Monday 20 September 2021

Long Living IS

Production of the IS-2 ended in June of 1945. The IS-3 replaced it in production. The IS-3 was supposed to replace it in service as well, and many considered the IS-3 to be a temporary tank as well. However, plans and reality are often different. Like its successor, the IS-2 stuck around for decades after the end of its production, undergoing two waves of modernization, which will be covered in this article.

Correcting mistakes

There are several reasons why the IS-2 remained in service for a long time. For starters, it was the most numerous heavy tank in history (if you consider just one model). The tank also stayed in production for a year and a half. The IS-2 changed significantly in this time. Its armament and sights were improved, the hull was modernized, an AA machine gun was introduced. Tanks produced in early 1944 and in early 1945 were very different in terms of combat characteristics. The quality of production also increased, although a high production volume (250 units monthly) made this more difficult. Improvements were made as a result of combat experience. Finally, half of all IS-2s (1626 units) survived the war. This number is correct as of late 1950 and does not include tanks that were written off due to wear and tear or transferred to the Polish (71 units) and Czechoslovakian (7 units) armies.
About half of all IS-2 tanks remained in service by the early 1950s. Unlike the IS-3, these tanks were already broken in and there were few complaints about them.

One major difference between the IS-2 and IS-3 was that the IS-2s were delivered straight to end users, while the IS-3s were largely shipped to warehouses, which is why design defects took a long time to pop up. Had this happened in 1945 or early 1946, design changes could have been made in time, but history doesn't know the word "if". The technical condition of the IS-3 tank became one of the most important topics of discussion in the Ministry of Transport Machinebuilding (MinTransMash) in 1947. A number of production issues resulted in breakdowns of tanks in service.

An attempt to solve this problem quickly failed. On December 12th, 1950, the Council of Ministers of the USSR signed decree #4871-2121ss, ordering ChTZ and Kirov Factory to modernize the IS-3 tanks. This modernization was known as UKN (correction of design defects). It included many changed to the design, not just to correct issues, but to make improvements. For instance, a modernized DShKM turret was introduced. The process of upgrading IS-3 tanks to the UKN standard began in 1951 and was completed in 1954. This tank is often called IS-3M, but that is a mistake. UKN-modernized IS-3s were still called IS-3. Unlike the latter modernization, the tank kept its V-11 engine. Suspension elements characteristic of the IS-3M began to appear later.
A Polish IS-2 tank that went through the second wave of modernizations in the late 1940s. Splash boards were added to tanks with a straightened front around this time.

Issues with the IS-3 took attention away from what was happening with the IS-2. Everything was not quite simple here either. While there were many IS-3s with defects, it was not so bad in comparison. About 300 tanks were modernized, judging by the serial numbers these were mostly 1945 production models. These tanks were not the most reliable to begin with, especially compared to 1946 production. 300 is a lot of tanks, but not that many considering the issues ChKZ had to endure in 1945 and the overall production volume. The claim that the IS-2 was much more reliable than the IS-3 is incorrect. The IS-2 had many issues of its own, for instance the track links. The stamped KV-1S-35-9 track links with alternating guide horns had a lifespan of about 800-1200 km. Even though ChKZ tried to replace it with the cast 237-35-15 track links, they were still issued until the end of the war. The same track links migrated to the IS-3. Both alternating guide horns and guide horns on every link can be encountered. There were also issues with cracks on turrets to the point where factory #200 produced new turrets for repair factories to install. The same gearbox problems experienced on the IS-3 were seen here, there were also issues with the main clutch and final drives. This was all routine maintenance for front line troops, and so there was no noise about it like with the IS-3. IS-2 tanks were also sent to repair factories rather than the original manufacturer for repairs or modernization, so only echoes of issues reached the MinTransMash. 
The IS-2 tanks could have both KV-1S-35-9 stamped track links and 237-35-15 cast track links at the same time in the second half of the 1940s. The latter are easy to distinguish by scallops in the sides.

