Saturday, 30 April 2016

Stalin's Maus Killers

Gun design is driven by the desire to be at least half a step ahead of its opposition. The creators of tank armour aim to provide tank crews with maximum possible protection from all existing weapons, and gun designers aim to create a gun that would penetrate the armour of any modern tank. In the spring of 1941, work on a new 107 mm anti-tank gun began in the Soviet Union, a gun that would not see a worthy adversary for several years. Both these guns and their likely enemies were built in metal, but neither one ended upon the battlefield.

Shooting at Shadows

At the start of 1941, Soviet anti-tank artillery evolved simultaneously in several directions. The most promising weapon was the 57 mm F-31, later indexed ZiS-2. This gun was developed at factory #92 under the direction of V.G. Grabin. In parallel, a "powerful 76 mm AT gun on the USV base" was developed, with the factory designation F-26. It was plagued with difficulties. Due to problems with the recoil brake, the gun was redesigned several times. Finally, in early January of 1941, under Grabin's initiative, an 85 mm anti-tank gun was also developed. It was based on the mount of the experimental 95 mm F-25 divisional gun with lengthened trails. In January, it fired 225 shots.

On March 11th, 1941, the chief of the Red Army General Staff Intelligence Directorate, Lieutenant-General F.I. Golikov, prepared a special report "On the direction of development of the armed forces of Germany and changes in its status". According to this document, German industry began constructing three new types of tanks. The most powerful one, called "Type VII" allegedly weighed 90 tons and was armed with a 105 mm gun. The military treated this information very seriously. The T-150, which was supposed to enter production as the KV-3, was cancelled. Instead, a new vehicle was developed on the basis of the KV-220. The new KV-3 would have a 107 mm gun, have armour about 120 mm thick, and weigh 67-68 tons. In addition, KV-4 and KV-5 tanks were developed in parallel.

Tanks weren't the end of it. By April 18th, 1941, a Council of Commissars decree titled "On tank armament, new anti-tank gun, artillery SPGs, and armour piercing ammunition" was prepared. The sixth point of the decree is most relevant:

"6. On the powerful 107 mm corps gun.
Order the NKV to develop and produce at factories #172 and #92 experimental guns with a 107 mm barrel with the muzzle velocity of 1100 m/s on the mount of the 152 mm mod. 1937 gun-howitzer mount.
Due date for delivery of experimental prototypes to the NKO: July 1st, 1941
By the same due date, produce a tank destroyer with a 107 mm gun of the aforementioned ballistics. The NKTM must build this tank destroyer on a lengthened KV-4 chassis with 60 mm of armour.
The NKB must develop and produce by that date 3000 armour piercing shells with tracer, phlegmatizer, and decopperer in the propellant."

The tank destroyer mentioned in the decree was to be developed in parallel with the SU-B-13 and "212" SPG. According to project documentation, development of the SU-B-13 began, a project was drafted, tactical-technical requirements were made, and the project stalled. The production of the 212 SPG ended during construction of the hull. Unlike these two, the project of the tank destroyer did not even reach the tactical-technical requirements stage. However, the "powerful 107 mm corps gun" made it further.

Preliminary calculations showed that the new powerful gun with a muzzle velocity of 1100 m/s would penetrate 188 mm of armour at a 30 degree angle at 1000 m. By early May, the characteristics changed. According to calculations for the 107 mm anti-tank gun, the muzzle velocity was reduced to 1020 m/s and the length of the barrel would be about 60 calibers. Even this data was not final.

Meanwhile, Grabin displayed his initiative and began developing a lighter alternative for the ZiS-24. This gun, indexed ZiS-23, used the mount from the 107 mm M-60 divisional gun, and its muzzle velocity would be 1150 m/s with a 70 caliber barrel.

The development of tactical-technical requirements for the new guns dragged on. The GAU Artillery Committee reviewed them only on May 22nd, 1941. According to the characteristics, the velocity of the 18.8 kg 107 mm armour piercing shell was kept at 1020 m/s and the penetration was decreased to 160 mm at 30 degrees. The barrel length was limited to 70 calibers. The rate of fire was set at 10 RPM with one piece rounds. The mount from the 152 mm ML-20 would be used as much as possible during development, including the sight. In addition to its AT duties, the gun would act as a corps gun, which is why it would also have an 18.8 kg HE shell fired at 730 m/s.

Draft project of the 107 mm M-75 antitank gun.

On the same day, requirements for a "special purpose 85 mm anti-tank gun" were approved. The gun would also serve as a divisional gun, and have a 9.2 kg 800 m/s HE shell. As for the AP shell, it was fired at 1150 m/s and weighed 10.5 kg, letting it penetrate 160 mm of armour at 1000 m at a 30 degree angle. The requirements included the use of the M-60 107 mm gun mount. Changes would be minimal and would include the gun shield.

Heavyweight Competitor

Until early June of 1941, design of high power AT guns proceeded only at factory #92, even though the decree mentioned two factories. This can be explained by Grabin's initiative, which was noticed at the top. Either way, factory #172 was too busy: it still had a large order for howitzer artillery and Supreme Command Reserve artillery to complete.

