IS-6 (Object 252)

Soviet Union (1944-1945)
Heavy Tank – 1 Prototype Built

After the conclusion of the Battle of Kursk in 1943, the Soviets sought to develop new heavy tanks for both a short and long-term, with the development programs being split into two main concepts, the IS-2 minor upgrade and IS-2 deep upgrade. Several factories would be assigned work for these projects, most notably the Experimental Plant No.100, which in spring 1944 began development of a deep IS-2 modernization that focused on using angled armor for improved armor protection without sacrificing mobility. This led to the development of the Object 252 or IS-6 by autumn 1944, and despite its moderately uneventful trials in the last months of 1944, the project was canceled, as ChKZ’s IS-3 and IS-4 tanks were further down the development stage. Instead, focus shifted to the electromechanical transmission Object 253 and later the IS-7.

Development

The aftermath of the Battle of Kursk in summer 1943 saw the German forces introduce a variety of new and potent armored fighting vehicles, including the Panzer VI Tiger, Panzer V Panther, and the Panzerjäger Ferdinand. These tanks were heavily armored and had powerful 75 or 88 mm guns. While the Soviets had introduced the IS into service on 8 September 1943, it was deemed not protected enough to withstand the German guns, and its 85 mm gun was underwhelming compared to its German tank counterparts. On the same day, the development of a new heavy tank was proposed by Commander of the Armored and Mechanized forces of the Red Army, Col.Gen. Y.N. Fedorenko. Consequently, the 122 mm D-25T gun was mounted on the IS tank and adopted into service on 31 October 1943, as the IS-2. Fedorenko was also of the opinion that the IS’ hull was not thick enough, backed up by the fact that the Panther’s 75 mm KwK 42 L/70 gun could penetrate the IS’ front. Subsequently, he unsuccessfully requested Stalin to allow the IS to be uparmored, increasing its weight to 55-60 tonnes.

Alternatively, in November 1943, the GABTU (Main Directorate of Armored Forces) laid down the technical requirements for a new generation of heavy tanks, to later replace the IS-2. The tank was to weigh 55 to 56 tonnes, be armed with a 122 or 152 mm gun, have a crew of 5, and frontal hull armor of 200 mm, hull side armor of 160 mm and turret armor of 160 mm. In terms of propulsion, it was to use an 800 to 1,000 hp engine, allowing the tank to reach a top speed of 35 km/h. These requirements were received at the ChKZ (Chelyabinsk Kirov Plant), which had already been working on such a tank and presented it on 10 December as the K-1. After approval from the Minister of Tank Industry V.A. Malyshev, this would be developed into the Object 701.

On 21 or 22 (depending on source) March 1944, the technical requirements were adjusted. The new heavy tank was to be armed with a 122 mm gun, a more powerful engine to increase the top speed to 40 km/h, and enough armor to be immune from the German 75 and 88 mm guns. Lastly, two transmission types were to be tested, a mechanical and an electrical system. While ChKZ continued work on the Object 701, the Experimental Plant No.100* together with engineers from the TsNII-48 (Central Research Institute No.48) worked together for the development of the new heavy tank for GABTU.

*The Experimental Plant No.100 was founded in 1942 by V.A. Malyshev at the ChKZ factory, and was renamed to the VNII-100 institute in 1949.

On 8 April 1944, decree No.5582ss was signed, stating that Malyshev’s NKTP (Ministry of Tank Industry), Zaltsmanns’s Kirov plant, and Chief Engineer Kotin had to develop and improve the IS-2’s protection, powerplant, and transmission based on the research from the new heavy tank proposals. Just five days later, on 13 April, the NKTP set order No.266ss, which requested a deep IS-2 modernisation from Plant No.100, based largely on the attributes of the Object 701.

