Object 715

Soviet Union (1946-1947)
Heavy Self-Propelled Gun – Partial Blueprints Only

The Object 715 was a heavy self-propelled gun based on the chassis of the IS-4 developed by SKB-2 at Kirov Chelyabinsk plant (ChKZ). A direct successor to the Object 704, in turn based on the IS-3, the Object 715 never even had full blueprint studies before the design was canceled due to the shifting in military philosophies, with heavy gun platforms falling out of favor with militaries across the world.

Background and Development

The end of war usually indicates a moment of peace and joy, and in tank-related terms, the cancellation of many designs and projects. However, the end of the Second World War could not have been any more different. The clear political and military tensions between the USSR and Western nations meant that tank development never really halted after the defeat of the Axis powers. As a matter of fact, the Soviets invested heavily into more powerful tanks. So much so that many programs overlapped with each other, such as the IS-3 and IS-4 heavy tanks for the SKB-2 (design bureau of the Chelyabinsk Kirov plant) alone.

The IS-4 (designation Object 701), like the IS-3, was meant to replace the IS-2 heavy tanks. Envisioned to be the pride of the Soviet tank forces, the IS-4 was given extremely thick armor, weighing 60 tonnes, and driven by a 750 hp V-12 diesel, allowing it to reach up to 43 km/h. However the IS-4 would end up becoming a very troublesome tank, with serious reliability issues throughout its development and service life, caused by its heavy weight, unreliable mechanical components and poor design.

Most importantly, the IS-4 had the same 122 mm D-25T gun like the IS-2 and IS-3. Production started in March 1947 and ended just a couple of years later, in 1949, when all activity on tanks weighing over 50 tonnes was canceled. The tank proved to be too heavy and sluggish and poorly constructed. Another small batch of 25 units was built in 1951 with some upgrades, called IS-4M. All previous tanks were upgraded to this level. In total, only 249 were built.

The idea of an self-propelled gun based on the Object 701 appeared in February 1945, after the positive conclusion of the trials of Object 701 prototype No.5, as it was proposed to enter production and that ISU-152 and ISU-122 SPGs be based upon it. In 1947, with the IS-4 entering mass production, the ChKZ plant began development of several derivatives based on the IS-4. One of these was the Object 715, a heavy SPG based on the hull of the IS-4. The SKB-2 design bureau had done something similar just a couple years earlier, designing the Object 704 SPG right after the IS-3 (Object 703). One prototype was built in spring 1945 and tested in the autumn of that year. State trials of the Object 704 revealed several issues with the SKB-2 SPG. The iconic sloped armor sides severely decreased the space available inside the crew compartment, hindering the loader from accessing all the shells properly, and the breech operator’s position was very tight, having a hard time extracting the explosive charges. This also led to a rather limited ammunition count. Trial officers recommended that the walls were to be made vertical again, like on the ISU-152.

An exact timeline of the development of the Object 715 is hard to establish. At some point during 1946, work began on the Object 715, when interest from the state in heavy SPGs armed with 152 mm guns grew again. SKB-2 would base this SPG on their new IS-4, which had gotten far in development. Interestingly, their rival, the Kirov plant in Leningrad, also got far with their designs on the Object 260 (IS-7) heavy tank and based their SPGs on this chassis.

Purpose

The versatility that heavy SPGs showed during the Second World War had greatly impressed the Soviets. Their large-caliber guns, whether 122 mm or 152 mm, were powerful enough to deal with almost any kind of threat. Casemates, machine gun nests, and other static defenses, would be deleted from the face of the earth, doubling down as iconic propaganda machines to this day. Even the heaviest German tanks would prove vulnerable to a direct hit from a 152 mm shell, cracking open welds and rupturing armor via pure momentum and force.

Deep Battle doctrine, a complicated and much-debated philosophy from the first Soviet use of tanks, proved to be effective after the Axis’ defeat. Here, self-propelled guns would prove important and critical in weakening enemy defenses prior to an attack, but support infantry and tanks with unparalleled firepower. Theoretically, such self-propelled guns would target remaining enemy troops after indirect fire shelling, which would allow the infantry to push along with the ‘regular’ tanks. But reality pushed these vehicles to adopt a more aggressive style of combat, acting a lot more like an assault gun. The ISU-152’s armor proved insufficient for such purposes, which was why the Object 704, at first designed at the beginning of 1945, focused greatly on protection, with thick, angled plates across the casemate.

