IS #1 and #2

"To the People's Commissar of Tank Production, comrade I.M. Zaltsmann

In accordance with the GOKO decree and your orders, the Kirov factory and factory #100 jointly produced two IS tanks, one with a 76 mm gun (IS-1) and one with a 122 mm gun (IS-2).

Both tanks were built in accordance with the tactical-technical characteristics outlined in the GOKO decree:

	IS-1	IS-2
Mass	Up to 38.5 tons (37,160)	Up to 39.5 tons (37,900)
Top speed	55 kph	55 kph
Transmission	8-speed gearbox, planetary turning mechanism, planetary final drives	Same
Armour	120 mm cast front 100 mm cast turret platform, turret 90 mm rolled sides	Same
Engine	V-2K 600 hp with electro-inertial starter	V-2K 600 hp with electro-inertial starter
Armament	76 mm F-34 gun 3 DT machine guns (coax, rear in turret, hull)	122 mm gun 2 DT machine guns (coax and hull)
Ammunition	80-85 76 mm rounds 32 machine gun magazines	30-35 122 mm rounds 32 machine gun magazines
Crew	4	5

The IS-1 tank was assembled on March 8th, 1943

The IS-2 tank was assembled on March 12th, 1943

The IS-1 tank completed factory trials on March 19th, 1943. It had travelled 305 km [cut off]

By March 19th, 1943, all changes required as a result of lengthy trials were made to the IS-2. The IS-2 is currently occupied in factory trials and will be presented to the state commission on March 23rd, 1943.

As a result of trials of the IS-1, the following drawbacks have been discovered:

1. Increased temperature of water and oil after long marches in high gear.
2. Slipping and breaking of the main friction clutch roller.
3. Weakening of the carrier disk of the final drive spline.
4. Gearbox slips out of 7th gear.
5. The roller bearing of the front road wheel and a crack along the reinforcement rib.

The main friction clutch slipping was caused by insufficient robustness of some components, which were strengthened. Further tests showed that this component worked without flaws.

The oscillations of the carrier disk were caused by low hardness of the roller of the final drive and weakening of the disk's position on the splines. The defect was corrected, and the harness of the roller was changed to the same value as on the KV tank, where this component is very reliable.

The slipping of the 7th gear was caused by deformation of the switching fork, which was not thermally conditioned. This was corrected.

Destruction of the roller bearing of the front road wheel was due to the incorrect choice of bearing (roller instead of conical). This design defect will be corrected on future vehicles by reinforcing the ribs and installing conical bearings that showed themselves well on the KV, a heavier vehicle.

As you can see, the aforementioned defects aside from the cooling are slip-ups of either production or design that could be easily corrected on both vehicles.

The only serious defect is the high temperatures after long marches on the highest speeds. The solution for this problem was found during factory trials of the first vehicle and consists of the following:

1. Increasing the cross-section of the air intake in the second step cooling fan.
2. Installation of a straightening brace on the fan casing, which improved air flow through the fan.

These measures significantly improved the function of the cooling system, which was proven in trials with driving at high speed. In parallel with the factory trials, work is being done on a test bench that shows results that decrease temperature even further. The issue is clear and a solution has been found.

Disassembly and inspection of the components of the IS tank show that all components (gears, rollers, etc) are in good condition. 

The measures taken to correct defects uncovered during trials and the condition of the components of the drive train allow us to make a conclusion regarding the reliability of the components and the vehicle as a whole.

Based on the above, the factory considers it possible to present the vehicles to the state commission for trials.

Based on our information, the state commission has the following wishes:

1. Increase the turret ring diameter by 150 mm.
2. Install a planetary gearbox.
3. Install English style MK-IV mirror periscopes.

Our opinion is as follows:

1. Increasing the turret ring diameter by 150 mm is not necessary to improve crew working conditions. The existing turret ring diameter is proven to be sufficient on the KV tank and increasing the turret ring diameter can make the tank up to 1000 kg heavier. It is much more reasonable to increase the ammunition capacity by adding to the dead weight, which we are doing.
2. The use of an 8-speed gearbox tested in toughest combat situations in combination with the planetary turning mechanism makes it impossible and unnecessary to install an untested planetary gearbox.
3. The MK-IV is a good mirror periscope, but this issue cannot be resolved by the factory. An optical factory must produce this.

Chief Engineer of the factory, S. Makhonin

Chief Designer of the factory, Zh. Kotin

Chairman of the Factory Trials Commission, Chief Tank Engineer A. Lantzberg"

CAMD RF 38-11355-1401 pp.18-21