Work on Soviet heavy tanks split into two directions in 1944. One was the development of a new heavy tank, called Object 701. The second was the improvement of the IS-2 heavy tank, which later split into a minor and a major modernization projects. The former led to the creation of an IS-2 tank with a straightened upper front hull. Finally, a third tank came up in the spring of 1944 on factory #100's initiative. This tank was called IS-6 and turned out to be the odd man out.
A new tank instead of a modernization
The first experience from the use of the IS-85 (IS-1) tank in combat was analyzed in the spring of 1944. It turned out that neither the tank's armour nor its armament are adequate for the contemporary battlefield. The GBTU already knew about the armament, and the IS-122 (IS-2) tank with a more powerful 122 mm D-25T gun was put into production in December of 1943. The armour, however, was an unpleasant surprise. It turned out that the German 75 mm KwK 42 L/70 gun penetrates the front hull and turret. For this reason, work to modernize the IS-2 already began in March of 1944. It was fully legalized by GKO order #5583ss "On production of new types of heavy tanks at the Kirov factory" issued on April 8th, 1944. The decree described a modernized IS-2 tank in addition to the Object 701. Three months were given to develop and produce an improved variant of the tank. The main requirements were the improvement of armour protection and improvement of the reliability of the suspension and running gear.
A modernized IS-2 developed by factory #100's design bureau in April of 1944. This tank was the foundation for a new tank project that the factory began working on in April of 1944.
The main driver behind the IS-2 modernization project was factory #100's OKB design bureau, led by A.S. Yermolayev. Work on the modernization began even before GKO decree #5583ss. In addition, the factory wasn't working on exactly a regular IS-2 tank. The project developed by N.F. Shashmurin was closer to a brand new tank designed according to the requirements composed for a 55-56 ton heavy tank. This was not a modernized IS-2, but rather a competitor to the Object 701. Shashmurin's tank also had a 5 man crew, the same as required of the new heavy tank. A lighter variant weighing 46 tons was developed based on this new tank. The shape of the hull with its sloped plates, running gear with large road wheels, turret with a maintenance hatch in the roof for the gun, and other elements were taken from the new tank.
Factory #100's OKB finished technical documentation for the project by late April of 1944. Several wooden models were produced and that was it. Even though the NKTP and GBTU actively corresponded regarding the final modernized IS-2, factory #100 did not work on this topic any more. It's not that the modernization died completely, as the factory still produced components developed at ChKZ's SKB-2, but the homegrown IS-2 modernization project was abandoned.
A dramatic episode of development that could be titled "Comrade Vovk and the Chamber of Secrets". This report shows how actively factory #100's OKB was working on a new heavy tank in the second half of April.
The first hint that factory #100's OKB was doing something other than modernizing the IS-2 was a letter from the military representative at the factory, Guards Engineer-Colonel A. Vovk. On April 18th, 1944, he reported that there was strange activity at the factory. He found a room that was under guard. The representative was barred from entering it, with the reason that Kotin's personal permission was required to enter. When questioned, Kotin claimed that this was his office.
In reality, "Kotin's office" was a workspace for N.F. Shashmurin, A.S. Schneidman, G.A. Turchaninov, and other designers. They had their own excuse: "this is a smoking room". The result was a scandal, and the factory was ordered to establish a normal relationship with military representatives. Nevertheless, work on unauthorized topics did not cease. The Chamber of Secrets became the birthplace of a rival for the Object 701. Since serious consequences could come from the NKTP over this, the work was done in strict secrecy. It's not hard to understand why Kotin allowed this. Experimental factory #100 turned from a cutting edge tank developer into a subcontractor, while ChKZ that was supposed to produce factory #100's designs got its own design bureau that was working on the Object 701. The relationship between factory #100 and ChKZ notably soured in late 1943-early 1944. It's not surprising that there was a competitor to the Object 701 in the works.
Better than the IS-2, worse than the Object 701
Work on a competitor for the Object 701 reached the practical phase closer to May of 1944. Even though the tank was directly related to the major modernization of the IS-2, factory #100's OKB only partially used components developed for this purpose. The modernization also came with a limitation that the IS-2's components had to be used. Meanwhile, the new heavy tank was going to be heavier, better armoured, and better armed.
The V-12 engine used on the IS-6. Its power output was reduced to 700 hp.
