The IS-1 was born out of a Soviet Army need for more formidable tracked weapon systems to combat the ever-growing power of German tanks - particularly the newer Panther and Tiger I series appearing by the end of 1942. Initially, the KV-1 and KV-2 series were a limited answer but these ultimately proved too cumbersome in the heat of combat and generally not highly regarded by their crews - particularly the high-profiled KV-2. Both lacked the necessary mobility needed for ever-changing fronts of modern warfare concerning World War 2 and proved expensive to produce in the large numbers required at the time. Therefore, Soviet engineers took to designing the "KV-85" tank as an interim solution while a new initiative was begun under the "KV-13" designation.
The KV-13 program produced the "IS-85" prototype (Objekt 233) and featured a more stout armor layout, thusly increasing protection at all angles, and a heavier main gun armament. The armor was specifically designed to counter the 50mm German anti-tank gun. Her weight limit was restricted to that of the original KV-1 model to help keep the design in check. Primary armament was an 85mm D-5T series main gun fitted to a traversing turret system atop a tracked, low-profile chassis. Six double-tired wheels were afforded to each track side. The drive sprocket was at the rear with the track idler at the front. Three track return rollers assisted the top portion of the tracks. External fuel tank stores were mounted to the rear hull sides. The turret was held well-forward in the design (as in the T-34 medium tank) and sported thick, curved armor facings with some sloping. A commander's cupola was fitted along the rear left side of the turret roof. A 7.62mm DT coaxial machine gun was fitted alongside the main gun and a defensive, rear-facing DT machine gun was mounted along the rear turret wall, offset to the left side of the turret. A further DT machine gun was provided in a bow mounting. Fifty-nine 85mm projectiles were stored throughout the vehicle with 2,520 rounds of 7.62mm machine gun ammunition. Power was provided by a single V2-IS 12-cylinder engine delivering 520 horsepower. Top speed was 23 miles per hour while operational ranges were limited to just 93 miles - both values factoring in ideal conditions at that. The IS-1 was crewed by four personnel to include a driver, tank commander, gunner and loader/machine gunner.
After hurried trials, the IS-85 system was accepted into serial production as the "IS-1" beginning in 1943. This also marked the start of the long line of successful and powerful "Josef Stalin" heavy tanks that would prove critical in the latter war years and even well into the Cold War. The Soviet need for capable heavy tanks was such that the KV-1 , KV-2 and KV-85 types continued to fight on through to the end of the war while the IS-1 lineage itself evolved as the war progressed.
The IS-1 was inevitably (and quickly) replaced by the more well-known IS-2 series and its 122mm A-19 series main guns. The IS-3 then appeared in late 1944 as an upgraded IS-2. Only 200 examples of the IS-4 followed and there was no IS-5 design. Instead, the IS-6 began an abandoned design project as did the IS-7. The line culminated in the Cold War-era IS-10 model, which marked the last of the Josef Stalin heavy tank lines as a shift to the Main Battle Tank had taken hold worldwide - making the concept of a heavy tank somewhat archaic. However, many lessons learned in the battles of World War 2 involving the IS series were put to practical use in the development of the new generation of Soviet tanks.
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