Saturday 2 December 2017

Less Armour, More Mobility

The gradual increase of a tank's mass is a normal and logical occurrence. Mass grows, first of all, due to thicker armour. During WWII, the thickness of tank armour grew several times over. However, there was one case where designers had to sacrifice armour. This case was the Soviet KV-1S tank, which was, in many ways, a necessary compromise, taken to resolve serious issues with the KV-1's reliability. The vehicle left a mark in the history of Soviet tank building. The KV-1S appeared as a result of a change of perspective in Soviet heavy tank doctrine. Mobility started to play a more important role. The new tank was not just a lighter KV-1, but had a large amount of new technical solutions. What is the history of the KV-1S, and why was its journey into production so difficult?

Diet plan

The problems with the KV-1 tank that surfaced in early 1942 were cemented in its design from the very beginning. The weight reserve of any given tank is limited. If this limit is passed as the tank gets heavier during modernization, reliability issues are guaranteed. Recall that the KV-1 was initially designed as a 40 ton tank. The mass production tank had a mass of 42.5-43 tons in 1940, and this was only the beginning. In 1941, its mass reached 46, then 47.5 tons.

The situation grew even more dire when the Chelyabinsk Tractor Factory (ChTZ, later ChKZ) began installing cast turrets instead of welded ones in the fall of 1941. They were thicker than welded turrets, and the tolerances were such that cases where the mass was heavier than it was supposed to be were common. Naturally, this affected the mass of the KV tank, which reached 50 tons.

This increase in mass could have potentially been just the beginning. The KV-1 was, if not yesterday's news, then a temporary solution. Keep in mind that the KV-3 heavy tank was the priority starting in the spring of 1941. This tank was only temporary as well. Either the KV-4 or KV-5 was supposed to replace it in production. Work on the KV-4 and KV-5 ceased after the start of the Great Patriotic War. As for the KV-3, its production was still supposed to take place at ChKZ.

By the end of 1941, it was clear that these plans will never come to pass, and a new idea arose: the modernization of the KV-1 to a level close to the KV-3. The armour thickness would reach 120 mm in the front (this thickness was also announced during discussion of the KV-7 assault tank). The tank would equal the KV-3 in protection. Work on the U-12 85 mm gun began towards the very end of 1941. The result would have been a "Soviet Tiger": the two tanks had similar levels of protection and firepower.

KV-1 #10033 on trials in February of 1942. It turned out that a more powerful engine did not result in significant performance gains. A more thorough modernization was needed.

The real history of the KV-1 took a different path. The increasing mass of the tank led to a drop in mobility. The changing situation on the front lines demanded greater mobility, rather than protection. Complains began to trickle in. This did not go unnoticed by the State Committee of Defense (GKO). In the evening on January 24th, 1942, the topic of the KV-1 came up during a phone call between Stalin and the Deputy People's Commissar of Tank Production, I.M. Zaltsmann.

There were several solutions for increasing the tank's mobility. The first was raising the engine power to 700 hp. This was a logical solution, but it was insufficient. The reduction in mobility also reduced the tank's ability to get through snow, which was directly stated by Stalin: "The T-34 can travel across snow well, like a flying swallow, but the KV fares poorly."

Stalin pointed out another solution to the mobility problem: the reduction of the KV-1's mass.

ChKZ choose to increase engine power, also somewhat modernizing the running gear. Three KV-1 tanks entered trials in February of 1942. One of them, #25818, was kept as a reference. The second, serial number #6728, received drive sprockets with fewer teeth. The gear ratios in the final drives were also changed. Tank #10033, in addition to these changes, received a V-2KF engine, turbocharged to 650 hp, and a Nastenko pneumatic regulator.

Trials showed that the tank with the more powerful engine exhibited the lowest speed. The engine overheated constantly, and the design changes did not radically improve the situation. Simple turbocharging gave no advantages. As a result, the running gear changes were approved (but never entered production). A note was made that the cooling system needs to be redesigned.

GKO decree #1331, February 23rd, 1942. The first instruction to decrease the mass of the KV-1.

Another deeply rooted issue surfaced in March of 1942. The initial KV-1 project had three transmission options. The model commission selected the 3-speed planetary gearbox designed by students at the VAMM (Military Academy of Mechanization and Motorization). Two more mechanical gearbox variants were kept in reserve. Out of the designs by Shashmurin and Alekseev, the second was chosen, and this was the only one built in metal and put into production. Alekseev's gearbox was based on N.L. Dukhov's propsal, which was, in turn, a further development of the gearbox on the SMK-1 tank. As for the planetary transmission, it was postponed every time. Modernized mechanical gearboxes, such as the one trialled on the T-220 tank, were also postponed. N.F. Shashmurin worked on new gearbox designs.

As for the KV-1, gearbox issues began with the trials of the first prototype. The tank was crossing a ravine during a trial run on September 25th, 1939, after which a noise was heard coming from the gearbox. The cause of this noise was damage to the idler gear and bent reverse gear bar. Gearbox issues resurfaced during factory trials in November of 1939, in addition to trouble with final drives. Gearbox issues cropped up one after the other. More than ten defects were recorded during factory trials of the U-7 prototype in August of 1940.

In addition to gaining weight, issues were caused in the fall of 1941 by factory evacuations and transition to simplified components. For instance, after October of 1941, the friction clutches were simplified. The Ferodo liner was removed. This caused overheating, metal buildup on contact surfaces, and breakdowns of the whole mechanism. The quality of gearboxes decreased, which was another strike against reliability.

"Group 21-212", designed by N.F. Shashmurin. This was one of the most important elements of the future KV-1S.

Investigations regarding to gearboxes began at ChKZ in January of 1942, but the real crisis began at  the start March of 1942. For various reasons, 22 KV-1 tanks from the armoured units of the Crimean Front became disabled. This incident became famous because the Deputy People's Commissar of Defense, Army Commissar 1st Class, L.Z. Mekhlis got personally involved. Lev Zakharovich demanded that new gearboxes be sent to perform repairs, and that Zh.Ya. Kotin should be sent to the front to investigate. Military representatives held a meeting on March 6th that was dedicated to maintenance, repairs, and detection of defective vehicles. A shocking number was announced at the meeting: the amount of defective gearboxes was as high as 40%!

GKO decree #1331 was issued on February 23rd, 1942, before this investigation, ordering the mass of the KV-1 be decreased by 1.3 tons. Decree #1332 was issued on the same day, ordering that the thickness of the side of tank turrets will be reduced to 90-100 mm starting on April 1st, 1942. Further decreased in mass was mandated by decree #1334, according to which the mass of the tank was to be decreased to 45-45.5 tons by thinning out the armour. The decree also proposed that the power of the engines, a fair number of which were also defective, should be increased to 650 hp. 

Diagram of the "group 21-212" gearbox.

Even all of these measures were insufficient. GKO decree #1472, issued on March 20th, 1942, ordered an increase in the quality of engines and gearboxes used on KV-1 tanks. At the same time, the issue of a modernization of the gearbox and design of a replacement was raised. In reality, only the order regarding improvements was completed. Work on new designs was slow. The first 8-speed gearboxes were sent to trials in April of 1942, but mass production deadlines were slipping.

Work on new gearboxes was only expedited after a meeting with Stalin on June 5th, 1942. This meeting was the starting point in the transition from the KV-1 to the KV-1S. The first tank with this index, the one that served as a test bed for the 8-speed gearbox, entered trials in April of 1942. Its  distinguishing feature was not a lighter design, but a more powerful engine. Now, ChKZ was instructed to come up with a deeply modernized KV-1, which was to be faster,  more reliable, but lose the heavy tank's main trump card: thick armour. The results of the meeting were cemented in GKO decree #1878 "On improvement of KV tanks", signed on June 5th, 1942.

Lightening and modernization

The tank was not just lightened and equipped with a new gearbox. There were many more complaints regarding the KV-1. One of the biggest ones related to the cooling system. The tank overheated constantly, which reduced its average speed. The observation devices in the turret were insufficient, especially for the commander. It's worth noting that steps were taken to solve this issue back in early 1941. The T-150 was designed with a commander's cupola. The same cupola would be installed on the T-222 tank (modernization of the KV-1). Now, there was an excuse to install it as a part of a more thorough general modernization.

Modernized radiators that helped bring the engine into normal operating temperature range.

One of the most important components of the modernized tank was the 8-speed gearbox. To be safe, two variants of gearboxes were designed in the spring of 1942. "Group 12-21" was designed by engineer Maryshkin. This gearbox, along with the V-2KF engine, was installed in tank #10279, which was indexed KV-1S. In reality, this was merely a modernized KV-1. It was equipped with KV-3 style water radiators and an oil radiator from the R-ZET airplane. The second gearbox, "Group 21-212", was designed by N.F. Shashmurin. This gearbox was installed on the second KV-1S, #10334. It had a stock cooling system with additional water radiators. The oil radiator was altered in the same fashion.

Trials began in late April of 1942. By then, the specially organized factory #100 was tasked with this experimental work. The trials showed that Shashmurin's "Group 21-212" design was more reliable, and it was chosen as the finalist. The final design was approved in June of 1942.

The KV-1S blueprint data reveals when work was started on the tank. The signature in the "senior engineer" field belongs to M.F. Baldzhi, who was responsible for work on a number of Soviet heavy tanks.

In addition to a new gearbox, the tank received a new friction clutch with four Ferodo disks, new gearbox control rods, a new fan, and a new "Vortox" air filter (also installed on late production KV-1s), later replaced with the more effective "Cyclone". The cooling system deserves a separate mention. It went through trials in July of 1942 in Tashkent. The new cooling system, which included leaf radiators, as well as a fan with stamped blades and a number of other improvements, was installed in KV-1 #10663. During trials involving three different tanks, #10663 performed the best. However, there was one caveat. It took longer than the rest, but the coolant still boiled. It took 7-15 minutes of driving in fourth gear. In other gears, the temperature remained normal.

KV-1S hull. Overall it was similar to the KV-1, but had a number of changes.

Development of the tank itself continued in parallel with trials of modernized components. Like the experimental vehicles, the tank was indexed KV-1S. According to GKO decree #1878, the mass of the tank was reduced to 42.5 tons. M.F. Baldzhi was appointed as the senior engineer, N.L. Dukhov was the manager, and the project was supervised by Zh.Ya. Kotin. L.E. Sychev, G.A. Mikhailov, A.N. Sternin, G.A. Seregin, A.N. Baran, and a number of other engineers from SKB-2 were involved in the project. A.S. Engineers Yermolayev and N.M. Sinev from factory #100 also made significant contributions. I.A. Blagonravov, a VAMM instructor who worked on tank transmissions, also helped immensely.

KV-1S turret. It has many similarities to the turrets of the KV-13 and T-34.

Development of the KV-1S hull began on the next day after the GKO decree was signed. Preliminary work was completed in late June. In general, the design of the hull was the same as the KV-1, but there were differences. For starters, the thickness of the armour was considerably reduced. The applique armour on the front was discarded, and the thickness of the lower front plate was reduced to 60 mm. The sides and rear were also thinned out to that level. Sloping the roof of the transmission compartment resulted in a small reduction in weight.

The KV-1S became the second Soviet tank to have a commander's cupola, after the T-50.

The turret was changed much more drastically. It was nearly redesigned from scratch. There was very little left from the old design. The new turret was closer to the one designed in the spring of 1942 for the prospective KV-13 tank. N.V. Tseits was the senior engineer on this project, which started in March. It is often classified as a medium tank, but in reality it was designed as a heavy from the beginning. There are also certain peculiarities about its name. SKB-2 called it KV-13, but factory #100 referred to it as IS-1. The first drafts of the KV-13 were ready by June of 1942, work was approved, but the development was delayed due to the urgency of the KV-1S project.

Nevertheless, the KV-13's turret (in turn, influenced by the T-34) became the starting point for the KV-1S. Naturally, it had to be redesigned, since the KV-13 had a two-man turret. Nevertheless, the overall design fit very well into the requirements outlined in decree #1878.

Visibility diagram of the KV-1S. There were some issues with dead zones in the front, but the visibility was greatly improved when compared with the KV-1.

An improved shape allowed the reduction of dimensions compared to the KV-1's turret while maintaining sufficient internal volume. As required, the turret received a commander's cupola. The commander's station moved to the rear left corner of the turret, along with the rear machinegun. One big drawback of the KV-1's turret migrated to the KV-1S: there was only one hatch in the turret. The commander's cupola had no hatch. A strange decision, but that was the design of cupolas in Soviet tanks in 1941-42.

Running gear of the KV-1S #15002. The lightened road wheels are visible.

The deadlines set in the GKO decree were only partially met. Two experimental prototypes of the KV-1S were completed by July 27th, bearing serial numbers 15001 and 15002. GKO decree #1958 issued on July 3rd, 1942, influenced the deadlines significantly. According to the decree, ChKZ was tasked with production of the T-34 in August. Because of this, not only did the factory cease production of the C-10 tractor, but had to slow down development of the KV-13 and KV-1S. Production of the necessary T-34 also reduced the volumes of KV-1 production. There were even suggestions to remove the KV-1 from production completely in favour of the T-34, but that proved unnecessary.

608 mm wide track links designed for the KV-1S.

Another serious difference between the KV-1 and KV-1S was the running gear. The changes are partially outlined in the GKO decree. The lightened tank received 608 mm wide track links with characteristic "diagonals". SKB-2 and factory #100 engineers did not stop there: they redesigned the suspension arms, return rollers, idlers, and designed a new lightened road wheel.

KV-1S with serial number 15004, driving along the swampy shore of Lake Sineglazovo near Chelyabinsk during comparative trials. August 1942.

There were certain concerns regarding the reliability of the running gear, especially the track links. For this reason, tanks #15001 and #15002 differed slightly. The two tanks can be distinguished by the rails on the turret: 15001 had them welded to the roof, but 15002 had them welded to the sides. Initially, there was another difference. 15002 was equipped with narrow tracks, and 15001 was equipped with regular 700 mm wide tracks used on the KV-1. The difference between a full set of old and new tracks was 200 kg.

Mobility trials on the swampy shore of Lake Sineglazovo, 18 km from Chelyabinsk, were performed in August. KV-1S #15004 with narrow tracks, KV-1 #10033 with wide tracks, and KV-1 #11021 with narrow tracks too part. KV-1S #15004 finished the course in the shortest amount of time. Even though additional trials were necessary, a decision in favour of narrow tracks was made.

KV-1S #15001 after installation of narrow tracks. The main difference, rails welded to the roof of the turret, is visible.

The main bulk of the trials was performed against the second prototype, #15002. This is not surprising, since the second experimental vehicle was supposed to be identical to the production model. The first prototype did not sit idle; after mobility trials, the tank was equipped with 608 mm wide tracks and used as a test bed for various components.

The same tank in three quarters view. The rear machinegun mount is visible.

The first stage of proving grounds trials for the KV-1S #15002 took place on July 28th, 1942. The average speed achieved in this trial was 22.5 kph. Mobility trials on paved roads lasted until August 5th. During that time, the vehicle travelled for 761 km along the Ufa highway. On August 26th, trials to measure the top speed were held, which lasted for another 40 km. The tank accelerated to 43 kph, which was respectable for a 42.5 ton tank. The fuel expenditure was 250-280 L per 100 km.

Another factor was much more important: reliability. The tank performed a 200 km march without a single breakdown. A small issue with friction clutches was detected the day before, but they were quickly corrected at the factory.

KV-1S #15002 during factory trials, end of July, 1942.

Trials at the Chudovo course were no less important. The KV-1S travelled for 553 km during their course. On August 9th, the KV-1S completed a 300 km dirt road test drive with an average speed of 20 kph, a signal that the tank was ready for production. Depending on the conditions, the tank consumed between 200 and 350 km per 100 km of travel. Another 673 km were travelled along the swampy shore of Lake Sineglazovo. Here, the KV-1S expended 300-350 L per 100 km, and demonstrated an average speed of 15.5 kph. The gearbox and cooling system functioned normally. The requirements for reliability were met.

KV-1S #15001 during trials in the winter of 1942-43. A new 650 mm wide track was tested on this tank. The new tow hooks are visible. They later migrated to KV-1S #15002.

Firing trials were completed successfully. The crew was dressed in constraining winter clothing, but despite that, a rate of fire of 6 RPM was achieved. The reduction in the size of the turret did not decrease the comfort of operating in the fighting compartment.

KV-1S #15002 during reliability trials, winter of 1943. The tank has new tracks and modernized road wheels.

After completing trials, both KV-1S prototypes were turned into test beds. Various components were tested on these tanks, including new track links and modernized road wheels. The future held a turbulent fate in store for tank #15002. It later turned into an experimental KV-85, then KV-122. This tank survived to this day and is on display on a pedestal in Avtovo (Saint-Petersburg).

Metamorphosis at a difficult time

Launching the KV-1S into production proved a difficult task. Zaltsmann, who was appointed as the People's Commissar of Tank Production as a replacement for Malyshev, received a high priority task to begin T-34 production at ChKZ. Malyshev was removed from his post for failing to meet T-34 production targets. It's not hard to see why Zaltsmann prioritized T-34 production over the factory's own KV-1S. Nevertheless, assembly of KV-1S tanks began in late August. The GKO decree's requirement to begin production of "KV tanks with commander's cupolas" by September 1st was satisfied.

Assembly of the KV-1S at ChKZ, September 1942. The tanks still have KV-1 road wheels and no rails on the turrets.

The first production KV-1S tanks were almost identical to tank #15002, aside from the changes requested in the trials conclusions. There were some changes on the inside. According to GKO decree #1878, the first 25 KV-1S tanks were to be built with 5-speed gearboxes. In reality, at least 39 tanks with this gearbox were built (this number includes several KV-1 tanks). These tanks have no external differences when compared with 8-speed gearbox tanks.

By the end of August, the tanks coming off the assembly lines looked different than the prototypes. Due to defects in casting, ChKZ was forced to install KV-1 road wheels on KV-1S tanks. This increased the tanks' mass by 390 kg. Malyshev, who remained on the State Committee of Defense after being fired, learned about this from the GABTU on August 19th. In addition, rails were no longer installed on tanks starting in September. 34 KV-1S tanks were built in August, and 176 in September.

This road wheel was designed in August-September of 1942, based on the KV-13 road wheel design. Due to issues with quality and robustness, it did not remain in production for long.

Issues with road wheels forced SKB-2 to come up with a new design. The design bureau didn't reinvent the wheel, and based the new design on the KV-13's road wheels. Even though, officially, work on the KV-13 was frozen, unofficially its components were not only developed, but tested. In July-August of 1942, a KV-1 tank was used to test KV-13 road wheels. The design of the road wheel was altered, and it was installed on KV-1S tanks in the fall of 1942.

GKO decree #2420, allowing the production of KV-1S tanks with KV-1 hulls.

Bigger problems began in the fall of 1942. They were connected with T-34 production. By October, UZTM, one of the two suppliers of KV hulls and turrets, switched entirely to T-34 hull production. As a result, factory #200 was left as the only producer of KV hulls, and it was incapable of dealing with the increased demand. Only the existing stock of KV-1 hulls mitigated this crisis.

A KV-1S with a KV-1 hull. This tank is from the 9th Guards Heavy Tank Regiment. Spring of 1943.

On October 15th, 1942, Molotov signed GKO decree #2420, allowing the KV-1S to be produced with KV-1 hulls. These tanks were built starting in the second half of October to early November of 1942. Fewer of these "centaurs" were produced than the authorized limit of 100. ChKZ understood perfectly well that the increased mass would impact reliability. KV-1S hulls were used as soon as possible. 70 KV-1S tanks with KV-1 hulls were built in total, which were sent to the 9th, 10th, and 12th Guards Heavy Tank Regiments, as well as the Ulyanovsk Tank School.

Knocked out KV-1S with a KV-1 hull, North-Western Front, February of 1943. This is likely a tank from the 12th Guards Heavy Tank Regiment.

Even though only a few KV-1S tanks with KV-1 hulls were built, they did not vanish without a trace. These tanks were caught in several shots by both Soviet and German photographers. One of these tanks, presumably from the 12th Guards Heavy Tank Regiment, survives to this day. The vehicle sank in early 1943. In 1998, the tank was raised from the swamp and installed on a pedestal in Parfino, