Saturday 28 November 2015

The Last Soviet Heavyweight

The last fighting machine named after Stalin would be the last Soviet heavy tank. It appeared at a time when the trend of constant increase in the weight of tanks came to a stop. Without the ability to grow protection and armament of tanks at the expense of weight, the designers of the IS-8 (T-10) used a number of creative solutions.

The end of an era of giants

Starting from late 1943, Soviet tank designers steadily increased the mass of their heavy tanks. Eventually, the initial stages of tank projects stepped over the psychological limit of 50 tons. The IS-3 was an exception, but it was essentially a deep modernization of the IS-2. Interestingly enough, there were doubts at the very top of the Main Armour Directorate (GBTU) about the suitability of the IS-3 for service. The Object 701, which hit a mass of 55 tons during the prototype phase, seemed much more desirable. Development of the future IS-4 was moving slowly: the first prototypes were ready for the summer of 1944, but the tank was constantly changed, and, despite being designed as an answer to the Ferdinand, only reached mass production in 1947. By that time, production of the IS-3 not only started, but also ended, netting a total of 1555 tanks.

The IS-4 was already yesterday's news in 1945. In the winter of 1945, the design of a new tank began, the IS-7. This designation, as well as the index "Object 260", covered three tanks that were significantly different in their design and characteristics. Designed as a counter to the German Maus, the IS-7 was the most perfect fighting machine of its class. The characteristics of the tank that was finalized in 1948 boggle the mind even today: with a mass of 68 tons, it could reach a speed of 60 kph, all with the armament and protection no less than that of the 189 ton German monster, even superior in some regards.

However, the increasing mass was a trap for heavy tanks. ChKZ managed to produce the IS-4 with great difficulty: only 55 tanks in 1947 and 155 more in 1948. The IS-7 was even more problematic: it needed a new engine, the production of which was yet to be mastered. The 68 ton weight meant that new recovery vehicles were needed, and the tactical mobility of the tank was questionable. Few bridges, even railroad ones, could carry such a monster. There was also an issue with railroad platforms that could carry sufficient weight. The problems that were bypassed in one way or another in 1941 caught up. The same issues would have to be resolved if the KV-3, KV-4, or KV-5 reached mass production.

Weighing all pros and cons, Soviet leadership made the only correct choice. On February 18th, 1949, the Council of Ministers of the USSR had a meeting which included the commanders of the Armoured and Mechanized Forces, as well as representatives from GBTU. The result of the meeting was decree #701-270ss, which stopped the development and production of tanks heavier than 50 tons. The IS-7 was doomed, as was the IS-4 (the last 19 tanks were amde in 1949).

The same decree ordered SKB-2 of ChKZ and the experimental department of factory #100 (Chelyabinsk) to begin development of a new heavy tank, no more than 50 tons in weight. The design had to incorporate components from the IS-4 to speed up development. At the same time, the armour layout and hull design were to be taken from the IS-3, as the most appropriate for this weight. The design team had very limitd time: the first three tanks were to be ready in August of 1949, and another ten in September, for military trials. In total, only half a year was allotted to the production of a new tank, which was yet to be designed.

Working together

In order to solve such a difficult problem, two competing design bureaus were involved. On March 19th, 1949, the factory #100 design group was included in the newly created VNII-100 (Leningrad). The first task given to the institute was the design of the new heavy tank. J. Ya. Kotin headed this new team. Leningrad was tasked with development, and workers in Chelyabinsk put their designs into metal. 41 members of VNII-100 were sent to Chelyabinsk. The vehicle also got a Chelyabinsk index. Vehicles developed in Leningrad were designated Object 2##, whereas Chelyabinsk tanks were Object 7##. For example, the IS-4 was Object 701, and IS-7 was Object 260. This vehicle received the code 730, or Object 730.

Some sources claim that the Object 730 was based on the IS-4, but that, to put it lightly, is contestable. The IS-5 was effectively a new tank, only inheriting the V-12 engine from the IS-4, and, initially, the cooling system with two large fans. Otherwise, the vehicle that was designed in April of 1949 was the product of a completely different school. The IS-4, especially its hull and turret, was the product of 1943 technology. Since then, Chelyabinsk produced the IS-3, which had almost identical armour, and Leningrad produced the IS-7, which left the IS-4 far behind. Other components were greatly improved over the previous 5 years. A large amount of work was done on the suspension, transmission, and other components during the design of the IS-7. Additionally, as mentioned above, the development was mostly based in Leningrad, and the experience of local tank designers was fully applied to Object 730.