Even so, the IS-2 slowly changed as well. This was mostly applicable to early model tanks. They gradually received DShKM machine guns and hardpoints to stow the machine gun during transit. In tanks produced before July of 1944, spare track links were moved from the rear of the hull to the front. Track links also changed. The Kirov factory in Leningrad mastered production of 237-35-15 cast track links. These can be distinguished by scallops along the sides. The lifespan of stamped track links produced at ChKZ also grew, and so these track links existed in parallel. A third type of track link was introduced in the second half of the 1940s. It is not known who produced it, but it wasn't ChKZ. This track link was first tested on the ISU-122 in the summer of 1944. These track links can often be seen on vehicles that went through one or two waves of modernization. Based on the stamps they were not Soviet in origin. This issue is not yet fully explored, but it's reasonable to guess that they could have been made in Poland. Early headlights were also replaced with the FG-10 and early turret handrails (typical of factory #200 production) were replaced with smaller ones.
IS-2 tanks on parade in Beijing, 1959. These tanks were delivered straight from Soviet storage facilities and were only modernized in China. They received DShKM machine guns but no splash boards.

These changes were made to the IS-2 tanks that ended up in China. 60 tanks were transferred in 1950, which became a big issue. The Chinese army received tanks from warehouses that were produced at various times. These including seasoned veterans from WW2 that saw events like the Lvov-Sandomierz Offensive. The tanks needed immediate repairs, as did the ISU SPGs that were given to China. 50 repair kits were sent to China in 1951. The tanks that survive in Chinese museums have signs of the first wave of modernization, but with some elements of the second wave (the ChKZ design bureau worked on it from 1948 to 1951, but it was implemented later).
DDR IS-2s. These tanks have new track links, FG-12 headlights, and new turret handrails. Not all tanks have AA machine guns.

The second wave of IS-2 modernizations began after the UKN program for IS-3 tanks was launched. Since a number of IS-2 and IS-3 parts were compatible, a program to improve the IS-2s also took place in 1953-1954. This mostly applied to internal components: gearboxes, main clutches, final drives, controls, etc. A number of components and assemblies in the driver's compartment changed, including the driver's seat. Drive sprockets (703-16-Sb.1), idlers (703-31-Sb.1), and suspensions (703-33-Sb.101) including swing arms were taken from the IS-3 tank. An extra 16-20 mm thick plate was added to the floor under the gearbox. These changes were made locally and were not universal, for instance some vehicles did not get the plate under their gearbox.
Some tanks received new track links after the war. Their manufacturer is not yet known, perhaps they were made in Poland.

Running gear changes were not major. In part, the road wheels received new bearings. A different road wheel design was later introduced that can be distinguished by hub caps attached with 10 bolts. This change was a direct consequence of the acceptance of the T-10 tank into service. A new drive sprocket cleaner (703-30-Sb.101) and gun travel clamp (703-51-Sb.109) were gradually introduced. Tracks were replaced with ones made from 703-35-Sb.1 and Sb.2 links. Both types had guide horns on every track link and were made from TVM steel. These track links were similar to the cast 237-35-15 type. The improved AA machine gun mount was also taken from the IS-3 UKN. The percentage of components with 703 blueprint codes kept increasing.
German IS-2s received new road wheels and drive sprocket cleaners from the IS-3.

Tanks received by the German Democratic Republic's army were modernized in this way. 47 tanks were sent there in total. Like the Chinese IS-2s, they were produced in different years. Like the Chinese tanks, these tanks had their running gear modernized. New road wheels and drive sprocket cleaners were introduced. One of these German IS-2s used to be owned by the Overloon museum in the Netherlands and is now in the US. This tank was built in March of 1944 and went through two and a half waves of modernization. Unlike Soviet IS-2s, these foreign IS-2s escaped the big modernization that took place in the late 1950s and noticeably changed the look of the tank.

Big modernization

The UKN program was just the first step. It improved the tanks' reliability, but their combat characteristics remained the same. Also, production of the superior T-10 heavy tank began in 1953. The T-10 itself did not sit still. The TVN-1 night vision device was introduced that could be installed instead of the driver's periscope. The lights changed as a result, now tanks had FG-101 and FG-102 lights. The IS-3 also had issues of its own. The V-11 was more reliable by the mid-1950s, but even the newest engines were getting to 10 years of service. The V-2-IS engines on IS-2 tanks and ISU SPGs were no younger. The issue of extending the lifespan of IS-2 and IS-3 tanks was a pressing one. No one was saying "don't worry, new tanks will come soon" like they were in 1945-1948. The T-10 was being produced much slower than its predecessors. Foreign heavy tanks were produced in even smaller amounts, but no one in the USSR knew that. The Cold War was in full swing, and some heavy tanks were located in the West, including the Soviet forces in Germany. One has to fight with what they have, rather than with what they want. The idea of a deeper modernization than just the replacement of parts compatible with the IS-3 came up.
A January 1944 production IS-2 modernized to the IS-2M standard. This tank is on display at Patriot Park.

