Technical progress moves in giant leaps during large wars. This rule applied to nearly all types of weapons and armament during WWII, including tanks. Every year of war raised the bar of requirements, and even the newest fighting machines quickly became outdated.
The heavy IS-1 tank was created as a response to the Tiger, but was obsolete by the time it entered production. The IS-2 that came to replace it had a much more powerful gun, but it too would need to be replaced soon. Today let us remember the tanks (IS-2U and other) that were supposed to replace the IS-2 in 1944. Not a single one was ever built in metal, but work on these projects resulted in the creation of the IS-2 with straight upper front hull armour and the IS-3.
The first battles of the IS-1 tank showed that the armour of new Soviet heavy tanks does not entirely meet the modern requirements for battle. On February 19th, 1944, tanks of the 13th Guards Heavy Tank Regiment came under fire from Panther tanks. The front armour of the IS-1, built to withstand fire of the 88 mm KwK 36, could be penetrated by the 75 mm KwK 42. Two tanks burned up and three more were knocked out.
Casting defects played a role in this result, but the main reason was that the armour was simply insufficient. This was the conclusion reached by Engineer-Major A.I. Shamin, senior assistant to the Chief of the NIBT Proving Grounds Experimental Department, who was attached to the 13th Guards Heavy Tank Regiment. He critiqued the shape of the front hull. The «step» in the armour and driver's hatch weakened the armour. Shamin's conclusions were included in the general conclusions to NIBT trials of IS-1 and IS-2 tanks built in January-February 1941. The first point in the summary was «the armour protection of the turret and front hull does not match the power of the tank's armament».
A proposal was made to improve protection of the hull and turret by improving the slope and quality of the armour. The T-34's hull shape was taken as a starting point. The NIBT proving grounds proposed changing the upper front hull armour. The thickness would remain at 120 mm, but the armour would be rolled, and the angle would be 30 degrees from horizontal. This would offer reliable protection from the 88 mm Pak 43 L/71 gun used on the Ferdinand tank destroyer. The designers considered the trials of this tank and another trophy from the Battle of Kursk, the Panther. Its 80 mm plate angled at 35 degrees from horizontal offered better protection than the IS-2's «step». The 85 mm gun couldn't penetrate this armour at point blank range, and the Panther itself could penetrate it at only 200 meters.
The results of firing on the Panther with a Pak 43 installed in a Hornisse SPG were also interesting. At a range of 600 meters the shell knocked out a 300×250 mm fragment and ricocheted.
The NIBT proving grounds specialists also proposed improving the side armour. The thickness was reduced from 90 to 75 mm, but rolled plates installed at 45 degrees would be used. The proposal also included increasing the front turret armour to 200 mm and the sides of the turret to 150 mm. The list of improvements also included replacing the 122 mm D-25 gun with a 100 mm gun that had a higher rate of fire. All of these proposals were grassroots initiatives.
From each according to ability
Precursor work to modernizing the IS series of tanks began in the summer-fall of 1943. Initially, the armament was the only thing that was improved. Work was aimed in three directions:
- Improving the firepower of the 85 mm gun
- Improving the firepower of the 122 mm gun
- Creating tank and SPG 100 mm guns
In the first and third case, the work reached the prototype stage.
More radical improvements to the tank's characteristics were also suggested. A draft report by Major-General Fedorenko to Stalin dated September 5th, 1943, contained a suggestion to increase protection of the IS-2 tank at the cost of raising the weight to 55-60 tons. This proposal was crossed out, but nature abhors a vacuum.
On September 28th, 1943, factory #627 proposed the TET heavy tank project with an electric transmission. The first variant weighed 50-52 tons, its front armour reached 150 mm, and its top speed was 35 kph. The A-19 gun (meaning the D-25) would be its main armament. The second variant was even more monstrous: its mass was 64-66 tons and it had 180 mm of front armour.
The GBTU composed requirements for a new heavy tank in late November of 1944. The mass would be 55 tons. Its crew would consist of 5 men. The front of the hull armour would be 200 mm, front of the turret would be 160 mm. The tank would either be armed with a 122 mm gun or 152 mm gun-howitzer. An 800-1000 hp engine would propel this monster. The transmission would be in the front. A planetary gearbox would be used. An electrical transmission was considered, but work never progressed past the draft stage.
A tendency to split work evolved out of the escalating rivalry between factory #100 and ChKZ. ChKZ's SKB-2 design bureau began working on a new heavy tank in the summer of 1943, which in many ways matched the aforementioned requirements for a heavy tank. unlike the KV-85 modernization, for which Zaltsmann and Makhonin received a dressing down from Malyshev, the People's Commissar of Tank Production approved this initiative. The 56 ton «K tank» turned into the Object 701. This defined two different approaches: replacement of the IS-2 and its modernization.
