ISU-152M

The beginnings of the ISU-152 came on January 24, 1943, when the first prototype of the SU-152 was unveiled. This was a fully enclosed 152mm gun-howitzer on the KV-1S tank chassis. It was designated *Объект 236* ("Project 236") . Project 236 was completed in Factory No. 100 in Chelyabinsk, and was tested successfully from January 24 to February 7, 1943. On February 14 the vehicle was adopted and put into production under the **KV-14** (*кв*-14) designation; in April 1943 the designation was changed to SU-152" (*СУ*-152).

Although the SU-152 was successful in combat, production of the KV-1S tank chassis was ending, which made the modernisation of the vehicle necessary, using the new IS tank chassis. On May 25, 1943, the administration of Factory No. 100 ordered the modernisation of the SU-152, which included increased armor protection and other improvements. Development began in July 1943, under the supervision of Josef Kotin, the chief designer of Soviet heavy tanks, and G. N. Moskvin as the main designer.

The new design, designated **IS-152** (*ИС*-152), was tested from September to November, 1943. Testing revealed a large number of deficiencies, which sent it back for further improvement. On November 6, 1943, an order was issued for adoption of the improved variant, under the **ISU-152** (ИСУ-152) designation, and in December production began at the Chelyabinsk Kirovsk Plant, replacing the SU-152.

## Design

Positions of crewmembers in the ISU-152: 1. Driver 2. Commander 3. Gunner 4. Breech operator 5. Loader.
On April 15, 1942 at the plenum of the artillery committee, where the development of assault guns for support of the infantry was discussed, the necessity of also developing assault guns capable of destroying fortified positions was acknowledged. It was intended these assault guns would be armed with a 152.4 mm gun-howitzer and used for penetration of the enemy defence in the offensive operations planned for 1942–1943. This resulted in the development of the Object 236, and eventually the SU-152, which concept was later continued and further developed with the ISU-152.

The ISU-152 followed the same design as most other Soviet self-propelled guns. The fully armoured hull was divided into two compartments: fighting compartment for the crew, gun and ammunition in the front of the hull, and engine and transmission in the rear. The gun was mounted slightly to the right of centre with a limited traverse of 12 degrees left and right. The crew consisted of four or five men placed in the superstructure. Three of the crew were to the left of the gun: driver to the front, then gunner and last the loader. The vehicle commander and lockman (who operated the gun breech) were to the right: commander to the front and the lockman behind. When the crew consisted of four men, the loading was carried out by the lockman.

The suspension consisted of twelve torsion bars for the six road wheels on either side. The drive sprockets were at the back, and the front idlers were identical to the road wheels. Each track was made up of 90 links. There were three internal fuel tanks, two in the crew area and one in the engine compartment. These were usually supplemented with four unconnected external fuel tanks. 12- and 24-volt electrical power supplies came from a 1 kW generator feeding four accumulator batteries.

For observation from the interior, all roof hatches had periscopes and there were two gun sights: telescopic ST-10 (СТ-10) and panoramic. For crew communication a TPU-4-BisF intercom was fitted, and for inter-vehicle communication there was a single 10R or 10RK radio. These were better than Soviet equipment at the start of the war but still inferior to German equipment.

The crew were given two PPSh submachine guns with 1491 rounds and 20 F-1 grenades for short-range self-defence.

The ISU-152 was armed with the same gun as the SU-152, but it used the hull of the IS-1 tank instead of the KV-1S. Later in the war the ISU-152 was further improved. It used the hull of the IS-2 or IS-2 model 1944 tank, the armour of the mantlet was increased, the gun was replaced by newer variants, a 12.7×108 mm DShK anti-aircraft machine gun was installed by the right forward hatch and later its ammunition capacity increased, the 10R radio set was upgraded to a 10RK and the fuel capacity was increased.

Some ISU-152s were equipped with even larger external fuel tanks, two tanks on the rear hull deck, in addition to the four external fuel tanks — 90 litres (20 imp gal; 24 US gal) each, maximum — or with two smaller additional external fuel tanks, on the hull rear. This option was probably available for the post-war ISU-152 variants.