Looking closely, the IS-5 is a creative reformatting of the IS-7 mod. 1948. The front pike with driver's periscopes on the side, the rear with its characteristic slope, the V-shaped bottom of the hull and curved sides, all this indicates the heritage of the Object 730. The turret, a combination of IS-3 and IS-7 solutions, also has nothing in common with the IS-4. Since the mass of the IS-5 was limited to 50 tons, the armour had to be reduced for the first time since the KV-1S. Thanks to scientific work to increase the effectiveness of armour without thickening it, a series of solutions were implemented, making the tank more protected than the IS-3. Compared to the IS-4, and especially the IS-7, the protection of the tank decreased. Such was the cost for lowering its weight to 50 tons.

Another regression was seen in the armament. The D-25T, which was installed in the IS-2, IS-3, and IS-4, was already not enough for the military in 1944. The IS-7 received a new 130 mm S-70 gun, but it was not possible to install it in an IS-5. As a result, the IS-5 received the same D-25T. The armour and armament of the tank was closer to the end of WWII, rather than the end of the 1940s.

A tank of compromise

The Americans faced the same problem around the same time. Starting with the 64 ton T-29 Heavy Tank, American tank designers managed to develop a final tank weighing in at 65 tons. While Soviet tank designers were working on the 50 ton IS-5, their American colleagues were working on the T43 heavy tank with a 55 ton limit. As a result, the M103 Heavy Tank turned out to be slightly heavier at 56.7 tons, but this was still significantly lower than the 65 ton heavy tanks of the late 1940s.

The cost of weight reduction was weak armour. The turret suffered the most. A significant advantage of the American tank was its 120 mm gun, but this made the vehicle very large. The characteristics of the FV214 Conqueror were similar, but it turned out to be even heavier than the M103, 65 tons. One can say that all leading armies of the world made the same step back, but the Americans and British managed to improve the tank's armament. The cost was increased size, and therefore weight.

Even though the armament and armour were steps back, it would be wrong to consider the IS-5 a failure. For the late 1940s, armour that was completely immune to the German 88 mm gun was standard. The D-25T was equally modern, as its enemies weren't German tanks, but those of the former allies.

Even the M46 and M47 Patton tanks that appeared in the late 40s and early 50s were not proper adversaries for the IS-3, let alone the new tank. Until the 105 mm L7 gun began appearing in foreign tanks, the front armour of Soviet heavies was reliably impenetrable. By the way, trials of the IS-5 hull performed in May-June of 1950 showed that the front armour withstands hits from 122 mm shells at 100 meters. The Soviet Army received a modern heavy tank.

As mentioned above, many technical solutions migrated from the IS-7 to this tank. This includes the novel bundled torsion bars. This system allowed the length of the torsion bars to be drastically reduced. Other elements of the suspension (road wheels, idlers, tracks) were further improvements of IS-3 components. The suspension looked similar, but the design was drastically altered and reliability improved. It was designed in such a way that it could be installed on other IS series tanks without significant changes.

In the mid 1950s, IS-2, IS-3, and ISU vehicles were modernized. This meant that the new tank and the old tanks had identical suspensions. The use of a 750 hp V-12 engine allowed for superior mobility compared to the IS-3. The maximum speed increased to 42 kph. Several types of planetary transmissions were developed for the IS-5. It's worth mentioning that some components were tested on IS-4 and IS-7 vehicles lightened to 50 tons.

Preliminary blueprints were shown to the state commission in April of 1949. By June, working blueprints were complete. For several reasons, there was a delay, and the first hull only reached ChKZ on July 30th, and second on August 9th. The first IS-5 was finished on September 14th. The tank had a 6-step gearbox and planetary transmission from the IS-4. Trials showed that the transmission and cooling system do not last for the required 2000 km warranty period. As a result, an 8-step planetary transmission developed by VNII-100 was installed, a design with roots in the IS-7. Instead of a fan-based cooling system, an ejector system was used, like on the IS-7. Two more tanks were build in December of 1949, which participated in the winter trials.


