Two Meters of Concrete
The KV-2 had many advantages compared to analogous SPGs that were designed at factory #185 to fill the same niche. For one, the KV-2 was built on the chassis of the KV-1, which was already accepted for service, the only difference being in the turret and the gun. Factory #185 was designing vehicles on the SU-14 and T-100 chassis. The SU-14 was abandoned in 1938, and the future of the T-100 was under question. The KV-2 was also smaller than its competitors, and had thicker armour.
The KV-2 also had plenty of drawbacks from the point of view of the artillerymen. In order to fit the 152 mm M-10 howitzer into the KV-2's turret, it had to be shortened, reducing its already humble concrete piecing effects. The ability to penetrate 90 mm of armour from a kilometer away was not enough when dealing with bunkers like the Finnish "millionaire". The KV-2 was not a replacement for factory #212's SPGs, but a supplement.
152 mm mod. 1935 gun, the best method of destroying pillboxes.
A proposal clarifying tank armament dated June of 1940 listed "self propelled heavy artillery for destruction of pillboxes" as point #3. The SMK heavy tank was suggested as the chassis, armed with a 122, 130, 152, or 180 mm gun. The most promising candidate was the 152 mm mod. 1935 gun, or the Br-2. The production of the T-100Y with a 152 mm Br-2 gun was suggested as a temporary measure, but there was no time to mount the guns on either the SMK or the T-100; in July the projects were cancelled. Only one of the suggestions from the proposal document was produced, namely the production of the "KV tank with a large turret". As it often happens, the temporary measure became permanent.
The fixation on the Br-2 was no accident. Factory #185 picked the 130 mm B-13 gun. It could penetrate 130 mm of armour at a range of almost 4.5 km. There was a problem with the quality of shells, but that could be observed with any Soviet ammunition of that time period. It seemed that the B-13 was enough to combat fortifications, but the artillerymen had their own idea.s Even though the fate of the Br-2 was a difficult one and the production volumes were miserly even compared to the B-4 high powered 203 mm howitzer, the gun had fantastic characteristics. It could penetrate that same 130 mm of armour at 5 kilometers, and calculations showed that a concrete penetrating shell from the Br-2 could penetrate a 2 meter thick concrete wall. That was the thickness of the walls of the bunkers on the Mannerheim Line. The presence of a "can opener" like this on a self propelled chassis could effectively solve the problem of well prepared echeloned defenses.
The T-220 heavy tank became the prospective chassis for the heavy tank destroyer after the T-100 and SMK were cancelled.
After the T-100 and SMK were cancelled, the bunker buster issue didn't go away. On July 17th, 1949, decree #1288-495ss was issued by the Council of Commissars, which ordered the Kirov factory to build 4 modernized KV tanks by December 1st, 1940. Two of them had 100 mm of armour, two had 90 mm. The armament was also different. A fifth vehicle was mentioned in the document which would have become the chassis for the 152 mm Br-2 gun.
The Kirov factory assigned the index "212" to the bunker buster project. The SPG is also called "object 212" in correspondence, but that is not an official title. There was also a second vehicle with that index, an engineering vehicle on the KV chassis that never passed the full sized model stage. The index "212A" is used in some sources, but it was not present in factory documentation. The 212 project was led by Zh.Ya. Kotin, and Ts.N. Golburt was the senior engineer on the project.
212 SPG, cutaway.
2 million rubles were allotted to the 212 project. According to the Kirov factory budget, 100,000 was spent on the design project, 25,000 on the model, 300,000 on blueprints, 75,000 on blueprint editing, 1,100,000 on production of the prototype, 100,000 on trials, and 300,000 on repairs. This budget did not include the cost of a gun.
The project was not a priority at SKB-4. Their main task was the creation of heavy tanks with more powerful armour and armament. Real work on the 212 project only began in August of 1940, after receiving tactical-technical specifications, and work on blueprints did not start until October. This was fully connected with the T-220 tank, which served as the chassis for the SPG. In October, the SPG began to take shape.
The 212's hull blueprints were finally approved by Kotin on November 5th, 1940. Note that there was no room for a hull gunner/radio operator.
