The SU-152 and Related Vehicles - читать бесплатно онлайн полную версию книги автора Yuri Igorevich Pasholok (CHAPTER 5. The Sverdlovsk Era) #7

CHAPTER 5. The Sverdlovsk Era

In early January, ideas of arming the KV-7 with weapons more powerful than the twin 76 mm guns or the 122 mm howitzer began making the rounds of GABTU management and were mentioned in a report about tests of the assault tank. An example of this kind of proposal can be found in a January 8, 1942, finding about the KV-7 signed by Maj. Gorokhov, chief of the 4th Branch of the 3rd Department of the Red Army’s Armor Directorate:

After reviewing the system and the firing conducted with it, I believe that this type of weapon is unacceptable to the armored forces for the following reasons:

1. The system is useless against tanks because it has little flexibility of fire; it can traverse only 15° to each side.

2. Nor can it be used against reinforced concrete bunkers or earth-andtimber emplacements because it is ballistically inefficient and its shells are insufficiently destructive.

I believe it would be better if the gun on this type of system were larger in caliber, approximately 152 mm, and mounted on a KV-2 tank—the M10.^{1}

Meanwhile, the situation with the KV-7 was becoming rather delicate. As the first assault tank prototype was undergoing firing trials, preparations were underway at Factory No. 200 to begin mass production of the hull. On December 30, 1941, Factory No. 200’s chief engineer, L. I. Eyranov, signed off on the specifications for manufacturing a preproduction batch consisting of 20 hulls. We do not know for sure how many hulls the factory manufactured, but judging by subsequent events, the technical specifications in Chelyabinsk did not place a limit on the number. The GABTU faced an unpleasant fact: the assault tank had gone from being a high-priority task to an experimental program, and something needed to be done with the hulls that had been built. In all fairness, this rarely happened in the Soviet tank industry, in contrast to the situation with the Germans, where Krupp regularly turned out turrets for canceled tanks.

The situation with the KV-7, however, was not that hopeless. The triple gun system was a failure, but that did not mean the effort was wasted. The superstructure for the KV-7 that had been developed for three relatively small tank guns was entirely suitable for mounting howitzers. The idea of mounting a 122 mm howitzer was dropped because there was a plan to place a similar weapon in a normal turret on a KV-9 tank. That left two larger guns: the 152 mm howitzer model 1938 (M-10) and the 152 mm gun-howitzer model 1937 (ML-20). Also, the idea of mounting the BR-2 152 mm gun on a vehicle was not going away.

^{ML-20 152 mm corpslevel gun-howitzer mod. 1937. Produced by Factory No. 172, this gun proved highly suitable for arming heavy SP guns (TsAMO).}

It should be noted that two enterprises were designing heavy SP guns at the same time in early 1942. The Bauman Institute of Mechanical Engineering in Moscow (now named the Bauman State Technical University (MGTU)) was tasked by the People’s Commissariat of Arms to work on conceptual designs for self-propelled guns, including bunker busters, armed either with the BR-2 (SA-BR-2) 152 mm gun or the B-4 (SU-B-4) 203 mm howitzer. The second enterprise was Factory No. 221 of the People’s Commissariat of Arms, the manufacturer of the BR-2 gun and B-4 howitzer. In March 1942, the factory’s design bureau offered designs for the BR-33P and BR-33G SP guns. Both designs were based on assemblies from the T-34 medium tank. According to the documentation, the BR-33P would be equipped with the BR-2 gun, and the BR-33G with the B-4 203 mm howitzer. Upon review of the designs, it was decided not to manufacture them because they did not meet the requirements for bunker busters. That seems a little strange, because the initial task was to mechanize corps-level artillery, not develop systems for destroying enemy fortifications by direct fire.

^{ML-20 in travel position (TsAMO).}

Compared with these designs, the idea of developing a bunker buster using the KV-7 hull looked much more reasonable. At the time, development of the twin 76 mm gun system assigned by the State Defense Committee’s January 6, 1942, Decree No. 1110ss was a higher priority task. Yet at the same time the requirement for an upgraded version of the KV-7 was issued, S. G. Ginsberg also drafted a requirement for a 152 mm SP gun based on the KV-7 chassis:

The 152 mm SP howitzer shall be designed using the KV-7 tank and shall be an artillery weapon for close-quarters destruction of bunkers in fortified regions.

I. Combat weight of the SP gun: 50–55 tonnes

II. Armor:

Armor protection for the hull and system: same as the KV-7 tank

III. Armament:

1. 152 mm howitzer: 1

- Twin gun traverse angles: +/-7.5°

- Elevation angles: +12° -5°

2. Number of DT machine guns

a) In the bow: 1

b) In the rear of the fighting compartment: 2

- Machine gun traverse angle: 30°

- Machine gun elevation angles: +15° -5°

IV. Basic load:

1. Projectiles for the 152 mm howitzer: at least 30

2. Number of machine-gun drums: at least 40

V. SP gun crew: 6

VI. SP gun hull.

The SP gun hull shall be the same as that of the KV-7 tank, except for parts directly associated with installation of the howitzer.

VII. Fighting compartment and armament installation

1. The fighting compartment must allow for convenient placement of the gun crew and have instruments for 360° observation.

2. The fuel tanks may be located on the bottom of the fighting compartment to increase the amount of ammunition that can be carried and its convenient location.

3. To increase the rate of fire and facilitate loading of the howitzer, it is desirable to at least partially mechanize the loading process (feeding the projectile and charge).

VIII. The tank’s mobility, engine, suspension and communications equipment must be similar to that of the KV-1.

Note: the factory may alter and supplement this operational requirement to improve the design in coordination with the People’s Defense Commissariat, the Armor Directorate of the Red Army and the People’s Commissariat of the Tank Industry.^{2}

^{ML-20 152 mm corpslevel gun-howitzer mod. 1937 in firing position. In combat, the ML-20 was frequently fired at targets, including enemy tanks, within direct line of sight (TsAMO).}

In the development plan for 1942 approved by People’s Commissar of the Tank Industry Malyshev on January 21, the system would proceed under the number 59. According to the plan, the Kirov Factory (ChKZ) would be responsible for the bunker buster’s chassis, and the Ural Heavy Machinery Plant would answer for the artillery system. The design drawings were expected by March 15, the prototype by May 1, and the production drawings by May 10. The amount allocated for the project was 300,000 rubles. That was not a large sum compared with the amount allocated for Project 212. However, it involved modifying existing KV-7 assault tank hulls, not developing a vehicle from scratch.

This project produced a mixed reaction at the GAU. People there continued to insist on developing a “BR-2 152 mm gun on a chassis incorporating assemblies from the KV tank.” The idea of developing an SP gun based on the KV-7 was considered lacking. That is evident, for example, from the letter GAU’s chief, Col. Gen. of Artillery N. D. Yakovlev, and GAU Military Commissar Kozlov wrote to L. P. Beria, Deputy Chairman of the Council of People’s Commissars of the USSR on March 25, 1942:

Our experience in this war and the war with Finland has revealed the following characteristics of modern military operations:

1. Massive use of highly mobile armored and mechanized equipment;

2. In-depth reinforcement of strategic lines with reinforced concrete bunkers and earth-and-timber emplacements.

These circumstances have given rise to new requirements regarding the mobility of artillery of all calibers and purposes and its capability for conducting direct fire.

