The new vehicle was based on the IS. The technical requirements for this project were: 100 mm of front armour, the same 152 mm gun, addition of a machine gun, improved optics, and improved ventilation. The designers were given until July of 1943, but finished much earlier. Mere weeks were spent on creating the blueprints. At the beginning of July, a prototype started construction, designated IS-152.
Various historians place the demonstration of the prototype vehicles on either July 31st or August 31st of 1943, on the Ivanovskaya Square in the Kremlin. Stalin, Beria, Molotov, Voroshilov, all came to familiarize themselves with this new vehicle. In order to guarantee the security of these important persons, all crew members were replaced with NKVD staff, excluding the driver. Stalin, very interested in the new SPG, decided to examine it closely. Looking inside the IS-152, Stalin asked if the problem with poor ventilation was solved. Obviously, the NKVD crewmen could not answer the question, since they were not very well learned in armoured vehicles. The driver piped up in time to report to Stalin that there is an additional fan installed to clear the crew compartment of gases. Satisfied, Stalin cleared the new vehicle, and it was accepted by the State Committee of Defense in November of 1943.
By that time, the first prototype of the SPG, given the name "Object 241", was undergoing field and factory testing. This vehicle became the template for the production SPG. The new vehicle saw combat under the name ISU-152. The design of the tank combines the existing solutions from the IS and SU-152 SPG.
The tank gave the ISU-152 its suspension: the same six paired road wheels, the same rear drive wheel, the torsion bars. The SU-152 gave its ML-20S model 1937/43 howitzer. The 152 mm gun was equipped with AP and HE rounds. When necessary, the SPG could be equipped with concrete piercing shells, for the destruction of fortifications. The single loader of the ISU-152 had a hard time moving around 40 kilogram shells.
The SPG had a diesel V-2-IS engine, with 520 hp. With it, the vehicle achieved road speeds of up to 35 kph and 10-15 kph off-road. It didn't need to set any speed records; the vehicle was not made for rapid marches.
The production of the ISU-152 started in November of 1943. The new SPG was very similar to its predecessor, and production speed was high enough that only one month was required to form the first heavy SPG regiment of ISU-152s. By the spring of 1944, the ability to produce the hull surpassed the ability to produce the gun. Surplus hulls were armed with a 122 mm gun. This gave birth to another heavy SPG, the ISU-122.
First seeing combat in the spring of 1944, the ISU-152 proved itself to be an effective and universal weapon. They were used as an assault gun, supporting tanks and infantry, and as tank destroyers. There were also instances of the ISU-152 firing indirectly from rear positions, but this was not widely done for two reasons. The gun elevation angle was not large, and the SPG could only fire in a very flat arc. Secondly, it had a small ammo rack, only 21 shells. Extra shells had to be placed near the SPG, and then, once the ammo rack ran dry, replenish it, which took about an hour. Alternatively, the shells could be handed to the loader one by one, further reducing the ISU-152's already non-exceptional rate of fire, resulting in lower effectiveness.
The separate loading process of the gun was a large weakness, due to which the ISU-152 could not be a very effective tank destroyer. However, it still earned the reputation of a feared opponent. In the Red Army, it was nicknamed Zveroboy (beast-slayer, hunter). The Germans called it Dosenöffner (can opener).
An example of the ISU-152's effectiveness can be seen in the battle of Katukov's 1st Guards Army near Nizhnuv Place in the Transcarpatians. The Germans, with 40 Panther tanks, penetrated the Soviet defenses and threatened the city of Chernovtsy, encircling Katukov's forces. In order to prevent this, a regiment of ISU-152s, deployed on a hill in the most likely direction of attack, spent several hours fighting the advancing Germans. In the end, the Germans retreated, leaving behind 30 tanks.
The SPGs also behaved very well in city battles. Powerful HE-fragmentation shells allowed liquidation of a dug-in enemy with only one shot. In order to protect the vehicle from Panzerfausts, ISU-152s were often used as a part of assault groups, with an infantry escort.
With all of its advantages, the ISU-152 had some disadvantages. The extra vent that Stalin was so interested in did not completely solve the problem of gas concentration. At peak rate of fire, it was hard to breathe inside the vehicle.
The loader's work was very difficult, with him having to move heavy shells by hand, in a relatively cramped space. The panoramic sight was inconveniently placed, making it difficult to aim at targets past 900 meters. The internal gas tanks increased the risk of the crew going up in flames along with the tank, or blowing up the gunpowder gases in case of a fire. Thankfully, according to documentation, it was very easy to put out fires on the ISU-152.
Even all those disadvantages did not overcome the positive qualities of the SPG. The ISU-152 remained a part of the Soviet Army for a very long time. The last time it saw battle was during the Hungarian Uprising in 1956. The commanders of the first stage of the operation did not show their best side, and about 10 ISU-152s were lost to incendiary bottles. The exact figures are still unknown. After that, the ISU-152 saw no more combat, but was used in training.
The last ISU-152s were decommissioned by the Soviet Army in 1972.
Original article available here.