Externally, the 212 resembled the enclosed version of the SU-14. The fighting compartment was in the rear, the engine compartment was in the center, the driver's compartment up front. There was only room for the driver in that compartment, shifted to the left. Some renders of the vehicle show a machinegun in the front plate, like on the KV It would have been difficult for the radio operator to fire it with the crankshaft passing through his spine.
Even through the resulting SPG was rather large, it seemed a lot smaller when compared to its predecessors, especially the SU-14. The rear casemate seems enormous, but in reality it was less than half a meter longer than the T-220 and a little bit taller. With such a large gun, it would not have been possible to make the casemate smaller. The V-2SN 850 hp engine, created by combining the V-2 with the AM-38 aircraft engine's mechanical supercharger, was proposed as the power plant. The same engine would go into the T-220.
The requirements given by the Artillery Committee caused fierce debates in December of 1940. Due to the late requirements and high priority of the T-220, the Kirov Factory could not present the 212 by December 1st. However, on December 10th, a letter from the Kirov factory was sent to the Artillery Committee, signed by Kotin, Fedorov, and Golburt. The letter contained many complaints about the requirements. The problem was that the required 75 mm of armour were completely incompatible with the maximum weight of 55 tons. This was not a problem with only the SPG. The T-220's mass started out at 56 tons and ended up being 62.7 tons.
The 212 was even larger. Rough calculations showed that with a mass of 55 tons, only 17.5 tons were left for the hull. The SMK, which was close in size but had thinner armour, had a 31 ton hull. The solution was simple: lift the weight limit to 65 tons. Even with this limit, the front armour had to be reduced to 60 mm.
After debates that lasted until the end of December, the artillerymen were forced to give in. According to a GABTU report on experimental work, the Kirov factory completed a series of components for the 212 by January of 1941. The design project was also completed, and blueprints were sent to the Izhor factory to build the hull. 1.5 million rubles were spent on the 212 by that point. According to a report from the Kirov factory, a full set of hull parts arrived from the Izhor factory on March 5th. Assembly was delayed due to a shortage of parts produced by the Kirov factory. This is where work stopped, as after March, the factory was too busy for a bunker buster.
Victim of a Phantom Menace
On March 11th, 1941, the General Staff received a report on the status of the German tank industry. Among other things, the report listed the production of new heavy tanks, the largest of which weighed 90 tons. The report radically changed all plans regarding heavy tanks. Instead of the T-150, set to be released under the index KV-3, requirements for a new tank indexed 223 were drawn up. Work on even heavier KV-4 and KV-5 tanks also began. All of these projects had a higher priority than the bunker buster. No progress was observed in May or April of 1941.
Supercharged V-2SN engine, the heart of the T-220 and KV-3.
This pause didn't mean that the bunker buster was cancelled. In April of 1941, new targets were thought up for the 212, the same heavy tanks that Soviet intelligence described. Guns other than the 152 mm Br-2 were considered for the role, described in Marshal Kulik's letter to Stalin, sent on April 17th, 1941.
"Create SPGs to fight bunkers and superheavy tanks using the following heavy artillery systems: 152 mm Br-2 gun, 130 mm B-13 gun, and the new powerful 107 mm gun. The 152 mm Br-2 gun can penetrate 155 mm of armour at 0 degrees from 2300 meters, the 130 mm gun penetrates 130 mm at 0 degrees from 4000 meters, the new 107 mm gun will penetrate 160 mm of armour at 30 degrees at 1000 meters.
The 152 mm SPG is already designed, and a prototype is being assembled at the Kirov factory. A lengthened KV-4 chassis is used. The mass of the SPG with a 152 mm gun is 65 tons. The SPG is protected by 60 mm of armour. Council of Commissars decree #274-120 issued on February 7th, 1941, tasks the Kirov factory with the production of 10 SPGs with 152 mm Br-2 guns this year.
It is necessary to instruct the NKTP to provide the pilot prototype on June 1st, 1941, and ensure that the rest of the SPGs are built during this year. Also instruct the NKTP to produce an experimental 130 mm SPG by September 1st and a 107 mm SPG by October 1st. The chassis will be the same as on the 152 mm SPG. The factory already has a 130 mm B-13 gun. Instruct the NKV to produce a 107 mm gun at factory #172 and send it to the Kirov factory by July 1st, 1941."