The mobility of equipment used to tow artillery lags far behind the mobility of modern tanks. The average speed of artillery prime movers is no greater than 10–12 km/h and agricultural tractors are no faster than 5 km/h, while tanks are capable of speeds in excess of 40 km/h.

In addition, corps-level and heavier artillery pieces are completely open, making it difficult to use them for the close-quarters destruction of bunkers.

We need to develop highly maneuverable artillery systems that are also adequately protected against short-range fire.

This can be self-propelled artillery incorporated into armored auxiliarypropelled hulls that share components with tanks currently in production.

Foreign armies possess mobile armored self-propelled guns.

I believe we need to have three types of self-propelled artillery systems:

1. Self-propelled artillery for destroying bunkers.

2. Self-propelled artillery for destroying tanks.

3. Self-propelled assault artillery for supporting mechanized forces.

4. Self-propelled antiaircraft guns.

Development of the following self-propelled systems must be organized at industrially capable artillery and tank factories:

1. A bunker buster mounting the BR-2 152 mm gun on a chassis incorporating assemblies from the KV tank.

2. A tank destroyer: 85 mm antiaircraft gun model 1939 on a chassis incorporating assemblies from the T-34 tank.

3. A self-propelled assault system: ZIS-3 or USV 76 mm gun on a chassis incorporating assemblies from the T-60 and T-70 tanks.

4. A self-propelled antiaircraft system: 37 mm automatic antiaircraft gun on T-34 and T-60 or T-70 tanks.

Operational requirements for development of these prototypes have been sent to the factories and the People’s Commissariat of the Tank Industry, but not all of the projects stipulated in the GAU’s plan have been acknowledged.

The People’s Commissariat of the Tank Industry has decided not to arm the bunker buster with the BR-2 152 mm gun, but rather to equip it with the ML-20 gun-howitzer, which is significantly less capable of penetrating concrete and armored turrets.

In addition, the People’s Commissariat of the Tank Industry is refusing to develop tank destroyers, citing development of the 85 mm tank gun for the KV tank as the reason.

Given the urgent need for development of these self-propelled systems, I hereby request that Comrade Malyshev, People’s Commissar of the Tank Industry, be ordered to undertake these projects.^{3}

However, the desires of the artillerymen in this case were at cross purposes with a number of obvious facts. Project 212 had been shut down by the spring of 1942, and the Chelyabinsk Kirov Factory had no plans to revive it. The same was true of the KV-3, the chassis used for the development of the bunker buster. Even if the Council of People’s Commissars insisted on this SP gun, there was absolutely no way a prototype could be built or the system could be put into production. Revival of production of the BR-2 152 mm corps-level gun was also unrealistic at that time. Whether the GAU liked it or not, therefore, the only option at the time was to develop a bunker buster based on the KV-7. In addition, although the ML-20 152 mm gun-howitzer proposed as the weapon for the new bunker buster was one-third shorter than the BR-2, it was still a very formidable weapon.

^{Factory drawing of U-19 heavy SP gun (TsAMO).}

On April 15, 1942, the GAU’s Artillery Committee met in plenary session to discuss the development of self-propelled artillery. In addition to members of the Artillery Committee, it was also attended by S. A. Ginsburg as the representative of the People’s Commissariat of the Tank Industry who oversaw self-propelled artillery issues. The plenary session came to the following decisions:

1. We hereby confirm that the requirements of the Artillery Committee of the Main Artillery Directorate of the Red Army concerning the need to have the following types of self-propelled gun systems in the inventory of the Red Army are correct:

1) Self-propelled guns for destroying bunkers;

2) Self-propelled guns for destroying tanks;

3) Self-propelled assault guns for supporting motorized infantry;

4) Self-propelled antiaircraft systems for escorting tank and motorized forces.

2. We consider it necessary to supplement the Red Army’s armament system with a self-propelled howitzer for combating earth-and-timber emplacements and concentrations of enemy personnel.

3. We believe it necessary to assign factories to manufacture the following self-propelled systems:

1) Self-propelled assault guns:

a) A USV or ZIS-3 76 mm division-level gun on a universal chassis incorporating assemblies from the T-70 tank.

Assign this project to Factory No. 37 (of the People’s Commissariat of the Tank Industry), to be assisted by Factory No. 9 (of the People’s Commissariat of Arms).

b) M-30 122 mm howitzer model 1938 on a T-34 chassis.

Assign this project to Factory No. 183 (of the People’s Commissariat of the Tank Industry) and Factory No. 8 of the People’s Commissariat of Arms).

2) BR-2 152 mm bunker busters based on a special chassis incorporating assemblies from the KV tank.

Assign this project to the Kirov Factory (of the People’s Commissariat of the Tank Industry), to be assisted by Factory No. 221 (of the People’s Commissariat of Arms). In view of the fact that chassis for this system are very difficult to obtain, we consider it advisable that this project be limited to system design.

For the time being, only mount the 152 mm gun-howitzer model 1937 in a KV-7 tank hull. Assign this project to the Kirov Factory (of the People’s Commissariat of the Tank Industry) and Factory No. 172 (of the People’s Commissariat of Arms).

3) Self-propelled antiaircraft systems:

a) Installation of the 37 mm automatic antiaircraft gun model 1939 on a universal chassis incorporating assemblies from the T-70.

Assign this project to Factory No. 37 (of the People’s Commissariat of the Tank Industry), to be assisted by Factory No. 4 (of the People’s Commissariat of Arms).

b) Install the 25 mm automatic antiaircraft gun model 1940 on the same universal chassis.

Assign this project to Factory No. 37, with assistance from Factory No. 172 (of the People’s Commissariat of Arms) (Comrade Loktev’s design bureau).

4. Due to the inability to rapidly obtain a special chassis incorporating assemblies from the T-34 tank for the 85 mm system with 360° traverse called a tank destroyer, discontinue further work on this project.

5. In order to accelerate the manufacture of self-propelled models according to paragraph 3 of this decree, we hereby request that Comrade Malyshev, People’s Commissar of the Tank Industry, and Comrade Ustinov, People’s Commissar of Arms, instruct the directors of the above-named factories to urgently conclude contracts with the GAU.

6. In order to increase the inventory of self-propelled artillery, we consider it sensible to modify captured vehicles for use as self-propelled guns mounting domestic arms.^{4}

Thus, the idea of building an analog to SP gun 212 was shelved. It is true that the ML-20 could not, like the BR-2, “gnaw away” two-meter concrete walls, but it was in mass production. As far as cooperation on an SP gun based on the KV-7 was concerned, changes were made. In place of Factory No. 172, work on the gun was assigned to the Ural Heavy Machinery Plant. Cooperation between the Ural plant and the Chelyabinsk Kirov Factory continued. The project that had been underway in Sverdlovsk since the winter of 1942 was assigned the factory designation U-18. As with the U-13 and U-14, installation of the ML-20 in the KV-7 was overseen by L. I. Gorlitsky.

