SU-100Y – The Soviet “Boxtank”

While fighting the Winter War against Finland the Soviets started development on a bunker-busting self-propelled gun named the SU-100Y. It was created by placing a powerful 130 mm naval gun on the chassis of the T-100 heavy tank.

Though technically designed to eradicate fortifications, the SU-100Y would have made light work of any German tank unfortunate enough to find itself in its sights.

Its an extremely simple design, with flat sided armor, no machine guns and it doesn’t even have hatches for the crew.

By the time the SU-100Y was complete the Winter War had ended and the Soviets had little use for this 60 ton beast. However the single SU-100Y built was reportedly pulled out of storage and used to help defend Moscow in 1941.

Some say that this mighty machine saw even more action, remaining operational until the war’s end.

Contents

T-100 Heavy Tank

As mentioned, the SU-100Y was built on the chassis of the T-100.

The T-100 was a large tank designed by Factory N°185, in the Bolshevik Factory in Leningrad in the late 1930s as a potential successor to the rather lacklustre T-35. It directly competed against the SMK, a similar looking and equally large tank from the Kirov Plant, also in Leningrad.

The SMK was named after communist politician Sergei M. Kirov (SMK).

The T-100 heavy tank.
The T-100 heavy tank, the basis of the SU-100Y. The multi-turret design added significant weight, and overcomplicated the commander’s job.

Both the T-100 and SMK had two turrets – less than the five-turreted T-35 – but probably still a bit too much. The use of multiple turrets was something of a Soviet fascination at the time, in part because of its theoretical capabilities (more guns = better, right?) and because of its propaganda effect.

In reality though a tank with multiple turrets will be much bigger and heavier, or have thinner armor to keep the weight down. The job of the crew is made harder, and the commander has to maintain control over where the tank is going, and what the turrets are doing.

On both, the larger turret contained a 76.2 mm gun, and the smaller sub-turret contained a 45 mm gun.

The T-100 weighed 64 tons, had a crew of seven and was extremely long. Because of the tank’s massive size it had poor mobility and unimpressive armor.

It was around this time that the Soviets began listening to designers, who knew multi-turreted tanks weren’t very good. On their own initiative, the SMK’s design team created another heavy tank in parallel to the SMK that only had one turret. It was smaller than the SMK and carried more armor all while weighing 8 tons less. This single-turreted tank would become the KV-1.

The SMK
The T-100’s competitor, the SMK. A modified version of this tank with a single turret became the KV-1.

The Soviets sent the SMK and T-100 prototypes over to Finland for a literal baptism of fire in combat, where their large size, cumbersome performance and overly complicated offensive armament limited their effectiveness.

Ironically neither the T-100 nor the SMK would enter production, with that achievement instead going to the KV-1.

While the Soviets weren’t happy with the T-100, a number of vehicles were proposed based on it. These were designated with the suffixes X, Y and Z.

Developing the SU-100Y

Experiences in the ongoing Winter War showed that there was a need for a tank capable of destroying fixed fortifications and concrete bunkers. A requirement was made for a vehicle containing a 152 or 203 mm bunker-busting gun.

To get a vehicle to the front as fast as possible, factory N°185 designed a turret containing a 152 mm gun for the T-100. This vehicle was named the T-100Z. However it was deemed inferior to the KV-2 and it never entered production.

Another proposal was the T-100X, a vehicle that was both a self propelled gun and an engineering tank.

The SU-100Y front quarter.
The only SU-100Y ever built at Kubinka. Image by Uwe Brodrecht CC BY-SA 2.0.

The T-100X had been ordered by the Soviet Ministry of Defense in early January 1940, but designers at Factory N°185 bravely decided to build a dedicated self propelled gun named the T-100Y (also known as the SU-100Y) instead.

A back and forth of confusing orders and cancellations among factories ensued (as was common in this era of Soviet tank design) before the hull of the SU-100Y was delivered in the final weeks of February 1940.

