The IS-7 was born in a strange time for the Soviet tank industry. The raising of the Red Banner over the Reichstag marked not only the glory of victory, but the problems of rebuilding for peace. Cities had to be rebuilt, factories returned from evacuation, defense industry repurposed.
The People's Commissariat of Tank Production was converted into the People's Commissariat of Transport Machinebuilding and later, when the commissariat system was deprecated, into a ministry of the same name. Wartime vehicles were modernized in the UKN program for longer use in the military during peacetime. Some work on future tanks was cancelled, other work progressed at a reduced pace.
One of those post-war projects was the Object 260. Work on it began in late 1944. The idea was that the new vehicle would combine all experience obtained during the design and use of heavy tanks. The design bureau of factory #100 headed by Joseph Kotin was tasked with this project.
Several variant of the new tank were created in the spring of 1945: Object 257, Object 258, Object 259. After the analysis of the advantages and drawbacks of each design, a final draft of technical requirements was made. The new vehicle received the index Object 260 and the letter code IS-7.
Over 20 institutes and science organizations were involved in the development of the tank. Over 25 unique technological solutions were used in the design, not found in any similar vehicles.
Working blueprints of the IS-7 were ready by September of 1945. The piked nose was inherited from the IS-3, but it did not protrude as far forward, and had more armour (150 mm compared to 100 of its predecessor). Despite its rather compact dimensions, the weight reached 65 tons.
For such a heavy tank, a 900-1200 hp engine was needed. Factory #800 was developing the engine, but did not complete it in time. As an alternative, factory #77 proposed a pair of V-16 diesels, but trials showed that it was unsuitable for the task. Jointly with factory #500, Kirov factory hurriedly developed the TD-30 engine based on the ACh-30 aircraft engine. This engine allowed the tank to reach a speed of 60 kph on hard ground and about 30 kph on a badly beaten stone road. This engine was installed on the first two IS-7 prototypes, although due to poor quality of assembly it needed more work.
The first variant of the Object 260 transmission had a six stage gearbox, a planetary turning mechanism, and hydraulic servos that made controlling the tank easier.
The suspension was designed based on suspensions of existing tanks. The IS-7 received a number of novel elements in domestic tank design: tracks with a rubber-metallic joint, bidirectional hydraulic shock absorbers, heavy duty internal rubber padding for its wheels, and bundled torsion bars.
A new 122 mm gun with a muzzle velocity of 1000 m/s was planned for the new tank, but work was not finished in time. Instead, on the request of Joseph Kotin, the Central Artillery Design Bureau designed the 130 mm S-26 gun with a slotted muzzle brake. Its characteristics were close to the B-13 naval gun with a similar caliber. The loading mechanism allowed the tank to fire at a rate of 6-7 RPM. Aside from the main gun, the tank had seven machineguns: one 14.5 mm KPV and six 7.62 mm RP-46.
The first IS-7 was assembled on September 8th, 1946. Before the end of the year, the tank travelled 1000 km during trials, and was approved as matching the tactical-technical requirements. The second prototype, assembled on December 25th, only travelled 45 km. Aside from the two experimental vehicles, Izhor factory also produced two armoured hulls and two turrets for ballistics testing with 88, 122, and 128 mm guns. The results were considered when designing the final layout of the tank's armour.
Over the next year, the Kirov factory design bureau worked on improving the design of the IS-7. The new tank retained most characteristics of its predecessor, but received some improvements. The width increased to 3.4 meters, but the overall height decreased to 2.6 meters due to a lower turret. The mass of the tank grew to 68 tons. As a result of a proposal from G.N. Moksvin, the tank received curved sides.
The TD-30 engine was replaced with a mass production 1050 hp M-50T naval diesel. For the first time in domestic tank production, it was cooled with ejectors made from milled armoured plate. The volume of fuel tanks was increased from 800 to 1300 liters. A new eight stage transmission was installed, designed in cooperation with the Bauman Moscow State Technical University.