The first drafts created at Plant No.100 envisioned a 46- to 47-tonne tank armed with either the 100 mm D-10 or 122 mm D-25T. Secondary armament consisted of the 7.62 mm GVG machine gun. While largely based on the IS-2, the hull was redesigned into more of an “UFO”-like shape, akin to the much later Object 907 and Object 279, presenting incredibly steeply sloped 120 mm armor from most relevant angles. Another noticeable feature was the introduction of large diameter roadwheels. The turret was also redesigned and bolstered to 150 mm thick cast armor. Plant No.100 then constructed two scale models for its tank proposals, one essentially being the same as previously envisioned, with the complex rounded hull and one with a much more simplified approach, where the frontal upper plate was split into three plates, a main plate and two cheek plates. They maintained a steep angle, which essentially allowed for similar protection as the “UFO” hull variant but without the considerable production issues. Top speed of this tank was to be of 50 km/h, suggesting that a more powerful engine was envisioned.

In parallel to these designs, N.F. Shashmurin, one of the engineers from Plant No.100, worked on his own deep modernization of the IS-2, based upon the requirements set on 21-22 March, which were different from the early April requirements. Shashmurin’s tank is now known as the IS-M, IS-2M, or other names. It featured a redesigned hull, with heavy sloping and a rear mounted turret. It featured both large diameter roadwheels but also regular IS wheels and had a weight of 55-56 tonnes and a 1,000 hp engine, later chosen to be the 1,200 hp M-40 engine. A self-propelled gun (SPG) variant was also drawn up, but Shashmurin’s tank never left the drawing board.

Development of Plant No.100’s heavy tanks was likely held in absolute secrecy, both for protection against espionage but also against the rest of the ChKZ, which were in the process of building the Object 701 prototypes. One peculiar incident caused by this secrecy occurred on 18 April 1944, when Guards Engineer-Colonel A. Vovk noticed a guarded room where he was not allowed to enter, being told that permission from Kotin himself was required, stating that it was his personal room. Engineers Shashmurin, Turchaninov, and Schneidman argued that it was their “smoking room”. Outraged, Vovk sent a letter to Engineer Major-General of the GABTU Afonin describing the situation. A large controversy was sparked surrounding the incident, especially considering that a military engineer and inspector of the plant was not allowed to enter said room. This incident further fueled the suspicion and tensions between Plant No.100 and ChKZ. By this point, both factories were working on heavy tanks completely independently from one another, despite being on the same industrial platform, with the GABTU supporting both projects.

In May 1944, the new tank was indexed as the IS-6 and had evolved beyond being an IS-2 modernization. The project was split into two different models, the Object 252 with a mechanical transmission, and the Object 253 with an electromechanical transmission. Initially, the IS-6 was to have an electromechanical transmission developed by the VAMM (Military Academy of Armored Forces), Dynamo plant, and Factory No.627, however, due to the experimental nature of the project, a mechanical transmission was to also be developed. I.Y. Kotin was the General Manager for the project, A.S. Ermolaev was the Chief Designer of the IS-6, G.N. Moskvin was the Senior Engineer, and Schneidman was tasked with armament incorporation.

On 6 June 1944, Malyshev would approve the development of both prototypes, with the Object 252 weighing 48 tonnes and the Object 253 weighing 50 tonnes. The tanks were to be armed with a 122 mm gun with muzzle velocity of 850 to 900 m/s and rate of fire of 4 to 6 rounds per minute. The tanks were to be immune to 88 mm German guns with impact velocity of 1,300 m/s. The tanks were to be powered by either the V-11U or V-12 diesel engines, offering a top speed of 40 km/h. To reduce development and production time, components from the IS-2 were to be used where possible.

