The capture of two Tiger tanks by the Red Army on January 18th, 1943, had a significant impact on Soviet tank building. Trials of one of the tanks revealed an unfortunate fact: the F-34 76 mm gun, the main weapon of Soviet tanks, could not penetrate the side. The reaction to this result was swift. Designers were tasked with developing a more powerful tank gun immediately. It was to be installed in the KV-1S heavy tank.
A new gun the easy way
Deputy People's Commissar of Defense, Marshall Kulik, raised the issue of the KV-1's gun being insufficiently powerful in mid-June of 1940. A decision was made during a meeting held on June 16th, 1940, to develop an 85 mm tank gun with the ballistics of the 52-K AA gun. The task of creating this gun, indexed F-30, was given to V.G. Grabin and his team. The gun was installed in the experimental T-220 tank, after which work on it stopped due to the development of the 107 mm ZIS-6 gun.
A second attempt to create an 85 mm gun for the KV-1 was made in late 1941. The U-12, designed by UZTM's design bureau under the direction of F.F. Petrov, did not even make it to the prototype stage. The third attempt, this time in spring of 1942, was also made in Sverdlovsk. The result was the same: the ZIK-1, meant for the KV-1 and T-34, did not proceed past the technical project stage.
The 85 mm S-31 gun in an IS-1 turret, May 1943.
The fourth and final attempt was made in the fall of 1942. The ZIS-25 gun was designed at factory #92. The gun was rejected, mostly because servicing he gun would have been far from ideal. The loader would have had the hardest time of all, since the 85 mm round was longer than the ZIS-5's 76 mm round. In addition, the KV-1 was no longer in production in the fall of 1942. The KV-1S took its place.
Thanks to all of these failed attempts, engineers in both Sverdlovsk and Gorky had extensive experience with 85 mm tank and SPG guns by April of 1943. However, both design bureaus underwent reorganization. F.F. Petrov's team was turned into the design bureau of factory #9, and factory #92's design bureau was moved to Kaliningrad, where it formed the Central Artillery Design Bureau (TsAKB). The existence of two artillery design bureaus, which often worked on similar projects, resulted in fierce competition. As it often happened, competition was good, and the GAU and GBTU had the freedom of choice. Often TsAKB and the design bureau of factory #9 had entirely different approaches to the same problem.
The TsAKB proposed this turret for the IS-1 (233) and KV-1S.
Officially, work on 85 mm tank guns began on May 5th, 1943, when GKO decree #3289 "On improving the armament of artillery armament of tanks and SPGs" was signed. The decree's text called for two KV-1S tanks and two IS-1 (233) to be armed with 85 mm guns.
Even though the decree gave the order to factory #9, TsAKB received an order for an 85 mm gun practically at the same time. They did not start from scratch, but developed the ZIS-25 further, especially since work on it never fully stopped. The return to the 85 mm gun happened in February of 1943. To continue his work, Grabin composed a request for tactical-technical requirements and documentation for the KV-1. In addition, he requested one KV-1 tank and one ZIS-5 gun to produce an experimental ZIS-25. The GBTU refused to give him a whole tank, since the TsAKB still had the KV-2 that was used to test the 107 mm ZIS-6 gun. A counter-proposal was made to send only the turret, and a KV-1S turret at that, since that is the tank that was presently in production. In addition, the TsAKB received documentation on the KV-1S and IS-1 (233).
Tactical-technical characteristics for a new 85 mm gun were composed by the GAU on March 26th, 1943. According to the requirements, the gun had to be capable of confidently penetrating 90 mm of armour from 500 meters. The gun would be installed not only in heavy tanks, but in medium ones, which caused protests from Grabin.
This work was done within the scope of the ZIS-25 program, since it satisfied the requirements. However, the order was sent not only to the TsAKB, but to factory #9. Because of this, decree #3289 was not a surprise for anyone. Work was already underway both in Sverdlovsk and in Kaliningrad when it was published. The requirement to defeat the Tiger only made the designers' work more complicated. To confidently penetrate its armour, the gun needed to have a barrel as long as the 52-K AA gun.
Installation of the S-31 gun into the stock KV-1S turret.
The chief of the 17th department of the TsAKB, P.F. Muravyev, as well as the chief of the 3rd department, Ye.V. Sinilshikov, headed the development of the gun, indexed S-31. TsAKB was not working on the same footing as factory #9. Petrov came to the conclusion that the turrets of the IS-1 and KV-1S were too small for an 85 mm gun, but the TsAKB worked with what they had. In addition, factory #9's specialists designed their guns based on old guns that they made themselves, while the TsAKB had to align their designs with what was already in production. According to the specifications, the new gun had to take most of its parts from the mass produced 76 mm ZIS-5 gun.
Cutaway diagram of the Object 238. This drawing differs slightly from the vehicle that was eventually built.
