Monday, 1 July 2013

Birth of the IS Tanks Part 1: IS-85

Stalin's name started appearing on tanks long before the start of the war. In 1935, the first modernized T-28A tank was named "Stalin", but it was just the one tank. The BT-IS (Iosif Stalin) in 1936 and BT-SV (Stalin-Voroshilov) in 1937 were projects to increase the mobility and armour of the BT tank series, respectively. Neither project was accepted by the army.

Another project is seen in 1942, and it's certainly optimistic. It's definitely more like a KV-4 or KV-5 than anything associated with a production IS.

CAMD RF 38-11355-935

This tank has a mass of 100 tons, armour 120-150 mm thick, was armed with 3 machine guns (one was AA) and a 152 mm ML-20 gun-howitzer. A small secondary turret contained a 20 mm autocannon. The hull would hold two flamethrowers. The tank would be powered by two diesel motors, with a combined 1200-1500 hp, accelerating the tank to 24 kph, both forward and backward. The tank was crewed by 7-8 crewmen. This project was very much ridiculous, and was declined. 

The birth of the "real" IS tanks, the heavy breakthrough tanks that are very well known today, started in 1942. By then, the advantages of the once invincible KV-1 have diminished. New German long-barreled 75 mm guns and APCR ammunition for 50 mm guns were capable of penetrating its armour. Its mass, and thus sluggishness, became a great disadvantage. A smaller, faster KV was in development. Its armour was cast and angled. It was smaller than the KV-1, and not as long, with 5 road wheels instead of 6. With the same armour protection, this new vehicle was much faster than the old KV-1. Its projected speed was 55-60 kph. The tank was named KV-13 (Object 233). A prototype was finished by May of 1942, and started factory tests. The tank could reach a speed of 51.7 kph (still pretty good for a heavy tank). An interesting feature was that it could use a drive wheel and tracks from both the T-34 and KV-1, a lifesaver for repair crews that had to handle both types of tank.

This first prototype still carried the old Voroshilov name, but its direct successors did not. A report from the Chelyabinsk factories announced the decision to name the new tanks after Stalin.

The two new Stalin tanks were direct successors of the KV-13. Many internal components were reworked, but the hull and overall layout remained the same. The old friction clutches were replaced with planetary gear mechanisms. The cooling system was redesigned. The tracks (the new tanks used KV-1 tracks) were lightened. The two tanks were equipped with a ZiS-5-IS 76 mm gun and a U-11 122 mm howitzer, in a KV-9 turret. They were named IS-1 (Object 233IS) and IS-2 (Object 234IS). While being superior to the KV-1 in speed, reliability, and simplicity, these new tanks brought nothing new in terms of firepower.

IS-1 (right) and IS-2 (left)

In February of 1943, news of Tiger tanks on the Tihvin front percolated through the ranks. All those 107 mm guns that were made for hypothetical superheavy tanks suddenly became a lot more relevant. Stalin called Grabin, who was ready to resume the production of ZiS-6 guns at factory #92 in 15-20 days. In April of 1943, while the new IS tanks were still undergoing trials, an order was received by the ChKZ factory for two new heavy tanks. These tanks must be at most as heavy as a KV-1, have more armour (at least 100 mm), and possess an 85 mm high power gun (with a turret ring diameter of at least 1700 mm) and a 107 mm tank gun (with a turret ring diameter of 1850 mm).

On April 15th, 1943, order #3187ss demanded new tank guns capable of fighting new German tanks and SPGs. These guns, with a caliber of at least 85 mm, must penetrate 120 mm of armour at 600 meters, and with a caliber of at least 107 mm, 120 mm of armour at 1000-1200 meters. These guns already existed. The 85 mm ZiS-25 with a muzzle velocity of 1150 m/s and 107 mm ZiS-26 with a muzzle velocity of 1012 m/s were created, and even built, at factory #92, but never passed any trials. The design of tank mounts for these guns would take as much time as making new guns from scratch.

Using these requirements, two new tanks were developed: IS-3 (85 mm gun) and IS-4 (107 mm gun). The extra mass of these larger guns required a longer hull, and six road wheels. However, nobody made ammunition for the 107 mm gun since 1942. Factories lacked machinery to make new 107 mm shells, and the cost of one such shell would be enough to produce 2 85 mm shells or 3-4 76 mm shells. Existing stocks of 107 mm ammunition were mostly pre-revolutionary. The IS-4 was not an option.

Several 85 mm guns were developed to compensate. All guns were more or less equal ballistically, and competed in trials against each other. IS-1 was armed with a D-5T-85 gun. IS-2 was armed with an S-31 gun. Two KV-1Ses participated too: one with a D-5T-85 in an IS turret, one with an S-31 in the KV-1S turret. Despite the similarities between the ZiS-5 and S-31, which would have made production cheap and painless, the D-5T was selected as superior based on smaller size and weight. The tank carrying it was indexed IS-85, and accepted into production. The KV-1S with the same turret was indexed KV-85, and also accepted, to serve while production of new tanks gets off the ground.


  1. Poor 107 mm guns, never saw a chance in battle despite being good all-around guns.

  2. Do you have any stats for the ZiS-26? Is it a separate development from the earlier experimental L/70?

  3. Correct me if I am wrong. There are 2 tanks that indexed IS-3s and 4s; one was a prototype if the original IS and the other one was an entirely different tank?

    1. Yes, that's correct. There was also an IS-4*, which was an IS-2 with a 100 mm gun.