Where does the ISU-122 fit in?

[lurrp]

So I was reading up on the various Soviet TO&Es to prep for RT, more specifically the Assault Gun Regiments. From what I can tell (correct me if I'm wrong), the 1944 Light AGR has 21 SU-76, the medium has 20 SU-122 or SU-85 plus a T-34, and the heavy has 20 SU-152 or ISU-152 plus an IS-2. Where does the ISU-122 come in, then? Was it just sort of attached to various formations?

[MikeyD]

Off the top of my head, I believe ISU-122 was interchangeable with ISU-152. ISU-122 came about simply because they had an excess of 122mm guns laying about. If I recall correctly ISU-122 unit use and organization was exactly the same as ISU-152. I'm no TO&E grog, that info just happened to fall under my gaze recently.

[JasonC]

The ISU-122 is a heavy and it goes in the heavy regiments. At first those had 16 plus a command vehicle in 4 batteries, later 21 in five plus a command vehicle, like the SU-152.

The ISU-122 gun was a better armor killer than the 152 was. Also had a higher ammo load, and superior range. The HE performance is slightly less, but still awesome. All around, it was simply a superior weapon to the ISU-152.

You would not find them mixed in the same formation, though. You would instead fine uniform regiments of ISU-122s, designated "heavy SU regiment", and typically assigned 1 to a tank corps (late war only, obviously).

[John Kettler]

lurrp,

Here's the short course on the ISU-122/ISU-152. They were breakthrough units and employed the way heavy tanks were in the attack, but they did their best work from ambush.

http://english.battlefield.ru/jsu-122-jsu-152.html

Regards,

John Kettler

[YankeeDog]

Yes; IIRC, by 1944 the Soviets had concluded that the ISU-122 was the superior system due to higher rate of fire and better armor penetration, but there weren't enough 122mm A-19 guns to fit all the tanks, assault guns, and artillery pieces the Red Army wanted, so they continued to produce some assault gun chassis as ISU-152s right through the end of the war.

In Red Army nomenclature, the ISU-122, the ISU-152, and the SU-152 were all considered "Heavy" Assault guns, and "Heavy" AG regiments were almost always homogenous of one of these types (except for the command tank). So you can't tell for sure which of these systems any given regiment was equipped with simply by the "Heavy" designation; you have to dig deeper.

What makes things even more confusing is that a "Medium" Assault Gun Regiment could be SU-85 or SU-122 (note the lack of the "I"). So if you just see the term "122mm Assault Gun", this could mean a Heavy ISU-122, or a Medium SU-122. Same caliber, but different chassis and different gun. The SU-122 was the first Soviet medium assault gun to go into service, and by late 1943 it was already being replaced by the SU-85, which was considered superior even though it had a smaller HE shell because it had a higher rate of fire and much better anti-armor performance. By Mid-1944, proportion of front-line units equipped with SU-85s was decreasing as well, he need for an 85mm Assault Gun having been largely supplanted by the availability of the T-34/85 in large numbers. However, there were still substantial numbers of both SU-122 and SU-85s involved in Bagration and in other combat through the end of 1944, and even into 1945.

The only one that's clear and simple is the "Light" Assault Gun regiments, which would pretty much always be SU-76M by mid-1944. Earlier in the war, some Light Assault Gun Regiments might be SU-76i (captured PzIII chassis converted into a 76mm Assault Gun), but I don't think any of these were still in front line service in 1944.

I don't remember off of the top of my head how all of these are designated in the CMRT editor; for purposes of readability and clarity, the game's designation system may not follow the historical Soviet system exactly.

[Fernando]

Zaloga says that the preferred Assault Gun was the ISU-152. The ISU-122 was done because the 122mm gun was available in sufficient numbers, but the 152mm had more punch against infantry.

The preferred tactic for the ISUs was using them behind the heavy tanks in order to support them and take care of any bypassed enemy positions. Tank destroying was not their primary task.

[YankeeDog]

Zaloga says that the preferred Assault Gun was the ISU-152. The ISU-122 was done because the 122mm gun was available in sufficient numbers, but the 152mm had more punch against infantry.

The preferred tactic for the ISUs was using them behind the heavy tanks in order to support them and take care of any bypassed enemy positions. Tank destroying was not their primary task.

I'm sure Zaloga has his support for this thesis, but production figures argue otherwise; far fewer 122mm A-19s were produced during the war than 152mm ML-20, and in fact towards the end of the war the Soviets began to shift some of the factories that had been producing ML-20s over to produce A-19s, presumably because the A-19 was in higher demand. Further, the A-19 was also needed for IS-2 tanks, while the ML-20 was only used in the SU/ISU-152 (in a motorized, armored mount; both were also used in the traditional towed artillery role). So it's hard to believe that they were producing the ISU-122s as a substitute for ISU-152s because they lacked enough ML-20s. If this were true, why were they converting ML-20 production lines to A-19s?

