Anti-tank potential and post-war service of self-propelled guns ISU-122 and ISU-122S
In 1942-1943, the USSR produced self-propelled guns armed with a 122-mm M-30 howitzer: the SG-122 and SU-122. The first vehicle was produced on the basis of captured StuG III self-propelled guns, and the second on the basis of a medium tank T-34.
SG-122
Between October 1942 and January 1943, 21 SG-122 self-propelled guns were delivered to military acceptance. Between December 1942 and August 1943, 636 SU-122 self-propelled guns were built.
SU-122
In terms of protection in the frontal projection, the SG-122 and SU-122 corresponded to the T-34 tank. These vehicles with guns that significantly exceeded the destructive effect of 76-mm guns were received better by the troops than the light self-propelled guns SU-76.
After considering the combat experience, it was considered optimal to use the SU-122 to support advancing infantry and tanks when the SPG was located behind them at a distance of 400-600 meters. During the breakthrough of the enemy defense, the SPGs suppressed enemy firing points with fire from their guns, destroyed obstacles and barriers, and repelled counterattacks. Thus, these SPGs repeated the concept of the German "artillery assault". It would have been logical to create a relatively lightly armored SPG with an M-30 howitzer, adapted for firing from closed positions. However, this was not done.
The anti-tank capabilities of combat vehicles armed with the self-propelled version of the M-30 howitzer were low. Even the presence of a BP-460A cumulative projectile with normal armor penetration of up to 160 mm in the ammunition did not allow fighting tanks on equal terms. The projectile, weighing 13,4 kg, had an initial velocity of 335 m/s, due to which the direct fire range was slightly more than 300 m. In addition, firing at fast-moving targets was a very difficult task and required coordinated work of the crew. Three people were involved in aiming the gun at the target. The driver-mechanic carried out approximate aiming with the tracks using the simplest sighting device in the form of two plates. Then the gunners came into work, servicing the vertical and horizontal guidance mechanisms. Given the low rate of fire of a separate-case loading howitzer, for every aimed shot from a 122 mm self-propelled howitzer, an enemy tank could respond with 2-3 shots.
The frontal 45-mm armor of the Soviet self-propelled gun was easily penetrated by German 75- and 88-mm armor-piercing shells at real combat distances, and direct clashes of the SU-122 with tanks were contraindicated for it. This is confirmed by combat experience: in cases where the SU-122 participated in frontal attacks along with linear tanks, they invariably suffered heavy losses. At the same time, with the correct tactics of use, good effectiveness of 122-mm high-explosive fragmentation shells against enemy armored vehicles was noted. According to reports from German tank crews who participated in the Battle of Kursk, they recorded cases of serious damage to heavy Tiger tanks as a result of shelling by 122-mm howitzers.
In addition, during combat operation, a number of significant shortcomings of the self-propelled guns with the M-30 howitzer were revealed. The crews of the self-propelled guns complained about cramped working conditions and heavy gas contamination of the fighting compartment, as a result of which after 2-3 shots, it was necessary to open the hatches for ventilation, which was unsafe on the battlefield. Due to overload, the front rollers of the SU-122 had a low service life, and the mileage before their replacement was 500-600 km. The large forward extension of the recoil devices and their armor obstructed the view from the driver's seat and did not allow for a full-fledged hatch to be installed on the front plate.
Creation and serial production of the ISU-122 and ISU-122S self-propelled guns
Based on the combat experience of the SG-122 and SU-122, it was concluded that the troops needed a better protected self-propelled gun, armed with a powerful gun with a long direct fire range.
Initially, by analogy with the SU-152, the option of creating a vehicle based on the KV-1S heavy tank, armed with a 122-mm A-19 cannon, was considered. But this did not happen due to a shortage of the KV-1S chassis. In turn, the appearance of the ISU-122 based on the IS-2 is largely due to a shortage of 152-mm ML-20S cannons.
When the ISU-122 was launched into serial production, the presence of heavy tanks and self-propelled guns armed with long-barreled 75-88 mm guns was taken into account, in connection with which the Red Army had a need for a self-propelled gun that would surpass German tanks and self-propelled guns in terms of firing range.
At the time work on the ISU-122 began, the only artillery system available in sufficient quantity to arm self-propelled guns was the 122-mm A-19 corps gun, which had very good characteristics for that time.
