Based on the KV-85
The origin of the IS-1 project came from numerous reports regarding the KV-1. Even in its faster version, the KV-1S, offered no greater fighting capabilities than the T-34, while costing much more and being more labor intensive. Stalin nearly cancelled all heavy tank development in 1943 but, in that summer, the Panther and Tiger were seen in action. Heavy tanks had to be improved, both in protection and firepower, to adequately cope with the new threats.
In March 1943, an order specified the rearming of all frontline tanks. One of the guns intended for this was an AA gun roughly similar in performance to the German 88 mm (3.46 in), of which one had been disabled, captured and the gun analysed. This resulted in the stopgap KV-85 heavy tank, only produced in limited numbers (143 machines). This vehicle set the tone for further improvements, being the basis of the long “Iosif Stalin” series. The KV-85 turret was brand new, well adapted to the new gun designs. It could hold three men, but the hull remained basically the same KV-1 flat-armored hull first designed for the multi-turreted SMK back in 1939.
The KV-13 program
The KV-13 project was launched by SKB-2, Chelyabinsk Kirov Plant’s design bureau, as early as late 1941. It tried to create a “universal tank”, a crossover of the KV-1 and T-34, resulting in the prototype Obyekt 233 tested in the spring of 1942. The tank was ultimately rejected as failing many specifications, mainly reliability, armor protection and for having a two-man turret design. However, the KV-13 program would lead to two other modified prototypes, which would serve to test the new IS-85 program in early 1943.
The IS-85 design
The new IS-85 was to address this issue with a brand new sloped hull design directly inspired by the KV-13. This was a stout armor layout designed to deflect or resist shots at all angles. However, the initial armor was designed to stop only Panzer III 50 mm (1.97 in) rounds, and needed to be massively thickened. The only strict design specification was to not exceed the weight of the original KV-1. The new armament was the same as the interim KV-85, the excellent D-5T 85 mm (3.35 in) gun, which had a far greater range and initial velocity than the previous F-34. The turret was also the same as the KV-85, housing three men, a commander cupola (rear left side) and a rear ball-mount DT machine-gun. A second one was mounted in the hull and the third coaxial with the gun, firing tracers which served to adjust the main gun. Ammo storage consisted of 59 85 mm (3.35 in) rounds and 2520 rounds for the three DT machine guns.
Production of the IS-1
The IS-85 (Obyekt 235) prototype started its successful but rushed trials in mid-1943. Production was assumed by Chelyabinsk Kirov Plant. The first IS-1 (definitive series vehicles) rolled of the line in October 1943, but it was stopped in January 1944, as the IS-1 was quickly replaced by the all-better IS-2. Because of this, only 200 to 207 were produced. However, it was a considerable improvement over the previous KV-85 and the T-34/76. The appearance of the new T-34/85 medium tank caused the IS series to evolve, and to be logically up-gunned. So, in January 1944, many IS-1s not yet delivered to the front were up-gunned. The IS-1 also tested a new 100 mm (3.94 in) gun and went into comparative trials with the IS-122, favorable to the latter. The new A-19 122 mm (4.80 in) gun had the punch to get through the armor of a Tiger at medium range.
The IS-1 in action
The first IS-1s delivered were issued to Guards Heavy Tank Regiments reforming after the summer’s heavy fighting. The IS-85 was issued to the 1st, 8th and 13th Guards Heavy Tank Regiments in Ukraine. These were heavily engaged in early 1944 at Starokonstantinov, Korsun-Shevshenkovskiy and Fastov Station west of Kiev. Later on, most IS-1s were found engaged in Slovakia, and were later given to Slovakian free units during the uprising.
IS-1 links and references
|Dimensions (L-w-h)||6.60 (8.56 with gun) x 3.07 x 2.74 m (28.08 x 9.84 x 8.99 ft)|
|Total weight, battle ready||43.54 tons (95,800 lbs)|
|Crew||4 (commander, loader, gunner, driver)|
|Propulsion||V2-IS 12-cylinder diesel, 2368 cu in/39 liters, 520 hp|
|Speed||37 km/h (23 mph)|
|Suspensions||Transverse torsion arms|
|Range (road)||150 km (93 mi)|
|Armament||85 mm (3.35 in) D5T
3 x 7.62 mm (0.3 in) DT machine guns
|Armor thickness||30 to 120 mm (1.18-4.72 in)|
IS-1, definitive production version in early 1944. Unknown unit.
1st Guards Breakthrough Heavy Tank Regiment from the 11th Guards Tank Corps, 1st Tank Army, 1st Ukrainian Front, Ukraine, March 1944.