It's certainly true, though, that neither was really designed as a tank killer; their primary use was direct fire against bunkers and other fortified positions. But ISU-122 was better in the secondary role of tank killer and this counted for something, especially in Mid-1944 when when the state-of-the-art in German armor was beginning to exceed the capabilities of the 85mm gun. Nevertheless, the ISU-152 was preferred for certain specific roles. For example, I have read that the ISU-152 was preferred in urban combat because the long barrel overhang of the ISU-122 made it difficult to maneuver on city streets.

The ISU-152 also had a longer service life post-WWII, but this had more to do with a shift in its primary role from direct fire support to indirect SP artillery, something it was more suited to than the ISU-122.

[JasonC]

For the record, most of the sources I've read are quite clear that the ISU-122 was the superior weapon. I think it is also obvious on the vehicles stats, especially the guns themselves, but also the ammo load.

The guns in both existed in towed form at the start of the war, pretty much. It was only the modified form needed for mounting in an AFV that was scarce later on. The 152s were based on the towed 152mm gun-howitzer and the 122mm on the towed 122mm gun. Gun, not howitzer. That was the standard army level counterbattery piece, with much longer range than the 152 - almost 30 kilometers in the indirect fire role, towed. The same muzzle velocity that gave it that superior range made it a far more effective tank killer. The 152 has a muzzle velocity around 600 meters per second, the 122 around 800 meters per second.

FWIW...

[MikeyD]

This sounds rather like the debate over the 'superior' tank gun. If you imagine killing other tanks to be your primary mission you'd pick one gun, if you think throwing HE downrange was the primary mission you'd pick another. British 17 pounder was really a pretty mediocre gun if the primary mission was throwing HE at the enemy.

[Grisha]

As stated above, the ISU-122/152 (and IS-II) were all breakthrough AFV's, tasked with supporting infantry through the first three lines of the enemy's tactical defense. Such units were assigned to tank armies too, and their use remained largely a form of mobile direct fire artillery.

As an aside, a significant portion of the artillery support in a Soviet front line assault was direct fire. That is to say 76mm, 122mm and even a few 152mm pieces were used. Given the extent of pre-op reconnaissance, their use was not for an extended period as they were usually assigned a specific target (bunker or other ID'ed position). Most indirect fire artillery assets up to army level were centralized and coordinated through the extensive and specialized network of an artillery division.

[Fernando]

Zaloga:

"There was no tactical distinction betweenthe ISU-122 or ISU-152. Both types where kept concurrently in production simply because of the availability of 122mm tubes and ammunition even though the ISU-152 was preferred for most assault roles because of its larger high explosive rounds. The better anti-tank perfomance and longer range of the ISU-122 were seldom take advantage of because of the ype of tactics used bythese units."

The ISU-152 was designd FIRST. The A-19 and ML-20 shared the same carriage an recuperator assembly, so LATER in 1943, Chelyabinsk developed a version of the ISU-152 with a 122mm gun, simply by switching the barrels and changing the internal ammunition stowage.

[MikeyD]

If you want an ISU that halfway compares to Jagdpanther that's got to be the ISU-122s. The late war type with the IS-2 D-25T gun in a big ball mount and nearly twice the ROF of the artillery cannon model. You haven't heard much about ISU-122s but its in the game showing up in September. In September you also get an electric turret drive for your T-34-85. Rotation rate is so fast that testers thought there was a bug until they looked up the stats.

[YankeeDog]

Zaloga:

"There was no tactical distinction betweenthe ISU-122 or ISU-152. Both types where kept concurrently in production simply because of the availability of 122mm tubes and ammunition even though the ISU-152 was preferred for most assault roles because of its larger high explosive rounds. The better anti-tank perfomance and longer range of the ISU-122 were seldom take advantage of because of the ype of tactics used bythese units."

I'm not disputing that this is Zaloga's opinion. I am noting that the production history for the gun systems doesn't support his opinion. If the ISU-152 was preferred, why were they converting ML-20 production lines to A-19 production lines in 1944? If they had wanted more ISU-152s instead of ISU-122s, all they had to do was keep some of those lines producing ML-20s.

The ISU-152 was designed FIRST. The A-19 and ML-20 shared the same carriage an recuperator assembly, so LATER in 1943, Chelyabinsk developed a version of the ISU-152 with a 122mm gun, simply by switching the barrels and changing the internal ammunition stowage.

Certainly true, but neither here nor that as far as which system was preferred. The T-34/76 preceded the T-34/85, but it does not logically follow that the T-34/76 was therefore the preferred system.

I'm sure if were were able to hire a psychic to hold a seance and interview the spirit of every Soviet heavy assault gun regimental commander in WWII, we'd find a range of opinions. Some would prefer the 152mm. Some would prefer the 122mm. Some would be more or less neutral. But at the least, I don't think that Zaloga's thesis that the 152mm was more often preferred should be viewed as proven; I certainly have read at least some primary source opinions to the contrary, and as noted above, there is technical data which runs counter to this thesis.