Between December 1943 and May 1945, 1,885 ISU-152s were built. Mass production ceased in 1947, with 3,242 vehicles produced in total.

Post-war ISU-152 modernisation included the installation of night vision sights, and the replacement of the V-2IS engine with the V-54K, the 12.7mm machine gun was replaced by a newer variant, the ammunition capacity increased to 30 rounds, additional armor, automotive improvements, and significant increase of the main fuel capacity.

## Variants

The initial variant was developed in 1943. The factory designation was Object 241 (Объект 241). It was armed with the 152.4 mm ML-20S (МЛ-20С) model 1937 gun-howitzer, with a barrel length of over 4.2 metres (27.9 calibers). This gun had a maximum range of 6,200 metres (6,780 yd). The armor-piercing round, weighing 48.78 kilograms (107.5 lb), had a muzzle velocity of 600 metres per second (1,969 ft/s) and a maximum penetration of 125 millimetres (4.9 in) of rolled homogeneous armour (RHA) at 90° at a range of 500 metres (547 yd). The rate of fire was 2-3 rounds/min. The ISU-152 carried 21 rounds of two piece (shell and charge) armor-piercing and high explosive ammunition. Later ISU-152 versions had a newer gun with a slightly longer barrel, up to over 4.9 metres (32.3 calibers), with a maximum range of fire of up to 13,000 metres (14,217 yd).

The ISU-152 had different versions, with changes in the gun (later versions), the number of hatches, or the hull (based on the hull of the IS-1, IS-2 or IS-2 model 1944 tanks). The IS-2 based versions had a thicker gun shield and larger fuel tank. Until May 1944, the main armament was the 152.4 mm ML-20 model 1937 gun-howitzer. The ISU-152 had a rate of fire of 2-3 rounds/min. The early versions had three hatches in the superstructure roof and one emergency hatch at the bottom of the hull behind the driver's seat, which had an armored cover. Later versions had a fourth, round hatch in the superstructure roof on the right, next to the rectangular hatch on the left.

### ISU-152-2

One prototype, developed in 1944. In April 1944, in attempt to increase the firepower of the ISU-152, a high-power variant of the self-propelled gun was developed in Factory No. 100, designated ISU-152BM (ИСУ-152БМ), sometimes referred to as ISU-152BM-1 or ISU-152-1. The factory designation was Object 246 (Объект 246). The "BM" ("БМ") in the designation stands for "High-Powered" ("Большой Мощности"). The main purpose of the ISU-152BM was to fight against heavily armored tank destroyers such as the Elefant and the Jagdtiger. It was armed with the 152.4 mm BL-8 (БЛ-8) long-barreled gun, which unlike the ISU-152's gun was not a gun-howitzer. The gun had a maximum range of 18,500 metres (20,232 yd), with the 43.56-kilogram (96.0 lb) high-explosive shell which had a muzzle velocity of 880 metres per second (2,887 ft/s). The overall length of the gun was over 8 metres (26 ft), with a barrel length of 7,620 millimetres (300 in) (50 calibers). The armor-piercing round, weighing 48.78 kilograms (107.5 lb), had a muzzle velocity of 850 metres per second (2,800 ft/s). During test firing at armour plates with different thicknesses, the ISU-152BM successfully penetrated a maximum of 203 millimetres (8.0 in) of RHA at 90° at ranges of up to 2,000 metres (2,187 yd). However, during trials in July 1944, the gun showed some deficiencies, such as being difficult for the crew to operate, unreliability of the muzzle brake and the breech block, and unsatisfactory performance of the shells. In addition, the gun, protruding far forward of the hull front, limited the maneuverability of the vehicle. The self-propelled gun carried 21 rounds of two-piece (shell and charge) ammunition, and had a rate of fire of 2 rounds per minute. It used the engine, transmission, running gear, and electric equipment of the ISU-122. In August 1944 the BL-8 gun was replaced with the improved 152.4 mm BL-10 (БЛ-10) long-barreled gun, with a slightly shorter barrel of 7.392 metres (24.25 ft) (48.5 calibers). This vehicle gun was designated **ISU-152-2** (ИСУ-152-2). The factory designation was Object 247 (Объект 247). It was also equipped with external fuel tanks. The gun had a modified muzzle brake and a semi-automatic breech block. It had a rate of fire of 3 rounds/min. The BL-10 had a maximum range of 18,000 metres (19,685 yd), with the 43.56-kilogram (96.0 lb) high-explosive shell. In December 1944 the ISU-152-2 underwent trials, revealing the barrel strength and the angle of horizontal guidance were unsatisfactory. The gun was sent for further improvement, but it was not completed before the war ended. The vehicle was never adopted. After the war, the final and most improved, third modification of ISU-152-2 was completed. The gun had a maximum range of 19,500 metres (21,325 yd), using a 48.5-kilogram (107 lb) high-explosive shell with a muzzle velocity of 880 metres per second (2,887 ft/s).