Under assignment from Kotin, Gorlitsky had been supervising a different heavy SP gun project, unrelated to the U-18, since April 1942. Another curious fact is that this project, which was assigned the factory designation U-19, had been sent to GAU and GABTU on August 12, 1942—three weeks before the U-18. And that was done despite the fact that installation of the ML-20 on the KV-7 was a high-priority task, and U-19 had not been mentioned. Moreover, a broad description of the U-19 dates back to May, and the drawings to mid-June of 1942. It is possible that this system was a response to the persistent demands of artillerymen for an “iron fist” to destroy bunkers by direct fire. This line of thinking is encouraged by the fact that the U-19 concept is highly reminiscent of Factory No. 221’s bunker buster projects and efforts by the Bauman Institute of Mechanical Engineering.

^{Sectional drawing of U-19 SP gun (TsAMO).}

The U-19 project involved mounting a 203 mm howitzer model 1931 (B-4) on a KV-1 tank chassis. The primary mission of this SP gun was to destroy fortifications that less powerful systems could not handle. Like the KV-7, the base KV-1 tank underwent minimal modification: its turret platform was removed, its engine compartment bulkhead made removable, and its fuel tanks and air intakes changed. The tipping parts and top carriage of the B-4 203 mm howitzer were adopted without modification in order to keep costs down. The turret was replaced by a massive superstructure that completely covered the gun, which was installed in the fighting compartment. The superstructure partially covered the top of the engine compartment, potentially making it difficult to service the engine. The superstructure’s front plate was 75 mm thick, and it had 60 mm of armor on the side, 40 mm on the rear, and 30 mm on its roof. For installation of the howitzer barrel, the superstructure had a large hatch in its rear that included an access hatch for the crew. The design called for the roof to be removable. The front of the howitzer was protected by a massive mantlet 75 mm thick. Because the main mission of the U-19 was destruction of enemy fortifications, the elevation angle was limited to 10°. The gun’s traverse angle was also kept to a minimum—no more than 4.3° to each side, which matched the specification for the B-4 howitzer.

^{Drawing of the U-19 heavy SP gun, section through the fighting compartment (TsAMO).}

^{U-19 heavy SP gun, 1:35 scale drawing.}

The military’s requirement that the chassis design changes be kept to a minimum while providing decent armor protection resulted in a very odd machine. The U-19 was calculated to weigh an estimated 66,190 kg, which exceeded even the SP gun 212A’s design parameters. Considering that the actual combat weight of manufactured models was usually somewhat greater than the design weight, the U-19 had the potential of becoming the heaviest Soviet SP assault gun, surpassing the armored version of the SU-14 for this dubious honor. In addition, putting the B-4 203 mm howitzer in an enclosed superstructure gave the system a height of 3510 mm, which was all of 50 mm lower than the armored version of the SU-14. The developers of the U-19 understood very well that these dimensions would make the SP gun an excellent target for the enemy. From the very beginning, therefore, the concept called for it to be escorted by conventional tanks.

The extremely mixed outcomes of the design process led to an appropriate conclusion. On September 9, 1942, the Ural Heavy Machinery Plant’s chief engineer, M. G. Umnyagin, received a letter signed by the chief of the GABTU’s Armor Directorate, Eng. Col. S. A. Afonin, that stated the following:

In response to your letter No. 3707/48s of August 24, 1942, regarding the U-19 self-propelled gun project with a 203 mm howitzer on the KV-1 chassis, I hereby inform you of the following:

1. With the U-19 weighing 66 tonnes and with retention of the KV-1 tank transmission, the V-2K engine does not reliably support movement of the SP gun at the assigned speeds. In addition, the KV-1 tank transmission and suspension are designed for a vehicle weighing 40 tonnes and cannot perform reliably when the weight is increased to 66 tonnes.

2. The SP gun would be a highly visible target due to its height (3.51 m) and the great width of the upper part of the vehicle hull (the fixed turret).

3. The armor protection of the turret is weaker than that of a conventional KV-1; therefore, it would not provide the necessary armor protection for the crew when receiving fire from close range.

4. If the running gear is disabled, the gun can only be fired within a narrow sector (9°) because the U-19 does not have a rotating turret.

5. The large weight of the vehicle appears to make towing of disabled vehicles from the battlefield problematic.

6. The SP gun’s mobility would be low, judging by its power-to-weight ratio (9 hp per tonne) and ground pressure (98 kg/cm²).

Accordingly, I believe it would be inadvisable to continue working on the design for the U-19 self-propelled gun.^{5}

Some paragraphs in the document are puzzling because projects like the 212 did not weigh less, had no better protection, and would be no less challenging to tow from the battlefield. Nevertheless, another bunker buster that the artillerymen wanted sank into oblivion without even reaching the mockup stage. The U-18 had no better luck, but for entirely different reasons.

Development work on installing the 152 mm howitzer in a superstructure on the KV-7 was delayed for a variety of reasons. The conceptual design was not finished until August 4, and it was proposed a month later—on September 3, 1942. A team of designers at the Ural Heavy Machinery Plant led by Gorlitsky tried their utmost to meet the military’s requirements while minimizing modifications to the KV-7. The reason for that is clear from the description of the U-18:

The self-propelled gun with the ML-20 152 mm gun is intended for destroying enemy bunkers and earth-and-timber emplacements.

The design objective for this self-propelled gun was to make maximum use of mass-produced assemblies from the KV tank and the ML-20 gun.

The most suitable base chassis for this system is the KV-7 tank. The Kirov Factory in Chelyabinsk has several dozen hulls with turrets for this tank that were intended for installation of one 76.2 mm and two 45 mm tank guns.

The regular tipping parts of the ML-20 gun did not fit in the existing KV-7 turret. Therefore, only the barrel and breech mechanism were taken from the ML-20 gun. The cradle and recoil mechanisms have been redesigned to have a shorter recoil length, which made it possible to fit the ML-20 system in the existing KV-7 turret.

The mounting parts were also redesigned. This solution presented advantages with respect to the turret weight and, especially, the system’s height.^{6}

The U-18’s artillery system comprised only 13 assemblies, and the KV-7 hull and superstructure were not modified. It appeared that the military’s requirements were fully satisfied and the system could be put into service, especially since extra hulls were available. But a problem arose from an unexpected corner.

^{U-18 heavy SP gun on KV-7 assault tank chassis (TsAMO).}

On September 24, 1942, the Ural Heavy Machinery Plant’s chief engineer, M. G. Umnyagin, received a letter signed by Eng. Col. Kovalev, chief of the 6th Department of the GABTU’s Armor Directorate:

In response to your letter No. 3655/48s of September 4, 1942, concerning the U-18 self-propelled gun project, I hereby inform you that a similar project proposed by Comrade Petrov between September 12 and September 14, 1942, was discussed at a meeting of the Technical Committee of the Council of the People’s Commissariat of Arms.^{7}

That put an unexpected end to the history of heavy SP gun development by the Ural Heavy Machinery Plant’s design bureau. Gorlitsky worked on no more heavy self-propelled systems. How did Petrov happen to get at cross purposes with Gorlitsky?