The requirements kept changing throughout the end of May and the beginning of June. The final variant was only sent to the factories on June 7th. By that time, factory #172 finally began the development of its own 107 mm gun. The latest changes put the factory in a difficult position, as they included the use of a mechanical rammer and variable rifling. In a letter to the chair of the Artillery Committee, Major-General Hohlov, acting director Kudryavtsev wrote that the design bureau would design a gun without these features. According to the bureau, the rate of fire of 10 RPM was attainable without a mechanical rammer. The gun designed at factory #172 was indexed M-75. According to preliminary calculations, its muzzle velocity was 1020 m/s and its penetration matched the requirements.

The gun shield that would be installed on all high power anti-tank guns.

Interestingly enough, development of an 85 mm gun began in Molotov (modern day Perm), and only fragmented information survives to this day. Information on it first comes in up mid June of 1941. Correspondence between factory #172 and the People's Commissariat of Ammunition reveals that not enough test rounds were produced for the ZiS-23 to also test the factory #172 gun. As a result, another 300 shells would have to be made: 50 by July 5th and the remaining 250 by July 15th. Further letters reveal the index of this gun: M-76. The letters also state that factory #172 received permission from the state to build a prototype. 

No tactical-technical characteristics for the M-76 were found. As for its competitor, the ZiS-23, its characteristics were finally confirmed in early July of 1941. To develop a muzzle velocity of 1150 m/s, the gun barrel was 96.8 calibers (8228 mm) long. The ZiS-24 also did not meet its barrel length restrictions. According to the final characteristics, the overall length of its barrel was 73.5 calibers, or 7843 mm.

The characteristics document dated July 4th, 1941, also mentions that factory #92's guns were finished and were currently at the stage of prototype assembly. Whether or not assembly was ever completed is not known. By June 31st, 1941, both guns were still being designed. Factory #92 had to send off two ZiS-23 prototypes (priority of the ZiS-24 was reduced) by August 15th. This did not happen, and furthermore, both guns are absent from a report on factory #92's experimental work. No additional information appears in subsequent documents. It is very likely that the ZiS-23 and ZiS-24 were never built.

Unexpectedly, a third competitor entered the race. On July 7th, 1941, a letter from Sverdlovsk arrived, addressed to the chief of the Orders and Production of Land-based Artillery Directorate (UVNA). The authors of the letter were the chief engineer of the Ural Heavy Machinebuilding Factory (UZTM) and F.F. Petrov (one of the creators of the ML-20 gun). In the letter, they described a draft of an anti-tank gun indexed UML-20.

Draft project of a 107 mm UML-20 anti-tank gun, July 1941.

The main feature of the project was that the gun was as unified as possible with existing guns, the ML-20 and A-19. The screw breech lock and barrel case were taken from the A-19. The breech and mount came from the ML-20. The A-19 also donated its shell casing. The possibility of using M-60 shells was mentioned, but it was not known if the shell would survive the higher pressure. The muzzle velocity was estimated at 1050 m/s with a barrel length of 59.5 calibers. In order to compensate for the altered center of mass, the barrel was extended by 235 mm.

Calculations for the UML-20 barrel. Unlike this gun, documentation for the ZiS-23 and ZiS-24 did not survive to this day.

UZTM's proposal was very reasonable. Thanks to the use of existing parts, development and, most importantly, production could have been completed quickly. However, the UML-20 project did not move past the design proposal stage.

In the end, the only high power anti-tank gun that reached the trials stage was S.N. Dernov's M-75.

Without a Worthy Target

Technical documentation for the 107 mm M-75 anti-tank gun was prepared by factory #172 by the end of June of 1941. The start of the Great Patriotic War did not influence its development. Unlike work on the KV-3, KV-4, and KV-5 tanks, no one cancelled work on the high power gun. Factory #172 began production of two prototypes in early July, and factory trials began by the end of the month. Muzzle velocities between 1017 m/s and 1047 m/s were recorded, which met requirements. The driving bands were not torn. Penetrations in the plate were in the form of a circle, meaning that the shells were stable in flight. The biggest problem during trials was the work of the semi-automatic mechanism. This was caused by the materials that were used to make the breech and breechblock.

M-75 on trials, October 1941.

On July 15th, a technical project was sent to UVNA chief, Military Engineer 1st Grade Komarov, developed in parallel with the production of prototypes. Overall, the gun met requirements. Its muzzle velocity was 1020 m/s, the barrel was 70.5 calibers long (7550 mm), and the design used the ML-20 mount. Nevertheless, the project was met with the following response:

"Instead of installing the new 107 mm barrel on the ML-20 mount, the factory performed an unfortunate "modernization" of the mount."

A more complete version of this appraisal was sent to factory #172 on July 28th, 1941, signed by GAU chief General-Colonel Yakovlev.

The same gun in firing position.

This reaction was caused by changes made at factory #172 to the design of the mount. Instead of the stock system to move the barrel from travel to firing position, a pulley was used, like the M-60. Aside from the change, GAU made a reasonable complaint about whether or not the addition would increase the speed at which the barrel could be transferred to and from the firing position. The designers of the M-75 also changed the recoil system and designed an almost completely new limber. In the conditions of war when German heavy tanks were expected any day, this was deemed unacceptable. Mass production M-75 had to be built on an ML-20 mount with a stock limber.

