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.

Renaming

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.

Experimental Guns

"Decree of the State Committee of Defense #4851ss
December 27th, 1943
Moscow, Kremlin

On the design, production, and trials of experimental guns for tanks and SPGs.

The State Committee of Defense decrees that:

1. On the 85 mm tank gun: GAU chief comrade Yakovlev and Armoured Forces commander comrade Fedorenko must perform trials of the 85 mm S-53 and S-50 guns installed in T-34 tanks (S-53 in a T-34 with a stock 1420 mm turret ring, S-50 in a tank with a widened turret ring) and the 85 mm LB-1 gun designed by factory #92 in a T-34 with a widened turret ring before December 30th, 1943.
   Report to GOKO on results by January 2nd, 1944.
2. On the 100 mm tank gun (comrade Grabin):
1. NKV (comrade Ustinov) and TsAKB (comrade Grabin) must provide the S-34 100 mm tank gun for trials by January 1st, 1944.
   GAU chief (comrade Yakovlev) and Armoured Forces commander (comrade Fedorenko) must perform trials by January 5th, 1944.
   Report to GOKO on results by January 7th, 1944.
2. NKV (comrade Ustinov) and factory #92 director comrade Yelyan must produce two S-34 100 mm guns according to blueprints from TsAKB, one for the IS tank and one for an SPG on the T-34 chassis to factory #100 by January 25th, 1944.
3. NKTP (comrade Malyshev) and factory directors of Kirov factory (comrade Zaltsmann), factory #100 (comrade Kotin) and Uralmash (comrade Muzrukov) must design and produce an IS tank and an SPG on the T-34 chassis with 100 mm guns.
   Perform factory trials of the IS tank and SPG with 100 mm guns by February 20th and provide the tank and SPG for proving grounds trials by February 25th.
4. GAU (comrade Yakovlev) and the Armoured Forces commander (comrade Fedorenko), in cooperation with NKTP and NKV, must, within five days, perform trials of the IS tank and SPG with 100 mm guns and deliver a conclusion to GOKO by March 5th, 1944.
5. TsAKB (comrade Grabin) must include the possibility to install an 85 mm barrel into the S-34 mount to achieve a muzzle velocity of 1000-1100 m/s and begin the production of this gun with the aim of producing an experimental prototype for government trials by April 1st, 1944.
6. People's Commissariat of Ammunition (comrade Vannikov) must produce ammunition (casing, propellant, AP and HE shells) for the 85 mm TsAKB gun with a muzzle velocity of 1000-1100 m/s and provide 1000 proof rounds as well as 250 each HE and AP rounds.
3. On the 130 mm tank gun (comrade Grabin):
1. NKV (comrade Ustinov) and TsAKB (comrade Grabin) must design by January 5th 1944 and produce by March 1st 1944 an experimental prototype of a 130 mm gun, with the ballistics of the 130 mm B-13 naval gun, to be used in an SPG on the IS chassis.
2. Director of factory 232, comrade Zakharyin, must produce a barrel with breech and other components, as well as cast and forged components, and deliver them to TsAKB by the aforementioned date.
3. NKV (comrade Ustinov) and TsAKB (comrade Grabin) must send one 130 mm SPG gun to factory #100 by March 10th, 1944.
4. NKTP (comrade Malyshev) and directors of factory #100 and Kirov factory must design an SPG on the IS tank chassis to use a 130 mm gun.
   Begin factory trials of the 130 mm SPG by April 1st, 1944, and provide it for proving grounds trials by April 5th, 1944.
5. GAU (comrade Yakovlev) and the Armoured Forces commander (comrade Fedorenko), in cooperation with the NKTP and NKV, must complete proving grounds trials in five days' time and report to GOKO with their conclusions by April 10th.
6. People's Commissar of Ammunition (comrade Vannikov) must produce 500 each of HE shells for the 130 mm SPG, AP and HE shells for the 122 mm gun with a muzzle velocity of 1000 m/s, and deliver them by March 20th, 1944 for trials.
4. On the increased muzzle velocity of the 122 mm tank gun:
1. NKV (comrade Ustinov), TsAKB (comrade Grabin), OKB-172 (comrade Ivanov) and factory #172 director (comrade Bykhosvkiy) must design and produce two prototypes of a 122 mm gun with a muzzle velocity of at least 1000 m/s using a 25 kilogram shell (one designed by TsAKB and one by OKB-172) to be used in an IS SPG.
   Submit finished prototypes to factory #100 for installation in vehicles by March 10th, 1944.
2. NKTP (comrade Malyshev), Kirov factory director (comrade Zaltsmann) and factory #100 director (comrade Kotin), in cooperation with TsAKB and OKB-172, must design IS SPGs with TsAKB and OKB-172 122 mm guns.
   Finished SPGs must be submitted for proving grounds trials by April 1st, 1944.
3. GAU (comrade Yakovlev) and the Armoured Forces commander (comrade Fedorenko), in cooperation with the NKTP and NKV, must complete proving grounds trials of the SPGs within 10 days and submit their conclusions to GOKO by April 15th, 1944.

Chair of the Committee of Defense, I. Stalin."

