Tuesday 20 December 2022

The AA Tank That Came Too Late

The story of a SPAAG on the T-60 chassis that was not fated to enter mass production.

SPAAGs were a sore spot of the Soviet tank design program. It’s not that there weren’t any of them by the start of the Great Patriotic War, but they looked nothing like what was originally envisioned. A class of SPAAGs on tank chassis was conceived and such vehicles were even built. The best example would be the SU-6, a SPAAG on the chassis of the T-26 light support tank. It was even accepted into service with a pilot batch built, but mass production never came. Development of SPAAGs on light tank chassis continued. The T-50 chassis was considered a candidate for a SPAAG, armed with either a pair of MP-6 23 mm auto cannons or a 37 mm 61-K. The actual result was quite sad: two experimental SPAAGs on the T-26 chassis were built in Leningrad and that was it. Mechanized units were covered against air attack by a variety of systems on a truck chassis.

The GABTU considered the installation of a T-90 turret on the T-60 to be a quick fix.

Further work went down the road of creating a universal chassis using T-60 tank components. The chassis was developed under code 31 at factory #37 in Sverdlovsk. The prototype designated SU-31 was quite a good vehicle for its time. The 37 mm 61-K auto cannon was an effective weapon against enemy aircraft. However, work dragged on and factory #37 was rolled into UZTM. Instead of the T-70, it was used to build T-34 tanks. Meanwhile, a German airplane was shot down in late June of 1942. A report was found on board detailing the trials of a Henschel Hs 129A ground attack aircraft. Its 30 mm MK 101 gun could penetrate even the armour of the KV-1 tank. The news that SPAAGs on truck chassis had poor off-road mobility were also bad. The combination of these factors gave rise to the idea of building AA tanks.

The SPAAG only entered trials in July of 1943. Compare this arrival to the due dates in the GKO decree above.
Work on a “dual 12.7 mm DShK AA mount for T-60 and T-70 tanks” began on August 8th, 1942. The full tactical-technical characteristics that appeared some time later described a SPAAG on the T-60 tank chassis that used the existing tank turret. This was the direction taken by I.V. Savin, the designer that led the development of T-60-3 and T-70-3 AA tanks at factory #37. The T-70-3 SPAAG followed the same principle of adapting the stock turret for a pair of DShKT machine guns. Trials held in the first half of December 1942 showed that the idea was incorrect. Both SPAAGs turned out to be too cramped. The T-90, a SPAAG designed at the Molotov GAZ, was a much better design. Lead engineer V.S. Solovyev decided to create a new turret in order to use available space more rationally. The T-90 was a clear favourite. It seemed the perfect time to put it into production, but the GABTU had other ideas. A draft GKO decree to accept the T-60-3 into service was made on December 14th, 1942. This was not exactly Savin’s tank, but rather a T-60 that had the turret from the T-90. It would be built at the newly revived factory #37 in Moscow. The tank wasn’t called T-60-3 in the end, but SZU.

The SZU turret was built nearly from scratch.

The logic behind this turn of events was not very hard to grasp. First of all, the volume of AA tank production would not be high. The T-90 was not considered interesting for wide scale production. The T-90 also ate up a portion of T-70 production. It was already known by December of 1942 that factory #38 would end production of the T-70 in favour of the SU-12 (SU-76). The T-70 and then T-80 would be built at factory #40 in Mytishi, but more time was needed to set up production. There were also already plenty of T-60 tanks around that were not useful for anything else. Production of the T-60 already ended, and so it was reasonable to convert the tank to a SPAAG. Factory #37 in Moscow was perfect for this. T-60 chassis would be sent for repairs and conversion there.

A turret bustle was added to house the radio.

This looked great on paper, but one minor detail remained: the conversion of the T-90 turret to fit the T-60. The Special Design Bureau of the GAZ Experimental Design Department (SKB KEO GAZ) took on the work. It quickly turned out that the GBTU’s ideas of a rapid turret swap were incompatible with unyielding reality. In addition to the tank itself being smaller, the turret had to be rearranged. Without it, there was no room for a radio. In order words, V.S. Solovyev was forced to design a brand new turret. This took extra time that was already in short supply.

Instead of the required 85 degrees, the maximum elevation was only 70 degrees.

Since the SKB KEO GAZ was busy with other projects, the SZU only came together in the spring of 1943. Only the general principle of the DShKT mount remained from the T-90. The turret was brand new. It was a heptagon with a prominent bustle that held a 12-R radio. The sides of the turret were 35 mm thick, making them a tougher target than the front of the tank. The turret roof could flip open in several parts. The gun elevation was reduced to 70 degrees, which the army later complained about. They expected the elevation angle to match that of the T-90 at 85 degrees. On the other hand, the traverse speed was satisfactory. The GAZ took previous feedback into account and increased the speed to 10 degrees per second. That was enough to track targets.

The turret roof opened up when firing.

The SZU was finished in June of 1943. Interestingly enough, the summary report from the factory mentions the tank and its name, but the report for 1943 omits it. Even though the turret of the SZU was heavier than the stock T-60 turret, the mass of the tank did not increase by much, only from 6200 to 6420 kg. Ammunition capacity was the same as the T-90, 480 rounds (16 magazines). The machine guns had an electric trigger plus a backup manual trigger. A special mechanism to rack the bolts was also installed, although trials showed that it only worked at angles of 20 degrees and less.

In the summer of 1943 it suddenly turned out that there were no chassis left.

The GABTU’s (by then the GBTU) decision to convert the T-90 turret to the T-60 cost them dearly. The trials took place from July 15th to July 25th, meaning that the “quick fix” lost them 7 months. This is a very long time in war. Trials also revealed a number of defects. In addition to the insufficient elevation, the turret turned out to be unbalanced, the TMFP sight was hard to use, and the roof panels closed in the wrong order. There were also issues with the electric triggers. In short, the SZU passed trials, but the amount of defects to resolve was higher than the SPAAG tested 7 months prior. There were also misfires during shooting.

The fighting compartment was not as comfortable as the one on the T-90.

The NIBT Proving Grounds proposed that the SZU could enter production after the drawbacks were resolved, but the situation developed into a tragicomedy. The Molotov GAZ factory was bombed to pieces in June of 1943 and had no time for the SZU. Factory #37 had already been building the SU-76I for several months and had no time for a SPAAG. There was also no chassis for conversion as no one wanted to return their T-60s from the front. The idea of using a refurbished chassis was also a questionable one, and factory #37 had a lot to say about using refurbished German chassis to build the SU-76I. There was no one and nowhere left to work on the SZU. The story of AA tank development lasted for a whole year and ended with nothing.

Original article by Yuri Pasholok.

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