Late Replacement

A sketch of an ISU-152 on a modernized IS-2 chassis, April of 1944.

The first attempt at modernizing the ISU-152 was a project designed by N.F. Shashmurin. Little is known about this design. The vehicle had a rear fighting compartment, which allowed powerful 122 and 152 mm guns to be placed more rationally. This vehicle would be built on the same chassis as the IS-2 proposed by N.F. Shashmurin. No work was done past sketches.

The situation became different in the summer-fall of 1944. The Chelyabinsk-based SKB-2 developed improved versions of the IS-2 and ISU-152. Only documentation related to the tank survived. Nevertheless, it's not hard to tell how the vehicle would differ from the production ISU-152. The changes to the engine and transmission compartments of both the modernized IS and ISU were the same. All the fuel tanks were moved into the engine compartment, and the batteries were m toved in their place. The cooling and exhaust systems also changed. Even though work on both vehicles started in August of 1944, the modernized ISU-152 only entered trials in October. By November 16th it had travelled 1000 km. The work ended there since SKB-2 was hard at work on an even more promising modernization of the IS chassis.

The first experimental modernized ML-20SM system. Despite the fact that the first prototype was ready in the spring of 1944, it was not installed in an SPG until a year later.

It was not just the chassis that was modernized. The GAU Artillery Committee began to consider the 152 mm ML-20S gun-howitzer as a suboptimal gun for SPGs towards the end of 1943. It is somewhat ironic that it was the Artillery Committee who rejected the factory #8 project known as the ZIK-20. F.F. Petrov, who led the development of the ZIK-20, proposed removing the muzzle brake and improvement of the system for installation in an SPG back in the summer of 1942. This idea earned no support in the fall of 1942, but returned in the end of 1943 as the ArtKom's idea. They proposed removing the muzzle brake, introducing a sliding breech, changing the cradle and the frame. 

The work was given to OKB-172, which began working on the ML-20SM project on January 13th, 1944. The work was led by the Chief Designer of Land Artillery of OKB-172, M.Yu. Tsyryulnikov. The GAU's requirements were partially met: the muzzle brake was removed, the cradle and recoil mechanisms were shortened, and the recoil brake was simplified.

The same gun installed in the Kirovets-2. Unlike the initial variant, this one has a coaxial DShKM machine gun.

The working documentation on the ML-20SM was completed by March 1st, 1944. By March 10th factory #172 finished the prototype. The first trials were held on March 11th, but the gun was removed from trials after 33 shots due to unsatisfactory function of the breech. Trials held in March-April revealed production defects. Nevertheless, on April 14th the ML-20SM passed high speed rate of fire trials, making 60 shots in 39 minutes instead of the allotted hour. In total, the gun made 249 shots in March-May of 1944, of them 196 supercharged. The rate of fire was up to 2.9 RPM.

The plan was to install the gun in an SPG after the trials, but this never happened. The trials of the gun continued at factory #172 until September of 1944. Meanwhile, the NKTP, GBTU, and GAU argued about the gun. Finally, a decision was made to send it to the Kirov factory to be installed in an ISU-152. The gun reached Chelyabinsk in mid-October, but it turned out that it was unfinished. The GAU instructed factory #172 to urgently send engineers to ChKZ in order to finish it. Representatives of factory #172 only arrived in Chelyabinsk in mid-February of 1945. Meanwhile, the situation with the ISU-152's modernization reached a new stage.

The fate of the ISU-152 was sealed by the time the modernized prototype finished trials. Assembly of a tank called 701-A was underway by November 16th, 1944. This tank, the lead developer of which was M.F. Balzhi, was a large modernization of the IS-2 tank. It kept the engine, elements of the transmission, and radiators. The factory design bureau made good use of the work done for the first modernization of the IS-2. Assembly was finished by November 23rd, the tank weighed in at 48 tons. "Specimen A", as it was referred to in correspondence, was later named "Object 703". By the end of December the tank earned the name IS-3, but factory documentation had another name: Kirovets-1. The same index later appeared in letters for the GBTU and NKTP.

