Monday 17 April 2023

Baby IS-3

The Second World War was a catalyst for the creation of many armoured vehicles, including heavy tanks. These vehicles were quite rare at the start of the war, but this changed radically in 1941-42. The Red Army was the most heavily invested in heavy tanks. At first, Soviet heavy tanks followed the classic path of gaining more and more weight, as a result of which the KV-1 became overloaded. The need for mobility led to the KV-13 and KV-1S. The result of this new branch of development was the IS-1 (IS-85) tank, which was quickly replaced with the IS-2 (IS-122). The tank combined powerful armament with sufficient mobility and high levels of protection. Nevertheless, work on modernizing the IS-2 began in the spring of 1944.

Kirovets-1 at the NIBT Proving Grounds, December 1944.

A new step in Soviet heavy tank development also began in the spring of 1944. At first, there were only two directions: further development of the IS-2 and work on a new heavy tank, the Object 701. The number of branches of development increased from there: factory #100 began to work on the IS-6 heavy tank and modernization of the IS-2 fell to SKB-2 at the Chelyabinsk Kirov Factory. A prototype of a modernized IS-2 tank was developed and built in July-September of 1944. At this point, factory #100 woke up and developed another modernization called IS-2U. This vehicle was overshadowed by the Kirovets-1, a modernized IS-2 tank. This tank was the direct ancestor of the IS-3 and the first attempt at using some solutions later realized on the IS-4.

A new tank with old parts

The cause for this change of direction in IS-2 tank development was predictable. After a certain point, further modernization of the IS-2 was senseless. Even if you discard the first KV-13 tank, the new Soviet tank concept was more or less formed by the end of 1942. The second prototype of the KV-13 was the direct ancestor of the best Soviet heavy tank of WW2. Meanwhile, growing requirements for protection meant that the hull and turret would have to be redesigned. The fact that the modernized IS-2 developed by factory #100 in the spring of 1944 looked like a completely new tank was no accident. The appearance of the IS-6 also hinted at the fact that the best way forward was a new tank, not a modernized old one.

The first mention of a tank called 701-A, mid-November 1944. The prototype is already being assembled.

ChKZ's SKB-2 followed the principle of "a bird in the hand" and dutifully modernized the IS-2 tank. The introduction of a straightened front hull in the summer of 1944 corrected the "birth defect" that reduced protection. According to N.F. Shashmurin's memoirs, Zh.Ya. Kotin was the biggest proponent of the "stepped" front, since according to him the driver didn't see well through periscopes. It's possible that the idea of an observation port in the straightened hull was his idea. At least, the early IS-2 and Object 701 didn't have the "mail slot". The idea was not a great one, as it both introduced a weak spot and didn't really improve vision. This only became obvious in the second half of 1944 when the modernized IS-2 tanks went into battle.

The new tank was also called Object A in correspondence.

By November of 1944 it was clear that a more radical modernization is needed. As a result, factory #100 began working on the IS-2U. They solved the issue of hull protection in a radical way, creating the "pike nose" that became the calling card of heavy tanks developed at factory #100 and VNII-100. The sides of the hull and the turret also changed. Factory #100 straight up ignored a number of requirements, which made SKB-2's IS-2 modernization preferable. Either way, Chelyabinsk was hard at work.

Not a lot remained from the IS-2.

Talk about a new tank began in November of 1944. At first it was called "experimental prototype 701-A". M.F. Balzhi was the lead engineer on this project. He was also the co-lead engineer on the Object 701 alongside L.S. Troyanov. N.L. Dukhov, the lead designer, clearly decided to play it safe. Object 701 was the priority, but this was a revolutionary vehicle that still needed a lot of work. To be fair, the Object 701 already had a competitor in the face of the IS-6, but it was in a bad spot having failed mobility trials with flying colours. The Object 701 was much better in this regard. Nevertheless, the idea of making an intermediate tank between the IS-2 and Object 701 was sound. Some solutions came from the old tank and some from the new one.

The Object A began factory trials by the end of November.

