ISU High Power Gun Projects

Author: Will Kerrs

Site: Tanks-Encyclopedia

Dedicated Beast Slayers

http://s44.radikal.ru/i104/0906/17/2fe7db9005ac.jpg

In mid-1944, the Red Army recognized that it might need tanks that could consistently and reliably destroy the Wehrmacht’s most well-armored tanks. The Red Army fielded few tanks that could destroy the King Tiger, Elefant, and Jagdtiger reliably from medium-long ranges. Although it is true that the ISU-122, ISU-122s, ISU-152, and IS-2 were capable of destroying German heavy tanks, their combat results were not consistent enough. As a result, starting in June of 1944, five “BM” (“High Power” – Russian: “высокой мощности”) guns were developed for the ISU chassis. The resulting vehicles were: Object 243 (ISU-122-1 with the 122mm BL-9 gun), Object 246 (ISU-152-1 with the 152mm BL-8 gun), Object 247 (ISU-152-2 with the 152mm BL-10 gun), Object 250 (ISU-130 with the 130mm S-26 gun), and Object 251 (ISU-122-3 with the 122mm S-26-1 gun). More gun projects were being developed at this time, but no others appeared to be mounted onto a chassis. The guns proved capable, on paper, of destroying tanks such as the Jagdtiger, but testing showed that they were simply not practical. Moreover, these projects took well over a year to refine, and seeing as though the war ended before they were complete, they were all dropped.

A table with comparative statistics is provided at the bottom of the article.

Context: Soviet guns against German armor

Mythbusting: The SU-152 and ISU-152 were “Beast Killers”

Soviet wartime propaganda suggested that the SU-152 and ISU-152 (SU being based on the KV chassis, ISU being based on the IS chassis) were “Beast-Killers” because they could destroy Panthers, Tigers, and Elefants. The ISU-152’s 152mm ML-20S howitzer was, indeed, capable of destroying heavy German armor, but this required a direct hit with an High Explosive (HE) shell. Such a direct hit could do one of three things to disable the tank: destroy the vehicle’s drive systems, kill its crew, or blow the turret / casemate / hull open (or even clean off, in the case of turrets). Armor Piercing (AP) and Concrete Piercing shells were developed, but these were expensive and complicated to make, hardly more effective than HE rounds, and thus were scarcely supplied – even at Kursk! However, the ISU-152 was not a dedicated tank destroyer – it was an assault gun designed for bunker busting and indirect fire. Needless to say, using an assault gun as a tank destroyer was risky business.

Firstly, the gun would need to be fired at short ranges against enemy tanks. This is because the ML-20S was a fairly low velocity howitzer, which would simply not be accurate enough to engage tanks from distances. Consider also that the vehicle had a maximum of 90 mm of armor, which meant that it whilst it was adequately protected from some German guns at long ranges, it simply was not thick enough to protect the vehicle in the short ranges it would need to operate in as a tank destroyer. For example, the 7.5 cm KwK 40 L/48, as mounted on StuGs, Panzer IVs, and Jagdpanzers, could penetrate 97 mm of armor at 500 m, and 87 mm of armor at 1000 m, at 30 degrees (the ISU-152 casemate was barely sloped at all). Also consider that the vehicle was simply not mobile enough to be engaging more nimble German tanks, and could probably be outmaneuvered anyway.

Yet another major issue of using the ISU-152 as a tank destroyer was that it could only manage 1-3 rounds per minute (depending if it had one or two loaders, and how experienced they were). This meant that in any type of ‘duel’, the ISU-152 would not only need to fire first, but also guarantee a hit, or practically any opposing German tank could get numerous shots off against it – as mentioned early, likely knock-out blows. Having said this, it did not seem to be a consideration taken into account when making the “BM” projects, as the guns all had equally as poor rates of fire.

In conclusion, the ISU-152 did not live up to its legendary name. Other Soviet field guns and tank destroyers had somewhat better results compared to the ISU-152, although the results were still not quite satisfactory.

Mythbusting: The 122 mm A-19S and D-25T / S were sufficient

It is a commonly held belief that the 122 mm D-25T / S and A-19S were sufficient at destroying the heaviest German armor. This belief is somewhat problematic, given the weight of the evidence.