Greater mass meant that a more powerful engine was needed. There were two candidates, the V-11 or V-12. The latter was also used on the Object 701. The AM-38 supercharger allowed it to reach 800 hp of power output. In practice this power could not be maintained for longer than 10 minutes, and the maximum power was later lowered to 750 hp. Factory #100's OKB considered the more reserved V-11 variant with only 620 hp (for a 50 ton tank this would have been enough). More interestingly, this tank was going to have an electromechanical transmission. This transmission was being worked on by a group at the VAMM led by N.I. Gruzdev jointly with the Dynamo factory and factory #627. Interestingly enough, the famous science fiction writer A.P. Kazantsev occupied a leading post at factory #627 at the time and had a direct input into the transmission's development. Work fell behind schedule and resulted in the EKV tank built on the chassis of a KV-1S tank in 1944. The transmission developed for the new tank at factory #100 was based on the EKV's transmission. Later, a mechanical transmission was also considered as a reserve variant. The tank with this transmission would weigh only 48 tons.
EKV experimental tank used to test an electromechanical transmission for Soviet heavy tanks.
Factory #100's OKB worked closely with NII-48 to provide their tank with protection similar to that of the Object 701. Institute specialists led by A.S. Zavyalov had significant experience in developing new types of armour and improving its resistance. Most likely the cooperation with NII-48 began earlier, during the development of modernized IS-2 hulls and turrets. The institute came to the conclusion that similar protection to the Object 701 could be attained by sloping the plates higher. A more powerful gun was also required. It had to have the same caliber as the 122 mm D-25T, but with a muzzle velocity of 850-900 m/s. No appropriate gun existed at the time.
A diagram of test hull #15 with interlocking joints. NII-48 considered this a high priority variant.
G.N. Moskvin was appointed as the senior engineer on this project. He had a lot of experience in tank and SPG development. The aforementioned Schneidman was responsible for the gun mount. Yermalayev was responsible for the project as the chief designer, and Kotin led the overall direction of the project. The tank was pitched to the NKTP in early June of 1944. As development was well underway, Malyshev approved further work.
Malyshev signed NKTP order #379ss on June 6th, 1944. This order gave the tank the index IS-6. The order pulled UZTM into the project as the producer of hulls and turrets. Detailed hull and turret blueprints for the tank with a mechanical transmission were expected by July 1st, the rest were due on July 15th. UZTM would then deliver two sets of hulls and turrets by July 25th, and the tanks were due on August 25th. Factory trials were to be completed by September 10th, the report was due on the 20th. The tank with an electromechanical transmission came later. Detailed blueprints were due on August 25th, two sets of hulls and turrets on October 10th, two prototypes would be finished by November 1st, and trials would finish on December 1st.
Test hull #16. UZTM opted for this system of joints.
The GBTU had its own opinion of the IS-6. The commission examining the project concluded that the tank's characteristics were not a step forward compared to the Object 701. On the other hand, it was a significant improvement over the IS-2, especially when it came to armour. The protection of the hull surpassed the IS-2 due to its sloping, but was still not as good as on the Object 701. The turret was in the same boat: it was up to 150 mm thick and thus better than the IS-2, but the Object 701's turret was up to 160 mm thick and had a narrower and therefore less vulnerable mantlet, although it turned out that the bulge for the sight was vulnerable during trials, so the mantlet had to be widened. The armament also received mixed reactions. The GBTU considered the 122 mm S-34 gun to be the optimal variant. The IS-6 was going to have a modernized D-25T with a muzzle velocity that was 50 m/s higher. The gun mount was criticized. A gun rammer and fume extractor were suggested. A recommendation was also given for a turntable for the fighting compartment. The running gear with large road wheels was also not a cause for enthusiasm.
Hull #15 under trial.
On June 12th Marshal Fedorenko sent Malyshev a letter proposing that the IS-6 be improved according to attached characteristics. There were some issues with these characteristics. The GBTU wanted a 50-55 ton tank that could not be penetrated by a 105 mm gun with a muzzle velocity of 1200 m/s from the front. No such gun existed in any German tank or SPG, but the GBTU decided to look into the future. This was only the start. The sides had to be immune to a 105 mm gun with a muzzle velocity of 1000 m/s. The tank itself had to have a 122 mm gun with a muzzle velocity of 1000-1100 m/s and a rate of fire of 4-6 RPM. The top speed had to be 40 kph, and the average speed was 24 kph. A 1000 hp engine and an electromechancial transmission were proposed.
Effect of shooting at the side. The sloped plates resulted in drastically improved protection.
Two months remained before the encounter with the Tiger Ausf.B at the Sandomierz foothold, so it was not clear who this armoured monster was supposed to fight. It was also unclear where such a powerful engine and gun could be found. The NKTP understood this. The GBTU sent them these requirements several times, highlighting the importance of an electromechancial transmission. The NKTP made some calculations and replied with a shocking conclusion. To meet the tankers' demands with the IS-6's hull configuration, the front hull had to be 130 mm thick and the sides 170-207 mm thick. The resulting tank would weigh 75-80 tons. The NKTP also noted that such a powerful gun or engine did not exist even on paper. The idea of single piece ammunition was also deemed strange, as this round would weigh 45 kg and be over a meter in length. Arguments around this issue continued for months. The original IS-6 was retained.
