Tuesday 30 September 2014

IS model 1940

Order of the Labouring Red Banner Voroshilov factory #174, department #1
July 22nd, 1940

#3794, Top Secret

To the Deputy of the People's Commissar of Medium Machinebuilding, comrade A.A. Goreglyad
Copy to the Chief of the NKSM Glavspetsmash, comrade Surenyan

On the issue of the heavily armoured IS tank.

I propose this heavy IS tank project, composed according to the orders of the NKSM.

Our factory agrees to produce the project, working blueprints, and experimental prototype, and considers it possible to produce up to 30 vehicles annually using the capabilities of the experimental division (formerly factory #185). In order to ensure such volume, cooperation among a series of factories is necessary, both from our Commissariat and others which supply our factory with raw materials and components. Additionally, the factory will need more equipment. An approximate calculation of required equipment and cooperation plans are included in the attachment.

Reply with your decision.

Attachment: explanatory note to the NKSM (two copies), to Glavspetsmash (one copy) and for the archive.
Preliminary cooperation plan for the NKSM (two copies), for Glavspetsmash (one copy) and for the archive.

Blueprints (two copies) for the NKSM

Factory director Markin
Chief engineer Zasovenko
Chief designer Ginsburg

Explanatory note for the heavily armoured IS tank project

The project completed by Voroshilov factory #174 fills the need of a breakthrough tank in a positional war.

The main tasks of this tank are the destruction of long-term reinforced concrete artillery emplacements, artillery, and heavy enemy tanks. These tanks can also serve as a powerful coastal defense gun, aiding in deflecting enemy landing attempts. The armour (main thickness: 110 mm) is thick enough to withstand all artillery installed in concrete emplacements, from 37 mm to 88 mm inclusive.

For instance, Germany uses an 88 mm AA gun with the muzzle velocity of 820 m/s. It is possible that this gun can be put in a bunker. It is unlikely that larger caliber guns will be placed in bunkers due to low rates of fire and difficulty in hitting moving targets up close. Additionally, high caliber artillery has no armour piercing shells, and its long barrels will reveal its location.

The tank is armed with a powerful 130 mm B-13 gun with a muzzle velocity of 880 m/s, using a 36 kg shell. Due to its powerful penetration (the gun can penetrate 150 mm of armour from a kilometer away), a shot that hits a bunker or pillbox will destroy it.

When firing a concrete piercing shell, up to two meters of concrete can be breached, with a fragmentation power approaching that of the 152 mm ML-20 shell.

The turret can spin in a full circle, and has a 2400 mm turret ring.

The presence of a semi-automatic breech, loading tray, gun rammer, and bore evacuator increase the rate of fire and make the crew's working conditions better. The hull can be made of rolled or cast armour. The tank has four machineguns for protecting itself from enemy infantry. Two of these are in turrets, which can quickly shift fire between land and airborne threats. One of these turrets solves the issue of granting the commander 360 degree vision. It's possible to use the turret for observation only, but then the advantage of rapidly shifting fire is lost. The tank can carry 50 shells for its main armament, which allows it to operate on the battlefield for a long span of time.

Instead of a B-13 gun, a 122 mm A-19 corps gun can be installed, with a muzzle velocity of 820 m/s and a 25 kg shell, or a 152 mm ML-20 corps gun with a muzzle velocity of 650 m/s and a 44.5 kg shell. The rate of fire of the 122 mm gun is close to that of the 130 mm gun and is significantly higher than of the 152 mm gun.

The vehicle uses the 24-cylinder V-24 engine, 1100-1200 hp, composed of a pair of V-2 engines in one unit. This engine uses parts produced by factory #75, and an experimental prototype can be built in 4-5 months.

Aside from the above engine, the tank can use the 12-cylinder MN-1 1000 hp engine, a batch of which was already produced by Ts.I.A.M. or a turbocharged V-2 engine at 850 hp, with a lower top speed.

The engine can grant the vehicle a speed of up to 35 kph, and the ability to traverse light obstacles and very rough terrain. As for off-road performance, it is difficult to achieve in this class of vehicle, as the ground pressure is limited by the size of railroads, maximum possible width, and internal volume.

With 800 mm wide tracks and 110 mm thick armour, given zeroth volume category (width: 3400 mm), the internal width of the hull ends up being 1560 mm, which is less than that of the light T-26 tank, where it is 1625 mm. This project has the ground pressure of 0.9 kg/cm^2.