A modification of the gun known as the A-122S was developed specifically for the ISU-19. The differences between the self-propelled and towed versions were that the aiming devices were moved to one side, the breech was equipped with a tray-receiver for ease of loading, and an electric trigger was introduced. Serial production of the improved modification of the gun, intended for arming the self-propelled guns, began in the second half of 1944. The modernized version was designated "122-mm self-propelled gun mod. 1931/44", and in this version, in addition to the free-tube barrel variety, monoblock barrels were also used. Changes were made to the design of the vertical and horizontal aiming mechanisms aimed at increasing reliability and reducing inertial load. Both guns had a piston breech. The vertical aiming angles ranged from -3 to +22°, horizontally - in a sector of 10°. The direct fire range at a target 3 m high reached 1200 m, the effective firing range at armored vehicles was up to 2500 m, the maximum was 14 m. The rate of fire was 300-1,5 rounds/min. The ISU-2's ammunition complement included 122 separate-case loading rounds.
The ISU-122 entered serial production in April 1944. Early production self-propelled guns had a solid cast frontal hull armor. ISU-122s produced since the fall of 1944 had a frontal hull armor welded from two rolled armor plates with increased shell resistance. These vehicles were distinguished by an increased thickness of the gun mantlet, larger fuel tanks, and the installation of a 12,7 mm DShK anti-aircraft machine gun, which was very useful during urban battles when it was necessary to destroy enemy infantry hiding among the ruins or on the upper floors and attics of buildings.
ISU-122
The mass of the self-propelled gun in the combat position reached 46 tons. The diesel engine with a capacity of 520 hp could accelerate the vehicle on the highway to 37 km/h. The maximum speed on a dirt road is 25 km/h. Cruising range on the highway is up to 220 km. Crew - 5 people. The thickness of the frontal and side armor of the hull is 90 mm, the rear of the hull is 60 mm. The gun mantlet is 100-120 mm. The front of the cabin was covered with 90 mm armor, the side and rear of the cabin - 60 mm. The roof is 30 mm, the bottom is 20 mm.
The frontal armor of the German Pz.Kpfw. V Panther tank could be penetrated by a 122 mm armor-piercing projectile at a distance of 2,5 km. But in terms of anti-tank combat, the A-19S gun had a major drawback - a low rate of fire, which was limited by a manually opened piston-type breech and separate-case loading. The introduction of a 5th member, a lock, into the crew not only did not solve the problem of rate of fire, but also created cramped conditions in the fighting compartment.
The rate of fire was increased by installing a 122 mm D-25S cannon on the SPG, which was created on the basis of the D-25 cannon, which was used to arm the IS-2 heavy tanks. The D-25 had a wedge semi-automatic breech and a muzzle brake.
The introduction of the new 122 mm gun required changes in the design of the recoil devices, cradle and a number of other elements. The D-25S was equipped with a two-chamber muzzle brake, which was absent from the A-19S gun. A new cast mask with a thickness of 120-150 mm was created. The sights and devices remained the same: the telescopic TSh-17 and the Hertz panorama. The crew of the self-propelled gun was reduced to 4 people, excluding the breech. The freed up space was used to place additional ammunition. The convenient location of the crew in the fighting compartment and the semi-automatic breech of the gun contributed to an increase in the combat rate of fire to 3-4 rounds per minute. There were cases when a well-coordinated crew could fire 5 rounds per minute, and in practice, the real combat rate of fire of the self-propelled gun was higher than that of the IS-2 tank.
ISU-122С
However, the ISU-122S was unable to displace in production the vehicles with the “122-mm self-propelled gun mod. 1931/44”, which was explained by the shortage of D-25 guns.
According to archival documents, 1944 ISU-945s were built in 122, of which 169 were lost in combat. By the end of April 1945, the industry had delivered 1335 ISU-122s and 425 ISU-122Ss. In total, 1945 ISU-1735s and 122 ISU-675Ss were delivered before production ended in August 122.
Use of the ISU-122 and ISU-122S self-propelled guns at the final stage of the war
Beginning in May 1944, some heavy self-propelledartillery Regiments previously armed with heavy self-propelled guns SU-152 began to be re-equipped with ISU-122. When regiments were transferred to new organizations, they were given the title of Guards.