A direct hit on the weld seam.
While the NKTP and GBTU argued about their dream tank, factory #100, NII-48, and UZTM were working on figuring out the optimal hull for the IS-6. G.I. Kapyrin led the work from the NII-48's side. Test hulls imitating several joints were developed by early August of 1944. They were indexed "model #15" and "model #16".
The hull survived a hit from the 105 mm AA gun (#3), but multiple hits from the Pak 43 led to penetrations and cracking.
The models were identical in terms of the thickness of the plates. They represented the hull designed at factory #100, or rather the front of it. The difference was in the joining of the plates. NII-48 preferred model #15 with interlocking joints. Model #16 had bevel joints with dovetailing where the upper and lower front plates met. Factory #100's OKB considered the second variant as optimal.
Destruction of weld seams were a key issue with wartime Soviet heavy tank hulls, especially when it came to UZTM. Model #15 was no exception.
The deadlines set by NKTP order #379ss were not met. This was largely due to work on the hull. The trials program for penetration testing of the hulls was sent out for approval on August 10th, 1944, two weeks after the UZTM was already supposed to have delivered hulls and turrets for the tank with a mechanical transmission (Object 252). The trials were only held in early September of 1944. Trials were held at the proving grounds at factory #9.
Results of firing at model #17. The trials were ended prematurely due to destruction of weld seams.
The results of the trials were mixed. On one hand, the idea of highly sloped armour plates (up to 65 degrees) significantly improved their resistance. The IS-2 tank with 90 mm thick front armour could be penetrated by the 88 mm Pak 43 from 450 meters, but the IS-6's hull could not be penetrated at all. The same results were accomplished by the German 105 mm AA gun and Soviet 85 mm gun firing APCR. Firing at the rear of the model hulls led to the same effect. The upper sides could be penetrated only by the Pak 43 at close range. The problem was that the weld seams were destroyed as the hull was shot up. Interlocking joints did not show improvement over beveled joints.
The finalized Object 252 (IS-6), late August 1944.
To confirm the results, model #17 was tested in December of 1944. This was a modification of model #15 from the front and #16 from the back. The hull was assembled from experimental I-Z armour plate. Trials showed that the armour was worse than what was used on previous models. As for the seams, they were destroyed earlier than expected and the trials were stopped.
Full scale model of the Object 252, fall of 1944.
Factory #100 continued working on the tank while the hulls were being tested. Its looks were finalized by late August of 1944. A close up look reveals influence from both the modernized IS-2 and Object 701. The tank itself was an original design, somewhere between the IS-2 and Object 701, as the GBTU noted. The idea of a driver's hatch in the upper front plate was somewhat strange. It was reinforced, but this solution hardly improved the resistance of the upper front plate. The tank was quite stout, the turret periscopes were only 2408 mm from the ground compared to 2480 mm for the Object 701 and 2735 mm for the IS-2.
Close up view of the engine deck.
The final variant of the tank had a V-12 engine reduced in power to 700 hp. The overall layout of the mechanical transmission was close to that of the IS-2. The tank had an 8-speed gearbox with two-stage planetary turning mechanisms. Unlike the production IS-2, the radiators were located to the left and right of the engine (a similar layout to that of the Object 701). Four fans in the engine deck were used to cool the radiators. The characteristic shape of the rear hull earned the tank the nickname "jewel-box". The running gear was nearly identical to the modernized IS-2 project. Large road wheels were used, making return rollers unnecessary.
The rear hull with the engine deck removed. The characteristic shape earned the tank the nickname "jewel-box".
Like the hull, the turret design showed signs of both the modernized IS-2 and Object 701. The lowered commander's cupola (or rather two of them) were borrowed from there, as was the idea of extracting the gun through a hatch in the roof. Aside from that, the turret was quite original. The issue of ammunition placement was also reconsidered. The Object 701 held ammunition stored in the turret bustle parallel to the gun, but the IS-6 had it perpendicular, like on the IS-2. The tank carried 30 rounds, same as the Object 701 and a little more than the IS-2. Factory #100 took the GBTU's advice to heart and added a fighting compartment turntable.
122 mm BL-13 gun developed at OKB-172 for the IS-6. This gun is often confused with the D-30.
