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The Most Numerous Heavy SPG

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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.

The People's Commissariat of Tank Production expected the first IS-152 SPGs from ChKZ on November 26th, 1943. Production was being set up as the last SU-152s came off the line. Assembly of the first IS-152s was difficult as the process was not yet ironed out. The designers and technologists too a part in assembly. The designers were needed since there were mistakes in the blueprints and parts made according to them could not be used. About 200 mistakes were discovered and corrected.

There were also issues with the finished vehicles. The driver's compartment was tighter than it was supposed to be, the efforts on levers and pedals were high, there were cases where the ball bearing separators on the planetary turning mechanism were destroyed. There were other issues that required correction.

The maximum gun elevation was 18 degrees instead of 20. At traverse of 7 degrees the panoramic sight pressed against the hatch edge. The gun was not balanced, as a result of which significant effort was required to elevate it. All of this meant that a lot of effort had to go into the QA process. Nevertheless, ChKZ successfully delivered their first five vehicles in November. 

The situation slightly improved in December. Out of 30 vehicles accepted, 19 were accepted after the first test run and 9 after the second. The vehicles were also reaching their end users. On December 31st 15 IS-152s departed for the artillery center in Mytishi and another vehicle was sent to Kubinka.

Production vehicles were not much different from the prototypes. The most noticeable change was the addition of external fuel tanks. 

ISU-152 front hull produced at UZTM. The characteristic casting marking can be seen.

The factory was overfulfilling its quota in the first month of 1944. In addition to 50 IS-152, two extra SU-152s were delivered. 65 were delivered in February, including 15 built in the previous year. A serious reorganization was taking place at the factory. Preparations for mass production of SPGs were underway. This had an impact on the factory's work, but the first positive outcomes were already seen in January. Assembly time for one vehicle dropped from 3 days to 24-30 hours for some teams. Quality of the vehicles also went up. Seven SPGs were accepted with no defects, nine with one defect, 18 with two defects, and 9 with three defects.

Not everything was perfect when it came to quality. This was confirmed by reports from end users. The factory knew about this and continued to take measures to improve quality. Changes were also made to the design, in part the connections for hoses were improved and bowden cables were given metallic conduits.

23 vehicles were delivered in the first 10 days of February, 45 in the first 20 days, and 75 for the whole month. The effect of reorganizing production was noticeable.

In order to avoid confusion between tanks and SPGs, the vehicles were called ISU-152 starting in February.

The rate of production continued to increase. GKO decree #4850ss "On increase of production of IS type heavy tanks and SPGs and supplying them with armament" called for 150 ISU-152 in March of 1944. Production of the T-34 tank at ChKZ ended on March 20th, which freed up production capacity for more SPGs.

The rapid rise in quotas made work harder, but the factory delivered vehicles reliably. Due to the end of T-34 production, a number of production stages (in part, the casemate and QA stages) were moved to workshop SB-34. Design changes continued. The right fuel tank was changed to make it possible to replace the idler crank.

A complete UZTM hull.

March was the peak of ISU-152 production. The cause was the production of the ISU-122 (Object 242) that started in April of 1944. There is a myth that these vehicles were built because of a shortage of ML-20S guns, but that is not the case. The Red Army wanted a tank destroyer with the effective 122 mm A-19 gun back in the spring of 1943.

The launch of ISU-122 production did not cause many issues, as the difference between the ISU-122 and ISU-152 was largely in the gun and ammo racks. Even the ML-20 and A-19 had many parts in common. ChKZ gradually increased production to 250 tanks and 250 SPGs monthly, as planned. ISU-152 production decreased. Only 130 were delivered in April. Changes were still being made to the design, for instance the latches on ammunition racks were improved.

Hulls en route to Chelyabinsk.

As mentioned above, only factory #200 produced hulls and casemates at the start, but UZTM joined in starting in November of 1943. At first they produced the front hull casting and mantlets, but work to set up full IS-152 hull production began in January of 1944. Factory #200 was ramping up IS-2 hull production. In January of 1944 they delivered 70 ISU-152 hulls and turrets, 56 in February, 35 in March, and zero in April. UZTM became the sole producer of ISU-152 hulls.