The Red Army leadership tried not to mix the ISU-122/122S and ISU-152 within the same regiment or brigade, but there were still cases of both types of self-propelled guns in some units. The different armament of the ISU-122/122S and ISU-152 led to difficulties with ammunition supplies.
In total, 56 heavy self-propelled artillery regiments with 21 ISU-152 or ISU-122 self-propelled guns each were formed by the end of the war (some regiments had a mixed composition). In March 1945, the 66th Guards Heavy Self-Propelled Artillery Brigade was formed (65 ISU-122 and 3 SU-76). Self-propelled guns with 122-mm guns were actively used in the final stages of the war.
In terms of technical reliability, there were no particular complaints about the ISU-122 from the active army. This was largely due to the fact that the main "teething problems" of the engine-transmission group and chassis were identified and eliminated on the IS-2 tanks and the ISU-152 self-propelled guns.
The ISU-122 was fully consistent with its intended purpose. This self-propelled gun was successfully used to destroy long-term fortifications and destroyed enemy heavy tanks and self-propelled guns at a distance of more than 1 km. Due to its significantly greater direct fire range, the ISU-122 had much greater anti-tank capabilities than the ISU-152.
The 53-BR-471 armor-piercing projectile had a fairly high armor penetration. Weighing 25 kg, accelerating in a 5650 mm long barrel to 800 m/s at a distance of 1000 m at the normal, it penetrated 130 mm of armor. At an angle of impact of 60° at the same range, armor thickness of 108 mm was penetrated. The 53-OF-471 high-explosive fragmentation projectile weighing 25 kg, containing 3,6 kg of TNT, demonstrated good results when firing at armored vehicles. German heavy tanks were often disabled after being hit by a 122 mm HE shell. Even if the armor was not penetrated, there was significant damage to the optics and mechanisms, as well as loss of combat capability of the crews as a result of concussion and damage from internal armor chips.
The results of the shelling of a captured German heavy tank Pz.Kpfw.Tiger Ausf.B from a 122-mm A-19 gun at a Soviet training ground, carried out in November 1944, are quite indicative.
The very first hit of a high-explosive fragmentation shell into the frontal armor led to an internal fire, as the fire penetrated through the machine gun mount. The armor-piercing shell, fired from a distance of 500 m, did not penetrate the 150 mm thick frontal armor, installed at an angle of 50°, but a 160×180×150 mm chip occurred on the rear side, and the seam between the upper frontal plate and the roof of the hull burst along the entire length.
The next shot, fired from a distance of 600 m, resulted in a penetration, but it was not counted, since the hit was close to the previous defeat. The armor-piercing shell accepted for counting did not penetrate the armor, but tore the seam between the upper and lower frontal plates. Thus, despite the fact that formally there were no penetrations, in practice there was a defeat. From hits of 122-mm armor-piercing shells, the seams cracked, the armor chipped off, thereby putting the tank out of action even without a penetration. A hit at a distance of 500-600 m in the front of the hull almost certainly disabled the gearbox.
The ISU-122S self-propelled gun with the D-25S gun was a very powerful anti-tank weapon by the standards of the Second World War, but it failed to fully reveal itself in this capacity. By the time the ISU-122S entered service with the Red Army, German tanks were rarely used for counterattacks and were mainly used in defensive battles as an anti-tank reserve, operating from ambushes.
Unfortunately, the potential of self-propelled guns armed with long-range 122-mm guns was not fully utilized during combat operations. Although the ISU-122 had sighting devices for firing from closed positions, this method of firing was episodic. This was due to the fact that the Red Army did not pay much attention to organizing the fire control of self-propelled artillery against visually unobserved targets, and also due to the fact that when firing from closed positions, the ISU-122 was inferior to the towed 122-mm A-19 gun, which had a vertical aiming angle of up to +65° and a maximum firing range of 19 m.
The ISU-122 self-propelled guns usually provided fire support to advancing tanks, moving behind them at a distance of 500-800 m, firing direct fire at enemy fortifications and firing points, destroying defensive nodes, or acting as a long-range anti-tank reserve, capable of fighting the heaviest German tanks and self-propelled guns.
As with the SU-100 tank destroyer, the ISU-122 self-propelled gun drivers also felt cramped in wooded and heavily rugged terrain, as well as on city streets. Maneuvering with a long gun sticking out several meters ahead turned out to be very difficult. In addition, the driver had to be very careful on descents so as not to "scoop up" soil with the gun.