RGASPI 644-2-254

Birth of the IS Tanks Part 3: IS-100 and Beyond

Even after the vastly successful D-25T gun was chosen for the IS, experiments with the armament of the IS continued. Immediately after the failure of the 107 mm ZiS-6, Grabin's construction bureau hedged their bets on a 100 mm gun based on the naval B-34, similar to the S-3 field gun already being developed. The resulting tank gun was indexed S-34. It also had a mechanism to ease loading and increase the rate of fire. The D-5T-85BM, a higher power 85 mm gun, was also in the running, as well as another 100 mm gun, D-10T. Since the IS-85 and IS-122 were renamed to IS-1 and IS-2 by this point, these new prototypes were named IS-3* (Object 244, with a D-5T-85BM), IS-4* (Object 245, with a D-10T) and IS-5 (Object 248, with an S-34). The IS-3* was rejected due to insufficient muzzle velocity, and thus penetration, of its gun. Initially, out of the remaining designs, the IS-5 was going to win the title of IS-100 and begin production, but both ended up being built. The IS-4* and IS-5 can be distinguished by the position of the commander's cupola: the IS-5's gun required a gunner to the right of the gun, flipping the internals of the turret. The turret also would contain a loading mechanism and a gun stabilizer, but they were not present in initial trials. IS-5 was deemed superior, due to better ergonomics of the loader and gunner, but both designs had some problems that had to be resolved. The D-10T gun was more reliable, but made the loader's job more difficult, and only allowed to carry 30 shells. The S-34 threatened to smack the commander when it recoiled. It was recommended to move him even further right. In October of 1944, the IS-5 turret was finalized. With its loading mechanism and stabilizer, it was superior to every existing heavy tank in terms of rate of fire and accuracy on the move. However, in Malyshev's letter to Beria on October 18, 1944, he wrote: "...[122 mm guns] can now fight any German tank at a distance of 1700 meters and greater..."

The following points supporting the IS-100 were outlined in the letters between Malyshev, Beria, and Fedorenko:

• Increased rate of fire (up to 8 rounds per minute)
• Increased ammunition capacity
• Lack of muzzle brake improves working conditions for the gunner
• The tank is 500-600 kg lighter, which allows for additional armour
• Smaller size, leading to a roomier turret
• Higher penetration
• Factory #9 was ready to produce D-10T guns along with D-10S

Points against the IS-100 in the same series of letters:

• D-25T is already sufficient against German tanks at all combat ranges
• Reduced caliber will reduce HE effectiveness against enemy fortifications and soldiers
• The increased ammunition capacity is negligible (29 rounds instead of 28)
• A large caliber, high rate of fire semi-automatic gun will lead to high concentrations of gases inside the tank

Additionally, in a report from the Chief of Experimental Machinery to the Chairman of the Technical Council, the superiority of the D-10T to the D-25T in armour piercing capacity is challenged:

"[The D-25T] penetrates a Panther frontally at 2500 meters, and that is not the limit. The 100 mm D-10T tank gun ... penetrates the front of the Panther at only 1500 meters, which is the maximum distance. ... The results are preliminary, as the D-10T has already fired 400 shells while the D-25T is new, but the difference is so drastic, that it is unlikely to change. "

The report explains this with the lower muzzle velocity of the D-25T: at such a large range, the heavy 25 kg shell has no problem penetrating the Panther's front, while its higher arc lets it impact the front of the Panther at a favourable angle. The D-10T's shells are faster, and therefore fly flatter.

The IS-5 was briefly a platform for very high velocity 85 mm ZiS-1PM (1000 m/s) and S-34-1B (1040 m/s), but those did not pass trials. From the same letter:

"Another question that is prompted by the experiments at Kubinka is the question of large muzzle velocities, namely of 85 mm guns with muzzle velocity of 1000-1100 m/s. Experience shows relatively poor effectiveness of the German 88 mm gun against the German Panther tank. At the same time, we know that such a gun would be of similar effectiveness to a 100 mm gun with muzzle velocity of 900 m/s. Work on these guns is conducted by TsAKB and factory #9, but their effectiveness against a real German tank is increasingly doubtful, especially considering that we cannot place this gun in a turret smaller than one that requires the D-10T or S-34 gun."

Moving on to the IS-6, its initial requirements were pretty lofty. Here are the project requirements:

CAMD RF 38-11355-2243

The mass was incredibly high compared to any IS tank before it, 65 tons. The armour was initially supposed to resist all German 75 and 88 mm guns, firing at up to 1500 m/s (I wonder where they managed to find those), but that requirement is struck out in the document. The tank was to be armed with a 122 mm gun, capable of 4-6 RPM, with one-piece ammunition. and 1100 m/s muzzle velocity. One 50 mm breech-loading mortar was planned (a feature experimented with on all IS tanks), but also crossed out. The tank would carry 35-40 shells for its main gun, 1500 rounds for its 7.62 mm coaxial machine gun, 500 rounds for its 12.7 mm AA machine gun, 1000 rounds for SMGs, 25 grenades, 27 signal flares, and 12 smoke bombs. All of this was planned to move at 40 kph, powered by a 1000 hp engine. 

The IS-6 didn't quite measure up to these requirements, but, several years later, the IS-7 did.