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The IS-1 is the first variant of the IS heavy tank family. The new IS-1 was intended to overcome the lack of firepower and protection issues of the KV series with a whole new sloped hull design. This was a strong armour design intended to deflect or withstand projectiles from all sides. The only major design requirement was that the weight not surpass that of the interim KV-85. The new armament was the same as the KV-85, an outstanding 85 mm D-5T tank gun with significantly higher range and muzzle velocity than prior Soviet heavy tanks. The turret was also identical to that of the KV-85, with three crews. The drivetrain was similar to that of the KV-85, with enormous tracks supported by three pairs of double return rollers and six double-tired wheels suspended by massive torsion arms. Extra fuel tanks were fitted to the rear of the hull, as with all earlier heavy tanks, and massive mudguards housed storage boxes. The engine was replaced with a new V2-IS 12-cylinder diesel engine producing 520 horsepower. The typical top speed was 37 km/h, and the practical range was only 150 km. Chelyabinsk Kirov Plant was responsible for the IS-1 manufacturing, and the first IS-1 rolled off the line in October 1943, but production was halted in January 1944 as the IS-1 was swiftly replaced by the all-round better IS-2. As a result, only 200-207 units were produced in total.
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