The experience with the ZIS-25 allowed the S-31 to be designed very quickly. An explanatory memo, as well as drafts of installation into the KV-1S and IS-1, were ready by May 14th. The initial vision of how the gun would be installed differed somewhat from what was eventually built in metal. This was especially true of the turret. The TsAKB did not demand that the size of the turret ring be increased, like factory #9 did, but the turret was still altered. The TsAKB considered the IS-1 a priority, but due to the same turret ring diameter the same turret could be used on the KV-1S. The turret bustle was enlarged, which allowed it to store 18 rounds of ammunition for the 85 mm gun. The commander's cupola and roof were altered to allow the loader to work with longer rounds. The cupola now stretched across the entire width of the turret. The turret also gained a turntable. According to the design, the tank would carry 76 rounds in total.
Perpendicular cutaway of the tank.
The design of the gun was an evolution of the ZIS-25. The gun had significant parts commonality with the ZIS-5: out of 449 parts, 350 were common, although some were slightly altered. The gun barrel, as requested, offered identical ballistics to the 52-K. The breech and semiautomatic mechanism were analogous to the design of the ZIS-5.
The project was reviewed by the GAU Artillery Committee on May 22nd. The turret, which was now not only larger, but also heavier, drew the most comments. The addition of a turntable made the crew's work easier, but also made it more difficult to access ammunition stored below. The fate of the turret was to be decided at a large meeting attended by representatives from the GBTU, People's Commissariat of Armament, GAU, the People's Commissariat of Tank Production, and the TsAKB.
There were also questions about the gun itself. Instead of the required maximum elevation of 30 degrees, the gun only elevated 25 degrees, but this was deemed acceptable. The fact that the requirements demanded a maximum of 350 mm of recoil, while the TsAKB's calculations showed that it would not be less than 520 mm, was much more problematic. The Artillery Committee instructed them to try and reduce the recoil length. To be fair, the recoil length of the 8.8 cm KwK 36 used on the Tiger was 600 mm, and that gun had the advantage of a muzzle brake.
Initially, the S-31 did not have a solenoid type firing mechanism. It was decided at the meeting that it should be added to the prototype.
Experimental Object 238 prototype, Chelyabinsk July 1943.
The fate of the new turret was decided at a meeting on June 7th, 1943. The Kirov factory was offered to design a turret based on the TsAKB's ideas. However, nothing of the sort was done in Chelyabinsk. By the time the decision was made no design work was happening anymore. The IS-1's fate was clear, and it was obvious that the tank would not go into production in its current form. In many ways this was connected with its turret, which was now being redesigned to fit the D-5T gun. The same turret was proposed for installation in the KV-1S by factory #9. This required a new turret platform, but resulted in the same turret being used by the IS and KV-1S. A third turret made no sense in this scenario.
In addition, the S-31 was far from perfect. It would seem that the GAU and NKV already had ideas about modernization, but the S-31 had no room for improvement. In addition, the D-5T was lighter and more compact. Its recoil length was only 430 mm. However, a final decision had not yet been made. The S-31 was installed in an experimental IS-3 (Object 237) and a KV-1S with a stock turret ring. Factory #92 was tasked with producing the TsAKB's guns, where they were indexed F-85.
View from the front. A late production KV-1S was used.
Work on installing the S-31 into the KV-1S' turret began in late May of 1943. Correspondence from the time indicates that initially work was being done to install the gun into both the stock turret and the IS turret. After the meeting in June, the S-31 with a new turret was cancelled, and the D-5T was installed in the IS-3 turret instead. The version with a new turret was jointly designed by factories #100 and #200, whereas the version with a stock turret was taken up by the Kirov factory's SKB-2. The KV-1S with a new gun received the blueprint index "238". In July of 1943 this index transformed into the official name of the vehicle: Object 238. The name "KV-85G" that appears in several sources was never used. This was a post-war invention. The lead engineer of the vehicle project was N.F. Shashmurin, G.N. Moskvin directed work on SKB-2's side, and Zh.Ya. Kotin oversaw the entire process.
The combination of the S-31 and the stock KV-1S turret was finally approved in mid-July of 1943. According to calculations, the mass of the tank increased to 44 tons, but its mobility would remain at the level of an ordinary KV-1S. The gun easily replaced the ZIS-5, since it was designed with its components. In addition to a new gun, the Object 238 had a slightly different gun mantlet, but the existing mantlet was preserved in practice. Another change was a redesigned ammunition rack, which now held 8 rounds. Unlike the mantlet, it was built in metal. The overall amount of ammunition was reduced to 55 rounds.
Object 238 from the side. In addition to a new gun, the tank had no differences compared to production KV-1S tanks.
Despite the fact that the design of the "238" was ready by May 25th, all further work was stalled for nearly 2 months. The tank that had the IS-3 (Object 237) turret and D-5T gun had a higher priority. That tank was indexed Object 239. According to the NKTP's orders, work on modernizing the KV-1S was of lower priority than Object 237. Another reason for the delay was the lateness of factory #92 in delivering the guns. It was proposed that Object 238 would be ready by July 1st, but the deadline was not met. However, two of these tanks were built for some reason, as evidenced by a report from factory #100 regarding experimental work performed between July 10th and 20th, 1943. Only one vehicle was used in trials.
The same vehicle from the rear.
