WW2 Soviet Heavy Tank Prototypes

K-1 (Object 701 First Draft Version)

Soviet Union (1943-1944)
Heavy Tank – Scale Model Built

In the summer of 1943, the Chelyabinsk Kirov Plant began the development of a new heavy tank out of their own initiative under the name K and index Object 701. By December, the drafts of the tank were presented to the state and further funding and development was approved. By March 1944, the project had evolved significantly and morphed into what is today the better known Object 701 design. The first prototypes of the newly drafted design were built in May. These would lead up to the creation of the IS-4 by 1946. As for the K tank, it remained just a model.

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The IS-2 is largely celebrated as one of the most powerful tanks deployed in combat by the Soviet Union, whether it be propaganda, or rightfully through its technical prowess. Its thick, sloped armor made it immune to a variety of weapons fielded by the Axis forces, while the powerful 122 mm D-25T gun could defeat most German tanks. Yet, even before it entered service in October 1943, several concerns were brought up, especially regarding its protection.

IS-2 during testing at NIBT, January/February 1944. Note the 122 mm D-25T gun with the German-style muzzle brake.
Source: Warspot

In September 1943, Marshal of the Armed Forces Y.N. Fedorenko requested in a letter to Stalin that the IS receive thicker armor and its weight be increased to 55-60 tonnes. Truthfully, the German Panther’s 75 mm KwK 42 L/70 could penetrate its frontal armor, especially due to its ‘stepped’ shape, caused by the driver’s position, which presented a much flatter frontal target. This issue was further highlighted after analysis of the battle of Kursk, which saw both the Soviet and German sides field new and powerful vehicles.

As early as July 1943, out of its own initiative, the ChKZ (Chelyabinsk Kirov Plant) heavy tank design bureau SKB-2 began the development of a new heavy tank to replace the IS-2. Alongside its own engineers, SKB-2 would employ designers from the Bauman Moscow Technical University for the development of the transmission and ballistic engineers from for the armament. Chief designer was the head of the bureau, N.L. Dukhov, Chief engineer was C.N. Makhonin and Senior engineer was L.S. Troyanov, later replaced by M.F. Balzhi. The main goal was to create a strong new tank for the military by mainly focusing on its protection, which was to be capable of withstanding fire from both the German 75 and 88 mm guns on the Panther and Tiger. As the project was a factory initiative, its funding came from the plant’s own budget.

In November 1943, the GABTU (Main Directorate of Armored Forces) laid down the requirements for a new heavy tank. It was to have a mass of 55 tonnes, 5 crewmen, 160 to 200 mm of frontal armor, an 800 – 1,000 hp engine, main armament of caliber 122 or 152 mm and top speed of 35 km/h. These requirements were signed by ChKZ factory director I.M. Saltzman on 3 December.

With these new requirements in mind, the new ChKZ heavy tank first draft was ready and signed by Saltzman on 10 December, under the name “K” (likely from Kirovets) and index Object 701. On the same day, the tank was presented to the People’s Commissar of Tank Industry V. A. Malyshev, who approved the project and gave additional funding. A scale model of the K tank was made, showing its unique features and a large step forward from the IS-2. Unfortunately, the blueprints were reused for other purposes in 1944 and then scrapped.

The model of the K tank, showing the vast design improvements in respect to protection compared to the IS-2.
Source: Domestic Armored Vehicles 1941-1945

Development of the Object 701 continued and a second model was presented. It was very similar to the original K tank, however, several features were changed, such as removing the muzzle brake, rear ball-mounted machine gun and altered gun mantlet. It is on this design that the first Object 701 prototype (No.0) would largely be based.

The Object 701 model, spring 1944. It was largely based on the K model presented in December 1943.
Source: Domestic Armored Vehicles 1941-1945


The K tank was revolutionary in terms of armor layout and profile. The hull was completely redesigned, with the front being divided into three segments, with the main upper plate being heavily angled. The rounded cheeks found on the IS-2 were replaced with steeply angled plates as well. The entire hull was lengthened, adding an extra roadwheel. In the engine bay, a “step” was added between the engine deck and lower hull, which allowed for the extension of the track length without excessively increasing the volume and mass. Instead of a large grille, engine cooling was provided by two large fans with circular cutouts on the engine deck. A large hatch for servicing the engine was provided. The steel roadwheels were sprung by torsion bars and the track was supported by three return rollers. The sprocket and final drive were kept in the rear of the hull.