Two days later, on 8 June 1944, Malyshev signed Order No.379 of the People’s Commissariat of Tank Industry, giving the official go ahead for the development of the IS-6 prototypes. Representatives A.S. Zavyalov and G.I. Kapyrin from TsNII-48 were assigned to help Plant No.100 during development. I.Y Kotin was appointed as the Chief Designer of the project, while the Plant No.100 Director Solodukhin, factory Chief dDesigner A.S. Ermolaev*, and Uralmashzavod (UZTM) Director Muzrukov and Chief Designer Gorlitsky would ensure the design, production, and testing of the prototypes. The reason why the UZTM plant, located in modern day Yekaterinburg, was to build the prototypes was due to the limited production capacity of the No.100 plant, which was more of a research and development institute rather than a factory and lacked the production capabilities. This was not the first time the ChTZ/ChKZ design bureaus had to resort to UZTM for prototype production, with one of the earliest examples being the Object 212. By 1 July 1944, blueprints and documents for the hull and turret were to be finished and submitted to UZTM, with the remaining documents to be finished by 15 July. The UZTM was to deliver two hulls and one turret by 25 July and assembly to be finished at Plant No.100 on 25 August. Factory trials were to be completed by 10 September with a report submitted 10 days later.

*The positions and roles of Kotin and Ermolaev seem to contradict and vary depending on different sources.

Ballistic Testbeds

To test the armor layout of the IS-6, a special testbed was created at UZTM, which would imitate the hull armor layout of the IS-6, named model No.15. One side of the testbed featured a frontal armor design made out of three plates, one main upper front plate and two angled side cheeks. On the opposite side, the testbed featured a pike nose. The “lower hull” of the testbed was made out of regular flat plates on the side. While most segments were meant to replicate the IS-6, some variations and adjustments were made to angles and joint types for several reasons, such as reducing the weight and dimensions of the testbed but also to vary the angles and thus armor protection. The 20 to 90 mm plates were rolled homogeneous armor plate of 42SM and 2P grades supplied by the Magnitogorsk Metallurgical Steelworks (MMK), while the 120 mm plates were of 66L grade and made at TsNI-48 with plates supplied by the MMK.

A second armor mock-up hull, model No.16, was designed and built. It differed from model No.15 only by its welding and armor plate joining technique, as this area was a big problem on several Soviet tanks.

Ballistic trials of models No.15 and 16 were held between 4 to 6 September 1944 at Plant No.9 in the Sverdlovsk region using German 105 mm (muzzle velocity of 830 m/s) and 88 mm (muzzle velocity of 1,000 m/s) guns firing armor-piercing (AP) rounds, Soviet 85 mm (muzzle velocity 1,020 m/s) guns, and small caliber guns. The report written after the trials concluded stated the following:

• The frontal section of the testbed, made out of 90 mm plates, was immune to the German 105 mm, 88 mm, and Soviet 85 mm guns.
• The pike nose section with 90 mm armor plates was also immune to the German 105 mm, 88 mm, and Soviet 85 mm guns.
• Lower plates on both sides were immune to the German 105 mm and Soviet 85 mm.
• Upper side plates withstood the German 105 mm and Soviet 85 mm (muzzle velocity 800 m/s).

As for the different joining techniques used, in both models, the weldings failed and no solution was deemed superior to the other.

A third mock-up, model No.17, was built and tested in December 1944. It was a combination of the previous two models, with the front of model No.15 and rear of model No.16. However, the armor plates failed even quicker and the welds cracked sooner, causing the trials to end prematurely.

Production and Trials

As for the IS-6, work proceeded slowly and the technical drawings and documentation were ready by the end of August 1944 and a full size wooden mock-up was built, equipped with the V-12 engine and BL-13 122 mm gun. One of the Object 252’s most noticeable and unique elements were the large-diameter roadwheels, originating from the IS-2 modernizations from spring 1944. To test the viability of these wheels, the Object 244 had its roadwheels replaced with the large diameter wheels, with the exception of the last one, due to the size mismatch. Trials were held between August and October, during which the tank traveled 1,875 km. During 1,589 km of these, the Object 244 was loaded with an additional 8 tonnes to bring it closer to the Object 252’s weight. The majority of the driving was done on asphalt and dirt roads (1,571 km), with 285 km on cobblestone roads, and 19 km off-road. The wheels were noted to be fine, however bolts would loosen, leading to cracks in the wheels, in turn leading to one wheel failing completely and one developing a 35 cm long crack.