The GBTU made the decision to modernize the IS-2 tank, which incidentally became the least reliable tank in service after the IS-3 modernization. This time the goal was not to just make the tank more reliable. The IS-2 had to be converted to take new devices, including the novelties introduced for the T-10. The T-10 was evolving quickly: mass production of the T-10A began in 1956, then the T-10B a year later, and the T-10M, its most numerous and superior variant, was produced starting in 1958. Experience in developing the T-10 was important, but the T-10 had a number of brand new components, especially in the engine and transmission compartment. The closest relative of the V-11 and V-2-IS at the time was the V-54. It had the same horsepower, but higher reliability.
The same tank viewed from above. The unditching log became an authorized item of stowage. It was carried on the right side of the hull.

Unlike the first wave of modernizations carried out by the ChKZ design bureau, the large modernization was developed at repair factories #7 (Kiev), #17 (Lvov), and #120 (army unit 48819, Kirchmöser, Germany). These factories were the main contractors for all subsequent work. Modernization was first tested on prototypes in the Soviet forces in Germany, after which it was applied to other IS-2 tanks. The modernization continued until 1959 and touched most tanks of this type in use by the Soviet Army. Only a handful of vehicles that were already in museums escaped this modernization, for instance the IS-2 tank in Sevastopol was modernized in the first wave, but not in the big modernization of the second wave.
The IS-2 received new fenders and toolboxes. Different factories had different lids for the toolboxes as well as different headlight guards.

It's easy to identify the tank that was officially indexed IS-2M starting with 1960. The fenders and toolboxes along the superstructure sides are the easiest way. An additional oil tank was placed on the left fender. The towing shackles were moved to the right. The box for the MZA-3 fuel pump was also added then. Clips for a pickaxe were added on the left toolbox and mounts for an unditching log were added to the right. The changes to the fenders and toolboxes were introduced thanks to experience with the IS-2 UKN. Differences in vehicles modernized at different factories are partially linked to this phenomenon. The IS-3 was modernized at both the ChTZ and Kirov factory, as a result of which some parts are different. For instance, the ChTZ toolbox lids have characteristic stampings on them, but the Leningrad tanks do not. The same thing happened with the IS-2M.

The second most noticeable change was the headlights. There were now two of them: the FG-100 on the left and FG-102 on the right. The headlights received guards that were different depending on the factory. IS-2M tanks with Chelyabinsk style toolboxes had headlight guards made from thinner wire that looked similar to those used on the T-10M. These distinctions were not systemic, and there were exceptions, same as with the IS-3.
The driver's visibility improved thanks to the introduction of an observation periscope.

The use of FG-100 and FG-102 headlights meant that the driver had a night vision device. It was built in as a periscope installed in a slot in the upper front plate, which was covered with a plug if necessary. Another periscope could be inserted in the daytime that resulted in much better vision. Both devices were unified with those used on the T-54 (Object 137), which was an advantage. This device was used on tanks with the straightened front. Visibility was not as much of an issue on tanks with the stepped front, and at night they could use the BVN-1 night vision device. Tanks with the straightened front also received splash boards as a part of this modernization. Running lights were also changed. A number of tanks kept the original ones, but usually more modern GST-49 lights were installed. Their positions also changed. The rear lights had new mounts and the front ones were moved forward.
Many changes were made to the floor in order to improve reliability of components.