The tale of the armament deserves a separate telling. According to the specifications, the IS-6 had a 122 mm cannon with a coaxial GVG (SG-43) machine gun. Another machine gun, a DShK, was installed on an AA mount. The 122 mm D-30 gun is usually specified as the armament. Initially this was the D-25 with a muzzle velocity increased by 50 m/s. In reality the D-30 ended up having the same muzzle velocity, but the gun was more compact and lighter. Neither a mechanical gun rammer nor a fume extractor were used.
The muzzle brake also remained unchanged. OKB-172 worked on a gun for the IS-6 in parallel with factory #9. Their gun was called BL-13 and was designed jointly with factory #100's OKB. Practical work began in July of 1944, and it's not surprising that this gun is present in IS-6 documentation. The gun's full length of 6002 mm gave its shells a muzzle velocity of 850 m/s, and the idea of increasing that to 900 m/s was also raised. A number of BL-13 components were shared with the D-25T. The gun would also have the same mechanical gun rammer that is often ascribed to the D-30, same for the fume extractor. The maximum rate of fire was estimated at 8 RPM.
One evolutionary step
Despite the delays in designing and producing the IS-6, factory #100 began to put their ideas into action in August of 1944. First of all, this had to do with the running gear. Large diameter road wheels (750 mm) were produced. The goal was to reuse as many IS-2 components as possible. As a result, there were only 7 parts with the index 252.
Road wheel designed for the IS-6.
Since there was no tank to put the road wheels onto, the wheels were tested on the Object 244. This was the name of the Object 237 tank converted to take the 85 mm D-5T-85BM gun. The large road wheel diameter meant that the sixth rear road wheel could not be replaced. The tank drove like this in August-October of 1944 for a total of 1875 km, 285 of which were done on a cobblestone highway, 1571 on packed dirt and country roads, and 19 off-road. 1589 km were driven in the stock configuration, then the tank was loaded with an extra 8 tons of weight, up to the Object 252's combat weight. Instead of 48 tons, the final tank weighed 51.5.
Object 244 with IS-6 road wheels loaded to a mass of 51.5 tons.
The road wheels worked well during driving, but the retaining bolts were weakened, which in turn led to cracks forming. As a result, one wheel broke after 1425 km of driving, another had a 350 mm long crack after 1480 km. Factory #100 reinforced their bolts to get rid of this defect.
Experimental IS-6 (Object 252) on trials, November-December 1944.
Assembly of the first and only Object 252 prototype began at UZTM on September 20th, 1944. The hull was built from medium hardness rolled armour, the turret was built from high hardness 70L steel. The plates were welded together using austenite electrodes, as any other electrodes gave bad results. The hull and turret were sent to factory #100 in October, where assembly began. Factory #100 tested components as they were finished, including the cooling system. Assembly was partially completed by November 8th, and mobility trials began. The tank drove for 319 km from November 8th to 10th (100 on a cobblestone highway, 219 on corrugated roads), the first 31 km of which were driven without a turret. The average speed on a cobblestone highway was 16 kph and 21 kph on the corrugated road. Issues with the VG-50 generator were found during trials, the road wheel bolts were weakened again. Design issues with the hull and turret made themselves known. The turret hatches were not large, and the driver's vision through the hatch was poor. The driver's head also pressed up against the front plate.
Trials revealed that reliability was low, especially for the road wheels.
The IS-6 drove for 825 km by December 8th, 1944, 315 on cobblestone highways, 420 on dirt roads, 90 off-road. The average speed attained was 22-24 kph. The gearbox heated up to 90-105 degrees at high speeds. Considering that the trials took place in November-December, one can only imagine what would happen in the summer. 60-65 kg of effort was required to depress the main clutch, but it was easy to steer the tank. Overheating of the gearbox was a small problem compared to what was going on with the wheels. 14 road wheels were replaced over 825 km of driving, on average a wheel lasted for 200-300 km. Factory #100 developed a reinforced road wheel, but this was not a complete solution. The VG-50 generator also gave big problems. There were also complaints about the K-73 DC generator. The fuel tank capacity (480 L) was enough for only 100-120 km of driving.
Object 252 with a modernized front hull, November 1944.
Factory #100 OKB developed a modernized Object 252 hull in the second half of November. It had a new front developed by V.I. Tarotko nicknamed "pike nose". This did not only improve the effectiveness of the armour, but allowed the designers to move the driver's hatch to the top of the hull. The IS-2 received a similar hull, giving the IS-2U tank. As for the IS-6, it was sent to Moscow on Kotin's orders. Drafts of the IS-2U and modernized IS-6 were sent along with it, but this was not much help. Trials of the Kirovets-1 aka Object 703 aka IS-3 took place at the NIBT Proving Grounds in December of 1944. This tank was not as revolutionary as the IS-6, but it was much more reliable (especially when it came to the running gear). The tank's armour was also significantly improved. The GBTU and NKTP made the decision to keep developing the Kirovets-1, giving ChKZ's SKB-2 the IS-2U and IS-6 blueprints. This was the death of the IS-6.