Trials requested by decree #701-270ss started with a half-year delay, in February of 1950. Three tanks were finished by April 5th and sent to Lomonosov, near Leningrad, where they underwent trials for the duration of April. Defects discovered during those trials were corrected by ChKZ, after which the tanks underwent more trials at Kubinka in October-November of 1950. Earlier, in June-July of 1950, the tanks underwent trials in Central Asia. In total, 13 IS-5 tanks were built in 1949-1950, and another 2 in 1951.

Changes were constantly made to the IS-5 to improve reliability. The tank served as a test bed for multiple systems, including an autoloader mechanism. The total sum of improvements was so great that it was decided that the tank should be renamed. In early 1953, the tank was named IS-8. This would be the name the tank would serve under, had Stalin not died on March 5th, 1953. In May, the tank was already referred to as only Object 730 in letters. It's possible that this was just a precaution to weather the power struggle in the Politbureau. On June 26th, L.P. Beria was arrested, and N.S. Khrushchev came into power.

The new heavy tank was accepted for service by a decree of the Council of Ministers on November 28th, 1953. This same decree assigned it the index T-10. The first 30 T-10s were built in 1954, and 190 tanks were built in the first series at ChKZ. Kotin's tank was the last to carry the name of a notable Soviet leader. 15 years prior, Kotin worked on the SMK heavy tank (Sergei Mironovich Kirov), which started the trend of naming heavy tanks instead of giving them numbers.

The T-10 was a temporary measure. In 1952, a program began to create a next generation tank, the result of which was the Object 777. The T-10 was destined to outlive its successor, which never moved past a scale model, as well as the next wave of heavy tanks, which birthed Objects 277, 279, and 770. In 1958, LKZ began production of T-10s, but only once did annual production surpass 200 units. In 1965, ChKZ built the last 60 T-10s, and that was the end for Soviet heavy tanks. A new type of tank appeared on the horizon, including the best parts of heavy and medium vehicles, the Main Battle Tank.

Original article by Yuri Pasholok.


  1. At the same time, the armour layout and hull design were to be taken from the IS-3, as the most appropriate for this weight.

    One of the problems with the IS-3 was that the welds for the "pike" nose would reportedly crack open due to the vibration of travel (if so, one would think that this might be a problem when struck by enemy AT shells too!). Was this issue resolved with the similarly "pike"-nosed T-10?

    Given the success of the IS-2, and how it remained in-service and reportedly popular with the Army for a long time (outlasting some of it successors if nothing else for its reliability, use-of maintenance, and good ergonomics) in my past readings I had supposed that Soviet designers, once freed from the urgency of war pressures, had gone off "the deep end" (to use English slang) and engaged in an orgy of impractical experimentation with heavy tanks, when what was really needed was a simpler upgrade/fix to the IS-2. Now I see that things weren't this simple. What was good about this post was that it compared how the US and British armies were also struggling how best to implement the heavy tank concept.

    Western tanks now often exceed 60 tons. So how do they handle operational mobility issues? Is this a case of logistical brute-forcing by the West?

    This article:

    Seems to confirm that the weight of western MBTs is indeed an issue.

    1. I wouldn't think so. I mean, honestly the transporting of heavies was, at the time, a matter of a lack of suitable bridges. For Russia, it was a lack of them in less-well-developed areas, and for the west it was a lack of big or strong enough ones. (This was also a problem for the Soviets.) In America it was a matter of width, as the train lines squeeze through tight tunnels and over narrow bridges. In England it was a matter of size and weight. While they had plenty of bridges, most of them were old (Some dating to Rome lol) and wouldn't take the load. (This was also a problem on the continent)

    2. These days however it's not much of a problem. Bridges are bigger and wider to hold way more traffic, etc.

    3. This comment has been removed by the author.

    4. Hello! Problems with seams IS-3 was only UZTM factory building, it was a marriage (the story of collapsed forehead from falling into the weld 100 mm shell). Factory building number 200 had no such problem.
      PS: I ask you to help the German material for the JS-2:
      1) tactical instruction to combat
      2) The results of the shelling, the test tank.

