Bunker Buster Project
In early 1942, it was clear that the KV-3 wouldn't take off in Chelyabinsk. Even if the tank and its engine were still listed as active projects, they were all but forgotten. The plans to build the 212A SPG on the KV-3 chassis were dropped in April of 1942, but the idea of a heavy assault gun remained. This time, the KV-7 served as the foundation, as its casemate allowed the use of a 152 mm ML-20 gun-howitzer. The project to mount this gun in a KV-7, indexed U-18, began at UZTM under the supervision of L.I. Gorlitskiy.
However, the compromise of using the KV-1 chassis and an ML-20 gun was not entirely satisfactory. The KV-1 chassis was acceptable, but the ML-20 was notably inferior in its destructive power to the 152 mm Br-2 high power gun, which was considered the gold standard for bunker busters. As a last resort, the B-4 howitzer was considered acceptable, since both could be installed on the same mount. Unlike the Br-2, less than 40 of which were made, the B-4 was a fairly common artillery system.
At the start of 1942, two organizations were involved in the design of heavy SPGs. The Moscow Baumann Machinebuilding Institute was tasked by the NKV to work on bunker busters armed with either a 152 mm Br-2 gun (SU-Br-2) or 203 mm B-4 howitzer (SU-B-4). The second organization was factory #221, the producer of Br-2 guns and B-4 howitzers. In March of 1942, the factory design bureau finished proposals for the BR-33P and BR-33G SPGs. Both were based on components from the T-34 medium tank. Documentation showed that the BR-33P would be armed with a 152 mm Br-2 gun, and the BR-33G would be armed with a B-4 howitzer. After a review, it was decided that these vehicles would not be produced, as they did not match the requirements for bunker busters.
The Ural variant
In April of 1942, the task of creating a 203 mm assault gun was given to the UZTM design bureau. Judging by correspondence, this vehicle was ordered by J.Ya. Kotin, Deputy People's Commissar of Tank Production at the time. It is interesting to note that, unlike the U-18, the U-19 was not present in reports. It is likely that the project had a low priority and was treated as a backup plan. Nevertheless, the U-19 was completed before the U-18. The overall description of the vehicle was finished in May, blueprints in June, and the project was sent to GAU and GABTU on August 12th, 1942, three weeks before the U-18. The backup plan was also thought out a lot more thoroughly than the main one.
The U-19 consisted of a 203 mm mod. 1931 howitzer (B-4) on the KV-1 chassis. The goal of this vehicle was the destruction of fortifications that were too tough for smaller guns. Like the KV-7, the KV-1 hull was altered: the turret platform was removed, the engine bulkhead was made removable, the fuel tanks and air intakes were altered. The oscillating part of the B-4 remained unchanged, in order to reduce the time required for conversion. The turret was replaced with a massive casemate, partially overlapping the engine compartment, which could cause difficulty when servicing the engine.
The thickness of the front plate was 75 mm, the sides were 60 mm thick, the rear 40 mm, and roof 30 mm. A large hatch was included in the rear for installing the howitzer, which had a smaller hatch in it for crew evacuation. According to the specifications, the roof was removable. Since the main purpose of the U-19 was the destruction of fortifications, the gun elevation was capped at 10 degrees. The horizontal traverse was minimal, only 4.3 degrees each way, same as on the B-4 howitzer.
The requirement to make minimal changes to the chassis while also providing a respectable level of protection resulted in an interesting product. According to calculations, the U-19 would weigh 66190 kg, which was even heavier than the 212A SPG. Since vehicles have a tendency to be heavier in metal than on paper, it was possible for the U-19 to be the heaviest Soviet assault SPG, surpassing even the fully armoured SU-14. Putting the B-4 howitzer in a closed casemate also required a height of 3510 mm, which was only 50 mm lower than the fully armoured SU-14. The creators of the U-19 understood that the large SPG was a perfect target for the enemy, which is why it was to be escorted by regular tanks on the battlefield.
A predictable outcome
The project led to predictable results. On September 9th, 1942, the chief engineer of UZTM, M.G. Umnyagin, received a letter written by the chief of the BTU, Engineer-Colonel Afonin.
"To your letter #3707/48s sent on August 24th, 1942, regarding the project "U-19 203 mm howitzer on the KV-1 chassis", I reply that:
- The mass of the U-19, equal to 66 tons, and the V-2K engine will not result in reliable movement at the required speeds. Furthermore, the transmission of the KV-1 is designed for a 40 ton vehicle, and cannot work reliably in a 66 ton vehicle.
- The SPG makes for a well noticeable target due to its large height (3.51 meters) and wide upper section (immobile turret).
- The armour is weaker than that of the regular KV-1 tank, as a result of which, the crew will not be protected at close ranges.
- If the suspension is damaged, the gun will only work within a 9 degree arc, as the U-19 does not have a rotating turret.
- Due to the mass of the SPG, it will not be possible to tow damaged SPGs from the battlefield.
- The off-road performance of the SPG, judging by its effective horsepower (9 hp/ton) and ground pressure (0.98 kg/cm^2) will be poor.