But after the war, the philosophy and environment changed. Raw armor thickness simply would not cut it. Battle tanks had increasingly more powerful guns and warfare got more mobile. To cope with these changes, designers had to increase the engagement distance of SPGs, in turn allowing for less armor. New medium and heavy tanks, both domestic and foreign, proved to be more and more self-sufficient in all areas. Self-propelled guns were no longer needed on the first line of attack and could afford to cut down on protection by engaging from as long ranges as possible. This would have required a seriously powerful gun.

Design

The design of the Object 715 was almost identical to that of the Object 704. The front of the vehicle was dominated by a large gun mantlet. This mantlet was a redesign of the one found on the Object 704, where water used to gather into the gap beneath the mantlet, meant to allow the mantlet to depress the gun. On the new design, the mantlet was depressed straight into the fighting compartment, and protected by an outer sleeve along the sides. The roof was also redesigned to allow the gun to depress more.

The most noticeable feature of the new SPG was the small volume of the casemate. The new armament had almost 3 times less recoil distance compared to the Br-2 howitzer. This allowed the designers to shorten the casemate significantly. The blueprints also show the lower hull of the IS-4 chassis, which has been altered to fit the casemate. Of great interest is the armor thickness of the lower plate, or lack thereof. Despite these changes, the blueprints of the Object 715 perfectly match and fit into those of the IS-4. The exact mass is unknown, but some sources claim 60 tonnes. While not impossible, this meant that it would have been heavier than the IS-4 itself.

Using the IS-4 chassis, most mechanical components remained the same.The engine was a V-12 diesel outputting 750 hp at 2,100 rpm, coupled to a 6-speed forward, 2-speed reverse manual gearbox.

Crew

Soviet tanks are not known to have vast and lavish crew spaces, and the Object 715 would have been no exception. This trend began with the pressures of the war in the 1940s, forcing designers to make sacrifices. This issue continued until the 1950s, from which point crew ergonomics improved greatly.

The crew likely consisted of 5: commander, gunner, driver, loader, and loader assistant (formerly breech operator), all cramped in a small casemate. They were positioned in the classical formation, with the driver on the left side of the gun, controlling the tank with two tillers. The driver had one MK-4 periscope for vision outside, increasing his reliance on the commander for directions. Communication within the tank was done via a TPU-4F intercom, with headset and microphone. While it is hard to prove, it is very likely that each crewmember had their own entry and exit hatch, like on the Object 704. Shared hatches would have been very cumbersome, as the gun took up a lot of space.

The gunner was seated behind the driver, on a small seat, attached close to the gun breech. For vision, the gunner had the main gun sight, a TP-47A, and a secondary horizontal (azimuth) ZIS-Z sight was also provided, for indirect fire. It protruded from the hull and was protected by an armored housing. Hand crank controls for elevation were also available for the gunner. Like on previous SPGs of this type, aiming the gun was a team effort, with the gunner controlling the elevation, and the traverse being controlled by the breech operator sat behind him. This was the unfortunate outcome of letting the individual tank factories design the horizontal traverse and not the gun factory (Factory No.172).

In previous vehicles, the breech operator was tasked with opening the breech, controlling the gun traverse and loading ammunition charges. The shells, although being two-part rounds consisting of charge plus shell, weighed between 43.56 kg and 48.78 kg. For this reason, it was deemed necessary to include a loader assistant that would remove some of the strain from the loader. Exact crew positions and the ammunition layout are not present in the drawing, but judging from previous similar vehicles, the ammunition was stored on the sidewalls, with the projectiles on the right wall and charges on the left.

The loader sat opposite to the loader assistant, on the other side of the breech. The loader was tasked with loading the shells and firing the coaxial machine gun, likely a 12.7 mm DShK, and operating the potential external anti-aircraft machine gun, also an 12.7 mm DShK (not featured on the partial blueprints).

Finally, the commander sat in front of the loader, to the right of the gun. The commander’s main task was coordinating the crew, scanning the battlefield, and communicating with other units via the 10PK-26 radio, with a stationary range between 20 km to 25 km.

