14. Development of the Iosef Stalin
The IS grew from an interim evaluation tank, a KV armed with an 85mm gun. The increased firepower was good, but the KV’s armor was inadequate, so the Red Army issued directives for a new tank design to be armed with an 85mm gun, but better armored than the KV. The result was the 85mm gun IS-1, which entered service in 1943, retaining the KV’s running gear and hull, but with greater armored protection thanks to a welded turret. In 1944, IS-1s were succeeded by the IS-2, armed with a 122mm gun that had separate shell and powder charges, resulting in a slow rate of fire of only 1.5 rounds a minute initially, later improved to 3 or 4 rounds. The huge shells and powder charges meant that fewer could be stored, and IS-2s were limited to 28 shells on board, usually 20 HE and 8 antitank.
Slow rate of fire and fewer shells were balanced by devastating power, as the 122mm gun could penetrate a German Panther’s front armor at 2700 yards, and its side armor from 3800 yards. By contrast, a Panther would have to close into 870 yards to penetrate the IS-2’s turret, and 660 yards to penetrate its front armor. Against the Tiger, testing showed that the IS-2’s gun could penetrate its turret at 1600 yards, and its front armor from 660 yards. The IS-2 was even more effective against infantry, as its 122mm high explosive shells proved murderous against strong points and bunkers. At a pinch, the 122mm HE shells could also be fired at German tanks, and if they struck, no matter the distance, the explosion could cause cracking and even tear off the front armor’s weld. The mechanical shock could disable the enemy tank even without penetrating its armor.