Kirovets-2 at the NIBT proving grounds, summer of 1945.

The trials resulted in a lengthy list of complaints that had to be dealt with on subsequent prototypes. The front of the hull also changed: the pike nose from the IS-2U designed at factory #100 was implemented. It slowly became clear that Balzhi's tank was the long awaited IS-2 replacement. ChKZ began preparing to produce an initial batch of the tank which was later accepted into service as the IS-3 on March 29th, 1945, after trials of the second prototype.

Work on an SPG on the chassis of the Kirovets-1 began even before that, in early 1945. Like with the ISU-152, the lead engineer was L.S. Troyanov. The vehicle was designated Kirovets-2 and received a blueprint index of 704 (also known as Object 704). Russian language literature sometimes calls it the ISU-152 model 1945, but this is a post-war invention. It was likely thought up in Kubinka where the Object 704 is still stored as an exhibit.

Like the IS-3, which was distinct from the IS-2, the SPG on the chassis of the Object 703 also differed from the ISU-152.

Kirovets-2 appeared in correspondence in February of 1945. Its development was reported on by S.P. Gurenko, chief designer of factory #172. He headed the very same commission that was awaited since December of 1944. The designers remained at ChKZ from February 14th to February 20th. Meanwhile, SKB-2 didn't just finish working blueprints for the Kirovets-2, but delivered them to factory #200. 

By mid-February the hull already arrived at ChKZ and components were being installed. The hull was radically different from the ISU-152, for which the ML-20SM was initially designed. This was also true for the gun mount. More changes had to be introduced into the gun that already spent several months at ChKZ. The lower corner of the cradle was trimmed down and the recoil brake was shortened by 70 mm. These changes were introduced by the Kirov factory, although they were approved by factory #172.

A technical meeting was held at ChKZ on March 3rd where the installation of the ML-20SM into the Kirovets-2 was discussed. It was decided that a new gun mantlet with an improved shape and 100-150 mm in thickness should be installed. Instead of the 52-TsM-504A panoramic sight the sight from the 76 mm model 1943 regimental gun was used, as it was more compact and simpler. The telescopic sight also changed. The smaller and more comfortable TSh-17 replaced the bulky ST-10. The Kirovets-2 also became the first Soviet heavy SPG with a coaxial machine gun. A coaxial DShKM machine gun was installed to the right of the cannon. The horizontal traverse mechanism also changed.

The Kirovets-2 had some differences from the Kirovets-1 and the production IS-3, especially when it came to the engine deck.

The issue was that the production ML-20SM also had to be changed. Kirov factory and factory #172 were in agreement, but the GAU had its own opinion. The GAU, GBTU, and NKV once more began a months-long discussion by mail. Precious time was lost and the ML-20SM was installed on the the Kirovets-2 in mid-June of 1945 despite the chassis being ready in the sprint. Even though the Kirovets-2 was built on the Kirovets-1 chassis, it was not an exact copy. The overall design of the engine deck was different and the exhaust pipes were taken from the Object 701.

The most important change was in the fighting compartment. The first goal was to increase its frontal protection to the level of the IS-3, which the designers accomplished. The thickness of the armour was 120 mm, which offered reliable protection against the 88 mm Pak 43 L/71. The sides were 90 mm thick and they were also at an angle. The gun mantlet was also seriously thickened. It grew to 160 mm, nearly twice as thick as on the ML-20S.

Inside the fighting compartment.

Not just the shape of the casemate was different, but its very concept. The driver was now placed in such a way that he didn't have any observation devices in the front plate. His station was moved as high up as possible, which allowed him to look out of the top hatch in his upper position, increasing visibility. In battle, he looked through a rotating periscope in his hatch. The commander had the same hatch. There were five hatches in the roof in total, one was meant for the panoramic sight. Thanks to widening the sides of the casemate the fighting compartment became a bit roomier, even though the overall height was 5 cm less than the ISU-152. Despite the significant increase in protection, the mass did not grow that much, only to 47-48 tons.

