Roof for the SU-76M

A number of questionable publications on the SU-76 created the idea that the SU-12 (SU-76) initially had a roof over the fighting compartment, but it was later removed, never to return. This is far from the truth. The SU-12 prototype and a large number of production vehicles had no roof initially. Stalin personally demanded that a roof be installed on every vehicle starting with the first one, but this was only put into practice in the second half of March 1943. The claim that roofs were removed on the front lines is not supported by photographic evidence. Vehicles without roofs always have their headlights and other stowage in different positions, same as the prototype. The roof was also present on the SU-15 prototype, although the improved SU-15M (SU-76M) didn't have it.

SU-76M with a rigid roof developed at factory #40 in the summer of 1944.

The cause for the disappearing roof is simple. The SU-12 had a number of defects, in part due to an overloaded chassis. The SU-15 chassis could handle the weight of the roof, but the NKTP and GBTU decided to cut a few hundred kilos just to be safe. The production SU-15M not only had no roof, but also trimmed sides and rear. The SU-15 weighed 11.2 tons and the SU-15M weighed 10.5. It seemed that everyone should be satisfied with this since there were no problems with weight or ventilation, but complaints began to come in at the end of 1943. It turned out that bullets and shell splinters can come from above as well and a roof was needed to keep them out. Some SPG regiments built their own improvised roofs. This added extra weight, but everyone wanted to live.

A firing port for a DT machine gun was added on the right side of the front plate instead of a commander's vision port.

The GBTU's SPG branch looked at these front line improvisations without enthusiasm. The increasing mass threatened to reduce the lifespan of the running gear and the transmission. However, the amount of complaints and front line modifications forced the start of official experiments on installing a roof on the SU-15M. The first such project was completed in the summer of 1944 by the Molotov GAZ factory. It was installed on the experimental SU-57. The design was very simple, just a plain roof with periscopes. The situation with fumes was acceptable while the rear hatch was open. This was unacceptable in combat, since the open hatch ran the risk of catching a grenade. With a closed hatch and engine turned off the conditions inside were uncomfortable. In any case, further experiments with roofs were not conducted in Gorky. Later SPGs built at GAZ had a raised rear, but no roof. 

The same port but without an observation slit was introduced in the left of the casemate.

The situation with the SU-15M changed in the summer of 1944. Until then, factory #38 in Kirov was the lead developer of this vehicle, but it was sent to Kharkov to shore up factory #75. Instead of working on the SU-15M and other light SPGs, M.N. Schukin's team moved to working on the T-44 and artillery tractors. Work on the SU-15M moved to factory #40. Factory #40 gradually became the lead developer of light SPGs. The first project of the factory design bureau led by N.A. Popov was the improvement of the SU-15M's fighting compartment. Work was headed in two directions. One is a topic for another article, but this one will focus on the variant with a roof. Work began in the summer of 1944 and the vehicle was tested at the NIBT Proving Grounds in October of 1944.

The same port was added on the right.

A production SU-76M was taken and altered. The roof was composed from two 4 mm thick steel plates: a top one and a rear one. This reduced the volume and therefore weight, but also the size of the fighting compartment. The rear door was enlarged to 820x630 mm. Two hatches in the roof were made for the crew and one for the PG-1 panoramic sight. Firing ports in the sides, rear, and front were also changed. They could now support the installation of a DT machine gun. The front DT machine gun port replaced the commander's vision device. An observation slit was added above it. Due to the altered layout of the fighting compartment, the commander and loader got separate seats instead of a shared bench, the PPSh rack and magazine rack were moved (the amount of magazines was reduced to 12 drums). A spring servo mechanism was added to the controls, which reduced the amount of effort required to drive the SPG. Two MK-IV periscopes were added to the roof in order to compensate for the decreased visibility.

Firing ports were also present in the rear. The sloped rear section of the roof can be seen.

The list of changes seems long, but the mass grew to only 10.68 tons, meaning that the alterations added less than 200 kg of weight. There was a key omission. The roof introduced an issue with ventilation. While the SU-12 at least had an open port for the panoramic sight, the SU-76M did not. One MV-12 fan would have been enough to clear the fumes, but factory #40 didn't install one. This was one of the key reasons for the vehicle's failure. The NIBT Proving Grounds checked the fumes first. With the engine shut off and hatches closed, the crew was feeling symptoms of partial CO poisoning after just 15 shots. Measurements showed that the concentration of fumes reached 0.31 mg/L, higher than the acceptable level of 0.2 mg/L. This factor alone meant that work on a fully closed fighting compartment was over.

The roof from above. The design was simple, but not successful. The biggest issues were concentration of fumes and poor visibility.

This was not even the only problem. Studies showed that the addition of just two MK-IV periscopes do not solve the issue of visibility. The vehicle had large blind spots and no way to look backwards at all. The NIBT Proving Grounds suggested installing a commander's cupola instead of the commander's hatch. The firing port in the front near the commander's seat also had issues, since it could obscure the sight at certain aiming angles.

Observation from the SU-76M with a roof. 1 indicates the commander's vision slit in the front of the casemate, 2 indicates the commander's periscope, 3 indicates the gunner's periscope.

Due to poor ventilation and insufficient vision the NIBT Proving Grounds couldn't recommend the introduction of a closed fighting compartment. The second backup variant of the SU-76M went into production. After some reworking, the SU-76M with raised sides went into production in April of 1945. 

Original article by Yuri Pasholok.