Monday 9 August 2021

Light Turretless Artillery Tank

The first work on SPGs began in the USSR in the 1920s. They began as just projects, but prototypes began appearing in the early 1930s. The first such vehicles were built in 1931, generally light ones. Even though the results were negative, this was not a complete failure. Soviet SPGs were produced in series, although small ones, and some of them still saw battle. This also applies to the "artillery tank", the AT-1. Much was expected of it, and the vehicle was indeed quite decent, but it had one serious issue: the AT-1 ended up without a gun.

Second simplification

The first 76 mm SPGs on a light tank chassis were developed using the MS-1 (T-18) as a starting point. This work died on paper. There were many reasons for this. The tank was too short and there was not enough place for a gun and its crew. The mass would also have been too much for the T-18 chassis, causing big problems with mobility and reliability. Finally, by the time this vehicle was developed the T-18's star was starting to set. It was either going to be replaced with a modernized vehicle (the T-20 small modernization or T-19 large modernization) or a Soviet copy of the Vickers Mk.E Type A. The British tank was a step back in some ways since it was only armed with machine guns, but it surpassed the T-18 in all other ways. The tank was accepted into service as the T-26. It replaced the T-18 at the Bolshevik factory in 1931.

SU-1: the first SPG on the T-26 chassis built in metal.

The T-26 was considered as a platform for SPGs from the very beginning. A decree by the RKKA Artillery Directorate signed in 1931 called for a whole slew of SPGs: a 37 mm SPAAG, a 45 mm tank destroyer, 76 mm and 122 mm close support SPGs. The most successful direction was the creation of a 76 mm infantry support SPG. The Bolshevik factory design bureau created the SU-1 where the 76 mm model 1927 regimental gun was located in a closed casemate.

Work was also underway on another vehicle: an artillery tank. The first such tank was built in 1932 under the direction of N.I. Dyrenkov. The vehicle was too cramped, and so an analogous one was developed at the Bolshevik factory (or rather factory #174, since the tank plant was reorganized into a separate organization as of February 16th, 1932) in 1933 on orders from the UMM. The OKMO (Experimental Design and Machinebuilding Department) was also formed around this time. The OKMO was headed by S.A. Ginzburg, who moved to Leningrad around this time. The first work on an artillery tank indexed T-26-4 dates to early 1933. A draft project was ready by February and work began on blueprints. A prototype was built in August and went through trials until late 1933. By the time trials finished the OKMO was no more. It was reorganized into experimental Kirov factory #185, led by N.V. Barykov, the former chief of the OKMO.

The 76 mm PS-3 gun was a priority in development of new tanks and light SPGs.

The OKMO developed a 76 mm tank gun indexed PS-3 under the direction of P.N. Syachintov. The PS-3 had a longer barrel and a smaller recoil length, 420 mm compared to 500 mm. The PS-3 also fired fixed ammunition from the model 1902 field gun. Some say that the T-26-4 used the PS-3 gun, but that's not exactly the case.

As with previous tanks, the T-26-4 used a 76 mm regimental gun with its recoil length reduced to 500 mm. The PS-3 was tested in this tank, but there were issues with the turret roof and robustness of the running gear when firing it. The PS-3 was a candidate for installation on the first production T-26-4 tanks, but of the five vehicles produced the PS-3 was used on at most three. Production of the T-26-4 ended there. The vehicle was the PS-3's first victim, since mass production of the gun never began. The last documents on this tank are dated 1935, at which point the OKMO's successor was working on a brand new vehicle. It first turned up in January of 1934.

The project named "PS-3 in an SPG on the T-26 chassis with a folding turret" was first shown at a GAU conference on January 8th. The project was indexed AT-1 (Artillery Tank 1). This was a return to the SU-1 on a new level. The project was led by G.V. Kruchenykh, Syatchinov was responsible for the gun. Ginzburg oversaw the design as the chief designer, and Barykov was in charge overall.

Cutaway of the vehicle as designed.

The preliminary project was examined and an order was given for development of detailed documentation. The deadline was set at April 1st, 1934, although the GAU decided to build a prototype on March 20th. It would be due by December 1st, 1934. As planned, the technical project of the AT-1 was ready by April 1st and sent to the Chief of the Artillery Mechanization Directorate N.F. Drozdov. The project was approved in May of 1934 and work began on blueprints. The plan was to approve them in July but really they were only approved on August 4th. The order to build a full sized mockup was also given on that day with a due date of August 25th.

