The Object 704 was the result of a Soviet mission to produce a vehicle that improved on the mighty ISU-152. This new vehicle was based on the IS-3 chassis and featured a complex armor layout and a massive 152 mm howitzer.
The Object 704 was one of the best protected of all Soviet assault guns, with its true shape hidden under layers of spaced armor.
Unlike many experimental vehicles the Object 704 turned out to be rather good. It never entered production, but this was a consequence of overly convoluted Soviet bureaucracy rather than any issue with the vehicle itself.
The SU-152 and ISU-152 proved to be fearsome machines on the battlefields of WWII. Useful for levelling city blocks, they also became famous for their ability to eradicate German armor, earning the unofficial nickname of “Zveroboy”, meaning beast killer.
The sheer weight of their 44 kg, 152 mm high explosive shells were terrifyingly effective against armor, but these vehicles were not tank destroyers. They were assault guns, designed to smash through enemy defenses.
While they were tremendously powerful in either role, they had a few drawbacks. Their enormous guns produced a significant muzzle blast that gave away their position and temporarily blinded the gunner.
Even more concerning was their armor, which was unable to comfortably resist the German 88 mm gun carried by the Tiger. These issues were raised as early as 1944.
An updated version of the ISU-152’s ML-20S gun, the ML-20SM, was produced with the intention of installing it into the vehicle, and there were even ideas of increasing its armor thickness.
However situation changed in late 1944 with the development of the IS-3; a powerful new heavy tank that improved upon the IS-2.
In early 1945 Factory No.100 Kirovskiy Works in Chelyabinsk started work on a self propelled gun based on the IS-3 that could replace the ISU-152.
It was to feature more armor, the ML-20SM gun and the same running gear, engine and transmission as the IS-3. The lead designer was L.S. Troyanov, the man behind the Object 279.
These developments ended the planned updates to the ISU-152.
Initially known as Kirovets-2, this tank would become the Object 704.
The Object 704 was similar to the ISU-152 in many ways, but carried massively improved armor in a more intelligent layout that combined thick plates with impressive angling.
The Object 704 was built upon the IS-3’s chassis, so it used the same gearbox and 520 hp V-2 V12 diesel engine. Not all IS-3 features were the same though, as it used a different exhaust and engine deck layout.
The most obvious advantage was the Object 704’s armor, of which there was plenty.
The armor was selected in such a way that the vehicle would have similar frontal protection to the IS-3 it was based on. The entire front of the vehicle was covered by a single 120 mm thick plate angled at 50 degrees.
At this angle, the 120 mm plate had an effective thickness of 190 mm. The gun mantlet was massively improved, with a thickness of 160 mm. This was almost twice as thick as on the ISU-152, an area which was considered a large weak point.
The side armor was 90 mm all over. The Object 704’s side armor was similar to other Soviet heavy tanks of the time, with the upper plates angled outwards and the lower plates angled inwards.
Also like other Soviet tanks, this shape was hidden under a layer of sheet metal.
The roof was 30 mm and the rear was 60 mm (more than the front of a T-34). Despite this hefty shielding the Object 704 only weighed 52 tons, almost identical to the ISU-152.
It could achieve a top speed of around 25 mph.
Inside, the Object 704 was similarly laid out to the ISU-152, with the crew compartment located in the frontal casemate and the engine compartment in the rear.
However due to the highly angled armor the fighting compartment was notably cramped. Inside was a crew of 5 (commander, driver, gunner and two loaders), 19 rounds of ammunition and the enormous breech of the 152 mm ML-20SM.
The less-than-ideal internal space was partially mitigated by the crews’ positions. The driver, for example, sat high up at the front of vehicle with his head protruding through a hatch in the roof. This provided great visibility in normal conditions, but was extremely limiting when buttoned down.
The commander and gunner also had relatively comfortable positions.
This was not the case for the two loaders though, who would have struggled to pass around the nearly 50 kg shells in a low fighting compartment.
To improve conditions, the Object 704’s crew compartment was fitted with fans and a heater.
The vehicle was mostly complete by spring 1945 but due to bureaucratic complications the ML-20SM gun was not installed until summer – a significant set back during wartime.
Although the war with Germany was now over, the vehicle still underwent a number of trials.
It was found to be a rather good vehicle, with adequate crew conditions and great armor. As it was constructed from mostly proven technologies it was also a rather reliable machine.
The ML-20SM gun, a muzzle brake-less improvement over the ISU-152’s ML-20S performed well with significantly reduced muzzle blast.
The performance of this gun was well known thanks to its use in the SU-152 and ISU-152, and this continued in the Object 704. It had a range of 2.4 miles, but it could also be used as an indirect fire weapon.
In this role it could launch a 152 mm shell 8 miles.
It had a few small drawbacks that would have likely been worked out had it entered production, but for a prototype, the Object 704 was relatively smooth sailing.
It was certainly a more capable vehicle than the ISU-152, with much better armor and a more modern chassis. However, because of delays and confusion caused by the complex Soviet chain of command the Object 704 would never enter production.
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By the time it was ready the war was over and there was no longer a need for such a machine.
Ironically, the ISU-152, the tank the Object 704 was meant to replace, ended up remaining in service with the Soviets until the 1970s. After all, a 152 mm gun is a 152 mm gun.
Only one Object 704 was built, and this prototype lives at the Kubinka Tank Museum.