A vehicle armed with the 130 mm B-13 naval gun appears several times in GAU and GABTU correspondence. According to requirements dated June 16th, 1941, the mass of the vehicles had to be 55 tons. Unlike the 212, the SU-B-13 was not a bunker buster. The requirements are for a heavy tank destroyer, which was hurriedly developed to fight German heavy and superheavy tanks. Its armour was only 30 mm thick, and the description "the gun is mounted without a turret, on the stock mount with necessary protection on top from diving aircraft" suggests that the SU-B-13 and 212 would have been significantly different. It's more likely that the result would have been an analogue of the German "self propelled gun mounts", like the 12,8 cm Selbstfahrlafette auf VK3001(H), which was designed in May of 1941, likely also as a means against Soviet superheavy tanks.
As for the 107 mm SPG, it didn't move past the proposal stage. No tactical-technical requirements were created. The armament would be the 107 mm ZiS-24 or the M-75. Grabin's gun was never built, but the M-75 was not only built, but tested.
107 mm M-75 anti-tank gun, one of the two candidates for the heavy tank destroyer.
Work on a lighter SPG didn't mean that the 212 was done for. Decree of the Council of Commissars issued on May 27th, 1941, illustrates that the artillerymen didn't give up on the bunker buster. It is listed in the first paragraph, but with a KV-3 chassis. Since both the 212 and the KV-3 were based on the T-220, nothing really changed. The first vehicle was due in August of 1941. The initial order was for 12 vehicles, later reduced to 10.
Without a Chassis
Some authors write that the 212 was cancelled after the start of the Great Patriotic War, but that is not so. In reality, the start of the war only suspended the work on the vehicle which hasn't seen any progress since March anyway. The bunker buster project was attached to the Kirov factory until August. An attempt to cancel it caused a furious letter from deputy chief of the GAU, Colonel-General Hohlov.
The project was only transferred to the Ural Heavy Machinebuilding Factory (UZTM) in late August of 1941. The factory had a wealth of experience building artillery systems. In 1941, the factory also became ChTZ's contractor for KV-1 hulls and turrets. The factory's design bureau was headed by F.F. Petrov, who developed many artillery systems, including heavy ones. However, neither the factory nor its design bureau had any experience with designing tracked vehicles.
Factory diagram of the 212, side view.
In early October of 1941, GAU received a letter from the chief engineer of UZTM, A.S. Ryzhikov. Ryzhikov requested all available information on the KV-3 and armament that was meant for the SPG. After that, activity on the bunker buster project died down for a while. The KV-3 chassis meant for the project was absent, and ChTZ, tasked with producing the tank, had better things to do in August of 1941. In addition, GAU's Artillery Committee was in no hurry on the bunker buster issue. All of these factors contributed to work being stuck. The topic of bunker busters arose once more in November of 1941. The issue was very interesting: due to factory evacuations and lifting of cancellation of the artillery tractor order at ChTZ, production of tractors ceased completely. There was a chance that soon, nothing will remain to tow artillery. A whole family of SPGs in the plan for 1942 was the solution. The first in the list was a 152 mm Br-2 gun on the KV chassis.
Reconstruction of the SPG's exterior.
In March of 1942, the subject of a 152 mm bunker buster on the KV chassis arises again. The chassis was from the KV tank, the gun was the oscillating part of the Br-2. The experimental factory #100, organized in March, was responsible for the chassis. Factory #8 was responsible for the gun. 1.5 million rubles were allocated to the project, and a prototype was due by July 1st. However, by the spring of 1942, the KV-3 was finally dead and another vehicle was explored as a chassis for a bunker buster.
The 212 never progressed past the construction of the hull, which was never fully assembled. Even if it was built, it's unlikely that its fate would have been a good one. The project was tied to production of the Br-2 gun, which stopped in 1940. Even the preliminary production volume was only 12 units, and the price of each vehicle was about a million rubles. The heavy bunker buster was a dead end branch in the Soviet heavy SPG tree.