In February 1942, artillery production was transferred from the Ural Heavy Machinery Plant to the People’s Commissariat of Arms Factory No. 8, which had been evacuated to Sverdlovsk in the fall of 1941. B. A. Fradkin stayed on as its director. In March 1942, Factory No. 8’s design bureau developed the ZIK-1 85 mm tank gun for the T-34 and KV-1 tanks. The ZIK-1 thus remained a project, but it became the first project of the design bureau headed by F. F. Petrov. Not one system with the ZIK designation (named after the Kalinin Factory) was ever put into service, but by 1943 the creations of Factory No. 9 acquired from Factory No. 8 in the fall of 1942 had begun destroying the enemy. These were the famous tank and self-propelled guns D-5 (SU-85, KV-85, and the initial T-34-85), the D-25 (IS-2, ISU-122, and IS-3), and the D-10 (SU-100), as well as the D-1 152 mm howitzer. The creations of Petrov’s OKB-9 design bureau beat out the handiwork of the famous designer V. G. Grabin in competitions. But that would come later. In 1942, the team in the chief designer’s department at Factory No. 8 was working in many different areas.

It so happened that a direct competition began in the summer of 1942 between Factory No. 8’s design bureau and Ural Heavy Machinery Plant’s design bureau for several projects, including a heavy SP gun. The situation was made particularly poignant by the fact that Gorlitsky’s design bureau was located on the fifth floor of the Ural Heavy Machinery Plant’s administration building, and Petrov’s design bureau was on its fourth floor.

Exactly when KB-3 of the chief designer’s department at Kalinin Factory No. 8 began working on the competitor to the U-18 is not known. According to documentation, task No. 5400072 for designing the “installation of the tipping parts of the 152.4 mm howitzer model 1937 (ML-20) in the KV-7 tank…” was issued by the GAU’s Artillery Committee on June 4, 1942. This date is somewhat at variance with a letter from GAU Artillery Committee chief Col. Gen. V. I. Khokhlov to Prof. E. A. Satel, chairman of the Technical Council of the People’s Commissariat of Arms. That letter was also dated June 4, 1942 and read in part:

The design department of Factory No. 8 is currently developing conceptual designs for the following self-propelled howitzers:

a) The M-30 122 mm division-level howitzer for installation in a T-34 tank;

b) The U-11 122 mm tank howitzer for installation in a T-34 tank;

c) The ML-20 152 mm gun-howitzer for installation in a KV-7 tank.

The timeline for development of these self-propelled systems, both the conceptual designs and the follow-on development stages, have not been established; they are working at their own initiative.

In view of the great importance of and urgency associated the development of self-propelled howitzers as a means of suppressing and destroying earth-and-timber emplacements and concrete bunkers, Factory No. 8 must be given strict deadlines, specifically for completion of the conceptual designs by June 15, 1942, and their submission to Moscow for consideration by the Red Army’s GAU Artillery Committee in conjunction with the People’s Commissariat of Arms and the People’s Commissariat of the Tank Industry.

A decision will be made and a general plan for implementation of these self-propelled systems will be drawn up after due consideration.

I hereby request that the director of Factory No. 8 be issued the appropriate instructions.^{8}

^{U-18 heavy SP gun, 1:35 scale drawing.}

^{ZIK-1 85 mm tank gun, spring of 1942. This version was designed for mounting in the KV-1 tank turret (SA).}

In other words, Factory No. 8’s design bureau had been involved in the heavy bunker buster project since the spring of 1942. As was the case with the U-18, the factory’s design bureau had been working on mounting the ML-20. Work on the chassis was assigned to the Chelyabinsk Kirov Factory. In contrast to the U-18, on which correspondence is almost entirely lacking, the Factory No. 8 project designated the ZIK-20 was much discussed in both the GAU and the GABTU. Work on the heavy SP gun was led by T. A. Sandler, Factory No. 8’s chief designer (prior to the evacuation he had headed up the quality control department at Factory No. 8). The absence of any reference to the designation ZIK-20 in correspondence caused confusion later. According to GAU Artillery Committee documents dating from the second half of 1943, the projects of the Ural Heavy Machinery Plant and Factory No. 8 came to be perceived as a single entity, and that perception was reflected in subsequent correspondence and the Ural Heavy Machinery Plant’s summary report. As a result, many researchers saw the ZIK-20 as the U-18, and much confusion arose because no graphic materials existed for the two SP guns.

The GAU also attempted to assign development of the much-desired bunker buster fitted with the BR-2 to Factory No. 8. This is particularly evident from a letter that A. A. Goreglyad, Deputy People’s Commissar of the Tank Industry, wrote to GAU chief Col. Gen. N. D. Yakovlev on June 23, 1942:

In response to your letter No. 538884 of April 23, 1942, I hereby inform you of our agreement to accept for implementation part of the experimental work on self-propelled artillery that you have proposed.

The self-propelled gun projects you mention have undergone a preliminary workup at factories supervised by our Commissariat, allowing us to clarify their possible implementation as follows:

<…>

3) Kirov Factory. The manufacture of a prototype self-propelled 152 mm gun-howitzer model 1937 (ML-20) using a type KV-7 hull can be accepted.

As regards your proposal to design a 152 mm self-propelled gun mounting the BR-2 gun, we consider even design work on this project to be inadvisable inasmuch as manufacture of this type of self-propelled gun would be an unrealistic undertaking for the near future.

The People’s Commissariat of Arms must be instructed to manufacture two models of the 152 mm gun model 1937 with muzzle velocity increased to 750–780 meters per second by lengthening the barrel and introducing a muzzle brake.

This version has been developed by the artillery design bureau of Factory No. 8 (Chief, Comrade F. F. Petrov).

To speed up the work and improve communications with the artillery design bureau, we consider it necessary to request that the People’s Commissariat of Arms assign the design work associated with the artillery systems and their modification for self-propelled artillery to Factory No. 8’s design bureau (Chief, F. F. Petrov), with factories of the People’s Commissariat of Arms to manufacture the artillery systems for the self-propelled guns at its discretion.

Upon receipt of your approval of these developmental self-propelled artillery projects, we will instruct the factories to implement them.^{9}

^{Sectional drawing of ZIK-20 SP gun (TsAMO).}

^{Top view drawing of ZIK-20 SP gun. Its crew of six had very cramped working conditions (TsAMO).}

Goreglyad’s proposal played a cruel joke on Factory No. 8’s design bureau. In addition to the ZIK-20, Petrov’s design bureau was working on a number of SP gun projects of all types at the same time. They included the ZIK-5 25 mm SP air defense gun on a chassis incorporating assemblies from the T-60 (a copy of Factory No. 37’s SU-32), the ZIK-7 76 mm SP gun on a chassis incorporating assemblies from the T-70 (a copy of Factory No. 37’s SU-31), and the ZIK-10 and ZIK-11 122 mm SP guns on the T-34 chassis (similar to the U-35 under development at the Ural Heavy Machinery Plant’s design bureau). And that list leaves out its impressive number of gun projects, from antitank guns to corps-level artillery! With a workload like that, it is not surprising that ZIK-20 development was significantly delayed. In fact, the 152 mm bunker busters completely disappeared from the development plans of the GAU and the GABTU after mid-summer. However, some progress was being made by the end of the summer. On August 14, 1942, Khokhlov sent a letter to Yakovlev:

According to the GAU Artillery Committee’s development plan, installation of the 152 mm gun-howitzer model 1937 on a chassis incorporating assemblies from the KV-7 tank should be completed in 1942.

