The acceptance of the KV-14 SPG, better known as the SU-152 "Beast Killer", didn't mean that further development of its design ceased. The use of an ML-20 gun-howitzer was, in a way, a compromise, and attempts to install more powerful systems continued. This article will discuss alternative armament for the heavy Soviet SPG: 203 mm howitzer and mortars. The SU-203 could have been one of these vehicles.
An intermediate type
The reason for searching for alternative armament is simple. During development of "bunker busters" it was assumed that they would use the high power Br-2 152 mm gun. This gun was chosen as the most powerful of all guns of this caliber used by the Red Amy. The most important thing was that the 53-G-551 concrete piercing shell could punch through up to 2 meters of concrete. This meant that bunkers such as the Finnish "millionnik" became vulnerable to SPGs. It's no wonder that the GAU tried to get a heavy SPG with this gun.
When designing his ZIK-20 SPG, F.F. Petrov had a variant with a reworked Br-2 in mind. The project, developed by the factory #8 design bureau, mounted a Br-2 barrel on the ML-20 mount. A massive two-chamber muzzle brake was installed. After factory #8 was split into two (factory #8 and factory #9) by GKO decree #2457ss on October 30th, 1942, Petrov continued his work, not as the chief of the factory #9 design bureau. The converted Br-2 was built as the experimental 152 mm D-4 gun.
203 mm U-3 corps howiter, 1942.
There was another alternative that never panned out. It consisted of installing the 203 mm U-3 corps howitzer onto the ML-20 chassis. This gun was designed in 1938 at UZTM under the direction of V.N. Sidorenko. It was built as a lighter and less powerful alternative to the B-4. With a shorter barrel (3250 mm), the U-3 was significantly lighter than the B-4 (9.4 tons compared to 17.7).
The U-3 was more promising than the rival M-40, but trials showed that it needed more polish. The start of the Great Patriotic War paused development, but the howitzer returned to trials in April of 1942. It failed again due to issues with the mount and the elevation mechanism. That led to the modernization on the ML-20 mount. According to the design, the mass of the system decreased from 9400 to 8100 kg. One drawback was the addition of a massive two-chamber muzzle brake and reduction of the maximum elevation angle from 75 to 65 degrees.
Installation of the U-3 howitzer on the ML-20 mount, October 1942.
The project description mentioned the possibility of a self propelled variant.
"As the 152 mm Br-2 shares its mount, cradle, breech, and recoil mechanisms, installation of the Br-2 on the ML-20 tank cradle opens the possibility, if necessary, of installing the monobloc barrel of the 203 mm B-4 howitzer, but it would be easier to install the 203 mm U-3 howitzer into a tank."
The barrel of the reworked U-3.
Not even a sketch of the installation of a converted U-3 into the ZIK-20 was made. Nevertheless, the story continued. In the spring of 1943, the factory #9 design bureau was working on installing the U-3 into another vehicle: the SU-152. The draft took many liberties. If such a vehicle was built, it would likely have the same mantlet as the SU-152. However, work did not move past this sketch. The issue was that the U-3 shell, identical to that of the B-4 special power howitzer, weighed 100 kg. It was not clear how ammunition would fit into the narrow and low casemate of the KV-14. It was also not clear how the loader would load them by hand, since it would not be possible to install a loading crane into the KV-14 due to the size of the casemate, like it was possible in the U-10.
The U-3's star set in 1943. The gun did not make it into mass production, and the possibility of it being installed in an SPG was more remote than ever. The idea of installing the U-3 on an ML-20 mount was never realized, even though the order to build a prototype came through in February of 1943. The U-3BM, a modernized version of the U-3 with a barrel lengthened to match the ballistics of the B-4, was never built in metal.
Draft of the U-3 in the SU-152.
The U-3's main competitor was the BL-39, designed in 1938-39 at the NKVD Independent Technical Bureau (OTB), the future OKB-172. The OTB was located inside the infamous "Kresty", and the letter index stood for "Beria, Lavrentiy". This was one of the infamous prisons where convicted engineers worked. In 1941-42, the OTB was evacuated to Molotov (modern day Perm), where it was renamed to OKB-172. Work on the BL-39 continued. Now the system competed against the M-40 high power corps howitzer, designed in 1938-39 at the design bureau of factory #172. The BL-39 lost to the U-3 during comparative trials, but it also did not enter production.