The turret was hexagonal shaped, with no flat surfaces, but rather various curves were implemented. The flatspot of the rounded IS-2 turret cheeks was removed by introducing a greater angle and by using a larger mantlet. The height was also reduced, with an extension of the turret roof to allow for higher gun depression values. A rotating periscope was mounted on the roof to be used by the gunner. The commander also received a cupola with a single rotating periscope. In the rear of the turret, the ball-mounted machine gun was kept.

Front view of the K tank. The frontal hull was completely redesigned, and the angled plates can be seen.
Source: Domestic Armored Vehicles 1941-1945
Rear view of the K tank, showing the redesigned engine roof area and rear hull. This “step” format would be kept even on the production IS-4.
Source: Domestic Armored Vehicles 1941-1945
Bottom view of the K tank model.
Source: русская-сила.рф


The crew layout remained largely the same as on the previous IS tanks, with 4 men. The crew consisted of a commander, gunner, loader, and driver.

The gunner was seated to the left of the main gun, with the commander behind him. The commander had a cupola with a service hatch and periscope. He was also responsible for operating the 7.62 mm DT machine gun in the turret rear wall, behind him. The loader was standing on the opposite side of the commander, to the left of the gun.

The driver sat in the hull, directly underneath the main gun. He had one periscope for vision and a service hatch, which could have posed issues in case of an emergency exit, as the gun could prevent the hatch from opening or the driver from exiting. Thus, an internal emergency hatch would have likely been placed behind his seat, as commonly found on other Soviet tanks of the time.


The front of the hull was to be 160 mm thick on the upper plates and 120 mm thick on the lower plates. The side armor was 160 mm thick on the upper hull and 150 mm on the lower hull. Rear armor plates were 120 mm thick. The turret was made out of cast steel and was 150 mm thick all around. The effective thickness was increased thanks to the angling.


The main armament would have been the same as on the IS-2, the 122 mm D-25T with two-part ammunition. The only apparent change was the replacement of the muzzle brake with a German-style one, developed after studies on German tank guns. The gun had a muzzle velocity between 790 m/s and fired armor-piercing BR-471 and high-explosive OF-471 shells. In the late stage of the war, a new AP round was fielded, the BR-471B. The amount of ammunition carried onboard is unknown.

D-25T Ammunition characteristics
Type & Name Weight Explosive charge Penetration (1,000 m at 90 ° )
AP BR-471 25 kg 156 g TNT 143 mm
AP BR-471B 25 kg ? 142 mm
HE OF-471 25 kg 3 kg TNT N/A

Source: Encyclopedia of Russian Artillery – A.B. Shirokorad

Diagram showing the D-25T mount in the IS-2 with the German-style muzzle brake. Note that the barrel was shortened on purpose.
Source: Amizaur via

It is also entirely possible that the K tank was to be fitted with an 122 mm S-34 from TsAKB, hence why artillery specialists from that bureau took part in the tank’s armament integration. This gun would be used on the second Object 701 draft and first few prototypes. Its main characteristic was the lack of a muzzle brake.


Despite the fact that the K tank was to be powered by an 800 to 850 hp engine, there were no working models at the time. Only later, in 1944, did plant No.76 design the V-14 with 800 hp output. A replacement for the Object 701 engine was the V-12 diesel engine, with 750 hp at 2,100 rpm. It was designed at the ChKZ beginning in February 1943 and was equipped with a centrifugal turbocharger from the AM-38 aviation engine.

The V-12 engine used on the K tank and its later Object 701 prototypes and even IS-4.
Source: Domestic Armored Vehicles 1945-1965

Later Object 701 prototypes and IS-4

On 8 April 1944, the ChKZ plant was given permission to build two prototypes of the new heavy tank and a ballistic hull testbed, which kept the index Object 701. Additionally, the ChKZ was also tasked with the development of an IS-2 upgrade. Chief designer was N.L. Dukhov and Chief engineer was M.F. Balzhi. In a meeting held on 4 May, the goal of the IS-2 upgrade tank was to strengthen its armor protection, reduce the fire hazard from fuel tanks in the engine compartment, as well as improve accuracy and rate of fire. However, work on this upgrade progressed very slowly due to the ChKZ plant heavily focusing on the Object 701 instead. The first prototype of the Object 701 was built in April, receiving index Object 701 No.0, followed by Object 701 No.1. After seven prototypes and rigorous testing in spring 1945, the Object 701 would enter service only on 29 April 1946 under the name IS-4.