The People’s Commissariat of Arms and the People’s Commissariat of the Tank Industry accepted these projects and assigned them to Factory No. 8 (Sverdlovsk) and the Kirov Factory (Chelyabinsk).

The design is now complete.

For manufacture of the prototype of the 152 mm self-propelled howitzer, I hereby request that you instruct the Chief of the GAU’s Artillery Equipment Supply Office, Maj. Gen. of Artillery Sokolov, to send Factory No. 100 (Chelyabinsk) one 152 mm gun-howitzer model 1937 without a limber and with the gun and battery kit of spare tools and accessories by September 1 of this year.^{10}

However, it was clearly too early to issue that instruction. First of all, the first ZIK-20 drawings would not be ready until early September and had not yet been reviewed by the GAU’s Artillery Committee. Second, the bunker buster as it was taking shape differed greatly from the U-18, in terms of both the gun and the fighting compartment. That is evident from the system description:

The following issues received particular attention during development:

1. Keep the design and location of the KV tank units and mechanisms and the artillery system parts and units unchanged as much as possible.

2. Make the operations of the artillery crew with the gun as convenient as possible within the existing dimensions of the tank and thereby increase its rate of fire.

3. Increase the number of rounds and make their location as convenient as possible from the standpoint of increasing the rate of fire.

4. Increase the artillery system’s traverse and elevation angles as much as possible.

5. Minimize any weight increases to the artillery system and thus the total weight of the tank.

6. Develop the best armor protection for both the artillery system and the gun crew, thus enabling the tank to be used for close-quarters direct fire on the enemy’s fortified lines and strongpoints.

7. Minimize the line-of-fire height. A height of 2170 mm has been achieved in this project, exceeding the line-of-fire height of the 76.2 mm gun in the KV tank with small turret by only 70 mm.

8. Facilitate assembly in a cooperative work environment to fulfill the task.

We mounted the ZIK-20 tank gun on a KV tank using a welded fixed turret of prismatic shape.

<…>

The turret wall thickness was taken to be 75 mm. Thus, in our understanding, we took the KV-7 tank as the base chassis; that is, not some prototype of the tank, but in the belief that any KV tank with a fixed turret is a KV-7 type tank.

The total weight of the “KV-7” tank together with the ZIK-20 system will be approximately 53 tonnes, i.e., it will be about 4 tonnes heavier than the KV tank with a small turret.

<…>

IV. Main features of the ML-20 howitzer system

1. The ZIK-20 152.4 mm tank howitzer was produced using the tipping parts of the ML-20 152.4 mm howitzer with a modified cradle, replacing the elevation and traversing mechanisms, and establishing constant recoil by attaching a counter-rod and removing the variable recoil mechanism and muzzle brake.

2. The design changes listed in paragraph 1 were made entirely by modifying the field gun into a tank gun, adding armor, and bringing the system into balance. The installation involves removal of the muzzle brake for reasons explained below.

3. The ballistics of the ZIK-20 152.4 mm tank gun are the same as for the 152.4 mm howitzer model 1937.

4. The ZIK-20 152.4 mm tank gun fires time-fuzed, high-explosive, and concrete-piercing shells.

5. The relatively powerful munitions, the strong armor (up to 90 mm in thickness), and the sight mechanisms that support both direct and indirect fire enable the ZIK-20 gun on the KV tank to be used for the following purposes:

a) Destruction of bunkers and trenches both at short range and from cover;

b) Engagement of enemy mechanized infantry units when operating as self-propelled artillery;

c) Suppression of enemy artillery fire.

<…>

VIII. General observations about the project

In the absence of a specific Operational Requirement for mounting the ML-20 on the KV tank, this project is based on existing performance specifications for similar models, with due consideration given to features specific to the ML-20 system.

Concerning the modifications made to the artillery system and several items inside the tank, the following can be said in conclusion:

1. We deem it necessary to remove the muzzle brake from the system; retaining it for a tank gun would be a mistake.

2. All changes to the cradle were made for the sole purpose of converting the field gun to a tank gun.

3. A practical loading test may show that the loading tray is unnecessary. Given the system’s small angles of elevation (15°) and the low height of the gun’s axis above the floor, and without a cradle extending beyond the breech face, which usually hinders loading, the crew will find loading easier without a tray, which would take time to readjust.

4. From a strength standpoint, the recoil mechanism parts are surely reliable (considering that firing will be done without a muzzle brake).

In practice, we adopted a constant recoil equal to the short recoil of the ML-20, thus eliminating the variable recoil mechanism. The front cradle cover could also be removed or the counter-rod attachment altered. However, resolution of this issue depends on the number of vehicles manufactured. If the number produced is small, the old hatch cover will be used. If the number produced is large, the counter-rod should be modified, which will reduce and simplify a number of cradle and recoil mechanism parts, and then the armor for the cradle can be made more compact.

An issue with the sight needs to be addressed.

We do not believe the decision to select and install the standard model 1927 sight is straightforward.

To give such a powerful system the capability of both direct and indirect laying, we believe it should have both a conventional sight and a panoramic sight such as the TOP tank gun sight.

The use of a TOP-type sight would make it possible to meet the important requirement for reducing the size of the sighting slit.

To resolve this problem, the factory needs to acquire drawings for current models of elongated tank sights.^{11}

^{Transverse sectional drawing of ZIK-20 SP gun. Dual-rack stowage clearly reduced the fighting compartment’s height (TsAMO).}

Thus, rather than simply installing an ML-20 in the hull of the KV-7, Factory No. 8’s design bureau was essentially proposing a new SP gun based on the KV-1. In addition, the ZIK-20 howitzer being mounted in the SP gun required even more modifications than the ML-20 under Project U-18. The SP gun’s superstructure was 17 centimeters higher than the KV-7, and the vehicle created by Factory No. 8’s design bureau weighed more than a KV-2. Given the completely new superstructure, the KV-7 hulls in Chelyabinsk were unusable.

^{Factory drawing of the ZIK-20 SP gun’s mantlet (TsAMO).}

The ZIK-20 project was reviewed at a meeting of the Technical Committee of the People’s Commissariat of Arms on September 15. Projects of the armor department at the Bauman Institute of Mechanical Engineering were also reviewed at the meeting. They included self-propelled mounts for the I-13-52 57 mm antitank gun and the B-4 203 mm howitzer. As designed, the SP gun with the B-4 had two ZIS-5 engines, and it had a top speed of all of 6.5 km/h.

