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IS with a Heavy Gun

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The tank that went into production under the name IS-85 was only a temporary solution. By the time GKO decree #4043 "On the production of IS tanks" was signed, the military already considered the 85 mm D-5T gun insufficiently powerful. Experience in fighting new German tanks at Kursk confirmed this. Because of this, the IS-85, also known as IS-1, was produced in limited quantities of just over 100 units. The role of the main heavy tank of the Red Army in the concluding period of the war fell to the IS-122, also known as the IS-2. Interestingly, work on this tank began long before Kursk.

The result of unification

The history of the IS-2's development began with a gun, and a towed one at that. The design bureau of factory #8 presented a number of draft systems on the same mount in October of 1942. The guns used the mount of the M-30 howitzer. One of them mounted the 122 mm A-19 corps gun barrel. The result was rather light and compact. Various variants were proposed, including one with a barrel shortened by 800 mm. All variants were equipped with a two-chamber muzzle brake.

A modified A-19 gun barrel on the M-30 mount. October 1942.

A month later, in November of 1942, factory #8 was split into two organizations. The design groups headed by F.F. Petrov, responsible for the aforementioned projects, continued their work at factory #9. In February, work on a new experimental 122 mm gun called the D-2 began. This gun was not put into production, but work continued until 1945. Unlike the initial project, the D-2 had a monobloc barrel. The length of the barrel was a little less than that of the A-19: 5520 mm vs 5650.

A similar unification took place in tank gun design. Work on the D-5 gun began in April of 1943. Factory #9's design bureau ensured that barrels with different calibers could be used. The highest priority was assigned to the 85 mm barrel with the ballistics of the 52-K AA gun and 122 mm barrel with ballistics of the M-30 howitzer. Nevertheless, Petrov and his team allowed the mount to support more powerful weapons. The 107 mm M-60, 122 mm A-19, and 152 mm M-10 were among the candidates for installation.

A hit from the A-19 shifted the Tiger's turret by half a meter.

The 107 mm caliber was discarded fairly quickly. No experimental work was performed. The idea to use the M-10 barrel later developed into the D-15 SPG, but it was never built. The A-19 progressed along a different path.

Trials of the German Tiger tank showed that the A-19 was quite a powerful weapon. The first hit to the turret from a range of 1500 m tore off a 580x230 mm chunk of armour. The turret was torn from its turret ring and pushed back by 540 mm.

GKO decree #3290 "On return of 122 mm 1931/37 corps guns into production and production of light corps guns" was signed on May 5th, 193. This was the start of the development of the BS-3, technically a field gun, but in reality an anti-tank gun. 122 mm D-2, M5 (designed by factory #172) and S-4 (designed by the TsAKB) were also designed to combat tanks.

Finally, in May-June of 1943 the design bureau of factory #9 began working on a tank gun with the ballistics of the 122 mm A-19. In order words, the results of the Battle of Kursk only gave an additional push to an existing program.

A draft of installation of a 122 mm tank gun into an IS tank. Kotin signed this document on July 17th, 1943, Shashmurin and others signed earlier, on July 14th.

The draft project titled "installation of a 122 mm tank gun into an IS tank turret" was prepared at factory #9 jointly with factory #100's design bureau by mid-July 1943. Work was done entirely on the factory's initiative. Kotin curated the work from the side of Chelyabinsk factory, Shashmurin acted as the chief engineer.

The gun had no name yet. Experience obtained during the D-2 project was extensively used in this project. This was one of the reasons why the tank gun had a different barrel length than the A-19. Like the D-2, the tank gun had a monobloc barrel. A massive T-shaped muzzle brake was installed to reduce recoil. The recoil resistance was similar to that of the initial D-5T, which was 900 kg lighter than the 122 mm gun. Even though the new system was much larger and heavier, the 550 mm recoil length allowed it to fit inside the IS tank turret freely.

Diagram of the first type of muzzle brake.

Even the GBTU was not yet aware of this project. A report made on July 20th, 1943, by S.A. Afonin mentions nothing about collaborative projects between factory #9 and #100. The document merely proposes the installation of a 100-122 mm gun into the IS tank in order to allow it to reliably penetrate the front armour of the Ferdinand tank. Only the work of the Central Artillery Design Bureau (TsAKB) was mentioned in connection to this, which was just starting work on the S-34. However, by mid-September work at TsAKB was progressing sluggishly.

The work at factory #9 and #100 was going much faster. By mid-September their project was officially approved. The system received the index D-25. The harsh deadlines set by decree #4043 reflected the situation of the time. By September 4th the draft project was coming to completion. All that was left was to build and test the gun.

Experimental Object 240 tank, Chelyabinsk, October 1943.

The tank, indexed Object 240, was not built from scratch. The second Object 237 prototype was used. However, a small delay happened with the conversion, as Stalin arrived to see the tank on September 8th. The experimental D-25 was expected on the 13th.