According to a Wa Preuf 1 (a Wehrmacht weapons research facility) report from October 5th, 1944, the 122mm A-19 (the A-19S being used on the ISU-122, the latest Soviet SPG at that time) could not penetrate the upper glacis of a Panther. However, it could penetrate the lower glacis from a distance of up to 100 m, the mantlet from 500 m, and the side of the turret from 1500m. This was still somewhat wanting, as the Red Army would prefer to engage such tanks from longer ranges, to prevent heavy losses of their own tanks.

The improved 122mm D-25T (which was used on the IS-2, the D-25S was essentially the same, and was used on the ISU-122S) seems to have fared much better against German armor. Testing of the gun on the IS-2 platform in Kubinka in 1944 suggests that a King Tiger’s turret (likely the side) could be penetrated from up to 1000-1500m. The welds of front hull seams could also be penetrated from 500-600m. Whilst these penetration statistics might make the D-25T sound more promising, they must be taken with some caveats.

Firstly, to score such hits would require a very skilled and very experienced gunner – especially to score a hit on the turret from a range of up to 1500m. Secondly, similar to the ML-20S, the D-25T could only manage up to 3 rounds per minute. Thirdly, the validity of these statistics has been called into question, because they come from Soviet sources. In the past, these statistics were often exaggerated for propaganda purposes. Finally, the very fact that the ISU “BM” projects were put into production suggests that the Soviets knew that the D-25T would not give consistently reliable results in AT duties.

Whilst, indeed, the D-25T was, in theory, capable of destroying the heaviest German armor, it was perhaps not as reliable as it needed to be in the field. Of course, it is true that penetrations were not required to disable the tank or kill the crew (both in the case of the ML-20S and D-25T), but one could not simply rely on non-penetrating hits to, in some manner, disable the tank.

As a result of these relatively unsatisfactory Soviet AT capabilities against the Wehrmacht’s heaviest tanks, in June 1944, Zavod Nr. 100 began developing new, high velocity 122 mm, 130 mm, and 152 mm guns to be mounted on the ISU (and perhaps IS and KV) chassis.

Object 243 (ISU-122-1)

Object 243 (ISU-122-1) with the 122 mm BL-9. This vehicle is distinguishable as its gun has no muzzle brake, and looks like an elongated A-19S. However, the gun replicator has been angled (see the rectangular plate on the mantlet below the gun), unlike a regular ISU-122.

• The Object 243 featured the 122 mm BL-9 gun – one of the infamous BL guns made at OKB-172. The vehicle can be distinguished by its gun and mantlet. The gun essentially looked like a longer version of the A-19S. The mantlet also had some tweaking to fit the longer and heavier gun – most notably, the tip of the gun replicator has been angled to one side (just below the gun).
• It could penetrate 204 mm of armor at 1000 m, with 2 rounds per minute.
• The gun’s muzzle velocity was 950 m/s with an 11.9kg AP shell.
• It had a range of 10,700 m, compared to the 6000 m range of the 152 mm ML-20S of the ISU-152 and SU-152.
• It could carry 20 AP rounds, the same as the ISU-152.
• Like the other “BM” guns, the BL-9 was likely too powerful for its mountings, which caused mechanical issues.

Object 246 (ISU-152-1)

Object 246 (ISU-152-1) with the 152 mm BL-8. This vehicle is distinguishable by its slightly longer gun than the Object 247 (15 cm, or 5.9 inches), but the same muzzle brake, and its unchanged gun replicator.

• The Object 246 featured the 152 mm BL-8 gun. This vehicle can be distinguished by its distinctive muzzle brake, and unaltered gun replicator.
• It could reportedly penetrate 203 mm of armor at 90 degrees from up to 2000 m away (dubious) ¹, with 3 rounds per minute.
• The gun had a muzzle velocity of 850 m/s with a 43.56 kg HE shell.
• It had a maximum range of 18,500 m.
• It could carry 21 rounds.
• Whilst these results sound excellent, trials in December 1944 showed that the crew found operating the gun difficult, the muzzle brake and breech block were unreliable, and the barrel strength and angle of horizontal guidance were unsatisfactory. Consider also that the very long gun would limit the maneuverability of the vehicle, much like the D-25S on the ISU-122S limited its maneuverability. As a result, the 152mm BL-10 was developed…

Object 247/ISU-152-2 by David Bocquelet

Object 247 (ISU-152-2)

Object 247 (ISU-152-2) with the 152 mm BL-10. This vehicle is distinguishable by the length of its barrel (it was slightly shorter compared to the BL-8), its muzzle brake, and its altered gun replicator.

