The Last breath of the IS-6
In the later years of the Second World War (‘The Great Patriotic War’ to the Soviets), there was a quest to develop a replacement for the IS-2 heavy tank. The development process resulted in the IS-6 (Object 252/Object 253) and IS-4 (Object 701). This program was as secret as it was ambitious, with two rival factories working on their designs in absolute secrecy in fear of leaking information to one another.
On paper, the IS-6 seemed superior to any Panzer in Germany’s arsenal at the time. However, due to its mechanical issues and overall poor performance, it lost to its competitor, the IS-4, which would go on to enter service in 1946. Despite this, a last ditch effort to revamp the IS-6 was made, with limited success. This renewed vehicle would become known as the Object 252 November improvement – more commonly known as the Object 252U.
The Object 252 upgrade from November 1944 might have never gotten an official designation. Yet modern historians and video game company Wargaming have called it Object 252U, with the ‘U’ probably coming from the romanized Russian word ‘улучшенный’ (uluchshennyy). For simplicity’s sake, we will refer to it as Object 252U for the rest of the article.
After the battlefront experiences of 1943, with the appearance of the new German heavy tanks and tank destroyers such as the Ferdinand, the Soviet Union quickly realised that a new heavy tank was needed. Thus, in November of 1943, the GABTU (Main Directorate of the Armed Forces) requested the development of a 55 tonnes (61 tons) heavy tank.
Two factories of the same organization (ChKZ) and the same city, Chelyabinsk, were assigned this task.
1. SKB-2, which was headed by Nikolai Dukhov, who had taken part in the development of the IS tank, and would later become the assistant of the chief designer of the Soviet atomic bomb plan. SKB-2 designed the Object 701, which was a planned upgrade of the IS-2 and later became the IS-4.
2. Experimental Factory No. 100, which was headed by Josef Kotin, who previously had done work on countless Soviet tanks. In contrast to their ‘opponents’, they developed a completely new tank, the Object 252 and 253.
From June to October 1944, Factory No.100’s Object 252 and Object 253 (IS-6) failed in comparison to the SKB-2 design. The armor was much thinner than that of the Object 701, yet it was still heavy, weighing over 50 tonnes (55 tons). The mechanical problems in the suspension and mobility were worse compared to its heavier counterpart. Kotin managed to get the IS-6 to Moscow, where it got tested against the Object 701, but to no avail. In late November of the same year, an upgrade attempt was made with the help of the NII-48 institute using a heavily angled pike-nose design for the armor and a new turret. Despite it being a dead-end when it came to the development of the IS-6, it was a turning point for future heavy tank designs. This is now known as the Object 252U.
Naturally, the design of the 252U was similar to that of the Object 252, as it was based on the same hull. Unmistakable are the large stamped steel wheels with a 750 mm (30 inches) diameter. These were first tested on the Object 244 (an IS-2 upgrade) to be used on the Object 252. The Object 253, although also an ‘IS-6’, used regular IS style wheels. The engine and the rear and side of the hull were untouched on the November improvement. The torsion bars suspension would remain the same too.
The engine would most likely have been the same as on the Object 252, a V12U diesel engine producing 750 hp at 2,100 rpm. The internal fuel tanks had a capacity of 650 liters (172 US gallons) but, in typical Soviet fashion, there would have been 4x 100 liter (26 US gallons) external fuel tanks on the sides.
Interesting to add is that the early designs for the IS-6 actually had a rounded frontal hull, similar to that of the much later Object 279, yet having a similar effectiveness to the Object 252U. The actual prototypes had a hull made of angled flat plates, probably because it was much easier and cheaper to produce compared to a large casting.
The most notable changes appeared in the hull and turret. The NII-48 institute strongly suggested that, in order to improve the protection yet not increase the weight, a pike-nose design should be used. This meant two diagonal plates were welded in the front, creating a pike-like shape and greatly improving the effective armor on the front of the vehicle from threats directly in front of the vehicle. This was designed by engineers G. N. Moskvin and V. I. Tarotko, the latter first incorporated this solution on the IS-2U and on the later Object 257 as well.
Previously, the Object 252 and 253 used the D-30T 122 mm gun. This offered very little improvement over the standard D-25T from the IS-2, yet the price was almost doubled. As a result, a new gun was planned to be added to the Object 252, although it ended up never being fitted. The new gun was the BL-13 122 mm, developed in December of 1943 by OKB-172 and was a combination between the D-25T and the BL-9 gun barrel. Further work was done between Factory No.100 and OKB-172, with the gun being ready by July 1944. The capabilities of this weapon are unknown. A ready rack for the massive projectiles was located in the rear of the turret, covered by a thin protective casing, an overall design similar to modern MBTs. A total of 18 warheads were stored here. The casing with explosive material was stored within the hull, on the exact opposite side. This made loading the warheads relatively easy, however, the placement of the cases required the loader to bend down, and manhandle them up into the breech. One scaled mockup was built of the design.