The height of the fighting compartment was just enough for a person to stand. This helped conceal the tank on the battlefield.

The design mounted the PS-3 gun on a pedestal in the fighting compartment. The mobility of fire was high. The gun could be aimed 15 degrees to the left or right, depress to -5 degrees, and elevate to 45 degrees. The T-26 was largely unchanged. The casemate was more or less the same in dimensions as the T-26's turret platform.

The mass of the vehicle was a little greater than the T-26: 9630 kg. This was because the gun was heavier and the AT-1 carried an impressive amount of ammunition for such a small tank: 41 76 mm rounds. The AT-1 was also more than 15 cm lower than the T-26. The crew was the same: 3 men. Since firing the gun produced fumes, the sides and rear of the casemate flipped out. This feature could also be used to help load ammunition, but the intended use was to flip them open when firing indirectly.

The PS-3 gun was installed on a pedestal mount. The weapon itself was nearly identical to the tank version of the gun.

This project was interesting to the GAU since it was considered a method of mechanizing regimental artillery. Unlike towed guns, the AT-1 didn't need to be deployed after moving. The vehicle could also defend itself. A DT machine gun was installed in a ball mount in the front. 29 magazines were carried on board. The PS-3 was also a more viable weapon against tanks than the 76 mm model 1927 gun. Using the stock model 1902 field gun shell the PS-3 penetrated 40-50 mm of armour at 500 meters. The AT-1 was a dangerous opponent for any tank in use at the time. The vehicle's height was also a bonus. 2038 mm was the maximum height, the height of the casemate roof was 1820 mm, a little taller than a person.

One can often hear the claim that the AT-1 was a competitor of the SU-5. This is incorrect, as the "Small Triplex" was designed with a different purpose in mind. The AT-1 had no place in the Red Army's system of tank armaments until 1935. This was because it was created to satisfy the artillery's needs. The reasons for rejection of the SU-5-1 SPG armed with the 1902/30 76 mm gun lie elsewhere.

Delays and delays

As mentioned above, the first AT-1 prototype was supposed to be completed in December of 1934. For a number of reasons (primarily the amount of other work assigned to the factory) this deadline was missed. Factory #185 was working on the T-29 and T-46-1, two higher priority items. Some publications claim that the AT-1 prototype entered trials in early 1935. That is not entirely correct. A letter dated May 19th, 1935, says that the prototype was just being assembled and is expected to be done in June. Ginzburg states in this letter that the vehicle still needs armament, including the PS-3 gun. A gun with factory number 107 and its tools were sent to factory #185 on May 22nd.

An experimental prototype of the AT-1 at the factory, summer 1935.

This time the deadline was more or less met. The AT-1 prototype was assembled and prepared for trials according to order #1006 (satisfying item #2 of Artillery Directorate contract #102471). There was a delay in payment since the factory did not correct several blueprints as required. This was again due to the overloaded design bureau.

There were some deviations from initial requirements. The DT machine gun ports on the sides were produced from mild steel due to an issue with subcontractors. Regular T-26 leaf springs were used instead of reinforced ones (since the production springs were already reinforced starting in 1934). A lot of attention was paid to the spent brass tray in correspondence, as there were issues with it. As required, the vehicle could carry 41 rounds of 76 mm ammunition, at least that's how many slots there were in ammunition racks. The problem was that the 41st round fouled the turning mechanism flywheel and could not be inserted fully into its slot. Finally, the prototype deviated from the blueprints in some ways.

The sides of the casemate are flipped down. They could be opened when firing indirectly.

The AT-1 prototype entered factory trials in June of 1935. Weighing showed that the mass was a little less than calculated, 9600 kg. The vehicle was broken in on July 3rd and 4th. This process revealed a whole slew of defects. The engine was hard to start since the sparkplugs were coated in oil spilling from the crankcase. The gear train from the clutch to the drive shaft produced excessive noise. Final drive clutches overheated. The overheating was dealt with by adjusting the turning mechanisms. The average speed of the vehicle was recorded at 16 kph, with a maximum of 27 kph on level ground.

In addition to helping with ventilation, the open sides made loading ammunition easier.

Gunnery trials began on July 25th. 60 shots were made in total. The crew positions were also studied. Trials showed that the effort to operate the traverse mechanism was too high, 20-26 kg. There was a method to reduce the resistance to 7-12 kg. To do this, the gunner (also acting as the commander) had to stand up from his seat. The gun had a pedal trigger, but it was too far and therefore uncomfortable to use. However, the overall verdict of the fighting compartment study was positive. The ventilation of the fighting compartment was good when the sides were open and the AT-1 was stable when shooting. One drawback was that the front fenders deformed from the muzzle blast. The elevation angle was also not quite the required 45 degrees at first, so some parts had to be filed down.