Results of firing at the IS-6 turret made from 70L steel.
Another strike against the IS-6 was trials held against turrets produced at UZTM from 70L and 72L steel. Trials held in January of 1945 showed that the 150 mm thick side of turret #5a could be penetrated by the 88 mm Pak 43 gun in 7 cases out of 12 from different angles. The situation with turret #5b made from 72L steel was worse. After 5 hits it cracked in half. This meant that the turret had insufficient protection from the gun it was designed to withstand. What's worse, the IS-6 turret was provided to factory #183 as an example when developing the turret for the T-54 tank. It was redesigned along the lines of the IS-3 without even holding trials.
Trials of new road wheels and the 122 mm BL-13 gun on the Object 244, 1945.
Such a sad end did not mean that work on the IS-6 ceased. Various mechanisms of the tank were tested. For instance, a hydraulic servo drive for the planetary mechanism was tested in the winter-spring of 1945. Work on modernized road wheels also continued. Their diameter increased to 800 mm and the design was closer to the IS-2's wheels. The first wheels were installed on the Object 244 in December of 1944 and it drove for 305 km that month and 393 km more in January. The BL-13 gun also did not disappear. Work continued and it entered trials in 1945. These new elements were tested on the Object 244. Work on the IS-6 was at this point destined for another tank. On February 19th, 1945, Vovk complained about "Chambers of Secrets" at factory #100 once more. With Kotin's blessing, factory #100 was working on a tank indexed Object 257, the first tank carrying the name IS-7.
Cutaways of the Object 253. Interestingly enough, the first shows a 122 mm D-30 gun with a Ferdinand style muzzle brake.
The tank with the electromechanical transmission was lost in the problems of the Object 252. This tank was designated Object 253 and had M.I. Kreslavskiy appointed as its chief engineer. Work on the Object 253 began in October of 1944, but they were accompanied with a warning. The G-73 generators heated up to 119 degrees during trials. Work on the tank dragged on due to issues that came up with the Object 252. Claims that the tank was tested in October-November of 1944 are false. Factory #100 reported nothing about any trials in this period. No mention of the Object 253 was made in January or February reports either.
Object 253 after the end of trials. Object 252 is behind it.
Initially the only difference between the Object 253 and Object 252 was the electromechanical transmission. It was developed jointly with the Dynamo factory, who had a lot of experience from building the EKV tank. The DK-302A and DK-302B electric motors were descendants of the DK-301 series of motors used on the EKV. The layout of the transmission also had a lot to do with the EKV's transmission. The main advantage of the electromechanical transmission was the ease with which one could steer, but the cost was weight: the Object 253 weighed 54 tons. Considering the issues with road wheels on the Object 252 factory #100 decided to not take risks and just use IS-2 road wheels.
The tank with the electromechanical transmission had some external differences.
The history of the Object 253's construction and trials is shrouded in mystery. This tank was supposed to have been finished in the first quarter of 1945, but exact information is missing from correspondence. The reason for this was bad results attained during trials which were only revealed in the designers' memoirs. Kreslavskiy wrote that a fire broke out in the engine compartment after 10 km of driving and the tank became uncontrollable. Trials of the Object 253 stopped there. This is the most common version of events.
The upper rear plate changed when the tank was repaired.
The story of the Object 253 had an ending. The cause was work on the IS-7 tank. Several transmission variants were considered, including electromechanical. A test lab was needed for this transmission, and the unlucky tank was remembered in 1946. 1.5 million rubles was issued to run tests and continue development of the tank. The work was done by the Leningrad branch of factory #100. The same branch received elements of the transmission taken from the Maus tank. The Object 253 was restored by January of 1947 and driven for 34 km. Further trials showed some issues, for instance the tank would pull to the side. Trials of the tank ended in June of 1947, by which point it drove for 1025 km. Photographs of this tank known today were all made after the conclusion of these trials.
Both Objects 252 and 253 were scrapped. These intermediate prototypes were victims of their design defects, but were never seen as candidates for mass production by the GBTU. The middle of the road characteristics putting the tanks between the IS-2 and Object 701 meant that there was no interest from the army that could lead to mass production. The appearance of the Kirovets-1 in late 1944 meant that the IS-6 no longer had any chance at life.