The modernization of the system was already being considered. The weakest link of the A-19 system was its low rate of fire (3-4 RPM), due to, in part, two-piece ammunition. The rate of fire would be even lower in a tank's turret. The issue of installing a sliding breech was raised in early September. Factory #9 agreed with the proposal, but it would take time. The Artillery Committee required two options: a vertical or a horizontal sliding breech. Lieutenant-General Hohlov proposed using a breech from the 152 mm NG howitzer, a Soviet variant of the 15 cm sFH 18. In any case, no progress was expected until October 15th.

Another solution could be mechanization of the loading process. TsKB-19, a developer of railway artillery systems, was tasked with this project, but work did not move past the discussion phase.

View from the front showing the first iteration of the muzzle brake.

The first experimental D-25 gun was ready by September 15th, 1943. The gun went through factory trials from the 16th to the 23rd, making 96 shots. The gun passed trials, but the muzzle brake did not. Engineer-Colonel Abramov, the military representative at factory #9, wrote that the muzzle brake deformed during firing. The gun was sent to Chelyabinsk on September 25th, and factory #9 began working on a new muzzle brake.

The installation of the new gun did not require many changes, and the outward appearance of the tank did not change much.

The installation of the D-25 was complete by September 30th, 1943. The rear machinegun was removed, but the mount remained in place. The weight of the tank increased to 45.5-46 tons, which was expected to reduce its mobility. According to calculations, the top speed of 37 kph would drop to 32.5 kph. The ammunition capacity also decreased, to 28 shots. The exhaust fan in the turret was replaced with a more powerful one to deal with a more powerful gun.

Another angle that shows the muzzle brake.

Factory trials took place after installation of the gun. The tank drove to the factory shooting range and back, 13 km in all. The firing trials almost ended in tragedy. The deformed muzzle brake was torn apart during firing. One of the fragments nearly killed Petr Voroshilov, Kliment Voroshilov's adopted son and an influential figure in the creation of KV and IS tanks. A pause in the firing trials followed. This was not the first such event for Petr Klimentyevich. Some time prior, the cap from KV-2 U-4's barrel flew past his head as well.

Increase in quality

Government trials followed factory ones. A commission led by Afonin, who by this point was a Lieutenant General of the Tank Engineering Service, arrived in Chelyabinsk. In addition to him and the aforementioned Petr Voroshilov, the commission consisted of Kotin, director of ChKZ Zaltsmann, head of the factory #100 special design bureau Yermolayev, representatives of the NKTP and GBTU.

These trials were a continuation of the first stage that led to the acceptance of the Object 237 into service. By the time they began, Object 240, formerly Object 237 #2, had travelled for 355 km. The first prototype, which had travelled for 750 km, also took part in the trials.

Interestingly enough, the first prototype was referred to as "IS" in the documents, the second as "IS-3". This was the second time this index was used to refer to IS family tanks.

In addition to the new gun, the Object 240 can be distinguished by a lack of rear machinegun.

This time the trials were rather unusual. The first step was a drive from Chelyabinsk to Zlatoust and back. The march began on October 1st from the gates of factory #100 and ended on the 4th in the same place. The tanks drove for 345 km, of those 178 on a highway. The average movement speed of the Object 240 was 18.1 kph, and 21.5 kph for Object 237. The average speed was the opposite: 10.7 kph for Object 237 and 15.4 kph for Object 240. The first tank stopped many times due to various breakdowns.

One of the two routes that Objects 237 and 240 travelled together.

Object 237 destroyed a wooden bridge near the Syrostan village. Object 240 had to work as a tow truck. The engine of the stuck tank would not start, and the second tank had to pull it off the foundation of the bridge and rotate it to be parallel to the river. The tank started up and pulled itself out of the trap.

The second unexpected situation with Object 237 took place near Kundrovy, where the tank became stuck in a ditch. Due to slipping tracks, the tank could not climb out on its own power, and Object 240 had to come to its rescue. Both tanks became stuck in the ditch, but got out with the help of logs tied to their tracks.

Object 240 as a tow truck.

The second stage of trials, the march to Sineglazovo and back, took place on October 6th. This time only 10 km of the total of 111 km were on a highway. The tanks also drove for 20 km on dirt roads. The rest of the march took place off-road. This time Object 240 performed more poorly from a technical standpoint. However, the trials showed that the fears of reduction in mobility were unfounded. The average speed of both tanks was the same on a dirt road, and their reliability surpassed that of the KV-1S. The designers achieved their goals. Despite more powerful armament, the reliability of the tank did not decrease, and the mobility was about the same.

Trials of the Object 240 continued. In October it drove for 1263 km, 714 on a highway, 486 on dirt roads, and 81 off-road.

Using an unditching log.