Armament: the M-31 and M-48 152 mm

At first, the armament was to be a 152 mm gun developed at factory No. 172, the M-31. The ballistics were mostly identical to those of the regular M1935 Br-2 howitzer, but with considerable upgrades in other areas. Firstly, the archaic breech block door was replaced with a more modern horizontal sliding breech block. It also received the famous TsAKB style slotted muzzle brake, which could absorb up to 70% of the recoil, decreasing the need of powerful recoil adsorption pistons. It still had two recoil absorption cylinders and two brake cylinders to absorb recoil, but these were considerably lighter, and in tandem with the muzzle brake, decreased the recoil from 1,400 mm (on Br-2) to 520 mm. Other improvements were an automatic barrel smoke evacuator, and the TP-47A direct fire sight, and ZIS-Z sight for indirect fire. Very noticeable is the sheer volume of the breech which was needed to offset the long barrel. One prototype of the gun was built and passed factory tests in the summer of 1948.

The M-31 gun was incorporated into the Object 715 in late 1947, however, the original plans of the Object 715 were rejected by the military, sending the SKB-2 designers back to the drawing board. The complaints came from the integration of the gun into the IS-4 chassis. It was seen as cumbersome, tight and hard to coordinate. Meanwhile, Plant No.172 developed an upgrade of the M-31, the M-48.

Plans for the M-48 were submitted for review to the Soviet state by plant No.172 on 9th August, 1946, which were approved in November. It increased muzzle velocity to 1,000 m/s and featured a slotted muzzle brake with an efficiency of 59%. A horizontal semi-automatic breech block was also designed in two versions, pneumatic and mechanical. The final draft of the gun was submitted on the 21st of June 1947 and approved for prototypes and testing. Three were built and tested. From here onwards, what happened to Object 715 is unclear. When exactly and how the M-48 was incorporated remains a mystery. It is possible that it was rejected once again, or the project was shelved for a future request, with work switching to other areas.

As secondary armament, the Object 715 had a 12.7 mm DShK heavy machine gun, loaded by the loader.

Armor

As the new weapons that were proposed allowed for greater engagement distances, the armor became less relevant. Any excessively thick armor, combined with the massive guns with great overhang, and the IS-4 chassis simply could not have handled the stress on the first set of torsion bars. Thus, the armor was made thinner but more angled. However, this was not to the extreme seen on the Object 704. The frontal plate was 100 mm thick angled at 45°. The lower plate was around 30 mm angled at -40°. Side and cheek armor values are unknown. The rest remained the same as on the IS-4 tank.

Fate

Plant No.172 agreed with the Kirov Chelyabinsk SKB-2 plant to use the M-31 in the Object 715, but also with Kirov Leningrad to use the same gun on their Object 261/Object 263 SPGs based on the IS-7, once again leading to a direct competition between the two rival factories.

In addition, Plant No.172 presented on 23rd August, 1948, a new upgraded gun, especially designed for the IS-7 based SPGs, which would eventually use the 130 mm S-70 gun. The IS-7 based SPGS only reached scale model stage before they were all canceled on 16th February, 1949, when the Council of Ministers of the USSR canceled all development and production of heavy tanks above 50 tonnes. This included the troublesome IS-4 tank subsequently cancelling all variants based upon it, including the Object 715. The concept of Heavy SPGs was once again considered with the Object 268 program, from where they would be sealed off for good.

Conclusion

While it was one of the most powerful direct fire SPGs ever designed, the shortcomings of the IS-4 platform and the shift from large guns on heavy chassis to other design philosophies, such as ATGMs, led to its demise. The Object 715 remains, to this day, one of the more mysterious Soviet post-war designs, with nothing but a half-drawn blueprint remaining.

Object 715 specifications
Dimensions (L-W-H)	6,682 (hull only) x 3.26 x 2.4 m
Total Weight, Battle Ready	circa 50 tonnes
Crew	5 (Commander, Gunner, Driver, Loader & Breech operator
Propulsion	V-12 Diesel engine, 750 hp
Speed	40 km/h (hypothetical) (25 mph)
Armament	152.4 mm M-31 howitzer or 152.4 mm M-48 howitzer coaxial 12.7 mm DShK heavy machine gun
Armor	Hull armor: Front top plate: 100 mm at 45° Front bottom plate: 30 mm at -50° Top: 30 mm Belly: 30 mm
Total Production	0; blueprints only

Sources

Domestic armored vehicles 1945-1965 Soljankin, A.G., Pavlov, M.V., Pavlov, I.V., Zheltov
Technics and weapons (TiV) 1996 06, M.V. Pavlov, I.V. Pavlov
Tank Archives: The Last Soviet Heavy Tank Destroyers
The genius of Soviet artillery. Triumph and tragedy of V. Grabin – Shirokorad Alexander Borisovich