The suspension of the new tank had 14 large road wheels with internal rubber liners. A group of engineers led by L.Z. Shenker developed bidirectional hydraulic shock absorbers, which greatly increased the smoothness of travel. Tracks with a rubber-metallic joint were complicated to make, but were very resilient to wear and very quiet during movement. The width of the tracks was 710 mm.
The Central Artillery Design Bureau perfected the S-26 gun. The replacement 130 mm S-70 gun fired 33.4 kg shells at a speed of 900 m/s. The electric loading mechanism was very compact. The sight was also improved for precise fire at long distances.
The tank received a second high caliber machinegun in the turret, attached to a special mount on the turret. The machinegun was remote controlled and could fire on ground or air targets without the crew having to exit the tank. Another three machineguns (one KPV and two RP-46) were located in the gun mantlet, two were on the fenders, and two more on the turret.
The IS-7 was equipped with M.G. Shelemin's automatic fire suppression system that could turn on three times in the event of a fire. It was installed in the engine compartment.
The IS-7's crew consisted of five men: a driver, a commander, a gunner, and two loaders, located in the turret. The commander was located to the right of the gun, and the gunner to the left. The loaders, aside from their primary duties, were also responsible for the machineguns.
In the summer of 1948, Kirov factory produced four IS-7 tanks. Factory trials showed that, despite its mass, the vehicle can easily reach a speed of 60 kph and has excellent off-road capabilities.
After factory trials, the IS-7s were sent to government trials. The tank had a strong impression on the commission members. The chief tester of the Ministry of Transport Machinebuilding, E.A. Kulchitskiy, wrote: "I had the great honour of being the first to put this amazing tank into motion. It's hard to put my feelings into words. At a speed of 60 kph, this huge machine easily reacts to the smallest tug on the levers or the pedals. The gears are switched with a tiny level, the vehicle is completely under control of the driver."
The IS-7 turned out to be almost invincible. It resisted shells from not only the German 128 mm gun, but its own weapon. In addition, trials on dogs showed high levels of protection. The animals were put into the tank instead of the crew, and then the tank was fired upon. The animals' condition was not impacted by these trials.
The trials didn't finish without accidents. During shooting, one tank lost a suspension arm with the wheel when a shell slipped along the curved side and hit the poorly welded part. Another tank had its engine caught fire, after expending its warranty resource. The fire extinguisher could not put out the fire, the crew abandoned the vehicle, and it burned up completely. Overall, however, the trials were very impressive.
It's hard to say how successful the IS-7 would have been if it was put into mass production. The order for 50 vehicles that the Kirov factory received in 1949 was never completed due to a lack of funding: the Soviet defense industry was transitioning to cheaper medium tanks. In addition, transporting the IS-7 over railroads would have been impossible, as the railroads of the time could only take a load of 50-55 tons. The military insisted on a lighter tank, and soon, work on the IS-7 stopped.
By its tactical-technical characteristics, the Object 260 surpassed both WWII era tanks and its post-war contemporaries. In the early 1950s, the most powerfully armoured and armed tanks were the British Conqueror and the American M103. Both tanks had 120 mm guns (L1 and M58). As trials showed, it was impossible to penetrate the IS-7's hull or turret with those guns. With similar mass, the Soviet tank surpassed both of them in mobility: 60 kph vs 34 kph. Tanks that could potentially fight it only appeared in the 1970s and 1980s.
The idea of an "ideal" tank began to seem outdated. In 1949, the NATO political bloc formed and the geopolitical opposition of the West and the USSR was changing. Nuclear weapons and aircraft became the go-to option in the "big game". Medium tanks were suitable for local conflicts.
The IS-7 was a mighty pinnacle of Soviet heavy tanks. It came ahead of its time and produced many experimental solutions, but turned out to not be in demand due to the realities of history.
Original article by Nikolai Nevskiy.