• The Object 247 fitted the 152 mm BL-10 gun, an improvement of the BL-8. This vehicle can be distinguished by its muzzle brake and slightly altered gun mantlet, whereby the rectangular tip of the gun replicating system had been angled, unlike the original ISU mantlet.
• It could penetrate 205 mm of armor from 1000 m.
• The gun had a muzzle velocity of 851 m/s with a 43.56 kg HE shell.
• It had a maximum range of 17,000 m.
• It could carry 20 HE shells.
• Testing revealed that barrel integrity and angle of horizontal guidance were poor.
• It was eventually deemed that there was no need for this work to continue, mostly because the war was over, and there was no need to combat heavily armored German vehicles.
• Consider also, that whilst the barrel length was a little shorter than the BL-8, it, too, would still suffer from maneuverability issues as a result.

Object 250 (ISU-130)

Object 250 (ISU-130) with the 130 mm S-26. This vehicle is distinguishable by its unique muzzle brake.

• The Object 250 (ISU-130) was built in autumn, 1944 and featured a 130 mm (5.12 in) S-26 gun. This gun is sometimes referred to as a naval gun, but this is not entirely accurate – the S-26 derived from a naval gun and featured a rectangular muzzle brake and horizontal wedges.
• It could penetrate 196 mm of armor from 1000 m.
• It had a muzzle velocity of 702 m/s, firing a 33.4kg HE shell, with 1.5-2.5 rounds per minute.
• It had a range of 15,000 m.
• It could carry 25 shells, which were smaller than 152mm shells, meaning that it provided similar ballistic results to the 152mm BM guns, but could carry more shells.
• In October 1944, the ISU-130 underwent factory trials, and the following month, trials were held at the Polygon.
• A major concern came from the caliber – 130mm. The issue was that the army would have to make special arrangements for the 130mm naval shells to be supplied to the army, and thus it was decided that a gun using current army-issue 122mm or 152mm would be preferable.
• Testing of the ISU-130 ended in 1945, and the gun was sent to the TaSKB for completion, but the war was over, and the project was disbanded.
• The ISU-130 is currently preserved at the Kubinka Tank Museum.

The ISU-130 preserved at Kubinka.

Object 251 (ISU-122-3)

Object 251 (ISU-122-3) with the 122 mm S-26-1. This vehicle is distinguishable by its round gun mantlet and its unique cylindrical muzzle brake.

• The Object 251 was derived from the ISU-130. It featured essentially a 122 mm version of the 130 mm S-26, which was designated the S-26-1. It had a round muzzle brake, different components, but the mantlet was the same shape.
• It could penetrate 204 mm of armor from 1000 m.
• It had very similar ballistics to the BL-9, but had a muzzle velocity of 1000 m/s, firing a 25kg shell.
• It could fire a disappointing 1.5-1.8 rounds per minute.
• It underwent field tests in November 1944, but according to sources, something (probably the mantlet and / or gun mechanism), was simply not strong enough to withstand firing the gun.
• The gun project was totally completed in June 1945, but was abandoned due to the war’s end.
• (Note that the ISU-122-2 was the ISU-122S with the 122 mm D-25S, hence the skip from ISU-122-1 to ISU-122-3).

Conclusion

The “ISU High Powered Gun Projects” were, in many respects, a failure. True, the guns were incredibly potent, particularly in the case of the S-26-1, which could penetrate 204 mm from 1000 m. They also had a very long range, only limited by the elevation of the ISU mantlet. However, they were simply not practical and mechanically reliable enough for their intended purpose, which was to knock out the thickest armored Wehrmacht tanks consistently, and from long ranges.

Even if the vehicles were put into serial production, how often these tanks would face off with the most heavily armored vehicles of the Wehrmacht is questionable. With Jagdtigers, King Tigers, and Ferdinands being so rare, the war was more likely to have ended before the ISU High Powered Projects saw combat with the vehicles they were designed to destroy. What had ultimately put the nail in the coffin for these guns was that the war had ended, and they were no longer necessary.

