object 238, taking place between the new IS tank (Object 237) and a KV-1S tank converted to take the D-5T-85 gun (Object 239). N.F. Shashmurin was appointed as the chief engineer on this project. The actual changes done to the tank were minimal.

Gun mount drawing. Note that the gun mantlet was altered.

The Object 238 was completed by July 17th, 1943, but the first drafts were dated May 25th. This was because Object 237 had a higher priority. Work on rearming the KV-1S was led by G.N. Moskvin under the supervision of Zh.Ya. Kotin. According to the prepared materials, the mass of the KV-1S grew from 43.4 tons to 44 tons. The extra weight came from the new gun and new ammunition racks.

One of the two Object 238 prototypes, late July 1943.

Changes to the KV-1S were minimal. Since the S-31 was based on the ZIS-5 gun, it was not difficult to install it. The gun mantlet and ammunition racks were taken from the initial project. The turret held an eight round ready rack. 47 more rounds fir into the hull: 6 on the right side, 11 on the left, and the rest on the floor.

Both tanks were converted production KV-1S tanks.

Even though the changes were minor, production of the Object 238 dragged on. It was expected that a prototype would be ready by July 1st, but the documentation was only approved on July 17th, even though factory #100 reported that work on the tank began in early July. The factory also called the S-31 "F-85" since it was built at factory #92. The Object 238 only comes up again in factory #100 correspondence in mid-July. There were two prototypes, both converted from KV-1S tanks produced in July of 1943.

The only difference between the two tanks were the casting numbers on the back of the turret.

The first trials of the tank took place on July 31st, 1943. Mobility trials continued until August 15th with a break for gunnery trials. In this time, the Object 238 only drove for 103 km, less than any other tank. There was no point in driving it for longer since the tank was not mechanically different from the KV-1S. The gunnery trials were more important. The commission decided that the tank could not be accepted into service. It was impossible to work with ammunition while loading and there was almost no space for the commander.

Object 238 at the Gorohovets Proving Grounds.

The Object 238 took part in artillery trials jointly with the Object 239 and two Objects 237 (one with the S-31 gun and the other with the D-5T-85). The first stage took place between August 2nd and August 4th, the second from August 21st to August 24th. The S-31 showed a recoil length of 480-570 mm, more than the D-5T-85. This was not so critical in the Object 237, but a major problem in the KV-1S with a smaller turret ring. Since a counterweight needed to be installed, the real gun elevation of the S-31 was less than 7 degrees. 

The tank had a low priority even before the trials began.

This was not the only problem. The conclusions made at the Gorohovets Proving Grounds about the fighting compartment were not as harsh as those made at ChKZ. Nevertheless, the rate of fire of 3-6 RPM (half as much as the other tanks) says a lot. The fighting compartment of the Object 238 was undoubtedly worse than the Object 237 or 239. Even though this was the simplest solution to improve the KV-1S' firepower, the Object 238 was rejected. The Object 239 was accepted into service with the Red Army as the KV-85 on August 8th, 1943.

Trials only confirmed the issues that ChKZ raised.

The Object 238 got lucky. One prototype was converted to test the aforementioned turret, the other was left as is. It was later sent to the NIBT Proving Grounds. The tank was not used in any more trials, but it was Kubinka's only KV-1S tank, and so it settled down in the proving grounds museum. Until recently, this was the only surviving KV-1S tank.

Layout of the fighting compartment.

It's hard to say that it led a calm life. The tanks at the proving grounds were put into running order in the 1960s "army style": ruthlessly and without purpose. All tanks and SPGs from the KV family had their original engines replaced with the most recent ones available. For some reason, Object 238 was equipped with wheels from the KV-1 and tracks from the IS-2M/IS-3M. The internal components including instrument panels were also "modernized". Various parts were welded onto the exterior, such as the bars on the air intakes.

Object 238 at Patriot Park.

The tank is still in running order. At the very least, it can move under its own power. Today it's displayed in Patriot Park as a part of the Battle of Kursk exhibit. This is not entirely incorrect, since it's practically a KV-1S, just with a different gun. 

The most interesting part of this whole story is that theoretically the KV-1S could have had an 85 mm gun with a normal rate of fire. A KV-1S tank with the S-28 gun entered trials a year later. It showed much better results, but the time of the KV-1S was long over. Nevertheless, the idea of equipping the KV-1S with an 85 mm gun turned out to not be hopeless.


  1. I find the parallel projects here (Object 238, 239) interesting, but I keep wondering why there was so much resistance to dropping in the K-3 76 mm AA gun, which could have been fitted into existing T-34s and KV-1Ss and would have improved their firepower markedly. Either option (85 mm gun or a K-3 gun) into the KV-1S is a temporary solution anyeay until the new IS tanks roll out, and the drop-in seems to be the much easier route.

    I know the Red army artillery department didn't like making ammunition for both the F-34/ZIS-5 and the K-3 guns, but the solution to that would have just to phase out the ZIS-5, and make the K-3, as the latter's HE round was also more potent.
    Maybe the artillery department favored the lower-velocity ZIS-5 because it was better in indirect fire uses in the 'pure' artillery formations?

    1. The S-54 in a T-34 was definitely a viable upgrade and I assume that the KV-1S would also have been able to take it without an issue, but then you would have to juggle a new type of round with the same caliber as an old one. With the 85 mm gun on the horizon, the extra complexity doesn't seem worth it.