Production of the first IS-6 prototype, the Object 252, began on 20 or 21 September 1944 at UZTM, and the hull and turret were sent to Plant No.100 in October for assembly. On 8 November, the hull was complete and trials began until 10 November. During this period, the tank drove 319 km, of which 219 km were on roads and 100 km on cobblestone roads. Interesting to add is that, for the first 31 km, the tank did not have its turret mounted. The average speed on asphalt roads was 21 km/h and 16 km/h on cobblestone roads. Already at this stage, some design issues were noted:

• The turret hatches were too small.
• The driver’s hatch offered poor visibility when opened.
• The driver’s forehead was resting on the upper frontal armor plate.

When exactly the Object 252 trials ended is slightly unclear, with M. Kolomites claiming 27 November, while Y. Pasholok claiming 8 December. Nonetheless, during the tests, the Object 252 prototype traveled 825 km with an average speed between 11.5 to 22 km/h and a top speed of 35.4 km/h. The tank covered 420 km on asphalt roads, 315 km on cobblestone roads, and 90 km off-road.

Several problems were noted, such as the clutch being very heavy to press, requiring between 60 to 65 kg of force. The gearbox was also prone to overheating. The largest issue were the new large diameter roadwheels, which wore out too quickly, with a durability of 200-300 km and 14 had to be replaced during the length of the trials. The electrical VG-50 and later K-73 DC generators also had faulty operation. Lastly, the 480 liter fuel tanks only lasted for 100 to 120 km of operation. To fix the issue of the failing roadhwheels, a new wheel design was made, this time of a concave shape.

Design

The general design of the IS-6 Object 252 was standard, much like previous Soviet heavy tanks, with the driver at the front, central fighting compartment and turret in the middle, and engine and final drive towards the rear of the hull. The upper frontal plate featured the driver’s service hatch, and above it was the driver’s periscope. To either side of the hatch was a headlight. Below the headlights were the tow hooks. The sides of the tank were heavily angled as well, and on them, the marker lights, tools, and a single (per side) spare fuel tank could be mounted. Much like the front of the hull, the rear was also split into three angled armored plates. Two hatches were mounted on the angled rear plate for engine access and maintenance. On the Object 252 mock-up, two small fuel tanks were mounted on the rear hatch, however these do not appear to have been added to the real tank.

The running gear of the tank consisted of six large roadwheels per side, with the idler and sprocket remaining the same as on the IS-2. The advantage of this layout was that there was no need for return rollers. For additional protection, thin sideskirts were added across the length of the hull. The Object 252 used 90 tracks per side, which were 720 mm wide.

The turret was made out of two main cast segments, an upper and lower part, creating a shape similar to the turret on the IS-2. Handrails were mounted along the sides and rear of the turret. Two machine gun ports were drilled in the sidewall as well. On the roof of the turret, there were two service hatches for the crewmembers and a ventilator for the fighting compartment.

Crew

The driver sat in the front of the hull, with the tillers and pedals for driving the tank. He also had the fuel distribution valve, manual fuel pump, compressed air cylinders (5 liters) for starting the engine, and four 24V main batteries. For vision, the driver had two forward mounted MK-4 periscopes protruding through the main armor plate. In front of him was also his entry and exit hatch. Underneath his seat was the emergency exit hatch, in the hull floor.

The rest of the crew’s positions were very standard for a Soviet tank. The gunner sat to the left of the gun, the commander behind him, and the loader on the opposite side, on the right side of the gun. Both the commander and loader had their own service hatches, however, the commander had two MK-4 periscopes as opposed to the loaders single periscope. The gunner also had a MK-4 periscope mounted in the turret roof. The commander was tasked with operating the 10R radio located in the rear of the turret. The loader, besides being tasked with loading the main gun, also loaded the coaxial machine gun and operated the AA DShK mounted on top of his hatch. The tank was equipped with a TPU-4-BIS intercom.