There was very little in common between the "701-A" that quickly changed its name to "Object A" and Object 701. Object 701 had sloped sides of the upper front hull, but the Object A had a flat front. There was also a cast bulge with three periscopes. They were installed in a hatch that could be opened when the turret was turned backwards. A similar solution was later used on the production IS-3 tank. This was SKB-2's solution to get rid of the "mail slot" and allow the driver to have good vision when travelling. His seat could be lifted to travel position when the hatch was open.

The components inside were similar, but the layout of the engine compartment was different.

Other parts of the hull were also different. The Object 701 had classic panniers, but Object A's sides were sloped. There was also a 16 mm thick spaced armour skirt. This increased the protection of the sides and reduced the tank's weight. The engine compartment was entirely redone. The V-11 engine was used instead of the V-2-IS. This engine was initially rated at 700 hp, but the power was later reduced to 520 hp in order to improve reliability. The air intakes were also changed, since Soviet heavy tanks hadn't conceptually changed their cooling since the T-28. Air exhausts were also changed. The exhaust pipes weren't moved to the rear as required, but the exhaust system was altered anyway. Like the modernized IS-2, the fuel tanks in the driver's compartment were moved to the engine compartment, which improved the driver's working conditions.

The tank was a ton heavier than its predecessor, but it had much better protection.

The Object A's most distinctive feature was the turret. Legend claims that Balzhi was inspired by a soap box, but if one dismisses the legend then it's clear that the turret was born from two requirements: the need to improve protection and improve ammunition storage. The new flattened turret was much tougher to penetrate from the sides. G.V. Kruchenykh led the development of the turret and made use of the benefits of casting. Depending on the slope, the thickness of the side armour varied between 90 and 175 mm. The same was true for the front of the turret. As required, the gun was extracted through a hatch in the roof. Ammunition was stored around the turret, which made the loader's job easier. An interesting idea was the use of large hatches with built-in observation cupolas that later migrated to the Object 701. 

The tank in travel position.

The result was less a modernized IS-2 and more a new tank that reused some components from the old one. The Object A was much better protected but only gained a ton of weight. ChKZ expedited the work on this project. It appeared out of nowhere and active work began when the modernized IS-2 was still undergoing trials. This rush is explained by the fact that factory #100 was actively working on the IS-6. They were aiming to compete with the IS-2 rather than the Object 701, even though the GBTU was quite skeptical about the Object 252's abilities. Trials showed that this skepticism was not unfounded, but the IS-6 was still a threat that ChKZ worked to preempt.

A replacement for the IS-2

When the Object 701-A first appeared in correspondence on November 16th, 1944, it was already almost finished. The chassis was assembled and preparation for bench trials was underway. The turret was also being worked on. Assembly finished on November 24th, at which point the tank drove its first 32 kilometer long run. The tank performed a 50 km long military QA trial on the next day. Despite some defects, the trial was a success. The tank showed an average speed of 25 kph and the temperature was within expected parameters. Meanwhile, trials of the modernized IS-2 had to be paused because of a breakdown.

The name Kirovets-1 was introduced in mid-December of 1944.

ChKZ actively prepared for proving grounds trials by the end of November. In addition to the tank itself, experimental hulls and turrets were sent to Kubinka for penetration trials. This was the correct decision, since the properties of the design could be obtained while it was still malleable. Meanwhile, the IS-6 test turret was submitted after the tank was finished. Issues with the robustness of its sides were only discovered later, after a similar solution was recommended for the T-54. As a result, the turret of the new medium tank had to be redesigned.

The tank was already called IS-3.

The tank and its components arrived at the NIBT Proving Grounds by December 18th, 1944. A number of events took place that influenced the fate of the Object A. As mentioned above, the factory #100 OKB was working on the IS-2U tank. It was worse than the Object A, but its front hull was better. Because of this, People's Commissar of Tank Production V.A. Malyshev signed decree #729ss on December 16th, 1944, combining the two projects. The pike hull and commander's fire controls were taken from the IS-2U. The new tank was called Kirovets-1. The tank that would later become the IS-3 had the same name, but to avoid confusion with indexes let us use this designation for the early tank.

During mobility trials.

