During the summer fighting of 1943, Kotin's design teams developed several versions of a successor for the KV1 heavy tank, the new KV-13, experimenting with new suspensions, new hull layouts and new turrets. As an interim step, a KV-1S was up-armed with an 85mm gun as the KV-1S-85, but this was not accepted for production because the turret was too small to accommodate the gun and its recoil. A new tank design was developed, based on the KV-13 research and named the IS1 or IS-85 (after Iosef Stalin; K. Voroshilov, after whom the previous KV tank was named, having been in disgrace since the beginning of the war).
This mounted an 85mm gun, and was as thickly armoured as the KV1 Modell 1942. However, by redesigning the hull and turret and by introducing transmission and powertrain improvements, the new Stalin tank IS1 was nearly as mobile as the lightened KV1S. The prototype was displayed to Stalin in August and won considerable admiration from the GKO. Production would take time to initiate, and there was some concern that the IS1 was only armed with the same gun as the new T34 - 85. To bridge the gap until IS1 production began, the NKTP ordered the Kirovskiy Works to build a modest number of KV1S with the more heavily armed turret of the IS1 as the KV85. In September and October 1943, 130 KV85s were manufactured. The Chelyabinsk team adapted two heavier guns to the IS1, the BS-3 100mm anti-tank gun and the A-19 122mm gun, and these were known respectively as the IS-100 and IS-122 heavy tanks. In November 1943, they were put through firing trials in Kubinka. A captured Panzer V Panther was used as the target and a 122mm round crashed through the frontal armour and clear through the tear armour as well. However, the 100mm gun had better armour penetration (185mm at 1,000m as compared to 160mm for the 122mm gun). But the 122mm gun was selected because there was a surplus of
122mm tube production facilities and an adequate ammunition base, while the opposite was true for the 100mm gun. A small number of IS1 with 85mm guns were completed before this decision was made, but with acceptance of the IS-I22 for Red Army use by the GKO as the Stalin tank IS2, the IS1s were re-gunned before being issued. A total of 102 IS2s and the prototypes were completed in 1943.
The type was in such great demand after its debut that in April 1944, the NKTP ordered construction of a new assembly facility at Tankograd to increase production. It first saw action with the 11th Guards Heavy Tank Regiment following theKorsun Shevchenkovskiy battles in the early spring of 1944.
The redesigned hull and layout of the IS2 gave it better ballistic protection than the KV, and maximum turret armour was an impressive 160mm. Far more appreciated by its crews was the anti-tank capability of the new gun. It delivered 3.5 times more kinetic energy on impact than the 76mm round, and even in the rare case where this was not sufficient to penetrate the armour, the force of the impact and explosion of the high explosive filler was usually enough to blow a turret off almost any tank. The most serious disadvantage of the IS2 was that its small internal size permitted stowage of only 28 rounds of ammunition, and these were of the split type which slowed the rate of fire.
The IS2 has been compared unfavourably to both the Tiger I and later Royal or King Tiger in terms of armour and firepower, though it should be kept in mind that in terms of weight, size and cost, it was much more comparable to the Panther medium tank than to the much larger and heavier Tiger. Indeed, the Germans' decision to adopt so large, heavy and complex a tank as the Panther for their medium tank was one of the factors which limited German tank production to levels far lower than Soviet production, and helped the Soviets to build up a decisive quantitative advantage in tanks during the final year of the war.
In the spring of 1944, the IS2 was further improved with the introduction of the IS2m (m: modifikatsirovanniy). Apart from fire control improvements and other internal changes, the main difference was the use of a new, simpler and better angled bow casting. Also the improved D-25T gun was introduced which was nearly identical with the A-19, but had a drop-breech which speeded loading. A number of experimental versions of the hevy tank were built, with various guns, the most drastic variant being the IS-2E with a new electrical transmission and a new road wheel configuration. None of these were accepted for production. While experiments with the heavy tank were being conducted, TsKB-2 was divided to permit a team under Dukhov to examine a radically re-configured IS2.
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