ISU-122

The ISU-122 (Istrebitelnaja Samokhodnaya Ustanovka 122) was a Soviet self-propelled gun used during World War II.

History
A prototype of the ISU-122 (in Russian ИСУ-122) heavy self-propelled gun was built at the Chelyabinsk Kirov Plant, (Chelyabinskiy Kirovskiy Zavod (ChKZ), Chelyabinsk, Russia), in December 1943. The design shared the chassis of the ISU-152 self-propelled gun and differed only in armament, having an A-19S 121.92-mm gun as its main weapon instead of the ISU-152's ML-20S gun-howitzer. Towed versions of these guns used the same carriage: 52-L-504A (Russian designation 52-Л-504А), so installation of an A-19 instead of an ML-20 gun was not a difficult task. After completing development of the ISU-152, ChKZ engineers mounted the A-19 gun on the ISU-152 chassis to create "Object 242" — the first ISU-122 prototype. It was successfully tested but not immediately launched into mass production.

At that time all ISU hulls were being equipped with the ML-20S gun-howitzer, but the production of hulls increased quickly and there was a lack of ML-20S tubes in the beginning of 1944. State authorities ordered these uncompleted hulls armed with an A-19 gun (specifically with the A-19S variant, slightly modified for the self-propelled gun mount). A further advantage of rearming the ISU was increased direct fire range against heavy German tanks. For these reasons the State Defense Committee adopted Object 242 for Red Army service as the ISU-122 on 12 March 1944. In April 1944 the first series ISU-122 left the ChKZ production lines.

The A-19S gun had a manual-piston breech, which reduced the rate of fire to 1.5 from 2.5 shots per minute. Soviet designers developed the D-25 by modernizing the A-19S's breech, creating a semi-automatic variant of the 121.92-mm gun. The D-25 gun was installed in IS-2 tanks as a priority, but in September 1944 became available for self-propelled guns. The prototype ISU vehicle, armed with a D-25S was designated "Object 249" and successfully passed plant and state testing. The fire rate was improved to 2 to 3 shots per minute and with two strong experienced loaders the rate of fire reached 4 shots per minute. Due to the muzzle brake reducing recoil forces the D-25 had a smaller recoil buffer than the A-19. This improved the crew's work conditions and allowed for a smaller, lighter gun shield with the same armour thickness.

After testing Object 249 was immediately launched in mass production as the ISU-122S (ИСУ-122С) self-propelled gun. However, the original ISU-122 remained in production (along with the ISU-152) due to a large stock of A-19 guns (the ML-20 and D-25 came directly from artillery factories). Mass production of the ISU-122 and ISU-122S ceased at the end of 1945. ChKZ produced 1,735 ISU-122 and 675 ISU-122S variants in total.

After World War II, many surviving ISU-122s were rebuilt as rocket launchers, very large calibre gun chassis or supply vehicles. The small number of ISU-122s which kept their original armament were modernized in 1958. This modernization was not as complete as that of the ISU-152. Most ISU-122s did not receive a new engine, only upgraded gun sights and radio sets. In the beginning of 1960, the ISU-122 was withdrawn from Soviet Army service (the ISU-152 served much longer). Some disarmed ISU-122s were transferred to civil organizations, to be used as emergency vehicles on Soviet railways or as tracked transport in Arctic areas of the Soviet Union.

Variants
Some vehicles were fitted with a 12.7 mm DShK anti-aircraft machine gun.

In later models the A-19 gun was modified to have a semi-automatic breech block. This gun was designated D-25S, and the vehicle mounting it ISU-122S. The modification increased rate of fire from 1.5 rounds/minute to 3 rounds/minute. The ISU-122S variant is recognizable by its ball-shaped gun mantlet and double-baffle muzzle brake.

Construction and design
Construction of the ISU-122 and ISU-152 is the same except for the gun mounting, sights and ammunition stowage. The A-19S or D-25S cannons of the ISU-122 had 18 degrees elevation angle and 30 rounds of ammunition (the ISU-152 had 20 degrees and 20 rounds respectively). The A-19S cannon was equipped with an ST-18 (СТ-18) telescopic sight and the D-25S cannon was equipped with a TSh-17 (ТШ-17) sight. Both types of sights had maximal exact targeting distance of 1.5 km (the ISU-152 could manage only 900 m). Maximum direct fire range of the A-19S or D-25S cannons was 5 km, much further than these sights' targeting abilities. For direct or indirect firing on distances over 1.5 km the gunner used the second, panoramic sight.

The crew of the ISU-122 was 5 men and their location and roles were identical to those of ISU-152 crewmen. The ISU-122S was used with either 4 or 5-man crews since the semi-automatic breech allowed for a reasonable fire rate with a crew of four; the place of the absent loader was often taken up with additional ammunition. With a five man crew (two loaders) the ISU-122S demonstrated a better fire rate than with four men.

Organisation
The organisation of military units utilizing the ISU-122 was the same as with the ISU-152. Soviet Army commanders tried not to mix the ISU-122 and the ISU-152 in one regiment or brigade, although there were some units equipped with both types. The different armament of the ISU-122 and the ISU-152 caused problems with ammunition supplies and logistics. Another disadvantage of mixed-type units was a doubling of the calculations required for massed indirect fire.

Combat history
The ISU-122 was used as a powerful assault gun, a self-propelled howitzer, and a long-range tank destroyer, the same as with the SU-152 and ISU-152 heavy self-propelled guns in general. However these vehicles differed in their combat use. The primary application of the ISU-122 was as a tank destroyer, while the SU/ISU-152 tended more towards the assault gun role. With the same hull as the IS-2 the ISU-122 had good armour performance but more importantly very good performance with high-explosive rounds. The 121.92-mm gun had great potential, although the gun's abilities were somewhat reduced by the available projectiles and its lack of accuracy. In 1944 the BR-471 was the sole armour-piercing round available (the Germans had armour piercing rounds also, including ballistic nose and sub-caliber variants). An improved version, the BR-471B (БР-471Б) was developed in early 1945, but was available in quantity only after World War II ended. In extremis the ISU-122 engaged enemy heavy armour with OF-471 (ОФ-471) high explosive projectiles. These shells had a mass of 25 kg, a muzzle velocity of 800 m/s, and were equipped with a 3 kg TNT charge. Mechanical shock and explosion was often enough to knock out enemy AFVs without any armour penetration.

For urban combat ISU-122's were utilized as assault guns, but with a lower efficiency in comparison to the SU/ISU-152. In general Red Army commanders viewed the ISU-122 as a good assault gun. The OF-471 projectile was powerful enough against unprotected and entrenched infantry, pillboxes, and fortified buildings. In urban combat the long barrel of the 121.92-mm cannon sometimes made maneuvering difficult.

Use of the ISU-122 as a self-propelled howitzer was rare, although its maximum range of fire exceeded 14 km. Usually the ISU-122 delivered indirect fire to the enemy during rapid advances when support from towed artillery was not available.

Surviving examples
Most of the ISU-122s produced survived the Second World War and examples remain, but only a few of the surviving ISU-122s kept their initial appearance. Many were rebuilt or scrapped in the mid 1960s. The number of ISU-122s preserved in museums and memorials is substantially lower than surviving ISU-152s. ISU 122s can be seen in the museums of the Russian Central Artillery and Engineer Troops in St Petersburg, the Kubinka Tank Museum, the Central Museum of the Russian Army, the Museum of the Great Patriotic War, Kiev and other tank museums and World War II memorials in Poland and Belarus.