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ISU-122 Frontline Impressions

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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.

ISU-122S in Soběslav, Czechoslovakia.

Yuri Pasholok's article clearly lists the reasons why the ISU-122 was produced with a 122mm gun. The separation of roles is confirmed by different tactics that the ISU-122s used. The ISU-122 was usually used as anti-tank or assault guns. The panoramic sight used when firing in an artillery role was often criticized.
"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." 

An ISU-122 column on the move in the Ukraine.

Despite this complaint, there were no requests to install a larger gun. The 122 mm gun's accuracy was praised for allowing it to hit targets at 2000+ meters. "The ISU-122 was used to fire at ranges up to 3.5 km, the result was always effective". 

In the last stages of the war focused fire from several SPGs or even several batteries proved effective. When assaulting German cities the ISU-122 literally swept aside any obstacles in the attackers' way.

Most of the complaints about the SPG had to do with firing the gun and servicing it.
"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."

Citizens of Łódź greet riders on an ISU-122 from the 1st Guards Tank Army.

The ISU-122's primary armament was satisfactory, but there were issues with secondary armament. Soviet historians liked to criticize the Ferdinand for its lack of machine gun, forgetting that Soviet SPGs didn't have it either. SPG crews often had to fight off enemy tanks and infantry on their own, and a lack of machine gun could result in tragic consequences such as a Panzerfaust to the side.
"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 DShK machine gun was used on the ISU-122 only towards the end of 1944. The heavy machine guns showed themselves well in city fighting where they were used to fire at top floors of buildings. The only complaint was about the amount of misfires after reloading.

An ISU-122 with a 12.7 mm DShK AA machine gun on the casemate roof, Danzig (modern day Gdansk).

The crew's weapons were a separate matter. Tankers abandoning their vehicle could grab a DT to go with them, but SPG crews only had their personal weapons. There was not enough room in the SPG to stow a PPSh or PPS for every crewman. In case of the ISU-122, the designers mainly worried about fitting a corps level gun in the fighting compartment of an SPG.

Many complaints were also made regarding the loader's working conditions. Of course, it was easier to work with the two piece ammunition than with a long single piece round, but even the separate pieces were not light or small. One report called for optimization of the ammunition racks, placing all the shells on the left and right walls and the propellant casings on the floor.

An ISU-122 crew rests in a suburb of Berlin. The graffiti says "Berlin will remain German".

Some SPG crews considered the driver to be less lucky than the loader.
"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. 

ISU-122 from the 59th Independent Breakthrough Tank Regiment, 9th Mechanized Corps, 3rd Guards Tank Army on a march through Western Ukraine.

In addition to armour, requests of improving observation devices were common. The driver and commander particularly wanted more visibility. One report indicated that fire correction was performed not just looking out of the gunner's hatch, but also from outside the vehicle. Another letter said that observation of the battlefield was done through an open hatch, and visibility greatly decreased when circumstances forced the hatch to be closed. 

Finally, there were a number of complaints about the reliability of the vehicle. Despite all of their advantages, the IS tanks and SPGs on their chassis were still raw designs. This was a pretty typical list of drawbacks and suggestions from SPG regiments.
"Drawbacks

  1. Rapid wear of the two-piece toothless track links. 
  2. Tearing of the road wheel nuts and bolts.
  3. ...
  4. Final drive gear trains leak.
  5. Engine retention bolts tear.
  6. The planetary turning mechanism brake band eyelets tear.
  7. Torsion bars break.
  8. Fuel gets into the oil system.
  9. The starter relay and locking wheel relay break.
  10. The idler and road wheel bearings crumble.
  11. The planetary turning mechanism jams.
  12. The suspension arms on the right side burst.
It is necessary to:
  1. Strengthen the road wheel retention bolts and nuts.
  2. ...
  3. Improve the linings in the final drives.
  4. Change the gearbox attachment method.
  5. Give the driver an observation periscope.
  6. Install two machine guns: in the front and in the rear.
ISU-122S in Koenigsberg, 3rd Belorussian Front, April 1945.

Communication issues were handled separately. Since SPG units were often pulled apart to reinforce other units, the issue of reliable communication was a pressing one. Good communication and timely support compensated for a lack of turrets.
"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.

Original article by Andrey Ulanov.


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