The ZIK-20 was the meeting’s main topic of discussion. Factory No. 8 design bureau chief F. F. Petrov reported on the project. His report included an explanatory note, drawings, and engineering analyses. The idea of eliminating the muzzle brake, which would give away the SP gun’s position during firing, was approved, but a suggestion was made to use a loading tray like the one on the SG-122 SP gun. Although Petrov raised the telescopic sight issue, the design did not include one (in contrast to the U-18, which had included a TOP from the outset.) In addition, mounting the ML-20 on the ZIK-20 rather than on the U-18 would locate the gun further forward, which threatened to increase the load on the front road wheels.

The layout of the fighting compartment generated many more questions from those present at the meeting. The sides of its superstructure were less sloped than those of the KV-7, which reduced the likelihood shells would ricochet. Although the height of the fighting compartment from the floor to the roof was 1895 mm, the crew would have to work in very cramped conditions. The double-row rack for shells on the floor took up 395 mm and would be very inconvenient to use. In addition, the attachment of racks to the sides of the superstructure increased the risk that the basic load would explode if the superstructure were struck by enemy shells. The decision to locate fuel tanks along the sides of the vehicle was questioned. A number of questions regarding the effectiveness of the fighting compartment ventilation and defense of the vehicle from the rear, which did not even have a submachine gun port (in contrast to the U-18, for which a stern-mounted DT machine gun was planned).

^{Factory drawing of the ZIK-20 SP gun (TsAMO).}

The exchange of views on Factory No. 8’s design led to the following conclusion:

1. The ZIK-20 project for a self-propelled gun mounting the ML-20 152 mm gun-howitzer model 1937 on a KV tank with a fixed turret proposed by Kalinin Factory No. 8 was particularly interesting as a practical solution to the problem of developing a heavy self-propelled gun with good armor protection to serve as a bunker buster.

2. The design is satisfactory and essentially correct from a technical standpoint, but it has a number of flaws, including the following:

a) The problem of proper placement of shells in the tank has not been solved satisfactorily;

b) Even though the crew space is reasonably roomy, the design falls short in terms of supporting a maximum rate of fire with convenient placement of shells and proper loading tray design, nor does it ensure adequate ventilation of the fighting compartment.

3. We concur with some of the modifications the design makes to the tipping parts of the ML-20 system, specifically the manufacture of new trunnion and rear rings and a new elevating arc, and we also concur with shortening the cradle frame and eliminating the muzzle brake from the barrel.

4. We concur with Comrade Petrov’s proposal to replace the existing counterrod with a spindle-type counter-rod, along with changes to the recoil mechanism parts connected to the counter-rod. This would eliminate the cradle cap.

5. In developing the engineering drawings, the design bureau of Factory No. 8 must consider the following remarks by those present at the meeting.

a) The loading tray: instead of the one proposed, use the loading tray from Factory No. 592’s 122 mm assault gun.

b) Modify the way the ammunition is stored, paying particular attention to facilitating the work of the gun crew.

c) Keep the standard sight for firing from cover and use the elongated TOP for direct fire.

d) To allow natural ventilation and move shells into the turret, provide a hinged hatch in the rear of the turret and make it as large as possible.

e) Provide for mounting a machine gun in the rear wall of the turret.

f) If possible, provide a ball bearing in the bottom support of the gun traversing mechanism.

g) Increase the floor-to-ceiling height to 1600–1650 mm, instead of 1500 mm as designed.

6. After the design is reviewed by the GAU’s Artillery Committee, a joint decision to continue development work on the project, prepare engineering drawings, and manufacture a prototype should be made.

Because a number of design issues need to be finalized, including the proper location of the ammunition and fuel tanks and simplification of the manufacturing process for the turrets, hatches, etc., and because assignment of lead for manufacture of the system as a whole depends on collaboration between artillery and tank factories, the Technical Department of the People’s Commissariat of the Tank Industry of the USSR must be engaged to make the final decision.

7. Allow the design bureau of Factory No. 8 to continue working to improve the design in accordance with the remarks and to develop engineering drawings.

The chief designer of Factory No. 592, Comrade Sinilshchikov, should immediately send Factory No. 8 the drawings for the 122 mm assault gun’s loading tray.^{12}

^{ZIK-20 heavy SP gun, 1:35 scale drawing.}

^{The KV-1S heavy tank replaced the KV-1 on the production line in September 1942. The designers of SP guns based on the KV-1 chassis had to modify their designs for the new chassis (TsAMO).}

P. F. Solomonov, chief of the Artillery Committee’s 2nd Department, who by that time had been promoted to Engineer Major, was engaged in tracking the ZIK-20 project after September 20, 1942. As had happened in late 1941, he had to visit the factories in order to personally supervise implementation of the new designs. He stopped first in Chelyabinsk, where he met with Kotin (who then was also serving as chief designer at the People’s Commissariat of the Tank Industry) and designers from Experimental Plant No. 100 of the People’s Commissariat of the Tank Industry:

At this meeting and in negotiations with Comrade Kotin, the following decision was made: by order of the Artillery Committee, Factory No. 100 and the Kirov Factory are to manufacture a self-propelled chassis and mount an artillery package and fighting compartment equipment on it. To integrate the work done at the two factories, the Kirov Factory must send two designers to Factory No. 8 in Sverdlovsk. The factory needs to be given an updated operational requirement, because the issue of full utilization of the KV-7 hull has not been made sufficiently clear. Since it has been proposed to make the hull larger overall than the KV-7, the designers at Factory No. 100 (whose design bureau chief is Comrade Yermolayev) believe it would be better to equip the lightened hull of the KV-1S with the prism-shaped turret that has been designed.

Then the Ural Heavy Machinery Plant should make the hull; the Kirov Factory’s production shops should install the engine, transmission, and running gear; and Factory No. 100 should install the artillery package and manufacture and install the ammunition racks. Drawings of the hull with the turret are being developed at KB-100 from sketches made by Factory No. 8 and the Kirov Factory.

Agreement must be reached in the People’s Commissariat of the Tank Industry and an order elaborating on the existing agreement must be obtained from the People’s Commissar with an appropriate assignment of work and establishment of deadlines.

The conclusion of a contract with the factory must be delayed until the wooden mockup is approved and the overall design is refined.^{13}

^{Wooden mockup of the ZIK-20’s gun system and superstructure, fall 1942 (SA).}

When Solomonov arrived in Sverdlovsk, he reviewed the drawings and issued the necessary instructions to the designers regarding modifications approved by the Technical Council of the People’s Commissariat of Arms. It was decided during the meeting held at Factory No. 8 to require the factory to finalize the drawings by October 25 and manufacture a wooden mockup of the system. A team of model makers and carpenters was detailed to construct the model by personal order of Factory No. 8 Director B. A. Fradkin. A designer named A. G. Usenko was assigned to oversee construction of the model. In order to ensure the project had enough designers and draftsmen, arrangements were made to return Factory No. 8 employees engaged in harvesting and logging operations.

