Staff Writer (Updated: 4/8/2014):
The IS family was born from the interim evaluation "KV-85" tank and its 85mm main gun. The new Soviet Army requirement called for a similar tank system with improved armor protection mounting an 85mm armament. The resulting "IS-85" designation marked the initial batch of 85mm-armed heavy tanks that shortly became the "IS-1" when Marshal Kliment Voroshilov had fallen out of political favor with his government superiors. The new tank series was therefore renamed to "Josef Stalin" in honor of the Soviet leader. IS-85 prototypes emerged in the fall of 1943 and shared the same hull and running gear as the KV series but its armor protection was vastly improved thanks to a new welded turret. Additionally, Soviet engineers managed to create a lighter tank product than the KV-85 which required a less powerful engine while promoting a higher top road speed. The IS-85 then formally became the serial production-minded "IS-1".
Work continued on ever-improved forms of the new IS series as the war waged on. This eventually produced the "IS-100" prototype mounting a 100mm BS-3 main gun. The IS-100 was completed and evaluated in relatively small numbers. Another armament alternative soon emerged in the 122mm A-19 main gun and both systems were tested head-to-head. While the BS-3 fared better in armor penetration qualities, it was not available in the numbers required and lacked maximum utilization of available HE (High-Explosive) projectiles. The A-19, on the other hand, was available in quantity while presenting adequate anti-armor qualities though able to deliver HE ordnance to great effect. It was then sanctioned that the A-19 should become the new main gun of the new IS tank variant. The pilot form of the new IS tank became the "IS-122", the designation denoting its use of the 122mm gun barrel. Upon entering serial production in October of 1943, the IS-122 became the "IS-2". Production of some 2,250 examples spanned from 1943 into 1945 out of the Kirov Factory while UZTM heavy industries was also tapped for manufacture to keep up with Red Army demand. First combat action was recorded in April of 1944 when IS-2, operating under the banners of the 11th and 72nd Independent Guards Heavy Tank Regiments, fared well against German Panther tanks - for all intents and purposes, the legacy of the Joseph Stalin tank had begun.
Design of the IS-2 was typical of the Soviet methodology concerning mechanized warfare - be it in World War 2 or in the Cold War years thereafter. A pair of track systems dominated the vehicle sides and worked in conjunction with six road wheels to a track side. The drive sprocket was at the rear with the track idler at the front and three track return rollers guided the upper portion of the track. The glacis plate was well sloped and covered over in thick armor. The hull roof was relatively low to which a rounded, thickly armored turret was set onto its bustle. The turret - situated well-forward in the design - sported a thick gun mantlet for maximum forward protection at this vulnerable spot and a long-barrel 122mm main gun fitting. The gun protruded out over the bow of the hull. The engine, a diesel-fueled, 12-cylinder Model V2 of 600 horsepower, allowed for top speeds of 23 miles per hour in ideal conditions and ranges out to 150 miles. The engine was mounted in a rear compartment within the hull. Additional fuel stores could be carried externally along the rear sides of the hull. The vehicle weighed in at 51 short tons and sported a running length of over 32 feet with a width of over 10 feet and height nearing 9 feet. She was crewed by four personnel to include the driver, tank commander, gunner and loader. The driver managed a position in the front center hull. Armor protection for the vehicle ranged from 30mm to 160mm across the various facings. Original turrets were welded whilst May of 1944 brought about a cast form. A 7.62mm DT machine gun was fitted coaxially to the turret. Another 7.62mm DT machine gun was fixed to the turret rear, meant to contend with enemy infantry attempting to assail the vehicle from the vulnerable rear. Only later IS-2 marks made use of the 12.7mm DShK heavy machine gun on the turret roof for anti-aircraft defense.