The first prototype of the KV-13 heavy tank entered trials on September 26th, 1942. The vehicle was quite unusual. With the protection of a heavy tank, it had both the size and weight of a medium. This later caused many hypotheses about a «medium tank with heavy armour». In reality, the KV-13 was never a medium. It was developed as a heavy tank from the very beginning, and the small size and mass were a direct consequence of the requirements issued by the GABTU. For a number of reasons, work on the KV-13, which also earned the name IS-1 during its design, did not continue past a second prototype. However, it was an important milestone towards the creation of a next generation Soviet heavy tank.
Weight gain trend
The KV-13 already did not meet its tactical-technical requirements at the moment of its assembly. A decision to require a three-man turret was made «at the top» in early June of 1942. A draft design was composed, which influenced the KV-1S's turret. However, in the scenario where the development of the KV-1S had a higher priority, and then urgent orders to set up T-34 production were issued, there was no time in Chelyabinsk for a KV-13 with a three-man turret.
One can confidently state that the experimental KV-13 appeared in spite of its circumstances. ChKZ was far too busy for it, and the tank was completely orphaned after the death of the project's chief engineer, N.V. Tseits. N.F. Shashmurin inherited the project, but he was inclined against it, as he did not like the concept of such a heavy tank. Nevertheless, factory #100 did not abandon the project. The orders of the People's Commissar of Tank Product, I.M. Zaltsmann, which instructed that all experimental work at the factory be shut down were partially ignored. Accusations of Zaltsmann trying to make the KV-13 into 'his own' T-34 look rather foolish. At the moment, Isaac Moiseyevich put all his efforts towards producing the T-34, putting the interests of his «home» factory below Stalin's orders. Even the creation of the KV-1S had a lower priority than the organization of T-34 production.
Despite a number of technical problems that plagued the KV-13 during trials, it's hard to call its development pointless. Yes, the tank turned out to be rather unpolished, and there was no point in bringing it to completion since the requirements changed, but the concept of a high speed heavy tank was proven to be feasible. An optimal suspension was discovered through trial and error. Using KV-1S designs proved to be the correct solution.
Overall, the work on the KV-13 and KV-1S continued in parallel. Successful solutions from one tank were copied over onto the other. Considering that the KV-13's lead engineer was Shashmurin, this was not surprising. As mentioned above, Nikolai Fedorovich was no fan of the concept, but he improved the design that he inherited, and quite successfully at that.
The results of trials showed that the tank needs serious improvements. Work on a reworked KV-13 began in November of 1942. They coincided with the first reviews of the KV-1S from its users, which were not all positive. For instance, the commander's cupola was criticised for lacking a hatch. As a result, there were cases where the crew died as they were unable to get out of the tank. The issue was even more pressing on the KV-13, since the driver didn't have his own hatch. The influence of the KV-1S on the design of the KV-13's road wheels.
The gearbox (with a few changes) was also taken from the KV-1S. At the same time, a number of completely new solutions were implemented. For instance, the turning mechanism, which was a topic that two institutions were working on: the Moscow Bauman Machinebuilding Institute (G.I. Zaichik's group) and Engineer-Colonel A.I. Blagonravov. Factory #100 had a good relationship with Blagonravov, and trials of his turning mechanism in KV-1S #15002 began on November 19th, 1942. Blagonravov's turning mechanism was used on the improved KV-13.
The design of the second variant of the KV-13 was completed by December 10th, 1942. That is how it was referred to by the ChKZ SKB-2. Factory #100 still called it IS-1. A wooden model was completed along with the technical documentation. The turret changed noticeably, even compared to the initial variant of the three-man turret. The shape repeated that of the KV-13. The 1590 mm wide turret ring, travel lock, and observation devices were also taken from there. The biggest difference was the commander's cupola. It was bigger, and more importantly, it had a hatch. However, there were still oddities. Like the T-150's cupola, the front had a widened section for the installation of the PTK-5 periscope.
Another difference was the gun mount. The second variant of the KV-13 had an F-34 gun instead of a ZIS-5. The gun was moved forward by 180 mm, which made the turret much roomier. The seats were also changed, which allowed for improvement of the working conditions. The ammunition racks were taken from both the KV-1S and the T-34.
The running gear was influenced by the KV-1S. Two kinds of tracks could be used: T-34 or KV-1S tracks. Zh.Ya. Kotin, who was supervising the whole project, discarded the idea of using T-34 components at a meeting on December 12th, 1942. Experience from the first KV-13 showed that it was better to use drive sprockets and tracks from the KV-13. The second prototype used 650 mm wide tracks, which were used by the KV-1S since November 1st, 1942.
The road wheels (blueprint 33-69), used on the KV-1S since December 2nd, also had an influence on the KV-13. The size was smaller (550 mm in diameter), but the similarities were noticeable. ChKZ's technologies proposed using KV-1S road wheels on the KV-13, but this never happened. To simplify production, the KV-13 received ball bearings for its road wheels, while the KV-1S used conical bearings.