Many changes were made to the rear of the tanks. The tanks that did not yet have a new travel clamp received one at this point. BDSh-5 smoke bombs with electric triggers were installed instead of the naval MDSh. Mounts for spare track links were also added on the lower rear plate. The track links did not have to be IS-3 UKN ones. Many tanks received 720 mm wide 730-35-1 track links from the T-10. They could easily be installed on the IS-2 or ISU, which reduced the ground pressure. This was an important thing to consider, as the IS-2M grew heavier to 47.5-48.5 tons. Additional fuel tanks were now directly linked to the fuel system, which improved the cruising range without manually refilling the main tanks. Small changes were added to the floor of the tank. A cutout was made under the gearbox and covered with an extra plate to increase the gap between the gearbox and floor. A socket for starting the engine with an external power source was added. Latches for locking the hatches in open position were also added.
Carriers for BDSh-5 smoke bombs were installed along with a new travel clamp and new spare track links. Note that an extra ventilation fan was added instead of a rear machine gun.

The external changes are nothing compared to what was going on inside. The V-2-IS (V2-10) engine was replaced with the V-54K-IS (the water pump was different than the one on the V-54). The position of the oil pump changed to make up for the difference in height. VTI-2 air filters replaced the VT-5. The front column of the gearbox frame was replaced, which improved the rigidity of the floor (a solution borrowed from the IS-3 UKN). The Kirov factory suggested ejection cooling that was used on the ISU-152K for the IS-2, but it was not used on the IS-2M. The cooling fan casing was improved alongside the gearbox column. The engine bed was strengthened, which improved the reliability of the engine mounting.
The running gear changed drastically, almost all elements were replaced.

The engine heating system was improved gradually. A heating system with thermal siphon circulation and a heater from the IS-3 UKN was used as of 1958. Starting with May of 1959 a system with forced fluid circulation and NIKS-1 heater was used instead. The cooling system had to change as a result of the new engine and lubrication system. A new expansion tank was added. The manual regulator was replaced with a regulation valve with a new MZN-2 electric oil pump. The electric equipment of the tank changed drastically, and the number of batteries increased to 4.
The external fuel tanks were linked to the fuel system.

Changes were made to the fighting compartment too. Ammunition capacity increased from 28 to 35 rounds thanks to optimization of the fighting compartment. The rear machine gun was removed as pointless (if anything, it got in the commander's way). To make better use of the space, a second ventilation fan was added. The turret traverse lock was replaced with the one from the T-54. The D-25T gun received an elevation mechanism with a recoil tension spring to prevent damage. The radio was replaced with the R-113 and the TPU-4bis-F intercom was replaced with the TPU-R-120. This resulted in some external changes, for instance the antenna port. The TPK-1 commander's periscope replaced the commander's MK-IV. Hooks for a tarp were added to the rear. The AA machine gun mount changed once more, now it was the same as the one on the T-54. There is also a claim that the new tanks received 100 mm D-10T guns. This looks like intentional disinformation that many researchers took for fact. Indeed, one such tank can be seen in Kiev, but that is a result of a high ranking officer performing inhumane experiments in shoving weapons into tanks where they don't belong. The IS-2M with a D-10T tank was only good as a museum exhibit.
IS-2M tanks in Cuba. This was the modernized tanks' first appearance.

The overall result of the major modernization in 1957-1959 was a drastic increase in reliability. Interestingly enough, the tanks were sent abroad almost instantly. 41 tanks of this type were sent to Cuba in 1961. Castro was the only operator of heavy tanks in the region aside from the US. Neither these tanks nor the IS-2M in general ever saw battle, but this is an interesting bit of historical trivia nevertheless.

Long Living Veteran

Work on improving the tank's fighting capabilities went on in parallel with the IS-2M modernization. Experiments with underwater driving finally progressed into mass production. This happened in 1957 when starting in September T-54B tanks were equipped with OPVT underwater driving gear. Heavy tanks received similar equipment. Council of Ministers decree #250-122 signed on March 3rd, 1958, instructed ChTZ to develop OPVT equipment for the IS-2 and IS-3.
Underwater driving equipment developed for the IS-2 in 1958.

Documentation was developed in the second half of June 1958. This work was led by D.F. Skvortsov. Interestingly enough, it was designed for IS-2 tanks that had not yet had the thorough modernization. According to documents, the engine compartment was sealed with a special pump, other compartments were made watertight as well. A cover was installed on the rear machine gun, gun mount, main gun, and commander's cupola. The air intake pipe was attached to the turret fan, which was equipped with a special "skirt" for that purpose. Changes and additions were made to six groups of components.
The IS-2M tank currently displayed at the Central Museum of the Armed Forces was equipped with the production variant of the underwater driving equipment.