Sidenote: Designations and identification through photos

In the writing of this article, it has been exceptionally difficult to pin down which photos correspond to which project. Indeed, some sources only mention four (in some cases, only three) High Powered Gun Project vehicles. It has been the author’s conclusion that there were five such High Powered Gun Projects mounted onto ISU chassis, as outlined in Solyankin’s book “Советские тяжелые самоходные артиллерийские установки 1941-45″. Most online sources, particularly non-Russian language sources, are incredibly inaccurate. Gun statistics were mostly provided by a Soviet data set as provided by Tankarchives.blogspot. However, these statistics sometimes differ with Solyankin.

	BL-9	BL-8	BL-10	S-26	S-26-1	ML-20S	D-25S	12.8cm PaK 44 L/55
Chassis	Object 243	Object 246	Object 247	Object 250	Object 251	ISU-152, SU-152	ISU-122S	Jagdtiger
Caliber	122 mm	152 mm	152 mm	130 mm	122 mm	152 mm	122 mm	128 mm
Penetration@90 deg	204 mm from 1000 m 157 mm from 1500 m 155 mm from 2000 m	203 mm1 from 2000 m (Dubious)	205 mm from 1000 m 202 mm from 1500 m 160 mm from 2000 m	196 mm from 1000 m 184 mm from 1500 m 156 mm from 2000 m	204 mm from 1000 m 157 mm from 1500 m 156 mm from 2000 m	125 mm from 500 m	147 mm from 500 m 138 mm from 1000 m and 129 mm from 2000 m	200 mm from 1000 m 30 degree angle PzGr.43 shell
Rounds per Minute	2	3 1	No data 1	1.5-2.25	1.5-1.8	1-3 2 (crew dependent)	1-3 2,3 (crew dependent)	2
Muzzle velocity	950m/s1(AP) 700 m/s (HE)	850 m/s (presumably HE) 1	826 m/s (AP) 851 m/s (HE)	898 m/s (AP) 702 m/s (HE)	1000 m/s (AP and HE)	600 m/s (HE)	800 m/s (HE)	950 m/s (AP)
Range	10,700 m	18,500 m 1	17,000 m	15,000 m 1	15,000 m 1	6000 m	5000 m	24,410 m
Shell Weight and Type	11.9kg AP	43.56kg HE	43.56kg HE	33.4kg HE	25kg HE	43.56kg HE	25kg HE	28kg HE 28.3kg AP
Ammo Capacity	20	21 1	20	25	24	20 (ISU-152)	30 1, 3	38-40
Overall length, chassis included	11.15 m (36.58 ft)	11.82 m (38.78 ft)	11.67 m (38.29 ft)	11.42 m (37.47 ft)	11.26 m (36.94 ft)	9.18 m (30.12 ft)	9.85 m (32.32 ft)	10.65 m (34.94 ft)

Notes:

¹ – Note 1 – Most data has been obtained from this Soviet data set. However, certain statistics have not been given and have been obtained from alternative (potentially dubious) sources. Where necessary, statistics provided by Solyankin have been used, but these are questionable.

² – Note 2 – Experienced crews were able to load much faster. Consider that the BM guns were likely tested by experts in test ground conditions, thus likely making their rounds per minute data higher than would be in the field, by more typical tank crews.

³ – Note 3 – Obtaining the ISU-122S’ ammo capacity has been difficult. It is reported that with a crew of four, instead of five (IE, one loader instead of two), then ammo capacity was increased, but the only given figure is 30, presumably with two loaders.

Sources:
“Germany’s Panther Tank: The Quest for Combat Supremacy”, by Thomas Jentz
“Sturmgeschutz & Its Variants”, by Walter J. Spielberger
“Советские тяжелые самоходные артиллерийские установки 1941-45”, by A.G. Solyankin
battlefield.ru
battlefield.ru (second page)
forum.axishistory.com
tankarchives.blogspot
tankarchives.blogspot (second page)
tankarchives.blogspot (third page)
armchairgeneral.com forums
WW2 AT Penetrations.pdf
alternatewars.com
alternatewars.com (second page)
ftr.wot-news.com
3.bp.blogspot