A regular T-26 was used to build the AT-1. Retention of the chassis was one of the main requirements for the artillery tank.

The final 25 km driving trial took place on July 26th. The vehicle drove well, although the noise in the gear train and oil leaks were not entirely resolved. 25 various changes were made to the design over the course of the trials, both to the chassis and the gun mount. The elevation mechanism flywheel diameter was reduced from 200 to 190 mm, the pedal trigger was moved, the spent brass tray was shortened, the ammunition rack slots in the front were lengthened. The AT-1 was deemed ready for proving grounds trials, with some caveats. Since the sights never arrived, trials had to start without them. The mechanisms making opening the sides of the casemate easier also worked poorly.

The gun at maximum elevation. The gun mount had to be filed down to achieve the required angle.

Work on the T-26-4 died quietly while the AT-1 was being worked on. The artillery tank remained, but on another chassis, the BT-7. The result was the tank often called BT-7A, although its real name was BT-7 Artillery. Trials of the AT-1 took place in parallel at the Rzhev Scientific Research Artillery Proving Grounds north of Leningrad. Trials began on October 1st, 1935, and ended on January 3rd, 1936. The trials paused several times for various reasons. The PS-3 gun made 259 shots, 117 of them supercharged.

AT-1 at the Rzhev NIAP, fall 1935. One can tell that the fenders are damaged from firing the gun.

Gunnery trials showed that the precision of the PS-3 was higher than that of the 76 mm model 1902/30 gun. This was one of its few advantages. Due to issues with the elevation mechanism, the crew missed a target at 2000 m twice. Firing on the move or from short stops was also difficult. Only one series of shots was undertaken rather than five, and only 4 shots hit the target out of 14. There were several reasons for this. One was the aforementioned issue with effort required to use the elevation mechanism. Another was that the TOP sight was located poorly and had no brow pad. The traverse mechanism flywheel was located so far that the gunner could not turn the gun and look into the sight at the same time. There were also issues with the loader's position. He risked being hit in the shoulder when teh gun fired. To prevent this, the spent brass tray height had to be increased on the right side. The tray was also too long and fouled the engine bulkhead.

The vehicle in travel position. The headlight shield is down, but the sight opening is closed.

The rate of fire was also under question. The gun could fire up to 9 times per minute provided that the aim was not corrected. If it had to be corrected, then the rate of fire dropped to 3 RPM. This was caused by issues with the design of both the PS-3 and AT-1. A big issue was that 32 rounds were stored on the gunner's side, and he had to pass them to the loader. There were also problems with the trigger pedal. The stability of the vehicle was deemed unsatisfactory, since the aim was offset after firing. The fighting compartment was considered too cramped. It had to be 200 mm longer and 20-30 mm taller. There was also an issue with the gun mantlet. It left a large slit through which bullets or splinters could enter the vehicle. There were also issues with the PS-3 gun. After trials the barrel retreated by 1.4 mm into its shroud. The problem with deformation of the front fenders also persisted.

The casemate sides are open. There were complaints about the mechanisms that aided with opening and closing them.

The AT-1 was also put through mobility trials. The vehicle travelled 520 km in total, half of which was on a cobblestone road. The top speed recorded here was higher than at the factory. In 4th gear the AT-1 accelerated to 37-39 kph, however at that speed seven road wheels on the rear bogies were damaged. There were also issues with the engine. Overall, compared to the number of complaints about the gun and fighting compartment the chassis was not criticized very much.

The gun at maximum depression. As a result of these trials, a request was made to replace the road wheels with more reliable ones with removable tires.

The conclusion was that the AT-1 and the PS-3 need more work. The only requirement not forwarded to factory #185 was the one regarding enlarging the fighting compartment. A technical meeting took place at factory #185 on February 29th, 1936. Ginzburg represented the factory's management. 13 items were discussed in total, including the new ammunition racks and correction of issues with aiming the gun. A ventilation fan was also required for shooting with the sides of the casemate closed. Road wheels with replaceable tires rather than the existing ones would be used. Pistol ports would also be introduced. A number of changes to the PS-3 gun were also discussed. The idea of using a different gun was first voiced at this meeting. The KT-28 was a natural choice, but it was soon dropped. The hull would have to be 280 mm longer and the maximum gun elevation would be only 30 degrees. These were the reasons for discarding this gun, at least this time.