203 mm M-40 howitzer on trials.
The M-40 was also unlucky. The gun failed proving grounds trials in 1940, since its wheels sank into the ground after firing. By 1942 the M-40 was not considered hopeless, but at the very least secondary. In October of 1942 orders arrived from the Artillery Committee of the Main Artillery Directorate of the Red Army to cease work on this howitzer.
Nevertheless, this was the weapon considered suitable at factory #172 for installation into an SPG. In the spring of 1943, the factory's design bureau used the M-40 in an SPG, which was given the index M-17. The M-17 was designed at the initiative of the factory, later approved by People's Commissar of Armament Ustinov. The project was presented for approval on June 5th, 1943 two weeks after OKB-172's failure with the SU-203. Unfortunately, only a written description and calculations related to this SPG survive. The concept of the M-17, designed under the direction of the chief of factory #172's artillery design bureau, V.A. Ilyin, was close to that of the SU-203. Minimal changes were made to the KV-1S chassis, and as many parts as possible were used from the SU-152. The gun mantlet, gun shield, and frame were taken without changes.
The M-40 was used since its barrel was the most suitable for installation into the M-17. The M-40 barrel was installed in the ML-20S cradle, which was minimally altered. In addition, the M-40 barrel received a two-chamber muzzle brake that was over a meter in length. The T-5 telescopic sight was used for direct aiming. The ammunition capacity of the SPG would have been 16 rounds. The shells were kept in two racks: 9 along the left side of the casemate and 7 in the front. The propellant was stored in two metal boxes, one of which was under the gun, the other in the right corner of the fighting compartment. In addition, the vehicle carried 25 F-1 grenades and 21 PPSh disk magazines.
The same crew, radio, hatches, and observation devices were used as on the SU-152. To allow for an angle of depression of 3 degrees, the roof slanted forward with that angle. The sides of the fighting compartment in the rear were also moved out to cover the entire width of the vehicle to increase space. The main fuel tank was widened, and the size of the fuel tank that was placed along the left side of the fighting compartment was reduced. To lower the mass of the SPG, the thickness of the gun mantlet was reduced to 65 mm, the front plate to 60 mm, and the sides and rear of the casemate to 45 mm. The mass of this SPG would have been 45.8 tons. A variant with the stock SU-152 armour thickness was also considered, in which case the mass would be 46.8 tons.
The muzzle brake was 1140 mm long.
The project was reviewed by the SPG Directorate of the GBTU, where it was decided to authorize a prototype. The GAU had a different opinion. Recall that the M-17's main objective was to destroy enemy fortifications, which is where an issue arose. Calculations showed that the penetration of the M-40's concrete piercing shell was only 4% greater than that of the ML-20 at 2000 meters, equal at 1000 meters, and 6% less at a range of 200 meters. Considering that enemy fortifications were usually engaged in direct fire, there was no point in building this SPG. Work on the M-17 ceased in the end of July of 1943.
203 mm SPG projects on the SU-152 chassis had one significant drawback. The fighting compartment of this SPG was already considered cramped, and fitting in 203 mm shells would not add any space. In addition, there was no room for equipment that would move the heavy ammunition. The SU-203 was unsuitable for this purpose. The only solution was to change the fighting compartment. This project was introduced in May of 1943.
203 mm M-4 corps mortar on trials, 1943.
The precursor for this project was another self-initiated project that rivalled the U-3, BL-39, and M-40. In March of 1942, the factory #172 design bureau began working on a 203 mm corps mortar. Work on the system, which received the index M-4, was led by S.P. Gurenko. The idea was simple. A shortened B-4 type barrel was installed on the ML-20 mount. The howitzer also donated the breech. On May 14th, 1942, the Artillery Committee approved the M-4 project.
The first trials of the mortar were held between May 18th and 23rd of 1942 when the mount was tested. A barrel made from the M-40 tube and the breech of a 152 mm B-30 gun was used. In August of 1942, a monobloc barrel was made for the mortar. The breech was taken from the B-4 howitzer, with some changes. Ammunition from this gun was also used. The mass of the M-4 in travel position was 8300 kg, an in combat position only 7500 kg, which was less than half a ton heavier than the ML-20.
