Monday 22 February 2021

ISU-122 Frontline Impressions

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.

  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.


  1. Finally some report which evaluate the armor protection. The fact that it not always protected from AT gun is just how many expected. There should be specified which gun with caliber of 88 mm can penetrate its armour at 1800 m. The german SPGs with armor protection level against the AT guns have the same or better armor protection than tank on which basis they are built on. For Soviet SPGs its in opposite direction except the SU-100/85 M. If they narrowed the nose of IS-2 to simplify the production and increase the protection, I think the similar thing should be done in case its SPGs.

    1. You mean they sloped the IS-2 hull front to get rid of that KV-legacy "step". Straightforward enough and made for few enough differences to the internals.

      Doing the same to the ISU casemate would have been a MUCH bigger deal requiring at the very least a nontrivial redesign of the gun mounting, and sloping the front enough for actually meaningful protection gains oughta have demanded revising much of the fighting compartement.

      Considering these were meant to be (relatively) "cheap and cheerful" standoff fire-support designs that brought more big guns to the frontlines on budget that doesn't sound like a very sensible project to embark on to me, especially with the end of the war and the replacement of the base tank already in sight.

      Compare the Object 704 which was designed from the ground up with proper sloping in mind and apparently still ran into working-space issues...

    2. I'm more with Crabtree, Kellomies. Given that the ISUs began entry in 1944 (the first regiments weren't formed until May, it seems) it's already clear that even when they were being developed, their armor protection was problematic. I've done some crude measurements with online protractors and scale diagrams and conservatively estimated that it should have been possible to construct a single plate stretching from the lower plate to top of the casemate of 45 degress (that's a slope multiplier of 1.69, and about 150 mm of effective armor).

      Now, doing this would have lost the 70+ degree sloped 65 mm of armor part of the original IS-2 'broken nose' armor scheme, which was tough nut at close range even for the Kwk43. But as this great protection only forms a small percentage of the overall frontal surface, so improving the protection of the rest would still be a plus. However the gun mantlet (100 mm, somewhat rounded) would still be a weak spot (though the ISU-122S slightly improved that). A frontal plate of 150 mm of effective armor would have protected the ISUs well against Kwk36s and Kwk42s at normal engagement ranges for this beast.

      The problem I see is that the ISU-122 began as a spin-off of the ISU-152, a replacement for the SU-152. The 152 mm gun variants were originally designed as infantry support bunker blasters that *could* also fight tanks if need be. The ISU-122 shifts this emphasis to produce a long-range heavy tank destroyer that *could* also blow bunkers to bits if need be. Each could fill in for the other, but there was a shift in emphasis.

      The armor protection of the ISU-152 was fine for a fight at medium range (c. 1000 meters) against infantry supported only by Pak40s or assault guns bearing the Pak40. It's not fine if your opponent is enemy tanks bearing better guns than the Pak40. When the ISU-122 was in the design phase, the Panther and Tiger I had been out for months and the Ferdinand and Kwk43s had come out at Kursk too. I find it strange, as I have said, that the T-34 team kept rejecting what I think were good proposals for improving the T-34's armor which would have made it almost impervious to the Pak40 at everything except maybe the closest ranges, and much more resistant to Kwk36s and Kwk42s, only because it wasn't good enough to stop the Kwk43, while the team working on the ISUs (and the ISU-122 was designed to take on those very same German heavies) were fine with armor that was 'meh' at best against those guns. That does not compute.

    3. Kellomies, we also know that not only hull front was redesigned but also the front of turret did changed its appearence quite well. From different gun shield with shifted left side and gun sight together with it to mayor change of gun attachment in the tank. I think this match up with the time and effort to redesignation of SPGs if it was reccommended over ISU-122 S modification. Change of a gun mount would be the biggest challenge but you would get more internal space if you narrowed the front without cutting from the roof and sides in case even better slope would be needed.

      Stewart Millen, that upper hull plate is hardspot to AP rounds even in case of 'door kicker' but probably not in case of other types of shells. One vehicle got two direct hits there from something which resulted in two holes and lost to Finn hands.

    4. Crabteeth: My reason for saying the part of the "broken nose" IS chassis, which was c. 65 mm thick and sloped at over 70 degrees, would resist even the Kwk43 at close range is found here:

      Specifically, this image:

      Thus 'scuff marks' on the 70 degree sloped plate are from Kwk43 rounds. While they penetrated (as one would predict) the lower plate (100 mm at 31 degrees), and the plate containing the driver's plug vision port (120 mm at 30 degrees), they did not penetrate the 70 degree-plus sloped plate.

      I can't explain the penetrations in the photo you showed, except to say (admittedly questionable from the viewing angle) that the plate penetrated doesn't look like it's anywhere near 70 degrees of slope.