While the tank was going through trials in Chelyabinsk, the issue with the muzzle brake was being solved in Sverdlovsk. The solution was based on a German design. Taking the muzzle brake from the 8.8 cm Pak 43 on the Ferdinand as a basis, Soviet engineers designed a new muzzle brake. It was called, predictably, "Ferdinand type". On October 10th the new muzzle brake was installed on the D-2 gun and performed trials over 30 shots. The new muzzle brake proved more reliable. Object 240 was sent to the Gorohovets Artillery Proving grounds on October 11th, and the new muzzle brake was rushed from Sverdlovsk directly to Mulino.

Diagram of the "Ferdinand type" muzzle brake.

The Object 240 arrived at the Gorohovets proving grounds on October 18th, 1943. The muzzle brake arrived on the next day.

The proving grounds report states that the D-25T consists of a shortened barrel of the A-19 on a D-5T barrel. That is incorrect. As mentioned above, the D-25 used the D-2 barrel. That is where the name of the gun came from: D-25 stands for D-2 gun on the D-5 gun cradle. The D-15 got its name in the same way: D-1 gun on the D-5 cradle. The ammunition, ballistics, and screw breech were the same as the A-19, but the tube was different.

Interestingly enough, the measurements for the full length of the barrel differ. The instructions say that the barrel was 5850 mm long, the report says 5924. The difference without the muzzle brake measured 130 mm, or just over a caliber.

Object 240 at the Gorohovets proving grounds, October 1943.

Due to the rush, the initial program plans had to change. Only 3 days were allotted for trials. To speed the process up, the propellant charges were picked out on the 16th. On October 18th the gun was taken apart and measured. Firing began on the next day. The gun was not new and had already fired 172 shots, most of them while the barrel was still in use on the D-2. 400 shots were fired between October 19th and October 21st. Firing was performed with regular and supercharged propellant, using AP and HE ammunition. It turned out that the muzzle velocity was 2.2-2.65% lower than on the A-19. This had to do with both the shorter barrel and the fact that it was used. The muzzle velocity decreased by 1% by the end of the trials, which was within norm. The number of shots fired from the new gun later reached 474: 214 using supercharged propellant, 213 using full charge, 47 using the third charge.

The same tank using the "Ferdinand type" muzzle brake. Due to the rush, it was not properly painted.

The gun showed itself well during trials, with the exception of the muzzle brake. A crack formed during the trials, but unlike the first type of muzzle brake, it did not break. The commission gave an order to improve the design.

Trials showed a rate of fire of 1-2 RPM depending on the position of the gun. The precision was high, no worse than that of the A-19. The recoil length was within calculated norms. Separate trials showed that ventilation was very effective.

Complaints were made regarding the precision when firing HE ammunition. There were also issues when firing on the move. However, considering the length and mass of the barrel, this could be expected. The aiming mechanisms were criticized. It took up to 25 kg of effort to turn the turret, and the turns were jerky. The commission also didn't like the position of the sight, as it was too close to the gun.

Cracked muzzle brake.

The fighting compartment was compared to that of the Object 237, which was studied at the proving grounds two months prior. The room in the fighting compartment decreased due to the larger gun. It was no longer possible to get around the gun from behind. The loader's work was made more difficult by the shortening of the space from the brass catcher to the rear of the turret ring from 450 mm to 200 mm. The shell and propellant fouled the rear rack, and additional movements had to be made to avoid that.

The sight was also somewhat offset during the loading. The screw breech was difficult to operate. It opened to the right, which further complicated the loader's job. The racks on the floor were also inconvenient. In trails held on October 22nd, even a trained crew did not attain a rate of fire of over 2-3 RPM. That was the cost of a more powerful gun.

Diagram of the fighting compartment.

Despite all complaints and issues, the commission concluded that the D-25 gun passed trials. Of course, changed had to be made to both the gun and the tank. However, the characteristics of the tank were not severely affected by the installation of a new gun, as was feared initially. Despite all of its drawbacks, the gun showed itself very well. The precision of the new gun was comparable with that of other tank guns.

The new gun could effectively combat the newest German tanks, primarily the Panther. It was already known by the fall of 1943 that the 85 mm gun could not penetrate the upper glacis of the new German tank. The D-25T had no problem with the Panther's armour out to ranges of over 1.5 km, which practical trials showed in December of 1943. The Ferdinand was also vulnerable at 800-1000 meters.

GKO decree #4479ss "On the heavy IS-2 tank with a 122 mm gun"

Successful trails were a trigger to launch the Object 240 into production. Stalin signed GKO decree #4479ss "On the heavy IS-2 tank with a 122 mm gun" on October 31st, 1943. According to this decree, production of the first tanks of this type would begin in December. As for Object 240, this tank continued trials. Like Object 237, it underwent various metamorphoses. 


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