The shape of the hull was also altered. The characteristic «hump» was much lower, and the engine compartment was altered. After the changes, the mass of the hull reached 14,600 kg, which was close to that of the KV-1S (15,700 kg). Overall dimensions of the hull did not change much compared to the first KV-13, but the armour was thickened. The mass increased as well. Preliminary calculations set it to 37.5 tons. There was no 'medium tank with heavy armour' here, but a heavy tank, without any doubt.
The tank's high mobility remained one of its most important parameters. The calculated top speed was 53.5 kph. At the time, the T-34's top speed was 49.6 kph. One of the most important features of the tank was that the time required for production dropped by more than half. The question of improving the tank's protection was already posed, and the KV-13 was superior to the KV-1S in this regard, as it had modernization reserves.
The transmission was also significantly different from its predecessor. In addition to the aforementioned planetary turning mechanism, it received a new gearbox. However, calling it brand new is a bit much. To ensure commonality of parts, Shashmurin used a slightly improved version of the 8-speed gearbox from the KV-1S. The first variant had the possibility of using the 5-speed T-34 gearbox on the KV-13 (and vice versa, the 8-speed KV-13 gearbox on the T-34). A prospective planetary gearbox was also considered.
The second KV-13 variant was approved in December of 1942, but nearly no work was done until late January of 1943. ChKZ and factory #100 were busy with a more important task: the KV-14 (SU-152) SPG. Around this time, a proposal was made to equip the KV-13 with a turret from the KV-9, which used the U-11 122 mm howitzer. This vehicle received the factory code of 234. In factory #100's documents, it is also called IS-2. This vehicle will be described in detail in another article, but for now let's just say that the information regarding the installation of the KV-9's turret on the IS-2 is false. A completely new turret was installed, with a number of visual differences.
Factory #100 reported that all components for the second KV-13 were ready by February 1st, 1943. The limiting factor was the absence of the hull and turret. The Ural Heavy Machinebuilding Factory (UZTM) was chosen as the subcontractor, but it had its own issues at the time, dealing with production of the T-34 and SU-122. In the meantime, work did not stand still, and some changes were made to the KV-13's design. A second gearbox variant, which was almost identical to the KV-1S, was introduced. This introduced parts commonality, which would make the tank easier to put into production in case the trials went well.
The new commander's cupola, another component influenced by the KV-1S, was no less important. The dissatisfaction with the cupola led to there being a whopping three variants for its replacement. The first was an improved version of the draft of the second KV-13. As an alternative, SKB-2 designed a cupola with T-34 style observation devices and no top hatch.
Finally, due to the insistence of the assistant chief of the 6th department, Engineer-Major Pestov, a third cupola was designed. It was born from the sketch of a cupola with five observation devices and a two-piece hatch. The hatch could rotate, and one of the pieces had a MK-IV periscope in it. In this case, MK-IV didn't mean 'Vickers Mk.IV periscope', but 'periscope taken from the British Mk.IV tank', as in the Churchill. Working blueprints of the cupola were approved by the end of January of 1943. A decision was made to test the prototype on the KV-13. Two variants were being built at that point: the basic (233) and howitzer (234).
The KV-13 project sat still in February of 1943 for the same reason: the hulls and turrets have not arrived from Sverdlovsk. Zaltsmann had to take administrative action. A letter was sent proposing immediate construction of the prototypes. The People's Commissar expected tat the KV-13 could be put into production to replace the KV-1S. Stalin approved his proposal, and GKO decree #2943 «On the production of experimental prototypes of IS tanks» was signed on the next day. The index KV-13 was no longer used.
Work on the IS tanks moved into the next stage after decree #2943. Zaltsmann signed NKTP order #104ss, ordering UZTM to deliver the hulls by February 27th and the completion of assembly by March 8th. In addition, Zaltsmann told the directors of ChKZ and factory #200 to prepare for mass production of the IS. The IS-1 (the official name of the second variant of the KV-13) was completed on March 8th, as required. Stationary trials took place on the same day. Weighing showed that the mass of the tank was 37.1 tons instead of 38.5. Overall, the tank was built according to the documentation written in the start of December of 1942. The changes impacted the gearbox, commander's cupola, and a number of other elements.
The first stage of factory trials began on March 9th. Slipping of the main clutch was observed during the 19 km march. On the next day, the tank drove for 74 km, showing an average speed of 17.4 kph and a top speed of 38-40 kph. Defects, unavoidable in initial trials, began to crop up. One of the most serious issues was the overheating of the cooling system, which forced the tank to slow down. The tracks also rubbed on the sides of the final drives. Openings had to be cut to reduce temperature. The main friction clutch broke on March 13th, and had to be replaced with an improved one. The factory trials, during which the tank drove for 305 km, ended on March 16th.
After the correction of small faults, the IS-1 was passed onto government trials on March 21st. They began on the next day. The first march ended with a breakdown of the main clutch, and the vehicle was towed to factory #100. During repairs, it was discovered that the breakdown was caused by a shifting of the engine in relation to the gearbox. The tank was taken on a 32 km march with an average speed of 21.4 kph after repairs, which took until March 27th. No defects were discovered on that drive. Further marches revealed issues with the engine frame, and cracks were discovered on seven track links.