    5. Please respond, if I'm not mistaken, you're familiar with these materials.

    6. AFAIK the Western heavies (that actually entered service) were intented primarily for relatively static defensive operations on the European frontline, and basically would be already on the scene if shooting started. That oughta made transportation inconveniences a rather less critical issue than for their Soviet peers, which were more general-purpose assault vehicles (and IIRC a whole lot more numerous).

      Ofc the USMC used the M103 too, but since at least in principle *they* largely traveled by ship...

    7. If I'm not mistaken they were largely for defensive purposes, fighting hull down and dug in, and were then supposed to advance supporting the mediums during a counterattack. Hence the "convergent evolution" of the Conqueror and M103. What's most ironic is a lot of the Soviet heavies wound up dug in as stationary gun emplacements once deemed obsolete.

    8. PS: I ask you to help the German material for the JS-2:
      1) tactical instruction to combat
      2) The results of the shelling, the test tank.

      I don't know if these questions were directed at me, or the blog author. But here goes what i do know.

      I have only snippets o"how to fight a IS-2" from various secondary sources: From the web--Guderian's advice:

      During one of these battles, near the settlement of Târgu Frumos, a single IS-2 was damaged and later examined by General Guderian himself, whom concluded that the “Stalin” was worth of its name. “Do not get involved in a fight with a “Stalin” without overwhelming numerical superiority in the field. I believe that for every “Stalin” we must account for an entire platoon of Tigers.” Any attempts by a single “Tiger” to fight a “Stalin” one-on-one can only result in the loss of a priceless war machine.” Soon, new tactical rules were devised to flank and surround IS-2s and get shots in its vulnerable sides, rear and the sensitive “shot trap” rear turret basket, and only at short range. Presumably German tactical superiority was again called for the task.

      Though I've read on WiT forums and elsewhere other translations of this, and I've seen no formal bibliography.

      This quote cited to Guderian is generally supported by the data published by Jentz, though:

      Tiger I vs. JS-122 ) JS-122 vs. Tiger I

      Turret 100 m 1500 m
      Mantlet 100 m 500 m
      DFP* 100 m 1300 m
      Nose 300 m 1000 m
      Turret 1000 m 2900 m
      Superstructure 1000 m 2900 m
      Hull 1500 m 3500 m
      Turret 100 m 2900 m
      Hull 300 m 2700

      These values were the result of German field testing (the data was extracted from a Wa Prüf 1 report dated 5th October 1944) This means it was obtained on a IS-2 model 1943, because the Germans never captures a model 1944 to my knowledge worthy of testing (the mod 1944 would be even tougher to penetrate). The Tiger I seems badly overmatched by the IS-2 by both accounts.

      For the Panther, Wikipedia cites (probably the same?) Wa Pru 1 report for the penetration of the Panther's 75/L70 vs the IS-2 (again, mod 1943);

      Mantlet: 400 m
      DFH (front plate: 600 m
      Non-mantlet front turret: 800 m

      Other than that, I've read in Zaloga and elsewhere the admonition in tactical instructions that Panther crews had to close to 600 m to "guarantee" penetration of the IS-2 (odd choice that some on the WoT forums make way too much of, because the penetration is not "guaranteed" at that range even by their own tests, nor by calculations). I have never seen the source for this (a captured training document, perhaps?). As the testing was done on an IS-2 mod 1943, the results with a mod 1944 would be less favorable to the Panther.

      As for the Soviet testing on an IS-2 with captured German weapons, I have nothing. Baryatinskiy describes the early 1944 tests using Soviet weapons on an IS-1 at Kubinka but only in general.

    9. as I understand it, from the Wa Pruf data are calculated and the estimated "shooting" was carried out at an angle of 30 degrees, if the Germans had hoped to normal, then I think the distance will increase more destruction. Although these findings are not far from the truth, in the German report from the recommended distance is 8 points defeat of the IS-2 500 meters, about 500 meters distance and Michael wrote Svirin

    10. Soviet sources give a figure for guns and Panthers Tiger 800-1100 meters on the defeat of the IS-2 1943, although the model 1944 to the vulnerable points is not much different, but their number has decreased: the new frontal part, 130 mm lower front part to the cast housings, extended kind of like a mask was also improved in terms of body armor, but I have no evidence.

