The Soviet ISU-122 heavy SPG is often overshadowed by its more famous siblings, the IS-2 tank and ISU-152 SPG. The similarity in the designs gave birth to the myth that the ISU-122 was an ersatz ISU-152 built due to a shortage of 152 mm guns. That is far from the truth. Let us try and figure out what crews of these tank destroyers valued the most, and what they would rather have changed.
"The ISU-122 is most often used in direct fire against visible targets."
"There is a number of inconveniences for the crew. The driver's workspace is cramped, when working with the levers he hits his right hand on the gun cradle. It's hard for the driver to enter and exit his station quickly as the OL-18 panoramic sight gets in the way."
"The panoramic sight gets in the way of the gunner when he aims the gun directly. When the gun is turned all the way to the left it's hard for the loader to extract casings from the ready rack and it's hard to open the breech after firing."
"Zeroing of the ST-18 sight is hard and sometimes imprecise."
"The AL-19 tank sight gets in the way of the driver, crews often remove it and do not use it in combat."
The powerful long-barreled 122 mm gun allowed the ISU-122 to successfully combat any enemy vehicles at any combat range. Interestingly enough, the toughest target according to the gunners was the Panther, not the Tiger. According to some, "when firing an armour piercing shell at the Panther's front from 2000 meters the latter ricochets and does not penetrate the armour."
"Due to radical simplification of the traverse mechanism a backlash forms after the first few shots."
"The hand trigger should be replaced with a pedal trigger or an electric trigger."
"The gun can jam when firing, there were cases where the breech jammed due to the casing expanding."
Crews also pointed out that the heavy SPG can have difficulties with mobility.
"It is difficult for the ISU-122 to cross trenches or water hazards with steep banks. In some cases it is made impossible due to the small gun elevation angle. The shift of the center of mass forward also reduces off-road mobility, and the SPG's tracks can slip, digging itself into the ground and bottoming out on difficult terrain."
"Drawbacks include a lack of machine guns to combat enemy infantry and aircraft."
"There is a lack of machine guns in ball mounts for the commander and for firing backwards."
"The driver's work is made difficult. For instance, the compartment is cramped, there is no forward hatch, the pathway from the driver's compartment to the fighting compartment is narrow. When in motion, the driver, especially very tall drivers, sit in a strained position."
"A few words on the design defects of the vehicles. As a rule, the driver dies when it is destroyed since the location of the left fuel tank and panoramic sight case do not allow him to quickly exit the vehicle in case of fire."
There were also general problems.
"If the periscopes are disabled, observation has to be performed through open hatches. The rear casemate hatch is difficult to open. The crew has to work by feel during rapid fire, as after 5-6 shots the turret is filled with smoke that the ventilator fan does not keep up with. There is no access to the servo mechanism.
90 mm of front armour was also not always enough to protect the crew from tank and anti-tank guns by the second half of the war.
"The steel armour is not powerful enough and can be penetrated by 88 mm guns from 1500-1800 meters. The armour pops when hit with HE shells or mines, splinters come off the interior side and strike the crew."
"The armour of the ISU-122 is insufficient and can be penetrated by 88 mm guns from 1500-1800 meters. 55-88 mm APCR penetrates the hull from 200-1500 meters. The gun mantlet, recoil brake plug, front armour above the driver's vision port, and lower front armour are the most vulnerable."
However, inspection of knocked out and destroyed SPGs showed that they were mostly hit in the side. As a rule, this was the consequence of throwing them into battle "tank style" with no regard for their special features. However, crews reasonably wanted more front armour from an assault SPG.
It is necessary to:
- Rapid wear of the two-piece toothless track links.
- Tearing of the road wheel nuts and bolts....
- Final drive gear trains leak.
- Engine retention bolts tear.
- The planetary turning mechanism brake band eyelets tear.
- Torsion bars break.
- Fuel gets into the oil system.
- The starter relay and locking wheel relay break.
- The idler and road wheel bearings crumble.
- The planetary turning mechanism jams.
- The suspension arms on the right side burst.
- Strengthen the road wheel retention bolts and nuts....
- Improve the linings in the final drives.
- Change the gearbox attachment method.
- Give the driver an observation periscope.
- Install two machine guns: in the front and in the rear.
"Radio communications. The drawback is as follows: in battle the regiment and battery commander can only issue orders until the shooting starts, as the commander can work with the radio only when there is no fire and he doesn't have to observe the battlefield. When fighting starts and the shooting begins, the commander cannot work with the radio."
To summarize, the ISU-122 was not a perfect work of art that a skilled SPG crewman would be unable to find fault with. Like any vehicle, it had its strong and weak sides. Crews and especially commanders sending these vehicles into battle had to be aware of them and keep them in mind. In skilled hands, the ISU-122 was a mighty and fearsome weapon, which played a big role in the concluding stages of the Great Patriotic War.