### Object 704

This was a prototype self-propelled gun developed in 1945. Also known as the Kirovets-2, it was developed on the chassis of the Kirovets-1, the initial prototype of the project that became the IS-3. The overall height of the vehicle was reduced to 2,240 millimeters (7.35 ft), which was compensated with an increased width of the superstructure. The factory designation was **Object 704** (Объект 704). It was armed with the 152.4 mm ML-20SM model 1944 (МЛ-20СМ обр. 1944 г.) gun-howitzer, with a barrel length of over 4.5 meters (15 ft) (29.6 calibers) and no muzzle brake. It had a maximum range of 13,000 metres (42,651 ft). The self-propelled gun carried 20 rounds of two-piece (shell and charge) armour-piercing and high-explosive ammunition. The armor-piercing round, weighing 48.78 kilograms (107.5 lb), had a muzzle velocity of 655 meters per second (2,149 ft/s). The rate of fire was 1-2 round/min. Object 704 had four hatches at the superstructure roof and one emergency hatch at the bottom of the hull behind the driver's seat, which had an armored cover. The self-propelled gun carried two external 90-liter (19.8 imp gal; 23.8 US gal) fuel tanks, not connected to the supply system. The secondary armament of the fighting vehicle consisted of two 12.7×108 mm DShK machine guns, one anti-aircraft, and one co-axial. The protection was increased by placing thicker Armor at more radical angles. In the area of the gun, where the mantlet combined with the hull front behind it and the housing of the recoil mechanism, the Armor thickness was 320 millimetres (13 in). Object 704 was the best protected of all experimental or production Soviet self-propelled guns of the Second World War. However, the radical incline of the superstructure walls combined with the increased recoil of the gun, due to the lack of a muzzle brake, significantly complicated the work of the crew, and for this reason, it was not adopted.

### Heavy assault gun

As a heavy assault gun, the ISU-152 was an extremely valuable weapon in urban combat operations such as the assaults on Berlin, Budapest and Königsberg. The vehicle's excellent armor protection finally provided the 152.4 mm platform with good protection from most German anti-tank guns, allowing it to advance into the face of direct anti-tank fire, while the huge, low velocity, high-explosive rounds were excellent at blasting open even the most heavily fortified and reinforced enemy strongpoints. Such actions would be much more dangerous and much less effective for a conventional towed artillery piece, with their high crew exposure and low mobility, or even a tank, with their smaller main guns. When supporting tanks, the usual tactics of the ISU-152 were to be used in the second line of the attack order, 100 to 200 metres (110 to 220 yd) behind the attacking tanks, which were usually IS tanks with equal mobility.

The ISU-152, like the earlier SU-152 and contemporary ISU-122, was employed by Independent Heavy Self-propelled Artillery Regiments. Between May 1943 and 1945, 53 of these regiments were formed. Many of them were re-formed tank regiments, and employed similar direct fire tactics as used by tanks when supporting infantry. Each of the heavy regiment had 21 guns, divided into four artillery batteries of five vehicles and the commander's vehicle. For support, these heavy regiments had some supplementary unarmoured vehicles such as trucks, jeeps, or motorcycles. In December 1944, Guards Heavy Self-propelled Artillery Brigades were formed, to provide heavy fire support to the tank armies. They were organized along the model of tank brigades, each with 65 ISU-152 or ISU-122 self-propelled guns.