This split was one of a few strange occurrences that took place in 1944 in Chelyabinsk. GKO decree #5582ss «On the production of a new heavy tank at the Kirov factory» signed on April 8th signalled an escalation of the situation. It describes the production of two experimental Object 701 tanks, but the third item of the document mentions a completely different vehicle.
«The NKTP (comrade Malyshev), director of the Kirov factory (comrade Zaltsmann), and chief engineer comrade Kotin must concentrate their attention on the improvement of the design and quality of the IS tanks and SPGs.
Within 3 months, perform the necessary work on increasing the IS tanks' armour protection, improvement of the transmission and suspension, using experience from new heavy tanks for modernization of the IS.»
This decree morphed into NKTP order #226ss signed on April 13th, 1944. The requirements for a deep modernization of the IS-2 were finally formed by May 19th, 1944. The list of improvements numbered 20 items. Some of them were based on the Object 701. The radiators, cooling system linkage, turret traverse mechanism, and turret ring design were taken from this tank. The front hull, 90 mm rolled armour or 100 mm cast, was based on the front armour of the Object 701. The turret design was also based on this tank: the commander's cupola and mechanism for extracting the gun through a hatch in the roof were the same. The exhaust pipes were moved to the rear plate, like on the T-34. This solution fully prevented exhaust fumes from entering the air intakes.
Interestingly enough, the design of this modernization was expedited not by ChKZ, but by factory #100. A draft project of the modernization of the IS-2 designed at this factory was presented on April 18th. The modernized tank weighed 46-47 tons. The hull was completely redone. Its shape was more reminiscent of a flying saucer. The hatch in the front of the hull was removed. The front and side armour was 120 mm thick. The turret, whose front was now 150 mm thick, was also radically redesigned.
The suspension was also altered. The tank was supposed to receive large diameter road wheels. The armament consisted of either a D-25T or a D-10T gun. A coaxial GVG (future SG-43) machinegun was used instead of a DT. No details on the engine or transmission are known, but two types of planetary transmissions were tested in early 1944: one designed at the Bauman Technical University, one desigend at factory #100. The engine is not known, but the fact that the tank's top speed was 50 kph hints at the fact that it would not have been a V-2-IS.
This project was not the only modernization developed at factory #100. Requirements for a heavy tank approved in November of 1943 were not forgotten. Revised requirements were introduced on March 22nd, 1944, which set the mass at 55-56 tons. The numbers for armour thickness were gone: the requirement was now only to protect the tank from the guns of the Tiger, Panther, and Ferdinand. The side armour would have to be immune at a range of 500 meters or greater. The tank would be armed with a 122 mm gun with a muzzle velocity of 1000 m/s and carry 30-40 rounds of ammunition. A 1000 hp engine would give the tank a top speed of 40 kph, or 24-26 kph on a dirt road. The tank would be equipped with an electrical or mechanical transmission. In the case of the latter, a 6-speed planetary gearbox with a 2-stage turning mechanism would be used.
These requirements were realized in the draft project designed under the direction of N.F. Shashmurin. The tank had a turret in the rear. The hull housed the driver and radio operator. The design of the turret, suspension, and partially the hull were used as starting points for the development of a less radical modernization of the IS-2.
The draft project that is sometimes called IS-2M was proposed in early April of 1944. It was forgotten quite quickly, but the 46 ton IS-2 modernization reached the wooden scale model stage. The development of the modernization ended here.
In April of 1944 factory #100 also began working on a new heavy tank project known as the IS-6. It was done in strict secrecy, and even some members of the factory management were unaware of its existence. The military representative at factory #100 even sent an official complaint to the GBTU. Work on the IS-6 incorporated elements of the modernized IS-2 project, and spelled doom for both the 46 ton IS-2 and Shashmurin's tank.
However, the situation remained odd. ChKZ and its neighbours were working on not only two brand new tanks, but two types of IS-2 modernization: large and small scale.
The situation with the engine is also worth mentioning. An order was given to increase the power of the V-2-IS engine back in 1943. A more powerful engine was also designed for the Object 701. Work in this direction moved far past requirements and draft projects. Three engines were designed based on the V-2: the V-11 by factory #75, V-14 by factory #76, and V-16 by factory #77. Of those, only the V-11 was mass produced, and even then only in 1945. Instead of the required 700 hp, it produced the same 520 hp as the V-2-IS.