Experimental prototypes were produced, but on September 20th, 1958, Council of Ministers decree #3099 cancelled the order. This is where the story usually ends, but work on OPVT gear for the IS-2M and IS-3M resumed in the early 1960s. Development took place in 1963-1965 and definitely did not jus end with prototypes. This new system was not so different from the one developed in 1958. The seals and air intake pipe were installed in the same way. Vehicles equipped with this gear still survive to this day, for instance the tank displayed at the Central Museum of the Armed Forces in Moscow. There are not many tanks with this equipment, but it's doubtful that this equipment was only experimental. A number of these surviving vehicles went through a second wave of modernization. 
Mass production variant of waterproofing the hull.

ChTZ's next project was a modernized ammunition rack for 6 3VBK5 HEAT shells. They were developed for the M-62 gun, but could be used with the D-25T as well. The plan was to carry 6 of these shells and reduce the number of AP shells to 12. Documentation was developed in April-May of 1969, the author does not know if work proceeded past that.
Location of 6 3VBK5 shells.

The IS-2M modernization was not the last. Repair factories continued modernizations in the 1960s and 70s. In 1960 ChTZ developed a mount for two 200 L fuel tanks in the rear of the hull. These were added later, in the late 1960s. The lights also changed around this time. GST-49 running lights were changed to GST-64, which had different caps depending on when they were produced. The FG-100 and FG-102 headlights were replaced with the FG-125 and FG-127 that had watertight casings. These were the last major changes to the IS-2M. As more and more T-10M tanks were issued, the IS-2M were gradually phased out to reserves or used as training tanks. Officially the IS-2M was only removed from service in 1997, but by then they were no longer used in their original role. They were instead relegated to immobile firing posts or targets.

The IS-2M tanks were modernized further. This photo shows tanks with FG-125/FG-127 headlights.

In conclusion, let us remark that the modernization of the IS-2 and its lengthy career was the correct decision. The presence of these tanks in reserves meant that a large amount of armoured vehicles was available in case of a new war. These tanks were kept in reserve and kept out of combat operations. Another benefit of the IS-2's long lifespan is that many of them survived and are quite often restored to running order. Sure, a modernized IS-2M is very different from a wartime IS-2, but let's be fair: this different is not noticeable to most people. The very presence of these tanks in parades and other events is a big plus. One can only praise the creators of the tank that made such a contribution to victory and remained in service for half a century.


  1. I was lucky enough to be at the exchange of the IS-2M for a Conqueror in 1988.
    The tank appeared to be from war reserve stock, if it had ammunition and crew it would have been ready for action.

  2. There were also issues with cracks on turrets to the point where factory #200 produced new turrets for repair factories to install.

    Wikipedia has the IS-2's armor being high-hardness and often brittle. The article quotes David Higgin's book "King Tiger vs IS-2: Operation Solstice" which is filled with many other factual errors (provable things like incorrect armor thicknesses, etc).

    I remember a Quora thread which (I think) had Higgins reply pointed to an often-reposted photo of an IS-2 model 1943 that had suffered an internal explosion and the upper glacis (120 mm @ 30 degrees) had a chunk of said plate missing as evidence for the brittle armor:

    However, I think instead if you look the lower plate has a penetration. That makes more sense, as that would have set the fuel tanks on fire and caused the tank to burn-out and suffer an internal ammo explosion. Then the missing chunk from the upper plate is the result of the explosion and is not the penetration.

    The reason I say this, is that I've seen a number of KO'ed IS-2s, most don't show any obvious cracking or shattering of the armor plate, even after suffering numerous hits. This one may have some cracking due to an edge effect hit near the gun point, but most are just round pock-marks.

    So, given the dearth of IS-2s that ended up in Western hands, I can't see how authors like Higgins down-rate the armor quality, in particular, because Western tests of the T-34 and T-34/85 showed Soviet armor quality to be at least as good, if not better, than their own domestic steel.