The rear of the casemate with the flap closed.

Even though the vehicle needed improvements, it was nearly accepted into service with the Red Army. Mass production would begin at factory #174. 10 vehicles were expected in 1936, which factory #185 had to provide documentation for. This was the theory, but in practice the issue was stalled. Factory #185 received a furious letter from factory #174 chief engineer G.O. Gutman and deputy chief designer I.A. Aristov on April 20th, 1936. They stated that factory #185 claimed they were working on the production blueprints, but not a single blueprint group was complete.

A.L. Dufur, factory #174 director, threw more oil onto the fire. He inquired how a vehicle with so many issues could be put into production at all. This question was posed to the ABTU leadership. Factory #174 had to be picky with what they received. On July 20th, 1936, 38 blueprints were accepted, 19 "could be accepted after corrections" and 18 would be accepted after they were tested on a model and a prototype. The gun mount would only be accepted after being tested on a prototype.

Ammunition racks on the loader's side. There were clearly not enough of them.

The financial side of the issue was interesting too. The AT-1 ended up being much more expensive than a T-26. It cost 155,000 rubles per unit, whereas a T-26 cost only 80,000. Bonuses issued to factory #185 staff were also a tricky issue. Barykov and Ginzburg were each awarded 2000 rubles (1000 of them for putting the vehicle into mass production), chief engineers Kruchenykh and G.V. Chunts received 3000 each, finally Syatchinov received 5000 rubles. In total Syatchinov received 33,000 rubles worth of bonuses for his work at factory #185, a record that would cost him dearly.

Gun and sight mount.

A torrent of correspondence between the ABTU and the factories broke out. Meanwhile, work on the prototype continued. Gun #23 replaced gun #107. A new semiautomatic mechanism was installed, the recuperator was strengthened, the tube attachment was changed, other changes were introduced. The improved prototype arrived at the proving grounds on September 11th, 1936. Compared to the gun, the vehicle was not changed much. Only the ammunition racks and gunner's seat were improved.

Trials of the improved prototype began on September 15th. 237 shots were fired rather than 210 initially called for. Officially there were no mobility trials, but the vehicle drove for 100 km. It was noted that the engine was hard to start (2-3 hours) and had to be push-started with the aid of a tractor. The tracks slipped off when driving off roads. The brake bands heated up during use and the cooling fan did not fully perform its intended function. Firing the gun broke the glass and lamps in the fighting compartment.

The main ammunition rack. It was located in front of the gunner/commander. Loading from it was a difficult process, which was confirmed by trials. The aimed rate of fire was rather low.

Results of gunnery trials showed an improvement. The maximum rate of fire was 12-14 RPM from standstill or 3-4 on the move. Precision was good, up to 60% of shots fired on the move at 12-15 kph hit the target. The aiming process was improved. The PS-3 gun improved as well, especially its robustness. There were still some complaints. The loader had only 18 rounds on hand instead of 20, the design of the racks also needed work. The ventilation fan had to be moved, and the pedal trigger had issues once again. Issues with the spent brass tray and gun mantlet remained.

Issues with the sights and aiming mechanisms were recorded.

The conclusions signed on November 17th, 1936, stated that the overall performance of the AT-1 was satisfactory. The same went for the PS-3. There were still some issues with individual assemblies, including the breech and traverse mechanism. All of these issues had to be corrected before the AT-1 could be permitted to enter military trials.

Gun starvation

The AT-1 met the end of 1936 in a difficult position. On one hand, the vehicle was prepared for production. On the other hand, this meant that the blueprints had to be produced, and their status was described above. Nevertheless, the ABTU signed a contract with factory #174 on January 28th, 1937, for the tanks and spare parts. Item #4 listed the "AT-1 SPG with the 76 mm PS-3 gun, TOD sight, PTK sight, DT machine gun, ammunition racks for the gun and machine gun, and toolkit". The contract called for 75 vehicles, 50 in September of 1937, and 25 in October. An agreement for an additional batch of 25 vehicles was signed on February 10th.

The L-10 became the AT-1's main gun instead of the PS-3 in early 1937. Production vehicles were supposed to receive this weapon.

