Iosef Stalin (IS)
The Soviet Iosef Stalin (IS) tank line, developed to replace the KV heavy tank family and address its shortcomings, were a success during the war and went on to set the template for Soviet tanks for decades. In 1941, the standard antitank gun on Germany’s dedicated armor-killing tank, the Panzer III, was 37mm, whose shells KV tanks simply shrugged off. The Germans quickly upped their firepower, upgraded their Panzer IVs with more powerful antitank guns, and in 1942, rushed the Panzer VI Tigers into service with powerful 88mm guns.
Against the Tigers’ thick armor, especially up front, the KV’s 76.2mm gun proved largely ineffective. Moreover, the slow KVs were unable to maneuver and close in quickly, like the T-34s could, in order to fire at the Tigers from closer ranges and at weaker spots so as to inflict damage. With a weak gun that could not damage the German heavy tanks, and armor that could be penetrated by the Tigers from long range, the rationale for the KV disappeared: it was far more expensive than the T-34, but lacked greater combat performance to justify the greater cost.
To cope with the tougher threat environment while fulfilling the KV’s designated role as a breakthrough tank, the IS line was designed with thick sloped armor to counter 88mm shells. IS tanks had thicker armor than the KV, but because of a better layout were lighter and faster than KVs, as well as lighter than Tigers and Panthers, and had a lower silhouette. IS tanks were equipped with powerful guns that, while intended primarily to fire high explosive shells at infantry strong points and bunkers, were also capable of taking out Tigers and Panthers.
The IS line was born 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 with better armor than the KV. The result was the 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, and armed with an 85mm gun.
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 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 was 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 in to 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.