According to correspondence, the Kirovets-2 was completed by mid-June of 1945. The vehicle was then sent to Moscow factory #37 and then to the NIBT proving grounds. The SPG awaited gunnery trials. There were no mobility trials performed, as orders were never issued. Nevertheless, the proving grounds composed a brief description of the vehicle. The testers remarked that the armour protection of the vehicle increased. Publications dedicated to the Kirovets-2 often mention that the NIBT proving grounds specialists found the fighting compartment cramped, but the reality in the summer of 1945 was different:

Due to various negotiations gunnery trials of the Kirovets-2 only took place in the fall of 1945.

The situation around the Kirovets-2 was rather comical, as the People's Commissariat of Armaments asked to send the vehicle to the Leningrad Scientific Research Artillery Proving Grounds (ANIOP). This direction was logical, since the gun that the development of the SPG started from still needed to be tested, but the Kirovets-2 remained in Kubinka. Even the trials program approved by the NKV and a letter from Ustinov dated late July 1945 didn't help. The GAU agreed to the trials in August and the Kirovets-2 was sent to the ANIOP, but no trials followed in August or September. This time the fault lay with the employees of factory #172 who did not travel to the proving grounds.

The representatives arrived on September 24th, only to leave shortly after, aside from one designer who did not have permission to conduct trials. Half a year was wasted due to various delays and negotiations. Usually these events don't signal anything good, as was the case with the Kirovets-2.

View from the side. The AA machine gun is covered up.

Due to the various delays and negotiations trials lasted from September 25th to November 13th, 1945. Kuznetsov and senior engineer-designer Nazarov arrived from factory #172 to take part. In this time 65 shots were made with a full charge and 244 supercharged. The gunnery trials were successful. Failures to extract were due to the casings being deformed and not the fault of the gun. One of the small issues with the ML-20SM was the need for introducing an access port in the mantlet for access to the recoil brake valve. 

There was far more positive feedback. The lack of a muzzle brake was a definite advantage, as the gun's position was not revealed when it fired. The blast did shatter the headlight, as a result of which a decision was made to put it on a buffer. The triggers, both the manual lever and electric button, were praised. The TSh-17 sight, much more comfortable than the ST-10, was also rated highly. The sight settings barely drifted during shooting. 

AA machine gun in fighting position.

The evaluation of the fighting compartment was the same as the NIBT proving grounds staff left. The driver's position was more comfortable than the one on the SU-152 or ISU-152, not just when it came to driving, but also to entering the vehicle. The commander's station was also praised. Unlike the SU-152 and ISU-152, he sat facing forward, instead of sideways. His observation was much better. The testers also praised the rotating hatch. The gunner's seat was also more comfortable.

The loader's lot was worse. The commission deemed that 12 rounds out of the 20, the ones located along the left side, were easy to load. The others were harder to reach. However, the ISU-152 had the same issue. The idea with sloped sides was deemed poor from the loader's point of view, as it reduced the size of the ammunition racks. The commission suggested that the sloped sides be disposed with. The breech operator's station was the least comfortable, same as with the ISU-152.

The vehicle from the back. The Kirovets-2 passed trials, but priorities changes by the end of 1945. The vehicle was no longer needed.

The Kirovets-2 and ML-20SM passed trials. The NIBT proving grounds planned to hold mobility trials, but precious time was already lost. The GBTU and GAU had new requirements both for heavy SPGs and for the guns that would be installed on them. The military dreamed of an SPG on the Object 701 chassis. Even the IS-3 was considered yesterday's news by the GBTU in the spring of 1945. Only issues with the Object 701 led to the rational idea of putting the IS-3 into production. In the spring-summer of 1945 the Kirovets-2 still had a chance due to the war, but the situation changed by the end of the year. Production of heavy SPGs was wrapping up and tank designers thought of the future. This future came in the form of the Object 701 chassis, accepted into service in April of 1946. The Kirovets-2 lost its chance at production. It later turned out that the IS-4 was not all it promised to be and the work on SPGs on its chassis did not progress past paper and scale models. As a result, the next heavy SPG was the Object 268, which partially repeated the fate of the Kirovets-2. Constant delays led to the vehicle losing any chance at mass production.