Saturday 16 March 2019

IS with a Heavy Gun

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 double baffle 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, 1943. 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 destroyer. 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. 


  1. Not to beat a dead horse, but I have been engaged in internet debates recently on German armor quality, and despite oodles of pics not only on Soviet testing, but actual knocked-out Soviet tanks (T-34 and IS and KV alike) and I can't think of any examples where you see such huge jagged holes and cracks from catastrophic failure like you see in the Tiger picture you show--even when the Soviet tank has been hit dozens of times. And mind you, it's not just in gunnery test photos, but in photos of the results of actual combat.

    Did the Germans put their tanks through anything like the performance trials Soviet tanks were put through? I am thinking of the first engagements of the Panther and Tiger II, where there were many mechanical failures. You'd think they'd have shown up in any trials like you described.

    1. They certainly used to, looking at the articles on the developement of the older kit on this site, which is no doubt a major reason why the pre- and early-war designs remained such reliable warhorses until the end. But there wasn't an awful lot they could do about the raw-material shortages and after around '42 the increasingly desperate war situation, and ever more Executive Meddling by Der Führer who was rapidly misplacing whatever marbles he now had possessed, obliged more and more cutting of corners in the entire process.
      Which was quite unfortunate given the rapidly inflating mass of AFVs in general during the war made all the worse by Hitler's growing infatuation with "corrupt gigantism" - and perhaps more excusably the need to rely on thicker armour plate to try and make up for shortage-related metallurgical weaknesses - with due greater mechanical stresses...

    2. Hitler's infatuation with 'corrupt gigantism' was part of a focus on impractical solutions, I agree, but I also think that there was a culture of corruption, self-promotion, and dishonesty der Fuhrer encouraged (he encouraged conflict between underlings rather than cooperation, 'let the best man win', he appointed people of dubious character and competency to key positions, his organizational tree had duplication of effort and crossed responsibilities, etc). Dividing up power and letting subordinates duke it out and scheme against each other might be a way to keep them (as potential candidates to replace you) weak, but it's not a very efficient way to run a country.

      Interestingly enough, I just saw today that there was a system of bribery that the Third Reich set up for senior military leaders. Almost every key military leader was getting these bribes (von Bock, Halter, Guderian, von Rundstedt, Model, and (possibly) Rommel. I had not reard that before.

    3. In all fairness most of those charges can be - and are - leveled against Stalin's USSR too (eg. pitting Konev and Zhukov in a race to capture Berlin); the important difference being that whatever his other flaws Uncle Joe wasn't *delusional* and knew where to draw the line so he didn't completely screw over himself and his empire.

      Playing underlings off against each other is really more or less par for the course for authoritarian regimes of all stripes, as those always need to worry about coups and routinely have to cope with varyingly shaky legitimacy. It rather comes with the territory and the question is more whether they can strike a working balance between the needs of staying in power and actually running the realm.

    4. Perhaps we should be more forgiving of corruption in Third Reich corporate circles. Anyone not involved in producing new military equipment was sent to the front line. By 1943 the Reich was already depleting it's supply of 18 through 32 year old men and was starting to draft anyone with a limp who wasn't vital to the war effort.

    5. Kellomies--true enough about Stalin and Hitler, but then "why was Hitler 'delusional'? My answer is that the Soviet underlings were (at least during the war) more apt to give him accurate information than were Hitler's. The culture of lying in Nazi Germany meant that all underlings tried to present the best picture (including what a 'great job' they were doing) to der Fuhrer.

      In the end Hitler is moving around armies that exist mostly on paper, but that's largely because, I believe, that his subordinates had both over-claimed their success and hid their failures (especially losses) to promote themselves, and so Hitler, not unexpectedly, thought these armies still had lots of men, guns, and tanks. So when those paper armies failed to achieve what he (not unreasonably) thought they could achieve (if indeed they had what he had been led to believe they had) then he lashed out at his military commanders, suspecting them of either cowardice, incompetence, or treason, or all three.

    6. That's a bit of a "chicken and egg" issue, but Hitler was certainly delusional from the get go - do recall that the fundamental reasons he started the whole war in the first place stemmed from lurid fantasies of an imminent racial apocalypse...

      The German military had a dubious track record about honesty as it was - see: the rank denialism and metric ton of never-my-fault excuses they spun about losing the *previous* war - but answering to a legitimate paranoid schizophrenic whose entire guiding ideology was built on voluntarist irrationalism can't have helped one bit. All the more so when that loon both held de facto absolute power and was capricious enough to fire commanders left, right and center if they offended his whims somehow.

    7. To be truthful, Kellomies, you have to remind yourself that Hitler never won any election--his best result was < 33 % (36.8 %, in a runoff) in anything like a free and fair election, and in 1933 even with the SA running the polls (!!) he got only 43.8 %. In modern parlance, his 'negatives' were too high.