The Kazakh Agricultural Institute (Alma-Ata) developed another engine for the IS-2, a very unique one. A steam engine, originally meant for the KV, took too long to develop, and was later designed for the IS. This development was discussed several times, and two variants with different dimensions were proposed. The engine was never produced, even though the funds for it were allocated several times.
Beefing up the archives
While factory #100 had enough time to develop and subsequently abandon its own IS-2 modernization project, ChKZ's SKB-2 had its own issues. For starters, they had to improve the design of the tank while its production was ramping up. Don't forget that the development of the Object 701 also had high priority. GKO decree #5583ss created a special group of 25-30 engineers and 10-15 technologists at the factory, whose job was the support and production of new heavy tanks. It's not surprising that the large scale modernization of the IS-2 was considered secondary. Only three months were allocated to this task.
Full fledged work on a large scale modernization of the IS-2 began only in the summer of 1944. This didn't stop the designers from reusing a number of solutions on the production tank. The most important was the introduction of a straight upper front hull section in August of 1944. The minimal distance at which the Pak 43 could penetrate the UZTM production welded hull increased to 575 meters, and the cast 100 mm front could only be penetrated at 255 meters. A downside was the reduced visibility from the driver's station and the weak spot formed by the Object 701 type observation device, but there were more advantages than drawbacks.
The GKO decree proposed that large scale modernization work would be immediately implemented in production. No prototypes would be built. As a result, a very precarious situation formed in the fall of 1944. A special request was made to L.P. Beria asking for authorization to produce IS-2 and ISU-152 prototypes with significant changes of their design. Work on the modernized tank stopped for some time, and was only limited to the creation of a wooden model of the turret. After permission was granted, work began once more, but it was clear that the factory will not meet its deadline. Work on improving the Object 701 had an adverse effect on this work, as the design needed a lot of work. Blueprints for the modernized IS-2 were only ready by the end of August of 1944. Kirov factory received the hull of the modernized tank on the 31st, and assembly was performed in September. The modernized ISU-152 was built in parallel.
Strangely enough, no photos of the modernized IS-2 or modernized ISU-152 survive to this day. Only scraps of documentation and correspondence are available. The development of this vehicle considered not only the requirements made in April-May of 1944, but also experience in combat and experience with building the modernized IS-2 hull. The large scale modernization of the IS-2 was perfomed with this type of tank as the starting point.
The biggest changes were made to the engine compartment. In addition to a T-34 type exhaust system, the modernized hull also had a significantly different air intake system, inspired by those used on the Object 701 and Panther. The fuel system was radically redesigned. The fuel tanks were moved from the fighting compartment to the engine compartment and became interchangeable. The former location of the fuel tanks was now occupied by new batteries. The total number increased from 2 to 4. The turret was also redesigned. It was now similar to the one designed at factory #100. The gun was removed through a hatch in the turret. This allowed the gun mount to be redesigned to be more robust. The turret front was 130 mm thick, which increased its resilience. The turret ring, commander's cupola, and ventilation fan were also changed.
The GBTU prepared a list of 12 changes to the design in October of 1944. However, they were never implemented. ChKZ began working on another variant of modernization in the fall of 1944. It received the index Kirovets-1 and blueprints number 703. The index IS-3 was assigned to it in trials. Additional work at factory #100 began in the fall of 1944. A project internally called IS-2U under the direction of A.S. Yermolayev began in November of 1944. L.E. Sychev was the chief engineer.
The IS-2U looked like a halfway point between the IS-2 with a straight front hull and ChKZ's deep modernization. The turret was also similar, but had weaker armour than the one designed in the spring of 1944. The rear was almost the same as on the IS-2.
The most interesting part of the project was the front of the hull, developed by chief designer V.I. Tarotko. It had a unique «pike nose» shape. The upper front armour was sloped at 25 degrees to horizontal and consisted of two parts. This shape of the front armour allowed the placement of a driver's hatch in the front of the hull. Unlike the Kirovets-1 or the experimental modernized IS-2, the IS-2U was never built. However, the project had a significant influence on Soviet tank design. The «pike nose» was used on later tanks, the first of which was the mass production IS-3. Tarotko also used it on the reworked Object 252, which evolved into the Object 257.
Translated by Peter Samsonov. Read more interesting tank articles on his blog Tank Archives.
- Central Archives of the Russian Ministry of Defence;
- Russian State Archive of Economics;
- Archive of Igor Zheltov;
- Archive of Sergei Netrebenko;
- Kirill Koksharov's archive.