Factory #178 produced gun mantlets from January to March of 1944. Factory #264 built casemates.

Since the hulls were assembled at only one factory, there was not a lot of variety in the way various ISU-152s looked. UZTM production hulls had a characteristic casting mark in the front of the hull and different handrails. Rather than long rods, individual small handrails were welded on to the casemate (two per side). The spare track links were moved from the rear to the front of the hull in June of 1944.

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.

Reorganization at ChKZ took place all spring, which had an impact on production. 135 ISU-152 were delivered in May of 1944. ChKZ's total output increased thanks to IS-2 and ISU-122 production. Only 12% of required vehicles were produced in the first ten days, and after May 20th workers from other workshops and even other Chelyabinsk factories were used to finish off May production. This extreme measure had an impact on quality, but there was no other way.

The design kept improving. The main clutch pedal was moved forward, the ventilation fan disk mount was strengthened, new straps for the driver's seat back were introduced. 130 ISU-152s were built in June, although overall production was going up. 20% of the monthly quota of 450 tanks and SPGs was delivered in the first 10 days.

The situation was made more complicated due to optics; there was a shortage of sights. Design changes continued. In addition to the moving track links, a stowage bin was added for the crew's belongings, the exhaust grille shutters increased in pitch, and a transverse gearbox adjustment was introduced. 100 ISU-152 and 175 ISU-122 were planned in July, but the plans changed because of a shortage of A-19 guns. Only 140 ISU-122s were produced. The amount of ISU-152s increased to 135.

DShK AA machine gun. It appeared on the ISU-152 in late 1944.

The factory finished delivering its June quota on July 10th with no spare parts buffer remaining. As a result, the July quota was only delivered by August 11th, and the quotas only increased. This had an impact on quality as well as deadlines. The SB-2 workshop rejected 8600 parts and assemblies. There were also complaints about hulls, which were not up to spec.

Design improvements continued in July. In part, drive sprockets with different teeth were introduced that month.

ChKZ reached the planned production volumes of 250 ISU and 250 IS-2 vehicles in August. The factory was now building three types of SPGs. In August, it built 100 ISU-152, 125 ISU-122, and 25 ISU-122S. Thanks to a second assembly line and other measures, the August quota was completed on August 31st. The quality of production rose again. Once more there were tanks and SPGs delivered with no defects at all, 16% of the total. Small changes to improve sealing of the hatches were introduced in August.

The hull with a welded front was introduced in February of 1945.

Rates of production began to stabilize starting in September of 1944. The factory delivered 250 IS-2, 100 ISU-152, 100 ISU-122, and 50 ISU-122S monthly. These numbers didn't change until the very end of 1944. In December, the factory surpassed quota, delivering 110 ISU-152s. The vehicle continued to improve, which impacted quality. In September, the number of road wheel bolts increased to 12. Locks were added to the escape hatches, th driver's switchboard got a protective cover, and the tachometer mount changed. The most noticeable change in late 1944 was the introduction of a DShKT AA machine gun. It was mounted on a pintle mount above the commander's hatch. The ISU-122 got these machine guns first, in practice the ISU-152 only started getting them in December. All new IS-2s and ISUs were equipped with these machine guns as of January of 1945. This was a useful new addition, as there were plenty of targets for the DShKT, both on the ground and in the air. Until its arrival, crews often had to fight off enemy infantry with personal weapons.

A late February 1945 production hull after penetration trials. Trials showed that the welded front hull was more robust than the cast one.

The ISU-152 production quota was once again surpassed in January of 1945. Due to a lack of A-19 guns, the factory produced 90 ISU-122 and 110 ISU-152. 100 ISU-152 were produced in February and March. As with the IS-2, MDSh smoke bombs were installed on the ISU-152 as of February 1st, 1945.

Typical ISU-152 produced at ChKZ in the spring-summer of 1945.

The front of the vehicle underwent noticeable changes. The IS-2 front hull was stamped at UZTM at this point, while the ISU front hull was still cast. A decision to assemble the front of the ISU hulls from rolled plates was made in February of 1945. Trials showed that the robustness of the front hull increased. The upper part could not be penetrated by the 88 mm Pak 43 L/71 at a range of over 250 meters. The front of the ISU-152 hull became welded in late February/early March of 1945. This change was the last major one in the vehicle's design.