Despite a requirement to use the ML-20 mount as much as possible, the M-75 prototypes had may changes in their design.

Further trials of the M-75 were performed at the Ural Artillery Proving Grounds (today the Nizhniy Tagil Metals Research Institute, NTIIM). The trials program was approved on August 16th, 1931. The robustness of the system, its ballistics, and penetration were tested. Gun #1 had a barrel #4 with 25 caliber rifling and gun #2 had a barrel #7 with 32 caliber rifling. Barrel #5 was shipped with the guns, also with 32 caliber rifling.

Gun in travel position. Instead of the stock system for moving the barrel from travel to firing position, a pulley like on the M-60 was used.

Documentation for the gun was delivered on August 28th, but trials only began one month later, on September 29th. The first stage of the trials was disassembly and reassembly, during which the guns switched barrels. Trials were complicated by a lack of equipment at the Ural proving grounds for various necessary work.

The first shots were fired only on October 5th from gun #1. On October 16th, after 105 shots, trials were stopped due to a suggestion from factory #172. This was caused by the need to make a new breechblock and several other components. Trials resumed on November 3rd. After 402 shots, barrel #5 was replaced with barrel #4 due to significant wear of the rifling. Firing trials ended on December 11th, but due to the busyness of the proving grounds staff with other guns, the report was only composed on January 2nd, 1942.

Gun in firing mode, from the front.

The results of the trials were not encouraging. Fist, the lifespan of one barrel was estimated at 250-300 shots, which was not enough. Second, there were serious problems with penetration. The gun penetrated 152 mm of armour, but only at 100 meters. No attempt was made to fire at one kilometer due to the weakness of the shell hulls. This was a problem with many Soviet shells already in production, and it was not solved immediately. The real rate of fire of the gun was 5-6 RPM. There were significant problems with extraction. The M-75 failed trials.

Shells for the 107 mm M-75 gun.

Factory #172 had the potential to improve the M-75. Many guns, including anti-tank guns, showed these drawbacks. Trials showed what had to be corrected. Also, as mentioned above, the Ural proving grounds were not properly equipped for trials, and many problems could easily be solved at the factory. Nevertheless, on April 22nd, 1942, a decree was issued cancelling work on high powered anti-tank guns.

"Since the experience of modern war did not confirm the need for such guns to fight tanks, further work on the M-75 gun and its ammunition is considered unreasonable. Reporting on the above, I ask for your permission to cease all work on the 107 mm M-75 gun and similar work on 85 mm and 107 mm ZiS-23 and ZiS-24 guns at Order of Lenin factory #92.

107 mm shells could penetrate armour, but cracked on impact.

The People's Commissar of Armament, Colonel-General Volkov, did not know that right when he signed that letter, Ferdinand Porsche's design bureau began work on the super-heavy tank that super-powered anti-tank guns were made for. Of course, neither the VK 100.01 nor the Maus ever reached the front lines. Nevertheless, the gun cropped up once more in 1943.

M-75 gun #1 on display at the future site of the Armed Forces Museum of the Republic of Kazakhstan, in Astana.

In July of 1943, at Ponyri, another brainchild of Porsche K.G. entered battle: the Ferdinand SPG. Its appearance was treated seriously by the Red Army. The M-75 appeared once more in the list of work on tank, anti-tank and self-propelled artillery dated September 15th, 1943. One of the reasons that the project was buried in 1942 was the use of the ML-20 mount. The idea of a tank destroyer with the M-75 had another chance at life two years after it was first discarded in 1941, especially now since it had a worthy target. 

The return to 107 mm artillery was brief. In the same letter that mentioned the M-75, Major-General Hohlov proposed discarding a number of projects, replacing them with a much more promising 122 mm gun with the muzzle velocity of 1000 m/s.

4 comments:

  1. I wonder, what technical methods were used back then to measure the muzzle velocities ? Do you have any knowledge of this, by chance ?

    ReplyDelete
    Replies
    1. High speed photography was already good enough to photograph the shell in flight and not only measure the speed, but watch the impact. DTIC has some American research papers on the topic.

      Delete
  2. The weakness of the shell was caused by using uncapped AP and APBC for very high velocity impact. Projectile break up -even against homogenious armour- is typical at impact velocities larger than ~800m/s with uncapped AP-shot.

    I presume, had they executed trials at 1km range, the perforation would pretty much have been not too different from the one at 100m. Projectile break up causes the penetration to plateau because more energy is lost by breaking up the shell and spreading out the force on the plate.

    You simply can´t make a monobloc AP-shot with the period level of metallurgical skill without AP-cap and have it survive a 1000m/s impact againt thick, homogenious armour, not even considering face hardened here.

    The germans run into the same issues, forcing them to re-engeneer their APCBC-HE in 1941 to 1942 to make them fit for longer barreled, highe velocity guns about to come in service by then.

    ReplyDelete