The IS tank, released in 1943, had 120 mm of front armour, compared to its predecessor's 75, in roughly the same configuration. However, while the KV's 75 mm rendered it nearly invincible against all modern anti-tank guns, the IS did not have such luxury. Work on an IS tank with superior armour began during that same year.
"K" Heavy Tank, a precursor project to the IS-4.
CAMD RF 38-11355-2189
The above is a fragment of the blueprints of the first Object 701, with a clear date: December 1943. Sadly, Kirov factory must have been short on paper, as they used the backs of early 701 blueprints for scrap in 1944.
Object 701 was a lofty goal indeed. It was designed to be impenetrable to the most powerful anti-tank guns of the day, the D-25 and KwK 43. As we've seen from a previous article, the tank eventually achieved that objective. However, early prototypes of the hull had a couple of weaknesses.
CAMD RF 38-11355-2411
"Photo #20. Penetration of the spherical cover of the TSh-17 sight with 88 mm armour piercing shells. Penetration of the driver's observation device with 88 mm shells.
Due to the spherical shape of the sight cover, a hit at a perpendicular angle, and therefore penetration, is possible at various angles. The rolled roof of the tank can be penetrated by 88 mm shells (see photo #19).
The cover of the driver's observation device can be penetrated by 88 mm shells (see photo #20). The factory used 8S steel, which cannot be properly conditioned when over 75 mm thick. Kirov factory representatives insist that use of this grade of steel was unavoidable.
The turret platform roof is penetrated by the shockwave of an HE-fragmentation 122 mm shell."
Well, the tank is mostly protected. Can't penetrate the upper glacis (shot 59 barely dents it), but the weak spots require a redesign. Let's look at how the rear armour fared.
CAMD RF 38-11355-2411
"Photo #9. Intact connections between the rear hull sides and hull rear, as well as the upper rear plate and lower rear plate after concentrated fire by 75 and 88 mm guns armour piercing shells."
Impressive! The guns cannot even penetrate the rear armour of the tank. The sides, however...
CAMD RF 38-11355-2411
"Photo #10. Rear side hull separated from the front side hull after welding seam was destroyed."
A lucky hit to the welding seam can penetrate the hull side, otherwise it performs very well. The tank's armour proves worthy, but the design of the hull needs some work.
That was Object 701 #1. Objects 701 #2 and #3 had improved welding seams and higher hardening of the turret armour. The test results greatly improved as well: "The armour of the Object 701 tank provides complete protection from 75 and 88 mm shells with muzzle velocity of up to 1000 m/s at any distance, in an arc of +/- 60 degrees from the front of the hull and +/- 30 degrees from the front of the turret." As a result of these trials, the hull and turret designs of the Object 701 were accepted. Object 701 #5 and #6 later had the front plate increased to 140 mm to counter potential 105 and 128 mm guns.
The drivetrain needed some work, though. Object 701 #4 received an improved transmission, which was further improved in #5 and #6 prototypes.
As for the armament, the gun was the same old D-25 that has proven itself against the toughest German armour.
"According to your requests, the Chebarkul GAU proving grounds tested the 122 mm tank gun model 1943 (D-25) in the new heavy tank (object 701), developed by the Kirov factory, between May 11th and 15th of 1945.
In total, 403 shots were fired. The trials established that:
- The precision of the gun at 500, 1000, and 2000 meters is satisfactory.
- The angle of incidence of AP and HE shells is the same, -2.9 minutes, nearly identical to the calculated angle (-3 minutes).
- The practical rate of fire is 2-3 RPM, and peak is 3-4 RPM.
The main complaints in the process of trials were about the fighting compartment equipment and gun installation.
- The three propellant casing holder on the right side of the turret is impeded by the signal flare box and machine gun ammunition box.
- The elevation mechanism carrier was crooked.
After the removal of the above deficiencies, especially after increasing the robustness of the elevation mechanism carrier, trials as to be repeated.
The 122 mm tank gun trialled in May of 1945 was produced in April-May of 1944. After that, the gun was substantially modified and improved. Most importantly, the gun's semi-automatics were improved, removing the possibility of breaking the breech opening roller pin. All other downsides of the gun can be explained by quality of production
Deputy chair of the AK GAU KA, Major-General of the Artillery Engineering Service, Zhevanik"
One of the modifications to the gun in that time must have been Kotin's loading assistance device, which increased the peak rate of fire on the IS-2 to 6 rounds per minute.
However, May 1945 marked the defeat of Germany. Fighting the Japanese would not require a powerful new tank, and it was unclear what it would be used for, if anything. Engineers and equipment was needed for peacetime duties. Here are some excerpts from a Kirov factory report for the first quarter of 1946.
"All the best and most precise equipment was moved from tank-building plants to tractor building plants.
The remaining equipment is from before the War, and did not have proper service since then, and therefore, 40% of it requires medium or major repairs.
The best tank engineers, plant managers, technologists, foremen, and highly qualified employees were transferred to tractor production. For example: out of 212 tank engineers, 112 remained by January 1st, 1945, and 66 remain as of January 1st, 1946. The 146 employees were transferred to other positions, including managers and production line foremen.
The remaining 66 engineers perform the following duties. 16 engineers develop new objects, while 50 of the less qualified engineers work on Object 701 and production of the IS-3 tank.
This is insufficient for preparation of the 701st for production. The number must be increased to 100 engineers.
There used to be 470 controllers and 7839 employees in the 11 tank building plants as of June 1st, 1945. These numbers are now reduced to 5 plants, 198 controllers, and 3358 employees.
The employees are new, and had to familiarize themselves with the new technologies in April-May, leading to lower quality of joints and installations of IS-3 components. "
CAMD RF 38-11355-3130
The IS-4 was cancelled after production of just over 250 units. The heavily armoured sluggish behemoth of old was incompatible with the modern maneuverable war, especially with the looming threat of nuclear escalation.