To minimize the risks of being knocked out by *Panzerfaust*-equipped units during urban operations, the ISU-152 usually acted in one- or two-vehicle detachments alongside infantry squads for protection. The infantry squad would include a specialist sniper (or at least a sharpshooter), some submachine gunners and sometimes a flamethrower. The ISU-152's heavy-calibre DShK machine gun was also useful for targeting Panzerfaust gunners hiding on upper floors of city buildings or behind protective cover and defensive barricades. Effective teamwork between the ISU-152 crew and supporting infantry allowed them to achieve their goals with minimal losses, but if such tactics were not adhered to, the attacking vehicles were easily attacked and destroyed, usually through the weaker armor on the roof or rear compartment.

### Heavy tank destroyer

Front view of ISU-152
The ISU-152 could also operate as an effective heavy tank destroyer. Though it was not designed for the role, the vehicle inherited the nickname *Zveroboy* ("beast killer") from its predecessor, the SU-152, for its ability to reliably kill the best protected German fighting vehicles; the Panther tank, the Tiger and Tiger II tanks, and even the rarely fielded Elefant and Jagdtiger tank destroyers. The sheer weight of the 152.4 mm shells resulted in an extremely low rate of fire, only one to three rounds per minute, and were not as accurate at long range as high-velocity tank and anti-tank guns. However, the massive blast effect from the heavy high-explosive warhead was capable of blowing the turret completely off a Tiger tank. A direct hit usually destroyed or damaged the target's tracks and suspension, immobilizing it. While the low-velocity 152mm shell did not generally penetrate heavy armor, it frequently killed or severely wounded the crew through spalling (splintering) inside the hull as well as injuries caused by blast concussion. Due to the brittle nature of German tank steel during the latter stages of the war, it was common for impacted armor plates to shatter from the concussive blast, or for weld lines to break, allowing the entire armor piece to fall away. Surviving crew were often left with an immobilized vehicle which had to be hurriedly abandoned before being destroyed.

For anti-tank operations following the July 1943 Battle of Kursk, armour-piercing ammunition was developed, with an eye towards giving the howitzer a more traditional anti-tank capability. However, these rounds were expensive, in short supply, and only moderately more effective than the standard non-penetrating high-explosive round. As a howitzer the ML-20S exchanged velocity and accuracy for throw weight and distance, and was not intended to compete with true anti-tank guns. Sometimes the concrete-piercing ammunition was used for the anti-tank role. A primitive shaped charge ammunition, with a 27.44-kilogram (60.49 lb) shell, was also developed. It had a maximum penetration of 250 mm of RHA at 90°, but it was not used during the war.

The ISU-152's 90 mm of sloped frontal armor, in contrast to the SU-152's 65 mm, provided excellent frontal protection from the 75mm KwK 40 gun of the ubiquitous Panzer IV and StuG family at all but the closest ranges, while also forcing the original Tiger I, with its vaunted 88mm KwK 36 gun, to close to medium ranges in order to successfully penetrate the vehicle, negating its traditional long-range superiority and exposing more of its vulnerable flanks to the 85mm ZiS-S gun of the Soviet T-34-85.

The ISU-152 was not a true purpose-built tank destroyer. It had a very low rate of fire compared with specialised tank destroyers such as the German Jagdpanther or the Soviet SU-100, which could manage a brief burst of five to eight rounds per minute. However, prior to the introduction of the SU-100 it was the only Soviet armored vehicle capable of tackling the German heavy tanks with any kind of reliability, and its ability to satisfy multiple roles meant it was produced in far greater numbers than the SU-100. Attention to camouflage, quick relocation between firing positions, and massed ambushes of four or five vehicles firing in salvo at a single target's flanks reduced the disadvantage of the low rate of fire.