The Izhora factory was responsible for producing the hulls and began working on a pilot batch in 1936. They received the very blueprints that were both approved and not approved. The result was predictable. It turned out that the blueprints contained a number of errors. Factory #185 sent engineer Piskunov, who made about 200 corrections to the blueprints, some of them quite major. The Izhora factory had finished 6 hulls and were working on 4 more by April 11th, 1937. However, a major change was made to the AT-1 by April.

Penetration trials of one AT-1 hull showed that it had a number of issues.

It was clear that the PS-3 was not doing well by early 1936. The gun was officially accepted into service, but had a number of issues. Because of this, Kirov factory designed the L-10 76 mm gun under the direction of I.A. Makhanov. Its characteristics were very close to those of the PS-3. The first L-10 was produced in the summer of 1936. For some time they were produced in parallel, since the Kirov factory was responsible for both guns, but the ABTU and GAU lost their patience by the end of 1936. The same gun was holding back four tanks (the T-28, T-35, T-29, and AT-1), plus Syatchinov also had the SU-14 that was still up in the air despite being officially accepted into service. Syatchinov was arrested on December 31st, 1936. He was charged with chapter 58 of the RSFSR criminal code (counterrevolutionary activity) and was executed on May 6th, 1937. Syatchinov was accused of espionage, but the real reason was his negligence regarding tank and SPG armament. While the T-28 and T-35 could be produced with the KT-28, the AT-1 had no alternative. It would have to be changed drastically to accept the KT-28. It's no surprise that work on the AT-1 was progressing slowly. Factory #174 could make the chassis, but it had no guns.

The welding seams survived the trials, but the rivets and bolts were dislodged when hit. The gun mantlet was unreliable and the machine gun ball was penetrated.

The Artillery Directorate figured out what was happening and issued requirements for an AT-1 with the L-10 gun in early February of 1937. The requirements called for minimal changes to the AT-1 and the gun. This included the ammunition racks. The L-10 had to have the same elevation and depression as the PS-3. Effort on the aiming mechanisms had to be no more than 5-6 kg. The mechanisms also had to be positioned comfortably. The gun would be equipped with the TOP telescopic sight as well as a panoramic sight from the regimental gun. The sights had to have additional armoured shutters. Extra attention was paid for the spent brass tray, which the AT-1 had issues with. 35,000 rubles were allocated for this work, in addition to the cost of the L-10 gun. Work on artillery tanks restarted in January of 1937. These would be built on the chassis of the BT-7 (this vehicle was never put into service) and T-46. In the latter case, the work quickly stalled since it was in no better condition than the AT-1.

The first AT-1 prototype remained the last. Mass production of the "artillery tank" never started.

Due to this situation, factory #174 stopped working on setting up mass production. One of the hulls (#1063) was shot up with 7.62 mm bullets. The results were mixed. The welding seams worked well, but bolts and rivets were dislodged when hit. Suspicions about the gun mantlet were confirmed. It could be jammed if hit. The DT ball mount also showed poor results. Its frame could be penetrated. All joints let through splash. The AT-1 hull did not provide adequate protection against even rifle bullets. An order to correct production hulls followed, which made the situation even more complicated.

The T-26 had similar issues and a modernization program was in effect. This was in addition to modernizations that were introduced after the experience in Spain. Since there was not much progress with the AT-1 and factory #174 was not meeting T-26 quotas, the order for 95 AT-1 SPGs was cancelled on November 29th, 1937, and the factory delivered 95 T-26 tanks instead. Nevertheless, work on the AT-1 continued. The vehicle and new gun remained an active topic in correspondence until the summer of 1938. The project was finally buried by Council of Commissars decree #198ss issued on August 7th, 1938. The vehicle was cancelled as not matching requirements. Ironically, L-10 production began around then. Only the prototype, 5 welded, and 5 riveted hulls were completed.

Just because the AT-1 was a prototype didn't mean it didn't fight.

After factory #185 was absorbed by factory #174 in 1940 the experimental AT-1 was sent to the NIBT Proving Grounds. The vehicle was classified as category 2 (used but functional) as of August 1941. The difficult situation in the fall of 1941 forced the mobilization of proving grounds vehicles. A separate tank company under the command of Sr. Lieutenant Semyonov was formed on October 8th, 1941. It included a T-40 tank, a T-29, an A-20, an AT-1, two T-26, two BA-10 armoured cars, and a BA-20. The company fought around Vereya. On October 14th, 1941, the T-40, AT-1, and A-20 were transferred to the 455th Motorized Rifle Battalion of the 151st Motorized Rifle Brigade. The AT-1's tracks end here.

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