Since Factory No. 8 lacked clear specifications for manufacturing a prototype of the system, Solomonov began drafting an operational requirement. This operational requirement, which was signed on September 25, 1942, was based on both the operational requirement for the bunker buster dating from April and on remarks about the ZIK-20 project:

I. Definition

1. The 152 mm self-propelled howitzer is a self-propelled gun made from the tipping parts of the 152 mm corps-level howitzer model 1937 (ML-20) mounted on a KV-tank chassis with a fixed prism-shaped installation.

II. Tactical role

2. The 152 mm self-propelled howitzer is intended for supporting mechanized and infantry units breaking through enemy defense lines and is used to destroy defensive fortifications, combat enemy artillery and tanks, and conduct artillery fire on assembly areas of counterattacking enemy groups.

3. The 152 mm self-propelled howitzer primarily executes fire by direct laying at close range while moving from point to point in short jumps. Fire is occasionally conducted from cover; firing from the move is expected to be rare.

III. Specifications

4. The self-propelled howitzer will be manufactured with minimal modifications to the service model of the KV tank (152 mm howitzer model 1937 and the KV-7 tank).

5. The vehicle’s overall dimensions—width, height, and line-of-fire height—must be as close as possible to the same dimensions on the KV-1 tank. The height of the tank and the line-of-fire height must not increase by more than 100 mm.

6. The vehicle together with ammunition, crew, and fuel must weigh 45–50 tonnes. It is desirable that this weight be met.

7. The main technical characteristics of the self-propelled howitzer are as follows:

Caliber: 152 mm

Angles of elevation: from -2° to +15–20°

Angles of traverse: +/-5° to +/-6°

Aiming rate: from 30 to 45 min. per flywheel rotation

Maximum recoil distance: no more than 850 mm

Resistance to recoil: no more than 30–35 t

Maximum rate of fire: 8 rds/min, including relaying the gun.

Effort on flywheels: 3 kg while turning

Basic load: 30 rounds

IV. Performance requirements

8. The howitzer is served by a crew of 6: commander, driver-mechanic, gunner, loader, breechblock operator, radio operator/machine gunner.

9. The tipping parts armor must allow free access for servicing the recoil mechanisms.

10. The gunner’s fighting compartment: the seat and the location of the gun laying mechanism flywheels and sight eyepiece must support easy and fatigue-free laying of the howitzer on a target both while on the move and while stationary. The effort required to operate the firing handle must not exceed 8 kg; it would be desirable to have the firing control on the elevation mechanism handle.

11. The tank commander’s fighting position must be equipped with the KV-1S commander’s cupola.

12. The howitzer must be equipped with a folding tray/barrier for preparing shells for firing that automatically locks the firing mechanism until loading of the howitzer is complete. The design of the folding tray must facilitate the loader’s job while requiring a minimum expenditure of muscle energy.

13. The fighting compartment height at the work positions of the breechblock operator and the loader must be at least 1700 mm.

14. The ammunition storage rack must be strong and non-warping, and it must allow free and easy removal of rounds in order to achieve the highest rate of fire.

15. The placement of rounds in the ammunition storage rack must conform to the division of labor during loading between the breechblock operator and the loader: the loader prepares and places a projectile in the chamber, and the breechblock operator opens and closes the breech and inserts the case into the chamber.

16. The hand ejector, fuze wrenches, and rammer must be attached to the turret walls.

V. Requirements for the turret and its equipment

17. The rear wall of the turret must have a hatch for the crew, for loading ammunition and for ventilating the turret interior in specific types of combat.

18. The turret walls must have openings with armor plugs for firing submachine guns and supplemental vision slits for observing the surrounding environment. It is desirable to have a ball mount for a DT tank machine gun in the rear turret wall.

19. The personal weapons of crew members must be located in secure and convenient racks near their work positions.

20. The turret roof near the commander’s position must have a port for signaling with rockets and a hatch in the rear for combat using hand grenades.

21. The turret illumination must support firing the weapon and monitoring the gun.

22. Radio and telephone communications in the tank shall be accomplished using a standard tank radio and a tank intercom.

23. Turret armor: at least 90 mm on the front and 50 mm on the front and sides; it is desirable to have the side walls sloped 30° from the vertical.

24. The commander’s position must facilitate use of a map and plotting board.

25. Each crewmember must have a comfortable seat for rest and for use while on the move. The seats of the breechblock operator and loader must be capable of being folded against the walls and must not interfere with them during combat.

26. The turret interior must have places for a first-aid kit, emergency rations, and drinking water.^{14}

^{U-3 203 mm corpslevel heavy howitzer during testing, spring of 1942}.

It appeared that the project for a 152 mm SP gun based on the KV tank chassis that had been stalled for half a year and had disappeared from the development plan had finally begun to move forward. Despite the fact that the base chassis was now the KV-1S rather than the KV-1, the overall SP gun concept had not changed, and the timetable was entirely achievable. Unfortunately, matters did not proceed as planned. Representatives of the Chelyabinsk Kirov Factory did travel to Factory No. 8 and became acquainted with the design documentation for the ZIK-20, and work on the project came close to reaching a practical stage. Another factor was that construction of the model stalled, and an event that had a serious impact on the entire project occurred at the very end of October. State Defense Committee Decree No. 2457ss of October 30, 1942, split Factory No. 8 into two enterprises. The first, Factory No. 8, was required to engage in antiaircraft production. The second enterprise became Factory No. 9, responsible for howitzer artillery and tank guns. L. R. Gonor became director of the new factory; P. I. Maloletov was appointed party organizer; and F. F. Petrov was made chief of Factory No. 9’s design bureau.

The splitting of the factory affected the timeline for constructing a fullscale mockup of the superstructure, but it had no impact on the activities of Factory No. 9’s new design bureau. Back in October, before the factory was split, Petrov had led a major effort to design a number of artillery systems.

Work to fine-tune the 203 mm U-3 corps-level howitzer designed by V. N. Sidorenko had been underway at the Ural Heavy Machinery Plant since 1939. The U-3 project surfaced again in the spring of 1942: between May and June, it underwent comparison testing with its main competitor—the BL-39 203 mm corps-level howitzer designed at Factory No. 172. The test resulted in a proposal to finalize the howitzer and a recommendation to place it in service with the Red Army. However, neither the U-3 nor the BL-39 went into production. The situation with the BR-2 152 mm gun that the artillerymen so desired to mount on a vehicle based on the KV chassis was no less sad.