      No, Hitler got into power because he was *invited* by the conservative government, which had been using Article 48 of the Weimar Constitution to govern the country in an emergency without Reichstag approval, and they figured that 1) they could use Hitler to boost their popular support, and 2) Hitler was a boorish, loudmouth, simpleton who they could control, intimidate, coerce, or dupe into doing what they wanted (Von Papen: "I'll have that little corporal so tight into a corner he'll squeak").
      But also, importantly 3) they didn't have a problem with much of Hitler's agenda. They too were for rearming Germany. They too were for expanding Germany's borders. They too were for "Lebensraum" (the term first started in WWI, not WWII). The German military, civil service, and judiciary that the Weimar republic inherited (and was not able to replace) was always hostile to the republic and was ready to welcome someone like Hitler.

      Of course, Hitler proved he was more cunning, more ruthless, and smarter than they had bargained for. It didn't take a year for him to reduce them to a state that they had never been under any Kaiser--a group of subservient, groveling, submissive toadies.

      But I say this that Hitler would have never come to power if his vision wasn't shared, partially at least, by a lot of those in the German military (this is not to say that "most Germans were Nazi; because in fact Hitler could never win a majority of their votes; rather to say "many important influential Germans in high places were ok with Hitler"). This is why in WWII, we did what we should have done in WWI--we fought to unconditional surrender, moreover we made the military sign the surrender papers (no 'the politicians lost the war' excuse) and then we went in and replaced (if not put on trial) most of the military, the judiciary, and the civil service.

    8. Let's not blame just Hitler for the beliefs of a racial apocalypse. The whole world was moving into a secular shift and the academic world itself was teaching students that that modern society was preventing the biologically week from dying out and thus passing on inferior genes.Furthermore it seemed logical that the different races like different animal species evolved at different rates and was supposed to fight it out to the death. Hitler was just smart enough to turn this into state policy as opposed to just something college professors ranted about. And as Stewart pointed out, those in the military and public looked forward to a larger German Republic.

  2. I doubt the A-19 could hit the turret on the first shot from 1500m. So to assure a hit they must have fired from a short range with a reduced charge shell. The 1931 1938 A19 firing tables were in error. Some time ago Peter posted here a post WWII revised and corrected firing table for the BR-471 D-25. Comparing the two firing tables it looks like what they thought was the striking velocity at 1500 meters was actually for about 700 meters.

    1. Not likely hitting on the first shot @1500 m is also true of the Kwk43. The "% to hit" value of 61 % @ 1500m cited by Jentz is what a 'calm gunner' who had already fired one shot and observed its tragectory could achieve.

    2. If we discuss about D-25T projectile trajectory- on one discussion about this gun on Polish forum, I found that D-25T point blank range for 3 meter tall targer (BR-471 ammo) that's 1130 meters. I think that D-25T trajectory it's good enought for typical combat ranges.

  3. @Stewart Millen, that 61% is arrived at by doubling the KwK 43 natural shell deviation. This incorrectly assumes the guns deviation error would be off by the same degree in both planes. When it is primarily the vertical error that is affected by ranging errors.

    @AKMS, You are correct that at 1130 meters the median deviation of the D-25 is 3 meters. This means that 50% of the shots will fall within 3 meters of the point of aim. If the point of aim is the center a 3 meter high tank the majority of the shots will be too higher or too low.

    Scattering of shells and its causes

    1. I don't write about deviation. I use term "point blank range", for maximumum distance, when maximum height of projectile trajectory don't be bigger than target height. In this case "point blank range" depend from flatness of trajectory and height of target. Ono drawing from my blog- this is point blank range in my terminology:

      On drawing we have 700 meters point blank range (maximum point blank range) for target which have height of standing soldier.

    2. OK, maximum height is 3 meters at 1130 meters and median deviation is 0.3 meters (not 3).

  4. So did the IS-2 always have the D-25T cannon or did the early models use the A-19? I was under the impression that only 1944 vehicle were equiped with the D-25T. The ISU-122 was equiped with A-19 and only in April 1944 was the D-25S used together with the ISU-122S.

    1. The IS-2 always had the D-25, the ISU-122 had the A-19 and then D-25S. The A-19 could be easily installed into the ISU-152, since it shared the mount with the ML-20, but the same trick couldn't be pulled with the IS-2.

    2. What about this article:
      Here it says:
      "1. Due to the outlines disadvantages of the 122 mm gun (two-piece ammunition, screw breech) (...)"
      I also remember reading a few article that claim that the RoF was improved on later version on the IS-2, which would indicate that the early version used the A-19 with interrupted screw breech which lowered the RoF.

    3. Early D-25s with the German style muzzle brake used a screw breech. Later D-25 with the domestic muzzle brake used a sliding breech. Both versions were called D-25.

    4. Was it just D-25 or D-25T? :)
      Also was this gun used in combat or was it only used during field trials?

    5. D-25T was the tank variant, D-25S was the SPG variant. I don't think there was ever a D-25S with a screw breech, but I could be wrong.

      The only type of D-25 to not be used in combat was the initial one with the T-shaped muzzle brake.