Armament

The main gun intended for the Object 252 is a slight mystery. Initially, the 122 mm D-30 was proposed, a modified variant of the D-25T used on the IS-2. The main differences were that the D-30 featured a gun rammer and a compressed air fume extractor. However, by July 1944, development of a new 122 mm gun began at Plant No.172 alongside Plant No.100, in the form of the BL-13. It also featured an automated gun rammer and bore evacuator. The gun could have reached a rate of fire of 8 rounds per minute, though other sources claim 12 rpm. However, the prototype produced and tested at the end of 1944 seems to have used a 122 mm D-25T. This was potentially done due to the lack of a BL-13 gun to mount. Nonetheless, the main gun had 30 two-part rounds stored onboard. The main gun had a depression of -3° and elevation of +20°, allowing for a direct firing range of 5 km and indirect range of 13 km.

Secondary armament was a coaxial 7.62 mm Goryunov GVT machine gun (both Pavlov and Kolomiets seem to agree on this but which machine gun this was specifically is unclear) and a roof mounted AA 12.7 mm DShK. Inside the tank, there were 500 12.7 mm rounds and 1,200 7.62 rounds.

Armor

The IS-6 was very well armored, with 120 mm of upper frontal armor angled at 65° from vertical, and the lower plate angled at 35°, also 120 mm thick. Side armor was 100 mm angled at 45°. Lower side and rear plates were 50 to 60 mm thick and roof and belly armor was 20 to 30 mm.

An interesting detail is that the hull was welded using austenitic electrodes with nickel, a very rare metal in the USSR at the time. The welding of the hull required up to 60 kg of nickel.

The turret consisted of castings, with the thickest section being 150 mm and tapering down to 100 mm towards the side and rear, and angled at 25° . The roof of the turret was 30 mm thick and welded on top.

Powerplant

The engine used on the IS-6 was the V-12 turbocharged diesel engine, outputting 700 hp (slightly reduced power output for improved reliability). To start the engine, a ST-700 15 hp electric motor could be used, or compressed air from the driver’s position. For starting in extremely low temperature, a steam heater was installed in the oil tank.

Within the engine compartment, to the sides of the engine, were the oil and fuel tanks, the latter of which had a capacity of 640 liters.

The cooling of the engine was done via four (two per side) cooling fans placed to the side of the engine, used for cooling both the water and oil in the engine. The air filtration system used two Multicyclone filters.

The transmission and final drive were located behind the engine compartment, with the multi-disc clutch, an eight speed synchronized gearbox, and lastly, the brakes and final drives.

The Object 252U and the End of the Project

In the second half of November 1944, Plant No.100 and the TsNII-48 institute developed a minor upgrade for the Object 252 based on the issues discovered during the trials. A pike-nose armor layout system designed by V.I.Tarotko was introduced, being one of the first cases of such a design layout on a Soviet tank. Additionally, with the reworked front, the driver’s position was adjusted, offering more room between the frontal plates and his hatch was put above him, offering better protection and structural integrity. A similar process was applied to one of the minor IS-2 modernizations on which Plant No.100 was working on at the time. These designs are often named Object 252U and IS-2U from the Russian word for improved, but this is likely a modern name, as the documents simply referred to it as “252”.

The Object 252 prototype and the documentation for the improved Object 252U and IS-2U were sent to Moscow for evaluation at Kotin’s command. However, the IS-6 was quickly dropped in favor of the Object 703, which would go on and become the IS-3 shortly thereafter, and the Object 252 was sent back to Chelyabinsk.

