Soviet heavy SPGs were initially designed to combat fortifications. When the war came, there turned out to be a much larger variety of targets for them. Wartime heavy SPGs were very different from what was initially envisioned. The Red Army needed universal vehicles that could both fight powerful fortifications and enemy heavy tanks. One of these vehicles was the ISU-152, a replacement for the SU-152 on the IS tank chassis. The ISU-152 became the most numerous heavy SPG in history.
Same thing on a new chassis
The KV-14 (SU-152 as of April 14th, 1943) was the best design out of the trio of SPGs designed in late 1942/early 1943. It didn't have the obvious issues with reliability like the SU-12 and it was more comfortable for its crew. However, there were still issues with the SU-152. The vehicle was improved over the course of production, but no radical modernization was performed. The desire to widen the fighting compartment resulted in a few drafts, but nothing more.
|Tactical-technical requirements for the ISU-152 based on the IS-1 (Object 233) chassis. May 1943.
The lack of radical alterations was due to the fact that the vehicle largely met requirements. Unlike the KV-1S, the SPG built on its chassis did not seem obsolete. Its armament was sufficient to defeat any German tank. It was thanks to this armament that the SU-152 outlasted not just the KV-1S but even the KV-85 in production. The last vehicles of this type were delivered when the IS-85 was already coming off the assembly line.
The fact that the IS-85 was accepted into service with the Red Army meant that the age of any vehicle on the KV-1S chassis was ending. GKO decree #4043 issued on September 4th, 1943, both accepted the IS-85 into service and set a requirement for an SPG on its chassis. A prototype was due by October 15th and trials were to be held by November 1st. The first five vehicles designated as IS-152 were to be delivered in November of 1943, and 30 more in December. These tight timelines are explained by the fact that the vehicle was designed on the IS-85 chassis which was not going to be significantly altered.
|Installation of the armament in the IS-152, late August 1943.
Decree #4043 gave an official green light for development of the IS-152, but really work began at experimental factory #100 back in August of 1943. The topic of a "SU-152 SPG on the IS tank chassis" appeared back in May of 1943. This was the IS-1 tank that is often erroneously called Object 233. The new vehicle would weigh 44-45 tons, have 100 mm thick front hull and casemate armour, and carry a V-2K engine. Since the IS tank went through significant changes after that, the requirement to develop a tank destroyer requested by a GKO decree was left on paper. The work on the SPG also stalled.
Towards the end of August of 1943 it became clear that the Object 237 was a success. Factory #100 began working on the IS-152. The project was lead by A.S. Yermolayev with G.N. Moskvin as the lead engineer. He had a lot of experience with heavy tanks and SPGs on their chassis. The design group included Ye.P. Dedov and A.S. Schneidman, who worked on the KV tank at the Kirov factory, just like Moskvin. K.N. Ilyin, another engineer with a lot of experience in self propelled guns, led the design of the gun mount at UZTM.
|The ISU-152's fighting compartment was larger than the SU-152's.
Development took the experience gained with the SU-152 into account. The fighting compartment ended up a bit roomier as a result of a vertical rear plate and altered sides. The slope of the side plates was reduced from 25 to 15 degrees. The rear side plates were lengthened, which freed up some room. The fuel tank was moved from the right side to the front left corner, next to the driver. This was a strange choice, but the fuel tank took up less space this way. This made the work of the commander and breech operator easier.
|The first IS-152 prototype, ChKZ, October 1943. No handrails are installed yet. The vehicle went through factory trials in this form.
The vehicle received the name IS-152 and blueprints index 241. It was also called Object 241 in correspondence. Blueprints from 14 groups out of 45 were ready by September 11th, 1943. Parts that could be taken from the IS tank were being selected in parallel. Production of components for the SPG was scheduled to be finished by September 25th.
Assembly of the prototype took place at factory #100, but it depended on subcontractors. The hull of the first vehicle only arrived from factory #200 on October 4th and it did not meet requirements. The front hull of the casemate was only 75 mm thick instead of 90. A suggestion was made to produce the front hull from 100-105 mm thick rolled armour rather than cast. This suggestion was declined. There was also a proposal to build two SPGs with Br-2 guns as a gift for the Red Army. This proposal was approved by Kotin and Br-2 guns were even issued for the project, but the SPGs were never built.
As for the IS-152 prototype, it was tested with dummy weights to increase its mass to the proper 46 tons. There were some issues that came up during the mounting of the gun. It could only traverse within a 10 degree arc rather than the required 14 degrees. The gun mantlet was also only 60 mm thick, like on the SU-152. This made the large component relatively vulnerable. A plan was made to thicken the gun mantlet, but this was never done.
|The same vehicle at the Gorohovets ANIOP. The handrails are already installed.
Despite all these issues, work was proceeding ahead of schedule. Assembly was due on October 12th, but factory #100 managed to finish a day early. Gunnery trials and 105 km of driving trials were held from October 11th to October 14th. The vehicle did not have handrails at first, they were only installed right before it was sent to proving grounds trials. The design of the handrails was similar to the ones used on the SU-152.
|The gun at maximum elevation. This opened a gap under the gun mantlet that could result in the crew getting struck by bullets or shell splinters.