The Object 701 No.0, built in April 1944. The similarities to the Object 701 model and in turn, the K model, are clear.
Source: Soviet Heavy Tank IS-4

The mysterious K-2

In the book by Russian historian Maxim Kolomiets, Soviet Heavy Tank IS-4 (2014), he presents the K tank as part of the IS-4’s development. In 2013, blogger Peter Samsonov made the same correlation in one of his blog posts. However, the tank model shown as proof is largely different to the K-1, using a different turret, as well as an entirely new frontal hull. Other changes include the service light and return roller position. This seemingly was a further development of the K tank, hence its popular online index “K-2”. Although, considering the similarities between the K tank model and the later Object 701 model, the “K-2” does not fit in the timeline. Rather, based on its design, it could have been a model for later Object 701 developments or early Object 703 design ideas, placing it somewhere between summer and winter 1944. In his newer book, Soviet Heavy Tank IS-4 (2019), Kolomiets replaced the photos of the “K-2” with one of the actual K tank.

The mysterious K-2 tank, often regarded as a further development of the K (Object 701). However, due to its radical changes, it is much more likely that it was a further development of the Kirovets-1, and predecessor to the IS-3.
Source: Soviet Heavy Tank IS-4


The K-1 tank featured a lengthened hull, thick, angled armor plates and new turret significantly improved the survivability of the tank compared to the IS-2. It was a pivotal step in the development of the Object 701, which would enter service with the Soviet armed forces in 1946 as the IS-4.

K tank illustrated by Pavel Alexe

K-1 (Object 701 first draft) Specifications

Dimensions (L-W-H) 6,520 – 3,220 – 2,360 mm
Total weight, battle-ready 56 tonnes
Crew 4 (Commander, gunner, loader, driver)
Propulsion 750 hp V-12 diesel at 2,100 rpm
Speed 35 – 40 km/h (hypothetical)
Armament 122 mm D-25T alt. 122 mm S-34
2x 7.62 mm DT machine gun
Armor UFP: 160 mm
LFP: 120 mm
Upper side hull: 160 mm
Lower side hull: 150 mm
Rear hull; 120 mm
Turret: 150 mm all around
No. Built 0, scale model only


Domestic Armored Vehicles 1941-1945 – A. Solyakin, I. Zheltov, I. Pavlov, M. Pavlov
Domestic Armored Vehicles 1945-1965 – Ivan Pavlov, Mikhail Pavlov
IS Tank – Igor Zheltov, Alexander Sergeev, Ivan Pavlov, Mikhail Pavlov
Stalin’s Superheavy tank IS-7 – Maxim Kolomiets
Soviet Heavy Tank IS-4 – Maxim Kolomiets
K-2 Heavy Tank – WikiWarriors
Tank 701 – from the first prototypes to the IS-4 | Stalinium armor, comrade! | Zen (

2 replies on “K-1 (Object 701 First Draft Version)”

1. The object 704 was obviously not based on object 701. It was based on object 703, even if some 701 parts were incorporated. Which you yourself even say in your object 704 article.

2. In what world is the “K-2” turret akin to that of the IS-3??? It’s clearly just a more sloped K-1 turret. It isn’t bar soap shaped. It isn’t round like the IS-3 turret. And the hull doesn’t share any similarities with the IS-3 (object 701-A/703) either. Neither the front armour layout, suspension, nor engine bay look the part. It’s clearly just a further evolution of K-1/701. Putting it in a timeline is still hard, but it’s clearly not connected to object 703.

Hello, thank you for the feedback. Yes, you are correct, the Object 704 was based on the Object 703, it was a typo.

As for the K-2, yea its a bizarre thing and hard to pinpoint exactly. I’ll reword the text to better highlight that.

– Regards, Pavel Alexe

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