While the IS-6 was virtually canceled by this point, with both the GABTU and NKTP choosing the Object 703, Plant No.100 continued its development, seemingly for experimental purposes. Yet another heavy blow came with the ballistic trials of the cast turret. The UZTM had constructed two turrets, one with 70L steel and the other with 72L steel. Trials held in January 1945 showed that the 70L steel turret’s 150 mm areas could be penetrated by the German 88 mm PaK 43, with 7 out of 12 hits penetrating. The turret out of 72L steel was even worse, having allegedly split in half after receiving just 5 shots!

Several other components were tested, with the large diameter roadwheels being reworked once again, increased to a diameter of 800 mm and strengthened with ribs, essentially becoming enlarged IS roadwheels. These wheels were once again fitted on the Object 244 in December 1944 and 698 km were traveled between December 1944 and January 1945. Lastly, the 122 mm BL-13 gun, which was developed in tandem with the IS-6, was also trialed on the Object 244 in 1945.

The Object 253 and the IS-7

The Object 253 was the second IS-6 variant, with the main difference from the Object 252 being the electromechanical transmission and regular IS steel roadwheels. Some Kolomiets claims that the prototype was built in November 1944. Pasholok disproves this, with no correspondence from Plant No.100 ever mentioning the Object 253 between January and February 1945. One of the most plausible reasons for this was the issue of secrecy. On 19 February 1945, Guards Engineer-Colonel Vovk once again complained about the guarded rooms. This likely had to do with the further developments of Plant No.100, including the Object 253, but also a new heavy tank based on new NKTP requirements, leading to the development of the Object 257, the first IS-7.

The Object 253 was likely built and completed in the early months of 1945 and trials commenced shortly after. According to the memoirs of the Object 253’s chief designer, M.I. Kresavksy, the electromechanical transmission proved to be more smooth and offer better maneuverability at the cost of a higher overall weight. However, the trials were cut short after the transmission caught fire, and further work was stopped. Yet, the Object 253 would still be used for testing electromechanical transmissions and, between 1946 and 1947, it was rebuilt and used during the IS-7’s development.

Both the Object 252 and 253 were then scrapped. The exact date is unknown, although this likely occurred with the cancellation of all Soviet heavy tanks weighing above 50 tonnes in 1949, after which the IS-6 prototypes lost whatever experimental value they still had left.

Conclusion

The IS-6 was born out of the GABTU and NKTP’s extravagant requests for various types of IS-2 modernizations. Although its simple yet refined design focused more on reliability, like the downgraded V-12 engine, it ended up being caught between the more reliable and crude Object 703 (IS-3) and the much heavier and better armored Object 701 (IS-4), making the IS-6 seem like a ‘nothing burger’. Despite its innovative armor upgrade and relatively successful trials, the Object 252 remained as a single prototype.

IS-6 (Object 252) Specifications
Dimensions (L-W-H)	7.025 (10.030 with barrel) x 3.43 x 2.408 m
Total weight, battle-ready	51.5 tonnes
Crew	4 (commander, gunner, loader, & driver)
Propulsion	700 hp turbocharged V-12
Speed	35 km/h
Suspension	Torsion bar, 6 bars per side
Armament	122 mm D-30/BL-13/D-25T 1x 7.62 mm GVT 1x 12.7 mm DShK
Armor (mm)	120 front 100 sides 60-50 rear 20 belly and roof 150-100 turret
No. Built	1 prototype built

Sources

Stalin’s Supertanks IS-7- Maxim Kolomiets
Domestic Armored Vehicles 1941-1945 – A. Solyakin, I. Zheltov, I. Pavlov, M. Pavlov
IS Tanks – Mikhail Svirin
Out of Amplitude | Warspot.ru – Yuri Pasholok
Modernization on paper | Warspot.ru – Yuri Pasholok
The Very First IS-7 | Warspot.ru – Yuri Pasholok
Tank Archives: D-30 Gun for the IS-6 – Peter Samsonov
Tank Archives: Wishlist Response – Peter Samsonov
“Elektrostalin” No6 | Warspot.ru – Aleksey Statsenko