Splash guards were later added to improve the driver's job off-road.

The IS-2 began to make way for its replacement, the IS-3, in the spring of 1945. This was not the case for SPGs on the IS-2 chassis. They were still in production, although in slightly reduced numbers. 95 ISU-152 were delivered in April, May, and June. 100 more were built in July. Due to a shortage of guns, 50 vehicles were delivered in August instead of 80 (and 70 ISU-122S instead of 50). 

ISU-122 production ended in August of 1945. In September, the factory delivered 75 ISU-152 and 30 ISU-122S. The ISU-152 remained the only SPG in October, when 85 vehicles were built. 100 ISU-152s were built in November, but the SPG's time was coming to an end.

UZTM delivered its last ISU-152 hulls in November and in December ChKZ delivered its last 65 vehicles. In total, ChKZ delivered 1100 ISU-152 in 1945 overall. It's worth mentioning a small change made in July of 1945: the splash guard. This change made it easier to drive through muddy areas since the driver's observation port was protected from mud.

An analogous vehicle at the NIBT Proving Grounds, summer of 1945.

It is worth mentioning the second factory where ISU-152 production began. GKO decree #5959ss "On organization of IS tanks, armoured hulls, and restoration of metallurgical production at the Kirov and Izhora factories in Leningrad" called for the Kirov factory to resume tank production in the fall of 1944. ISU-152 SPGs were built here as well. Tank production was not as successful. The first IS-2 was assembled here from a kit in 1944 but only accepted in the spring of 1945. In total, only 10 IS-2s were built in Leningrad.

The situation with the ISU-152 was also looking grim at the start. The Kirov factory and its hull supplier, the Izhora factory, did not complete their quota in 1944. Some progress was seen in the spring. The factory delivered five IS-2s with great effort, and even these were made from hulls produced at factory #200 in the first half of 1944.

Assembly of the first ISU-152s at the Kirov factory, April 1945. These vehicles have welded hulls.

The ISU-152 had no backlog of hulls to draw from. They all came from the Izhora factory. The overall design of Izhora hulls was the same as at UZTM. The Izhora factory did not produce cast hull fronts and used rolled plates from the start. This made production much simpler, since the factory was having a hard time with castings.

Early Leningrad vehicles had a problem with quality.

The first five ISU-152 built in Leningrad were delivered in April of 1945. As of May 1st, the factory had 16 more vehicles that were assembled but not accepted by the customer. The factory also produced 15 gearboxes and turning mechanisms, 16 final drives, and other components that month. There were significant issues with some of them, which explains the miserly deliveries. The quality of ISU-152s built in Leningrad left much to be desired. 

Because documentation transfers were late, the ISU-152s produced at the Kirov factory did not have DShK machine guns or MDSh smoke bombs. The factory planned to begin equipping its vehicles fully after the 50th vehicle, but the GBTU refused to accept any more than 15 vehicles like this (5 in April, 5 in May, and 5 in June). All subsequent deliveries had to be equipped just like the Chelyabinsk-made SPGs.
A Leningrad-built ISU-152 on trials. The vehicles looked like this from the summer of 1945 until the end of production.

The Leningrad ISU-152s were more expensive than the ones built in Chelyabinsk, 300,000 rubles versus 230,000. That was the cost of setting up relatively small scale production. Nevertheless, the small stream of vehicles coming out of Leningrad increased. The factory delivered 5 vehicles in April, May, and June. This number was taken off the ChKZ quota. 10 vehicles were built in Leningrad in July and August, 15 monthly in September through November, then 20 in December. 100 ISU-152s were built in Leningrad in 1945.

Kirov factory continued to build ISU-152s even after production stopped at ChKZ. 200 ISU-152s were built in 1946 and 50 more in 1947. In total, the Kirov factory delivered 350 ISU-152s. To compare, the closest equivalent to the Soviet heavy SPGs, the Jagdpanther, was only built in a series of 314 units with the combined efforts of three factories. 

2815 ISU-152s were built from November 1943 until the end of 1947. This was the most numerous heavy SPG in history. The second place is occupied by another Soviet vehicle, the ISU-122.


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