The NIBT Proving Grounds approached the issue of nomenclature in its own way. The tank was called IS-3, the second tank to go by this name after the Object 237. The order to conduct trials was made on December 15th, 1944, and the trials took place from December 18th to December 24th. Trials were conducted in two stages. The main 500 km trials took place first, then 1000 km long warranty trials. This was the correct approach, as was the production of a pilot batch. Presumably, someone remembered how two new SPGs were accepted into service back in 1942 before they even passed proving grounds trials.

The mobility of the tank was equal to the IS-2.

The tank drove for 502 km during the first stage of trials, 103 on an icy highway, 358 on snow-covered country and forest roads. 41 km were spent in obstacle trials. To make the trial as representative as possible, a driver with a medium qualification was chosen for this task. The trials were performed in difficult conditions, but the performance was good. The Kirovets-1 showed an average speed of 30 kph on the Minsk highway, which is quite a good result for a heavy tank and 3 kph faster than the IS-2. The fuel expenditure was higher: 213 L per 100 km compared to 203 L on the IS-2. The average speed on country roads was the same: 17.2 kph. This time, the Kirovets-1 burned less fuel: 285 L per 100 km compared to 300.

Reliability was also similar.

The Kirovets-1 could not complete the warranty trials in their entirety. The cooling fan broke on the 810 kilometer. This was not a disaster, since the production IS-2 tank had similar issues. The most important part was that the engine didn't run any hotter than on the IS-2. The tank could make a nonstop 50 km march without any issues. The driver's work was more comfortable, although there were still complaints about his position. There were 10 suggested improvements overall for the driver's compartment and 13 more for the engine and transmission. These included the suggestion to develop removable grousers for the tracks to help the tank drive on ice. The same suggestion applied to the IS-2.

Location of ammunition. It was positioned comfortably, but the layout of the fighting compartment was constricting.

The layout of the fighting compartment was another issue. The shape of the hull was a good one for protection, but made the crew's working conditions worse. Complaints were made about the positions of all three turret crewmen. The gunner's seat was decent, but the periscope was uncomfortable to use. There were also complains about the electric turret traverse and elevation. The report describes the commander's station as "insufficiently comfortable", but that was putting it mildly. The commander's station made it difficult to both look around and work with the radio. The loader's station was even worse and didn't allow him to do his job properly.

Vision from the tank.

It is not surprising that the rate of fire trials (37 shots) gave some food for thought. The rate of fire was 1.6 RPM, the same as on the early IS-2 tanks with a screw breech. The ammunition racks were well placed, but the loader just didn't have enough room to work. The precision of the gun was high. There were also questions about the observation devices. The commander and loader couldn't make use of their cupolas. The NIBT Proving Grounds suggested installing a cupola similar to the one on the M4A2(76)W tank. In total, 21 suggestions for improving the fighting compartment were made.

The fighting compartment was uncomfortable and cramped.

More issues were raised after penetration trials held on December 21st, 1944. An 88 mm Pak 43/41 and a 122 mm A-19 corps gun were used. The front armour of the Kirovets-1 was tough and could not be penetrated by the 88 mm gun at a range of even 100 meters. It was not an accident that ChKZ liked Object 701 style hulls so much. The turret, on the other hand, had problems. The front could be penetrated by the German gun, meaning that its protection was insufficient. The proving grounds suggested that the design of the turret and sides of the hull be changed.

Penetration trials showed that the turret needed improvement.

Many complaints were made about the Kirovets-1. Nevertheless, the NIBT Proving Grounds recommended it for production after the necessary improvements were made. It was clear that the tank won't see service as is, but it arrived at Kubinka as an intermediate step to begin with. The most important thing was that the tank was quite reliable, unlike the IS-6. The trials that were carried out resulted in a large amount of improvements. The due dates for the pilot IS-3 batch were moved back and the second iteration of the Kirovets-1 had a lot more to offer than just a pike hull.

1 comment:

  1. Loading the Kirovets-1 photo into an online protractor tool determined the side turret vertical slope was mostly 45 degrees at the bottom, thickest area to going over 60 as you get to the top (War Thunder's summary says the side turret slope goes from 33-69 degrees). I'm surprised the 88 went through even at a 45 degree offset angle of firing!