By the middle of 1942, the Soviet Army - now on the offensive against the German/Axis invaders - was in dire need of a self-propelled, anti-siege weapon system for which to defeat the network of fixed fortifications along the Eastern Front. Up to this point in the war, the Army was still relying on horse- and vehicle-drawn howitzers for the role but this exposed the crew (and transport vehicles) to all manner of battlefield dangers. It also became increasingly difficult to transport such weapons over soft terrain in the Soviet winter/spring and furthermore these existing guns were Non-Line-of-Sight (NLoS) howitzers through-and-through - they could not be effectively used in double-duty as armor-defeating weapons.
The Soviet State Defense Committee began to research solutions and eventually put out a requirement for a heavy assault gun that could be built in quantity and also in a short amount of time. The project began on December 31st, 1942 and, in just a period of twenty-five days, results were already being witnessed. Soviet engineer Joseph Kotin combined the 155mm ML-20 (Model 1937) heavy gun-howitzer to the chassis of the existing KV-1 Heavy Tank - a vehicle which was nearing the end of its battlefield life and production cycle. This produced the "Object 236" identifier from the No.100 factory at Chelyabinsk. The pilot vehicle was evaluated during early 1943 and placed quite quickly into serial production as the "KV-14" soon after. In April of 1943, the vehicle was redesignated to the better-remembered "SU-152".
The SU-152 retained the automotive drive components of the original KV-1 including its track-and-wheel arrangement. Instead of the traversing turret however - which could not take the 155mm gun - the SU-152 was finished with a thick, fixed boxy superstructure set over the hull. The massive gun protruded a distance away from the frontal panel which measured 75mm thick for maximum protection. The sides were completed in 60mm thickness and the roof was protected in just 20mm of armor. As the SU-152 would be called upon to engage both fortified enemy positions as well as well-armored tanks, the frontal protection was a stated requirement of the design.
Internally there was a crew of four or five operators and point-defense against aircraft, light-armored vehicles and infantry was from an optional 12.7mm DShK heavy machine gun (set over the front-right side of the hull superstructure. The vehicle was powered through a Model V-2K engine of 600 horsepower output which was installed at the rear of the hull. Suspension was by way of a torsion bar arrangement, providing some cross-country support, and the vehicle could hope to make speeds of up to 27 miles-per-hour on roads.
Since the size of the main gun (and its accompanying breech) meant that it was to be slightly offset-right from centerline, the driver's position was left of it. The gunner and ammunition loader had positions behind the driver along the left side of the hull superstructure with quick access to ready-to-fire 155mm projectiles. The vehicle commander, as well as the breech operator, were positioned to the right of the hull superstructure. All told, the fighting compartment was a cramped, crude workspace with few creature comforts for the crew.
By this time, the German Army had begun fielding its mighty "Tiger" heavy tanks in greater numbers along the East Front and this terrorizing vehicle was both well-protected and well-armed, the latter through the classic "German 88" - a proven 88mm armor-defeating weapon that began its service career as an Anti-Aircraft (AA) weapon. Armor protection on these machines reached 100mm but the series proved itself mechanically unreliable and was not available in the numbers ultimately required to change German fortunes.
With the arrival of the SU-152, the Soviet Army finally had a weapon system capable of leveling the playing field for its 155mm gun could also fire an effective Armor-Piercing (AP) round to defeat the Tiger's stout armor at range. In some instances, the Tiger's turret was completely blown off by the massive 107lb round. The SU-152 crew could hope to reach a rate-of-fire of about 1.5 rounds-per-minute, such was the size of these projectiles and, beyond its armor-defeating value, the weapon could still operate as a howitzer to render lighter-armored vehicles and fortified positions useless even with near-misses from High-Explosive (HE) shells.
Despite its successes on the battlefield, it was clear that there was room for improvement in the SU-152. The KV-1 chassis was well on its way out of the picture and a more modern solution was in order so this came in the form of a revised version of the gun-howitzer weapon being fitted to the chassis of the new "IS" ("Josef Stalin") heavy tanks coming online. The SU-152 was already under consideration for replacement as soon as mid-1943 when development began on this new form and the pilot vehicle, once it had its issues ironed out from testing that occurred in September-November 1943, was adopted as the "ISU-152". Production then began before the end of the year. This vehicle is detailed elsewhere on this site.