^{U-3 203 mm corpslevel heavy howitzer elevated to 45 degrees. This gun was developed to replace the B-4 howitzer and was proposed several times for installation in an SP gun (TsAMO).}

^{U-3 203 mm corpslevel heavy howitzer in travel position (TsAMO).}

^{Factory drawing of U-3 203 mm corps-level heavy howitzer (SA).}

Petrov’s proposals were bold; their basic idea was to place the barrels from heavier systems on the carriages of the M-30 122 mm howitzer and the ML-20 152 mm gun-howitzer. He proposed six projects in all; however, only two of them were of interest. The reason is clear from the project description:

Placement of the BR-2 152 mm gun model 1935 and the U-3 203 mm howitzer on the ML-20 carriage, however, significantly increases the weight on the wheels, but there can be no doubt about the combat utility of these systems for the following reasons:

a) Not even the ML-20 152 mm gun-howitzer model 1937 or the A-19 122 mm gun model 1931/37 can currently traverse swampy terrain freely. Their bottom carriage and trails become badly bogged down. But this carriage, even with the extra load from the BR-2 gun or the U-3 howitzer, moves well over other types of ground, including plowed fields.

b) ML-20 is currently being converted from 1250×300 cast wheels to twin KMP 1250×200 wheels, which will reduce ground pressure.

c) The springs in both systems are being strengthened, which poses no difficulty because the ML-20’s spring leaves are simply being enlarged.

<…>

An even more striking result is obtained if, on the one hand, the 152 mm gun model 1935 is combined on the ML-20 carriage with a monobloc barrel and a cast breech having a combat weight of 9000 kg and a travel weight of 9800 kg, and, on the other hand, the same gun on the B-4 203 mm howitzer carriage is combined with a triple-layer barrel with a combat weight of 18,260 kg and a travel weight of 24,000 kg.

Both the service/combat and industrial/economic benefits of this are particularly high.

If the BR-2 152 mm howitzer is placed on the ML-20, then installation of the gun with restricted traverse in a KV tank becomes entirely possible without modifying the tank.

Under our ZIK-20 project to mount it on a KV tank, the installation would use almost the entire cradle and all mounting parts from the ML-20.

The weight of the self-propelled gun would increase relatively little—1800 kg.

In any event, this solution for developing a self-propelled gun is significantly simpler than the approach taken before the war, which involved developing a new and larger chassis for the BR-2 based on the KV tank.^{15}

Petrov’s idea appeared a bit bold, to put it mildly, only when viewed from the sidelines. As the experience gained in building the M75 107 mm antitank gun demonstrated, the ML-20 carriage was fully capable of mounting heavier systems. As it later turned out, the same could be said of the M-30 howitzer’s carriage. Of the six proposals, only one became metal. The M-10 152 mm howitzer on the M-30 152 mm howitzer carriage was placed in service with the Red Army as the D-1 howitzer during the summer of 1943. The debut design of Factory No. 9’s design bureau has been so successful that it is still in service in a number of countries.

^{Installation of BR-2 152 mm gun on the ML-20 gun-howitzer carriage, a Factory No. 8 design, October 1942 (TSAMO).}

Factory No. 9’s design bureau had another project in addition to these six gun systems. Petrov’s idea for mounting the BR-2 in the ZIK-20 was translated into a conceptual design. It was no less valuable for being based on the version of the ZIK-20 revised as suggested by the Technical Council of the People’s Commissariat of Arms. The SP gun had a rear hatch, telescopic sight, a rack for stowage of ammunition in a single layer on the floor, and the commander’s cupola from the KV-1S. In addition, the BR-2 installation project included ZIK-20 specifications based on both the KV-1 and the KV-1S.

^{Factory drawing of BR-2 152 mm gun on the ML-20 gun-howitzer carriage (TsAMO).}

^{Factory drawing of BR-2 152 mm gun on the ZIK-20 SP gun (TsAMO).}

^{BR-2 152 mm gun on the ZIK-20, 1:35 scale drawing.}

1. The Technical Council of the People’s Commissariat of Arms and the GAU’s Artillery Committee have approved the ZIK-20 project for mounting the ML-20 152 mm gun-howitzer model 1937 on a KV tank with a fixed (welded) turret. The drawings for this installation are almost finished.

If the BR-2 152 mm gun is mounted on the ML-20 carriage, the barrel of the BR-2 gun with a muzzle brake made of armor steel can easily be placed on the ML-20’s cradle.

The barrel would only require modification of the cast breech. Its weight would increase and its weight distribution would change. The modification would be required to achieve balance and provide the needed firing angles.Only the profile of the recoil throttling rod would change as compared with the tank-mounted ML-20.

2. This installation would result in a self-propelled gun with the following characteristics:

No.	Parameter	GAU’s operational requirement	ML-20 152 mm gun-howitzer model 1937	BR-2 152 mm gun model 1935
1.	Caliber, mm	152	152	152
2.	Shell weight, kg	49	43.5	49
3.	Muzzle velocity, m/s	880	600	880
4.	Charge type	Bagged	Separate loading	Bagged
5.	Weight of entire SP gun using:
a) KV tank, t	Not more than 65	53	55
b) KV-1S tank, t	Not more than 65	46	48
6.	Number of shells	47	30–40	40–47
7.	Displacement of center of gravity relative to a KV tank, mm	–	360	390
8.	Armor thickness, mm:
a) Glacis	60	KV: 75, KV-1S: 60	KV: 75, KV-1S: 60
b) Side	60	KV: 75, KV-1S: 60	KV: 75, KV-1S: 60
c) Turret	60	75	75
9.	Traverse angle	+/-4°	+/-6°	+/-5°
10.	Elevation angle	+15°, -3°	+15°, -3°	+15°, -3°
11.	Distance barrel extends beyond vehicle, mm	–	1700	3000
12.	Line-of-fire height, mm	–	2190	2206

3. As the table and the drawing of the KV tank installation show, the BR-2 installation differs little from the ML-20 installation in the same tank, but it is more than twice as powerful.

It is necessary to accept the greater extension of the gun barrel forward of the tank as compared with the ML-20 because we are dealing with such a powerful barrel, which, together with the muzzle brake, is about 8000 mm long.

If there is a need to develop a self-propelled gun of this type, this solution is the most practical, the simplest, and the most constructive and technically correct of all the alternatives.

4. Since the BR-2 152 mm gun shares a carriage, cradle, breech, and recoil mechanisms with the 203 mm howitzer, placing the BR-2 on the ML-20 tank cradle also offers the opportunity, as needed, of placing the B-4 203 mm howitzer’s monobloc barrel on this cradle with similar modifications and, of course, it would be even easier to place the U-3 203 mm howitzer in a tank.^{16}

The idea of mounting the BR-2 on an ML-20 cradle was entirely feasible; moreover, similar projects were implemented in 1944. On the other hand, it was much more important towards the end of 1942 to complete work on the ZIK-20 superstructure, and the situation with that project was far from rosy. The transfer of SP gun projects to Factory No. 9 delayed work on the design even more. The situation was aggravated by the fact that the model shop at the new factory was less capable. As a result, the full-scale wooden model of the ZIK-20 was not finished until December 15.

A review of the design documentation and model were scheduled for January 3, 1943, by decision of Col. Gen. of Artillery Yakovlev, Chief of the Main Artillery Directorate; Zaltsman, People’s Commissar of the Tank Industry; and Ustinov, People’s Commissar of Arms. It took place at the Chelyabinsk Kirov Factory, and it went beyond a simple finding based on the model to involve competition with projects proposed by the Kirov Factory’s design bureau. Competition to the Sverdlovsk SP gun emerged from Chelyabinsk because the ZIK-20 design was delayed and collaboration between the two factories was lacking.