In order to reduce the ground pressure on soft terrain, it is possible to replace the 800 mm wide tracks with 1 meter wide tracks, at which point ground pressure decreases to 0.75 kg/cm^2. The narrower tracks can be equipped for railroad transport and use on solid ground or in winter conditions. The size of the vehicle fits within the zeroth category. According to NKPS instructions (TransZhelDorIzdat, 1938), a load of these dimensions can be transported without restrictions on Soviet railroads.

The combat mass of the tank is 75 tons. Considering that the mass of the vehicle without ammunition, equipment, fuel, and crew is no more than 70 tons, they can be transported on 60 ton train cars with acceptable overloads. If the turret and hull are transported separately, existing platforms (even 50 ton ones) fully enable the transport of this tank.

The project is completed with two variants.

Variant 1 (blueprints IS-S1, IS-S2, IS-S3, IS-S4, IS-S5, IS-S6, IS-S7) has the classical tank layout.

The transmission and engine group is in the rear of the tank. The cooling fan is on the axis of the crankshaft, and is the two-stage axle type. The air is sucked in at the top of the vehicle, and exhausted on the bottom, near the tracks. The transmission is mechanical, with five speeds forward and one back.

In order to make the transmission as small as possible, the friction clutches are inverted and placed behind the final drives above the suspension, in a box that is the continuation of the turret platform. The turret is in the middle of the vehicle. The ammunition is placed in the perimeter of the turret, in the hull, and in the turret bustle.

Fuel tanks are located in the fighting compartment, and act as a floor for the gun crew. The engine and transmission can be accessed through special hatches and passages.

As it is difficult to protect the suspension from artillery fire, there is a divide between the protection of the hull and suspension.

Variant 2. This variant has significant differences in its design (blueprints IS-S-1, IS-S2-1, IS-S3-1, IS-S4-1). In order to protect the suspension, a separate track is used. Each side has two independent tracks, each of which is driven independently. To achieve this, the transmission and engine group is in the middle of the vehicle, and each side has two drive wheels. If one track per side is destroyed, the tank can still continue on its course. In addition, the front idler can be modified to be used as a mine roller. If the front track is hit and falls off, it protects the rear track from being damaged by explosives. In order to obtain a sufficiently large surface, the tank's hull is increased in length by 600 mm. That and the increase in weight due to the suspension results in a tank weight of 79 tons. In order to achieve the desired weight of 75 tons, the main armour thickness must either be reduced to 100 mm or the turret must be transported separately.

Due to a reduction in track surface area, the ground pressure increases to 1.05 kg/cm^2 without sinking and 0.85 kg/cm^2 when sunken by 100 mm.

The crew is separated, and can exit trough two emergency hatches underneath the tank.

Two-track Variant
Four-track Variant
Mass (tons)
Size (mm)
Length of tracks
Height (to roof)
Height (to highest point)
Length of the track's gripping surface
Track width
Distance between track centers
Armour (mm)
Sides, turret platform, front and rear, turret side
Main gun
130 mm B-13
Muzzle velocity (m/s)
Shell mass (kg)
Horizontal traverse range (deg.)
Gun elevation (deg.)
Gun depression (deg.)
Turret ring diameter (mm)
Optional armament
45 mm gun
Machinegun bullets
Gun shells
Engine power (hp)
Speed, kph
5th gear
4th gear
3rd gear
2nd gear
1st gear
Reverse gear
Traversible obstacles
Trench width (meters)
Grade (deg.)
Water depth (meters)
Ground pressure (kg/cm^2)
L/B ratio

The cost of the vehicle when produced in small batches is no more than 3 million rubles, which compares favourably to the cost of the 100-Y vehicle (the cost of one is 3.3 million rubles).

If the project is approved immediately, the first prototype will be ready by June-July of 1941, and the first batch will be ready in the first quarter of 1942.

Factory director Markin
Chief engineer Zasovenko
Chief designer Ginsburg"

V. Lehn Collection


  1. Ho? What vehicle is that?
    No blueprints available?

    Very very interesting !!

  2. if i had to guess its prototype 103.
    A google lead me to the SU-100Y wiki page where prototype 103 is mentioned.

  3. Yes, indeed, but that one has less armor IIRC. Also the Text says that 2 of the MGs are in turretS.

  4. Well like i said if i had to guess.

    I did read the KV-4 was planned to potentially carry the B-13 gun but that was at a later date.
    I guess we would have to see what peter has to say about this.