[JasonC]

Let's see, I can have 33% more muzzle velocity, much better armor penetration, superior accuracy, a higher rate of fire and a 50% larger ammo load, or I can have --- bumpkis. Yeah, I will take the long 122, every day of the week and twice on payday.

[MikeyD]

I wouldn't call an 88 pound HE round (compared to 55 pound for 122mm) 'bumpkis', especially since the beast's *job* is to throw HE.

[Michael Emrys]

I wouldn't call an 88 pound HE round (compared to 55 pound for 122mm) 'bumpkis', especially since the beast's *job* is to throw HE.

On the other hand, either one is substantially more powerful than the ~75mm round that by this stage of the war was more or less the standard for most AFVs. I suspect that there wasn't a great deal of dissatisfaction with a 55 pound HE round, and the superior AT performance of the 122L was appreciated on those (rare? I don't know) occasions when it was called for. Not to mention the higher ROF and larger load out. Both were impressive weapons that I would not have wanted to see coming my way. I don't doubt that the 152 would have been better for some jobs, but the 122L offered greater flexibility for more jobs.

Michael

[JasonC]

MikeyD - what lives through a direct hit from a 122mm HE shell again? Also, you get 50% more rounds and a higher rate of fire.

[MikeyD]

50% more rounds... each of which contain roughly 50% less HE. So it sounds like twice as much effort to put the same amount of HE on the target.

[Fernando]

Zaloga again:

“In November 1943 they (Note: “they” were an IS-100 and an IS-122, the up gunned versions of the 85mm IS-1) were put through firing trials at NIIBT in Kubinka. A captured Panther was used as the target and a 122mm round crashed through the frontal armor and clear through the rear armor as well. However the 100mm gun had better armor penetration (185mm at 1,000m as compared to 160mm for the 122mm gun). But the 122mm gun was selected (note: for the IS-2) because there was a surplus of 122mm tube production facilities and an adequate ammunition base, while the opposite was true for the 100mm gun.”

I other words, the 122mm A-19 gun was selected for the IS-2 not because it was the best performer, but because it was available in more than sufficient numbers AND the ammunition could be easily supplied. I guess, and Zaloga says, it happened the same for the ISU-122. The preferred gun was the 152mm, but it was possible to increase ISU production by using another quite good gun and make good use of already available resources (122mm gun facilities and ammo production ), so let’s put the 122mm gun on the ISU. This way we will have more ISUs on the field. Some of them won’t the desired 152mm version, but we will make good use of the 122mm gun production capabilities.

It looks like a typical Soviet decision, doesn’t it?

[Fernando]

I'm not disputing that this is Zaloga's opinion. I am noting that the production history for the gun systems doesn't support his opinion. If the ISU-152 was preferred, why were they converting ML-20 production lines to A-19 production lines in 1944? If they had wanted more ISU-152s instead of ISU-122s, all they had to do was keep some of those lines producing ML-20s.

The A-19 was needed for the IS-2 tank which was in great demand after its debut. It was so demanded that in April 1944 the NKTP ordered construction of a new assembly facility at Tankograd to increase IS-2 production.

I guess it made sense to shift some 152mm production to the 122mm gun production in order to ensure adequate supply in excess of the needed numbers for the IS-2 , then use the surplus guns for the ISUs. They would share the same ammo, so supply it to the units would be easier as well.

However it doesn’t mean the 122mm gun was the desired one for the ISU or the IS. A 100mm gun was better for the IS-2 and a 152mm for the ISU, but they mounted the 122mm one in the end because it was the available gun and had a more than adequate performance. The decision was easy because the A-19 was not a bad gun at all, but most of the reason for choosing it were purely logistical.

[womble]

On the other hand, either one is substantially more powerful than the ~75mm round that by this stage of the war was more or less the standard for most AFVs.

And both remain more powerful than the 105mm that had been being used by the Germans for a year in the StuH and was coming into use by the Allies in the Sherman 105, as well.

[lurrp]

So will time period and unit quality affect the type of assault gun fielded, or is it just random? Or can you choose?

[Michael Emrys]

So will time period and unit quality affect the type of assault gun fielded, or is it just random? Or can you choose?

I would expect that we would continue to be able to choose. But note that cost and rarity might be different for the different models.

Michael

[Dadekster]

When I first saw the question of the OT I though it was a variation of the following joke.

Q- Where does a 800lb. gorilla sit in a movie theater?

A- Wherever it wants.

[Amedeo]

It's obvious that, during WW2, the ISU-122 (especially the ISU-122S) was better suited to anti-tank operation than its 152mm counterpart. But during the post-war era, when the necessity to have the heavy assault guns doubling also as stopgap TDs waned, the ISU-152 was the preferred vehicle. It was mantained in production longer than the ISU-122, it was used in frontline units until the early '70s (when replaced with the new generation of SP guns) and it was subject to extensive rebuilds and modifications (the ISU-152K and ISU-152M variants, the latter sporting also IR sights).