Redesign of the IS-1 in order to work out problems but also to reduce the weight, improve the armor, provide an improved shape, and simplify production.
Well-armed and well-armored, its chief shortcoming was the slow rate of fire of the main gun, the consequence of separate projectiles and cartridges.
Early in the autumn of 1943, the Soviets were completing the first of three prototypes of their new IS-1 (IS-85) heavy tank using design experience gained from their previous KV heavy tank projects. These new IS (Iosef Stalin) prototypes were systematically demonstrated before the special commission from the Main Defense Commissariat and with the completion of general factory trials the IS design was approved for production. Although the first production vehicles mounted the 85mm gun also seen on the T-34/85 medium tank, these initial IS-1 tanks were shortly converted to carry the 122mm M1943 D-25 gun before they were provided to the tank troops. The new IS-2 tank weighed little more than its predecessor KV tank, but it had thicker and better-shaped armor that provided vastly improved protection. The overall weight was kept low by using a more compact hull and component design, as we shall see later. Once the 122mm gun was installed and series production continued, the new "Tiger Killer" was officially named the IS-2 Heavy Tank, although its weight and armor characteristics closely coincided with the German Panther medium tank. By the end of 1943, the Kirov Factory had produced a total of 102 IS-2 heavy tanks and they were used for the first time in February of 1944 at Korsun Shevkenskovsky. Although there were a number of external improvements to the IS-2 during its production and field use, there were relatively few internal changes made over the years.
The IS-2 hull was designed after ample combat experience with the KV tanks, and you can see that the hull actually overhangs the tracks. It is an interesting hull design in that the bow casting is welded directly to a circular casting for the base of the turret. Additional rolled armor plates are then added to form the rest of the hull sides, ending at a sloping rear plate that covers the engine and transmission. Both heavy armor castings and rolled plate are utilized in the hull, and the castings in the bow provide around 122mm (4.7in) of frontal armor. This initial bow design was based on the earlier KV-13 tank layout.
Although the first IS-2 vehicles used this curved and gently sloping front bow casting, it was later replaced in 1944 with either a new casting or a welded plate nose, both of which had a straight 60-degree slope from glacis to the top of the hull. The Uralsky Factory of Heavy Machinery (UZTM) plant made the welded noses, while factory #200 made the cast types. The IS-2 was one of the first production Soviet tanks to remove the traditional second driver/hull machine gunner from the bow of the tank, providing additional space for fuel tanks. In place of a ball mounted MG on the front plate, a DT machine gun was mounted on the right side of the hull behind the driver and up near the turret ring. It was fired via a remote control firing cable from the driver's position.
The new tank design illustrates the Soviet's mid-war combat strategy reorientation from using tanks for infantry assault to tank hunting and killing machines. As a result, the second driver in the hull was deemed unnecessary and the crew was reduced from the traditional five soldiers to only four, the driver then placed in a central position in the bow. The commander is located inside the turret at the left rear, the gunner is to the left of the main gun, and the loader is to the right. Both the gunner and loader are provided with over-head periscopes and the commander has a non-rotating turret cupola incorporating vision blocks that provided a 360-degree field of view. He also has a rotating periscope in his cupola hatch. A round roof hatch that is flush with the turret roof is provided for the loader on his side of the turret.
Initially, the 122mm L/43 gun mounted in the cast turret retained its original interrupted screw breech, showing its ancestry from the already proven D-19 field gun. But the screw field gun breech was replaced (by early 1944) with a horizontally sliding block, semi-automatic type, and of course the recoil cylinders and elevating mechanisms were altered from the field gun to fit into a turret. Because the 122mm ammunition rounds were so huge, they were provided in separate pieces, a projectile and a charge cartridge, but even so only 48 complete rounds could be stored inside the tank. Although a number of ready rounds were strapped into easily reached racks in the turret, most of the ammo was stored in sheet metal boxes down on the hull floor and, as we have seen in the T-34/76, these boxes were often covered with rubber floor mats. There was no turret basket in the IS-2; the turret crew seats were either suspended from the turret and rotated along with it (commander's) or the seats were supported on tubes that rose from the center of the floor and also rotated with the turret (gunner and loader).
By the time the IS tanks were being manufactured, the Soviets had plenty of technical experience with casting large pieces of armor, and the IS-2 turret became one of the biggest castings they manufactured during the war. Although Western writers have tended to criticize the coarse standard of Soviet armor finish, the urgency of tank manufacture in 1943 did not warrant lavishing extra time and energy on unnecessary refinements. Over the course of its production, the turret was gradually changed. The early IS-2 tanks that were manufactured in 1943 were originally designed to have installed a D-5T 85mm gun in their turrets, and they had a narrow opening for the telescopic sight just to the left of the gun. When the 122mm D-25T gun was placed inside these same turrets, it was very hard for the gunner to use his telescopic sight, as it was so close to the gun. So, in mid-1944 a new turret with a larger sight opening that was also shifted slightly to the left was produced. Also at this time the thickness of the turret's mantlet was increased, along with the lower hull sides. The new turret also moved the commander's cupola slightly to the left and the gunner's PT4-17 periscopic sight in the turret roof was changed over to a Mk.4 type. About this time a Model 1938 12.7mm DShK anti-aircraft machine gun was installed outside on the commander's cupola to provide some protection from strafing German aircraft.
Production dates: April 1944–June 1945
Number produced: 2,250
Manufacturer: Factory No. 100,
Kirovskiy Works (Chelyabinsk)
Armament: 122mm gun D-25T; 1 x 12.7mm DShK machine gun; 2 x 7.62mm DT machine guns
Weight: 101,184 lbs.
Armor: maximum 129mm; minimum 60mm
Ammunition storage and type: 28 x 122mm; 945 x 12.7mm; 2,330 x 7.62mm
Power plant: V-2-IS (V2-K) V-12 600-hp diesel engine
Maximum speed: 23 mph
Range: 100 miles
Fording depth: 4’3”
Vertical obstacle: 3’2”
Trench crossing: 8’2”