If we do not take into account the increased dimensions of the self-propelled gun compared to a heavy tank, caused by the gun protruding from the fighting compartment, then the mobility of the ISU-122 was approximately equal to the IS-2. At the same time, heavy tanks and self-propelled guns based on them did not always keep up with the T-34-85 medium tanks and SU-100 self-propelled guns on soft soils.
Post-war operation of ISU-122 and ISU-122S
Unlike the IS-2 tanks, which after modernization served in the Soviet Army until the mid-1980s, and were retained as firing points in fortified areas until the mid-1990s, the life of the ISU-122S was not so long. Also in the post-war period, the ISU-152, which had the same base as the ISU-122 and a similar layout, turned out to be more in demand.
In the early 1950s, all ISU-122s armed with the self-propelled version of the A-19 gun were withdrawn to the reserve, where they remained for about 10 years. ISU-122S self-propelled guns with D-25S guns continued to serve in combat units until the mid-1960s.
However, unlike the SU-100 and ISU-152, the ISU-122S that remained in service were not subject to deep modernization. Most of the vehicles did not undergo major repairs in the post-war years. The most radical innovations were the use of new R-113 radio stations and the TVN-2 night driving device, which worked together with the FG-10 headlight equipped with an IR filter.
Unlike the 100mm tank guns of the D-10 family, a large variety of new shells were not created for the 122mm D-25S and D-25T guns in the post-war period.
Components of rounds for the 122 mm D-25S cannon: a cartridge case with a propellant charge, a high-explosive fragmentation projectile OF-471N, an armor-piercing BR-471 and an armor-piercing BR-471B
In 1946, the ammunition of self-propelled guns and heavy tanks armed with 122-mm guns was supplemented with the 53-BR-471B armor-piercing tracer projectile, capable of penetrating 500-mm armor at a distance of 155 m at normal angle. At a distance of 1000 m, armor penetration was 148 mm.
The ISU-122S self-propelled gun remained a very relevant tank destroyer until the mid-1950s, until the USA and Great Britain began mass production of the M48, M60 and Chieftain tanks, against whose frontal armor the 53-BR-471B shell was already weak.
The armor penetration of 122-mm guns used on Soviet heavy tanks and self-propelled guns could have been increased by introducing new subcaliber and cumulative shells into the ammunition loadout, as was done for the 100-mm D-10. But by that time, the Soviet military-political leadership had become too carried away missile technology, and it was considered inappropriate to allocate resources to improve the weapons of “outdated” tanks and self-propelled guns.
A certain number of decommissioned ISU-122s ended up at training grounds and served as targets during testing of new anti-tank weapons and during training shooting.
Self-propelled ISU-122/122S units that had not exhausted their service life were used as chassis in the creation of experimental self-propelled artillery systems of high and special power, as well as for self-propelled launchers of tactical missiles. Units with dismantled guns and welded holes in the front plate of the cabin, known under the general name ISU-T, after major repairs were used as tank tractors, mobile command posts and well-protected vehicles of forward artillery observers. Staff vehicles and mobile observation posts were equipped with R-111 and R-130 radio stations.
ISU-T
In 122, the LKZ design bureau created the BTT-122 tank tractor based on the ISU-1962/1S. The vehicle was intended for evacuating heavy tanks of the IS and T-10 families, pulling out stuck tanks, and restoring damaged (faulty) tanks.
BTT-1
Unlike the ISU-T, the BTT-1 tractor had dampers for pushing a damaged tank using a log, coulters, a platform above the engine-transmission compartment and a crane boom with a lifting capacity of up to 3 tons.
A significant number of ISU-T and BTT-1 were transferred to various civilian departments in the 1960s-1980s, where they were used as tractors or transporters in difficult terrain. Disarmed and re-equipped self-propelled guns were part of the emergency recovery trains of the USSR Ministry of Railways.
Having been installed on railway platforms, they have been preserved in very good condition.
Quite a few ISU-T and BTT-1 have survived to this day, and several tractors have been converted back into fake self-propelled guns. For example, in Bryansk, a tractor with an attached imitation gun stands on a pedestal in the form of a monument.
To be continued ...
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