The first trials of the Object 238 were performed at the Gorohovets proving grounds from August 2nd to the 4th. The trials were comparative: in addition to the Object 238, one Object 239 and two Objects 237 with different guns took part. The precision of all four was identical, but then the 238's shortcomings started showing themselves. The smaller turret ring made working in the stock KV-1S turret, which was already a source of complaints, even more difficult. As a result, the rate of fire of the Object 238's gun was 5-6 RPM, while the Object 239 fired at 10-12 RPM. The S-31 also proved itself unreliable.
Issues with the tank version of the S-31 were not significantly different from the SPG variant. The cramped turret only exacerbated them. The result was predictable: Object 239 with the turret from Object 237 was accepted for service. GKO decree #3891ss "On production of KV tanks with an 85 mm gun (KV-85)" was signed on August 8th, 1943.
Diagram of crew and component placement in the Object 238's turret.
One of the two Objects 238 survived to this day. This vehicle is mostly a mass production KV-1S tank (#30751) produced in July of 1943. The only difference is a new gun. It is not known what was done with this tank. It has road wheels taken from the KV-1 and "foreign" return rollers. The track links were taken from the T-10, and bars are welded onto the air intakes.
Work on 85 mm guns did not cease at TsAKB after the victory of the D-5T. More than that, discussions of producing the TsAKB's turret continued until the fall of 1943. However, no work was done. Nevertheless, Object 238 was not the last KV-85 from the TsAKB. The next attempt was made a year later thanks to continuing work on medium tank guns.
KV-1S with the S-28 gun, August 1944.
As mentioned above, the initial requirements for the 85 mm gun were to use it in both heavy and medium tanks. Despite Grabin's protests, the S-31 was also compatible with the T-43 medium tank. After factory #183 modernized the T-43, it received a turret with a 1600 mm turret ring. This was done to allow it to carry an 85 mm gun. Due to the loss of the S-31, the altered T-43 turret used a D-5T gun.
Nevertheless, work on this topic did not go to waste. Muravyev and Sinilshikov designed several more tank guns in the second half of 1943. Among them was the S-50 "triplex", which could use 57 mm, 76 mm, or 85 mm barrels. LB-1 and S-53 85 mm guns followed.
The same vehicle from the side. A winter-spring production KV-1S tank was used.
Initially, the S-53, which was tested in a T-34 with a stock 1420 mm turret ring, showed itself no better than other guns of the TsAKB. It did not pass trials, Nevertheless, this gun was a lucky break for the TsAKB. Initially, the modernized T-34, called T-34-85, used the D-5T gun. This was logical, as this was the same gun used on the SU-85, KV-85, and IS-85. On the other hand, the KV-85 was out of production, and the days of the IS-85 (IS-1) were numbered. It was replaced by the IS-122 (IS-2) with a much more powerful 122 mm gun. The S-53 was trialled again in late January-early February of 1944, after which a decision was made to put it into mass production.
Overall view of the S-28 with gun mantlet.
After many defects were discovered in the S-53, the gun was improved by the TsAKB, after which it received the index ZIS-S-53. The gun was tested in April of 1944, the discovered defects were corrected again, which was confirmed in July of 1944. The ZIS-S-53 was put into production, but in August of 1944 another tank with the same gun arrived at the Leningrad ANIOP. This was a KV-1S, and the gun it carried was indexed S-28.
Unlike the Object 238, its mantlet was seriously altered. The mantlet was the same as on the ZIS-S-53, with the whole system fitting comfortably into the gun port on the regular KV-1S turret. The biggest advantage of this design was that the firepower of the KV-1S could be easily improved. In addition to the gun, other components of the turret were changed. Racks for 85 mm ammunition were added in the bustle and on the right. The overall capacity was 40 rounds. The racks in the hull still needed work.
S-28 from the rear.
Trials of the S-28 took place from August 8th to August 20th, 1944. A.S. Chasovnikov represented the TsAKB at the trials. Based on his work, he was a specialist in rearming tanks. Chasovnikov was also one of the authors of the ST-1 and ST-II projects. The presence of a representative from the Main Tank Repair Directorate, Engineer-Colonel Gavrilov, also hinted at the goal of this design. It would seem that tanks already in service would be rearmed.
Overall, the trials were successful. It turned out that the crew conditions in the turret differed little from those in the T-34-85. However, there were some complaints. The field of view of the TSh-15 sight was small, the rack in the turret broke during testing, the rate of fire was only 4-6 RPM, and the precision was deemed unsatisfactory. In addition, it was proposed that a ventilation fan should be added to the turret. Nevertheless, the overall verdict was positive.
Ammunition racks. One of the slots in the side rack was torn off during trials.
Despite the results of the trials, the S-28 remained a prototype. The career of the KV-1S was coming to an end in the summer of 1944. These heavy tanks were replaced by the IS-2. The T-34-85 would have surpassed the KV-1S armed with an S-28 gun in effectiveness anyway. Too much time and effort would have to be put into rearming existing vehicles. It was easier to gradually replace the KV-1S with new tanks.