The overheating was solved in a much more radical way. The finned horseshoe shaped radiator was replaced with a piped one in early April. This design proved itself very effective during stationary trials. The road wheels were also redesigned during the first ten days of April. It turned out that the ball bearings were destroyed after 200 km. They were replaced with the same conical ones that were used in KV-1S road wheels.
A preliminary conclusion of the government commission was prepared after the first stage of trials on April 5th. A proposal to improve the vision of the driver was among the commission's suggestions. Instead of observation devices along the perimeter of the casemate, two MK-IV periscopes were suggested. Another periscope for the loader was to be installed in the turret roof. The commission remarked on satisfactory reliability of most of the transmission components, but the low quality of the engine. Overall, the engine was swapped out three times during IS-1 trials.
The IS-1 returned to trials on April 6th, travelling 24.5 km to the Kopeisk proving grounds. The average movement speed was 30 kph. The tank achieved a top speed of 55 kph during trials. Trials continued until April 19th, during which time the tank travelled 536 km over a highway, 390 km over dirt roads, and 80 km off-road. The replacement of the radiator was clearly effective, as the water temperature only reached 50 degrees during driving in 8th gear with external air temperature at 15 degrees.
The replacement of the road wheel bearings improved reliability, but this time there were more fundamental complaints about the road wheels. The smaller number of road wheels than the KV-1S had reduced the tank's mobility on soft soil. The commission instructed the increase of the number of road wheels on subsequent IS tank models. There were also complaints about the commander's cupola and gun mount. Another serious issue was the fact that the air intakes became clogged with mud when the tank drove off-road. This didn't mean that the IS-1 was destined go to the way of the KV-13, but serious design changes were needed. Meanwhile, ChKZ and factory #200 were preparing for a trial batch of 10 tanks.
The IS-1 did not sit idle after the end of factory trials. In May, the IS-1 travelled another 243 km. Protection of the air intakes from mud was tested, as well as a reinforced engine frame. Multicyclone air filters were also tested, which later became the main type of Soviet air filters. However, it was clear by early June that a number of serious changes awaited the IS-1 before it could be put into production.
The Tiger's first victim
On January 18th, 1943, near Worker's Village #5, two German Tiger from the 502nd Heavy Tank Battalion were captured. Tanks with turret numbers 100 and 121 were delivered to Kubinka, where they were studied from all sides. In April, tank #121 was subjected to penetration trials. The results weren't encouraging. It turned out that the 76 mm F-34 and ZIS-5 tank guns did not penetrate the German tank from the front or the sides. The most effective weapons were the 57 mm ZIS-2 anti-tank gun and 85 mm 52-K AA gun. The ZIS-2 only confidently penetrated the side from 800 meters and couldn't penetrate the front from 500 meters. The 52-K, however, confidently penetrated the front of a Tiger from a kilometer away.
GKO decree #3289 «On the improvement of armament on tanks and SPGs» was signed on May 5th, 1943. According to the first item of the decree, the IS-1 prototypes will be equipped with 85 mm guns with the ballistics of a 52-K AA gun. A competitor cropped up unexpectedly in mid-May: V.G. Grabin's TsAKB. Taking the 85 mm ZIS-25 gun designed for the KV-1, a group of designers headed by P.F. Muravyev and Ye.B. Sinikshilov designed a tank gun that was indexed S-31. At factory #92, where the gun was later built, it was called F-85. The project, which was mostly led by Sinilshikov, was presented on May 17th, fewer than two weeks after the signing of the GKO decree.
As per tradition inherited by the TsAKB from factory #92, the biggest possible gun was fitted into the IS-1's turret. 85% of the S-31's parts came from the ZIS-5. However, the turret had to be enlarged, to make loading the gun easier. roof was elevated in the loader's section. The ammunition capacity would consist of 71 rounds. According to calculations, the mass of the IS-1 with the S-31 would increase to 39.5-40 tons.
This idea did not progress past a draft. On June 6th, 1943, GBTU chief Colonel-General Fedorenko sent a letter to Molotov. In it, he wrote that a large number of issues arose in the installation of the stock 85 mm gun into the IS-1 turret. The turret was too small, and it was too hard to work with the gun. As a result, Kotin gave an order to develop a new tank with an 1800 mm wide turret ring. This meant that the hull had to be changed, and lengthened by 420 mm. That was the birth of the IS we know today. Assembly of the first tank began in mid-June.
As for the IS-1, June was the last month when it underwent trials. It travelled 724 km, demonstrating the same issues with clogging up the air intakes with dust and dirt. Having driven for 2294 km overall, the tank took its place next to the KV-13 at the factory courtyard. The time came for another tank bearing Stalin's name.
Translated by Peter Samsonov. Read more interesting tank articles on his blog Tank Archives.
- Archive of Gennady Malyshev;
- Russian State Archive of Economics;
- Russian State Archive of Socio-Political History;
- Central Archives of the Russian Ministry of Defence.