On October 15th the vehicle was loaded onto a train and shipped to the Gorohovets ANIOP. The trials program was approved back on October 1st. 481 shots were planned, 177 of which were supercharged, 259 at full charge, and 45 with a special charge. Since the trials were urgent, they were expedited to take place from October 21st to October 23rd. The number of shots was reduced to 303: 1 with reduced charge, 187 with a full charge, and 115 supercharged. Firing trials showed that the precision of the IS-152 and towed ML-20 gun-howitzer were about equal. Firing on the move was ineffective, but firing from short stops was satisfactory. The aimed rate of fire was measured at 2-2.5 RPM.
|IS-152 on mobility trials at the NIBT Proving Grounds in Kubinka.
The crew's working conditions were evaluated separately. The ammunition racks were evaluated to be convenient and it was comfortable to service the gun. It took 12-15 minutes to refill ammunition during the day or 17-18 minutes at night.
There were also complaints. One extractor fan was not enough to get the gun fumes out of the fighting compartment. The guard rail around the gun was also insufficient and the commander could be injured. The gunner's station was uncomfortable, especially his seat. A suggestion was made to make his seat folding and lower it. The mount for the panoramic sight also needed to be changed.
The breech operator had another problem. The breech was tight to open and could not be opened at all at certain aiming angles. A proposal was made to install a sliding breech. Covers over the control rods and wiring meant that the height of the fighting compartment varied from 1760 to 1600 mm, which the testers didn't like either. Most of these complaints also applied to the SU-152. Compared to that SPG, the fighting compartment of the IS-152 was better. These complaints did not mar the overall picture. The Gorohovets ANIOP commission deemed that the IS-152 passed gunnery trials.
|The production IS-152 was almost identical to the prototype.
Mobility trials took place at the NIBT Proving Grounds at Kubinka from October 28th to October 31st, 1943. By then the vehicle had already travelled 175 km. The IS-152 drove for another 410 km, 232 of which were driven on a highway and 178 on dirt roads. The average movement speed was 23.4 kph and fuel expenditure was 225 L per 100 km on paved roads and 18.2 kph with 330 L burned per 100 km on dirt roads. Without external tanks, this gave the vehicle a cruising range of 220 and 150 km respectively. The suspension and engine group were sufficiently reliable. As with tanks, the main issue was the lifespan of the track links.
|Location of the spare track links. They remained on the rear lower plate until the summer of 1944.
The vehicle also passed mobility trials. The commission attached a list of required changes to their report, but this was not a significant obstacle in the acceptance of the IS-152 into production.
The SPG that outlived the IS-2
Stalin signed GKO decree #4504ss "On the heavy ISU-152 SPG with the ML-20S gun-howitzer" on November 6th, 1943. The document called for the start of production while keeping the total numbers the same as outlined in decree #4043. The first vehicles were very expensive. Each one cost 400,000 rubles, considerably more than the SU-152. The price dropped to 350,000 in Q1 of 1944 and 295,000 starting in April.
Preparations for production proceeded at a glacial pace in October of 1943, especially when it came to hull production. The only factory supplying hulls at the time was factory #200, which was already at the limit of its production volume. It was supposed to deliver 70 SU-152 hulls and casemates, 57 KV-85 hulls and turrets, and 15 hulls and turrets for IS-85 tanks. The factory concentrated on finishing its SU-152 and KV-85 deliveries without producing a single IS-152 set, even though five were due in October. Interestingly enough, correspondence traditionally called the casemates "turrets".
Mass production of the IS-152 began in November. The factory delivered 30 sets of armour components with a quota of 26 and 46 more sets in December. December was a pivotal month, as the mess with production of 3-4 different chassis simultaneously ended. The IS-85 and IS-2 only had different turrets and could be considered one tank for the purposes of production.
|A typical ISU-152 produced from late 1943 to Q1 of 1944. The characteristic handrails indicate that the hull was produced at factory #200.
|ISU-152 front hull produced at UZTM. The characteristic casting marking can be seen.
|A complete UZTM hull.
|Hulls en route to Chelyabinsk.
|Typical spring-summer 1944 production ISU-152. The spare track links were moved from the rear to the front, but not on all vehicles simultaneously.
|DShK AA machine gun. It appeared on the ISU-152 in late 1944.
|The hull with a welded front was introduced in February of 1945.
|A late February 1945 production hull after penetration trials. Trials showed that the welded front hull was more robust than the cast one.
|Typical ISU-152 produced at ChKZ in the spring-summer of 1945.
|Splash guards were later added to improve the driver's job off-road.
|An analogous vehicle at the NIBT Proving Grounds, summer of 1945.
|Assembly of the first ISU-152s at the Kirov factory, April 1945. These vehicles have welded hulls.
|Early Leningrad vehicles had a problem with quality.
|A Leningrad-built ISU-152 on trials. The vehicles looked like this from the summer of 1945 until the end of production.