Despite a number of issues, the M-4 was suitable for mass production. On June 12th, 1943, Stalin signed GKO decree #3564ss "On preparation for production of the 203 mm M-4 mortar". This triggered the idea to make an SPG with the M-4. According to documents, the initiator of this project was Major-General of the Engineering Artillery Service A.A. Tolochkov, who held the post of the chief of experimental works at the Technical Council of the People's Commissariat of Armament. OKB-172 verified the preliminary requirements that would form the foundation for the project with him.
It was not clear when the M-4 heavy SPG program was launched. Judging by the dates on technical documentation, work was already underway in April of 1943.
General view of the SU-203 SPG.
On May 12th, 1943, two draft projects were sent to the chairman of the NKV Technical Committee Satel and the chairman of the Artillery Committee Lieutenant-General Hohlov. One of them was the SU-2-122, a double barrelled SPG on the T-34 chassis. The other was an SPG indexed SU-203. According to the description, the following requirements were considered during development.
- KV-14 type layout.
- Maximum ammunition capacity.
- Crew comfort.
- Simple loading procedure without the use of great force, rate of fire of 1-1.5 minutes per shot.
- Loaded mass of about 46 tons.
- Telescopic main sight, regular normalized sight as installed on the M-4 howitzer. PTK commander's panoramic sight for observation.
- 70 mm of front armour, 60 mm of side armour, 20-25 mm roof and rear armour.
- AA machinegun and pistol ports as additional armament.
Cutaway drawing. A redesign of the casemate allowed for the installation of lifting equipment.
The note of "KV-14 type" is very descriptive of the result of the efforts of A.F. Smirnov's team. Only the gun shield and mantlet remained from the original project. The rest was a completely new SPG on the KV-14 chassis. The project did not require that the SU-152 casemate be used, and so OKB-172's designers had a certain freedom allotted to them. The result was a project with great attention paid to the crew's working conditions.
Despite the increase in caliber, there was plenty of room in the fighting compartment.
Since the M-4's aiming mechanism took a lot of room, it was moved to the right. To make the driver's job easier, the gun was moved to the left. However, there was the option of leaving the driver in place was also explored. In this case, the breech mechanism had to be made more compact. The location of the commander, who sat to the right of the gun in the SU-152, was solved simply: he was made the gunner as well. The SU-203 was not equipped with a radio, which was another argument for combining the functions of the gunner and commander.
The number of crewmen did not decrease, since there were now two loaders. The loader to the left of the gun lived like a king. He also had access to a commander's cupola with five observation periscopes taken from the KV-1S. The right loader was also fairly comfortable. He had a hatch with a built in pintle mount for a DT AA machinegun. The breech operator also had a PTK periscopic sight. The commander only had the telescopic ST-10 sight and panoramic sight for indirect fire.
Lifting equipment that was necessary in such an SPG.
14 shells lined the sides of the fighting compartment, 7 per side. The fuel tanks had to be taken out of the right side to accomplish this. A special mechanism was developed to move such heavy weights. To fire, the nearest shell was taken from its slot, after which another shell was rolled over in its place. Propellant was kept in individual containers that were housed in the ammunition racks. Two more shells were placed in bustles that went outside the casemate. In addition, two more shells could be carried in loading trays. The six aforementioned shells were carried as backup ammunition that had to be loaded into the main rack as that ammunition was expended.
Thanks to the dense layout, the SU-203 was only 10 cm taller than the SU-152. The increase in height was necessary to fit lifting equipment, without which heaving around shells that weighed 100 kg would have been difficult. Smirnov's design group deserves praise. Unlike the U-19, monstrous in both mass and dimensions, the SU-203 was as compact as possible. At the same time, there was plenty of room to fit the crew comfortably inside.
The ammunition racks on the SU-203 were well thought out.
The conclusions regarding the SU-203 was made on May 21st, 1943. There were no complaints about the design, but the issue was external. The problem lay in the M-4 gun. It turned out that the ML-20's shell penetrated 1200 mm of concrete, while the M-4 could only penetrate 800 mm. It turned out that the gun was not as good at fighting enemy fortifications. Considering that its rate of fire topped out at 1.5 RPM, there was no point in building and producing the SU-203. Of course, the HE effect of the 203 mm shell was much better, but that was not what was needed from the SPG.
An SPG on the KV-1S chassis with a special power gun was eventually built, but the layout was completely different.