The SU-100Y was completed on March 14th. Ironically, the Winter War ended the day before.

The SU-100Y

The SU-100Y was built on the hull of the second T-100 prototype. The T-100’s turrets and associated systems were removed and replaced with a 130 mm B-13 high-velocity gun.

This gun was extremely powerful, especially for the time. Originally a Navy weapon – used on cruisers and in coastal batteries – the gun was overkill against most armored vehicles the Soviets would face during the war.

With the right ammunition, the B-13 could punch through 160 mm of armor 2,000 meters away. At closer ranges it could go through 200 mm of steel.

Side view of the SU-100Y, showing its gun.
The SU-100Y’s mighty 130 mm gun was originally a naval design. Image by Uwe Brodrecht CC BY-SA 2.0

This was less important in reality though as the SU-100Y’s primary purpose was to reduce fortifications to rubble. This was where the B-13 really shined.

Its high-explosive rounds contained 2.5 kg of explosive filler, which was excellent against infantry, soft skinned vehicles and, of course, bunkers.

Armor plating was placed around the gun, creating a casemate structure on top of the hull. The casemate was 60 mm thick on the front and sides, although it was positioned with very little angling, creating a boxy appearance.

This lack of angling decreased effective armor thickness, but increased internal space (which was still reported as being rather minimal).

The SU-100Y's boxy casemate.
The SU-100Y’s armor isn’t exactly what you’d call “angled”.

The SU-100Y had a pretty unusual crew arrangement, with a driver, radio operator, two loaders, a commander and a gunner.

The commander assisted the gunner by controlling the gun’s vertical movement. The two loaders were necessary due to the large two-piece ammunition.

Somewhat strange is the SU-100Y’s lack of roof hatches on top of the casemate, which likely severely limited the crew’s quality of life and situational awareness. The crew would have entered and exited the fighting compartment via a door at the rear of the casemate.

It also lacked any defensive close-in weaponry, with no coaxial or hull mounted machine guns.

These missing features aren’t surprising – it was a pre-production vehicle after all. It’s safe to assume that these sorts of items would have been added had it entered production.

SU-100Y close
The single example is rather sparse on features, but this is to be expected for a prototype vehicle.

In the back of the SU-100Y was the Mikulin GAM-34 – a remarkably powerful engine for 1940 standards. The engine was a derivative of the Mikulin AM-34 aero engine that powered some of the Soviet Union’s biggest aircraft, including the Tupolev ANT-20 and Kalinin K-7.

This 46.9 litre (2862 cu in) diesel V12 had an output of 890 hp and could move the 60-ton self-propelled gun to a top speed of 22 mph (35 km/h) on road and 10 mph (16 km/h) off road.

Speaking of weight, the exact figure varies between 55 and nearly 70 tons depending on the source.

Fate

The end of the Winter War and its lengthy procurement process meant the SU-100Y was not ordered into production. The sole example was moved to the Kubinka proving grounds in mid 1940.

From here the story gets a little hazy. There is a common belief that in late 1941 the SU-100Y was brought into Moscow to defend the city against the Germans.

Supposedly it then went on to fight at Kursk and other battles throughout the war. On the other hand, some say that the vehicle spent most of WWII in trials or storage.

The SU-100Y prototype.
It is logistically unrealistic for the SU-100Y to have fought throughout the war. Image by Alan Wilson CC BY-SA 2.0.

Without clear evidence it’s difficult to verify which series of events is true. It is unlikely that the SU-100Y served throughout WWII, as consumable supplies like ammunition, service items and spare parts would have been hard to come by for the lone machine.

However, with the extremely desperate nature of Moscow’s defense and the city’s close proximity to Kubinka, it is possible that the SU-100Y really did take part in that particular battle.

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The SU-100Y survived the war and can still be found at the Kubinka Tank Museum today. It is significant as the only SU-100Y ever made, but it should also be remembered that this vehicle is the last surviving remnant of the T-100 heavy tank project.