As the Object 252U was, for the most part, very similar to the IS-6, the crew compartment was mostly identical. It had a crew of four, a commander, gunner, loader and driver. The gunner sat to the left of the gun with the commander behind him. They seem to have shared the same entry and exit hatch, which could be fatal for the gunner in case of a need to evacuate quickly, as he would have to wait for the commander to exit first. The commander had no cupola to look out from, rather just two periscopes (one pointing forwards and the other backward) on top of his hatch. This would have made the commander almost blind when buttoned up. The commander was also in charge of the radio. The loader, as discussed above, had a tough time when loading the main gun. As the cases for the rounds were in the hull, he would have needed to get off his chair and lift them up. Since the turret had no basket, the crew members in the turret could either sit on seats attached to the turret or stand on the hull floor. The loader seems to also have been tasked with the operation of the 12.7 x 108 AA DhSK heavy machine gun. To operate it, the loader had to expose himself, by partially climbing out of his hatch. It is also entirely possible that he was also responsible for loading the co-axial machine gun, a 7.62 mm SGMT. The driver was located in the hull, with the hatch directly underneath the gun, potentially making it very difficult to exit, if the gun was pointing forward.
Since the Object 252U put such a large emphasis on protection, crew comfort and ergonomics were sacrificed. The angling of the hull armor plates made storing ammunition and overall life inside cramped and claustrophobic. An awful thought for many western tankers, Soviet tank design doctrine often sacrificed crew comfort for protection or a low silhouette.
If the IS-6 or Object 252U would have entered service, it would have most likely received a series of upgrades to fix the problems mentioned above, like the periscopes. The Object 701 had its periscopes altered and more added several times until full scale production. However, the armor profile and no turret basket were still features of future Soviet heavy tanks.
It was regarding armor that the IS-6 struggled the most in comparison with the IS-4. The front was one 100 mm (4 inches) thick flat plate angled at 65°. The lower front plate was 120 mm (4.7 inches) thick, yet only angled at 52°. The side armor at the thickest, was 100 mm. Testing was done in Kubinka, Moscow with captured German 88 mm and 105 mm guns, which could not penetrate the upper frontal plate from 50 m (55 yards). The 120 mm lower plate, being less angled, was penetrated from a “shorter distance”. These results, while being better than the IS-2 and IS-3, fell short of the protection offered by the IS-4. In this regard, the new pike nose design of the Object 252U came into play, as the lower plate was still 120 mm thick, but more sharply angled at 28°. The two upper plates forming the pike nose were 100 mm thick, yet angled at 16° from the side. This increased the effectiveness of the armor significantly. The rest of the hull remained the same as that of the regular IS-6.
With the help of NII-48, Factory No.100 also designed the IS-2U simultaneously with the Object 252U, making an IS-2 with a pike-like front hull. An improved version of the IS-2, it featured a new gun, and the pike-nose frontal armor. Like the Object 252, it was rejected, in favor of SKB-2’s Kirovets-1, which later became the IS-3.
Fate and Conclusion
The Object 252 November upgrade got as far as a mockup, as the fate of the IS-6 had already been sealed. Yet, despite being unsuccessful, the design was not in vain. Instead, it served as a basis for the Object 257, which in turn led to the IS-7 heavy tank, the heaviest Soviet tank ever built. More importantly, it was one the first Soviet designs to implement the pike-nose design, which became famous with the IS-3, and was implemented in the majority of Soviet heavy tanks until their discontinuation.
Object 252U specifications
|7.50 x 2.4 x 3.3 meters
(25 x 7.8 x 10.8 feet)
|Total Weight, Battle Ready
|4 (Commander, Gunner, Driver & Loader)
|V12U diesel engine, 750 hp at 2,100 rpm
|35 – 50 km/h (hypothetical)
(21 – 31 mph)
|122 mm D-13 2-part ammunition gun
12.7 x 108 mm DShK heavy machine gun on roof
co-axial 7.62 mm SGMT machine gun
Frontal plates: 100 mm forming pike nose at 16°
Lower plate: 120 mm angled at 38°
Upper side plates: 100 mm angled at 45°
Lower side plates: 100 mm at 90°
Upper rear armor: 60 mm at 60°
Lower rear armor: 60 mm at 30°
Upper hull armor: 30 mm
Floor armor: 20 mmTurret armor
Front: 150 mm
Side: 150 – 120 mm
Rear: 100 mm
Top: 30 mm