    11. The tower remained vulnerable at a distance 1000-1100 m. I have information that the manufacturer's works there are still reports of shelling factory JS, and they are slightly different from those of firing military.

    12. I understand it, from the Wa Pruf data are calculated and the estimated "shooting" was carried out at an angle of 30 degrees

      I've read it was plus/minus 30 degrees, which I'm not sure what that means (some averaged metric within a 30 degree cone?). It may not be "exactly 30 degrees". I also think it's calculated at 50 % penetration probability.

      The reason why I'm saying this is that, looking at the situation in reverse, the Tiger I seems terribly vulnerable to the IS-2's 122mm gun for this to an exact 30 degree test using 'certified penetration" criteria. When I plug in the IS-2 122 mm BR-471 round against the Tiger 1's armor using the Western metrics of the WWII gun vs armor calculator, which is done at the 50 % penetration probability, I get values actually *less* favorable to the 122 mm than cited in the Wa Pruf 1 report, even at zero degrees.

      I did my own back-of-an-envelope calculation, based on Russian recommendations for uparmoring the IS-2 turret, and calculated the IS-2 turret front "cheeks" to be vulnerable to the Panther's 75/L70 at about 1000 m at zero degrees, 50 % penetration probability, as opposed to the 800 m tabulated in the Wa Pruf 1 report. That would suggest that the Wa Pruf 1 figures aren't much more restrictive.

    13. Soviet sources give a figure for guns and Panthers Tiger 800-1100 meters on the defeat of the IS-2 1943

      The only figure for "800-1000 m" I get on the vulnerability of the IS-2 to the 88 mm gun was from the report of the 71st Guards Heavy Tank regiment in its fight with King Tigers at Sandomir., in August 1944. That of course raises the question of "which German 88 mm, the 88/L56 or 88/L71"? The report is ambiguous, it could be interpreted either way (because it also covers the period between July 14th-August 31st).

      We know this regiment had encountered King Tigers, but did they encounter Tiger Is in this period? I recall seeing the breakdown of Soviet claims of the 31 tanks they destroyed around Sandomir by type, and can't recall whether any Tiger Is were listed. Encounters. between IS-2s and Tiger Is weren't that common. On the face of it, "800-1000 m" seems overly optimistic for the IS-2's frontal resistance to the King Tiger's gun (save on the upper hull) but overly pessimistic about the IS-2's resistance to the TIger I's gun.

    14. With that, I do have a paper copy of the breakdown of IS-2 losses of the 72nd Guards Heavy Tank regiment, with cause of loss. To summarize these:

      1 destroyed by a "Ferdinand" at 600-700 m, in the flank

      1 destroyed by a Tiger I at 1000-1100 meters, by a hit on the lower plate (the weakest point of the IS-2's front armor). This appears to have been only a partial penetration, as the driver was wounded from a fragment of the armor plate, not from the shell, and a fuel tank in front was set on fire. The crew bailed out.

      1 damaged by a Tiger 1, "frontally at 400 m", repaired.

      1 burned out by a hit from a Tiger 1, hit in the flank, at 500 m.

      1 hit in the hull flank by a Tiger 1, at 800-1000 meters, burned out.

      1 refuffed three shots from a Tiger 1 frontally and ranges > 1000 m, but was set on fire by another Tiger one "from the right side" at 400-500 m.

      1 destroyed by a Tiger 1 by a hit on the left side at 500 m which set the engine on fire.

      1 destroyed by a Tiger 1 at 800-1000 m by a hit in the right flank which set the fuel tanks on fire, killing the driver, the crew evacuated.

      1 destroyed after "2 rounds hit its side", no gun or distance specified.

      1 destroyed by 2 hits--1 in its turret (front, side or rear not specified, probably the side by the location of the next shot). The second shot hit the side into the engine compartment. All but the driver were killed. No range or gun caliber specified.

      1 damaged by a hit from a "Ferdinand" at 800-1000 m, front, side, or rear hit not specified. First shot failed to penetrate the turret ring but the 2nd shot hit the engine and set it on fire (so likely a side shot).

      1 damaged by a hit on the gun barrel.

      The reason I am listing all this is that it's not clear that the "800-1000" meters vulnerability specification doesn't include flank shots. All but two of the above-cited losses appear to have been due to flank shots, at medium range or closer, which makes sense considering to Guderian's suggestion.

    15. although the model 1944 to the vulnerable points is not much different

      The upper hull of the IS-2 mod 1944 is just about impenetrable to almost everything, save maybe the 88/L71 at point-blank range or the German 129 mm. That alone is a significant difference.

      As for the turret, the 100 mm-thick rounded "cheek" armor (vulnerable to the Panther's 75/L70 at 800 m in the Wa Pruf 1 report). area is significantly reduced in size in the IS-2 mod 1944. It's not a big target and would be not an easy weakness to take advantage oft. Shots near the gun itself are also likely to not penetrate, as that is over 200 mm. The probably "best target" considering thickness, angle, and size on the front turret is the rounded 80 + 80 mm armor to the right of the gun where the gunner sits; this is more resistant in the mod 1944 than it was the mod 1943 but not hugely so.

      The lower front plate is the same, and if you could hit it, that would be the place to shoot.


    16. The upper hull of the IS-2 mod 1944 is just about impenetrable to almost everything, save maybe the 88/L71 at point-blank range or the German 129 mm. That alone is a significant difference.

      upper frontal part model 1944 made its way out of 8.8 cm. Cast Plant number 200 from 450 meters, the plant UZTM 600 meters.

    17. The lower front plate is the same, and if you could hit it, that would be the place to shoot.

      130 mm in the lower part of the body cast I think for Tiger and Panther guns is not an easy target.

    18. upper frontal part model 1944 made its way out of 8.8 cm. Cast Plant number 200 from 450 meters, the plant UZTM 600 meters.

      I'm not doubting what you wrote--it appears in the post on the IS-3 development here on this site--but I have a hard time explaining it. In every gun versus armor references I've I've tried, in the gun vs armor calculator online, plugging in the 88/L71 APCBC round versus the IS-2's upper hull results in "no penetration". By Soviet metrics, the 88/L71 only penetrates 168 mm of armor at 100 m, and the IS-2's hull armor by cosine function alone is effectively more than that. By Western metrics, the 88/L71 can penetrate as much as 250 mm at point-blank range (239 mm in the online calculator) but that is at most equal to the effective thickness of the IS-2's hull (by US Army standards, at 60 degrees it's 2.5 x nominal thickness).

      The only way I can get these calculators to give anything like the results you describe is that I have to lower the IS-2's hull thickness to 95 mm or less, and lower the slope to 55 degrees instead of 60 degrees plus assume cast hulls (and some were welded). Even then it's not a "probable penetration" but a "possible penetration". I also tried teh 88/L71's APCR, not much better (if at all).

      On some Soviet tables that have been posted there, there are figures for "possible penetration" (20 % probability) instead of "guaranteed penetration. I know that penetration on paper doesn't always equal that in actual combat (though there are more cases of under-performance than over-performance with that). But could one explanation for this be that we're talking about "possible penetrations"?

    19. 130 mm in the lower part of the body cast I think for Tiger and Panther guns is not an easy target.

      Yeah, but the Tiger I/s 88L56 gun should be able to penetrate that out to about 1300 meters at zero degrees, and it's bigger (and likely weaker) target than the 100-mm rounded turret "cheek"

    20. I know that penetration on paper doesn't always equal that in actual combat (though there are more cases of under-performance than over-performance with that

      Examples include British 17-pounder APCBC and US HVAP rounds bouncing off and/or shattering against the Panther's glacis, even at very close ranges, where these guns could punch through that armor up to 1200 m.

      Also, of course, we have the problem of the 122L46's performance; German testing indicated that at plus/mine 30 degrees it couldn't penetrate the Panther's upper hull at all, whereas IS-2 crews reported problems with the BR-471 round but only past 600-700 meters. Here I am more wont to believe the reports of Soviet tank crews if only because you'd expect significant overmatching with a round as large as the 122 mm versus the Panther's 80 mm of armor.

    21. Examples include British 17-pounder APCBC and US HVAP rounds bouncing off and/or shattering against the Panther's glacis, even at very close ranges, where these guns could punch through that armor up to 1200 m.

      Oops, the last sentence should read "where these guns should punch through that armor at up to 1200 meters".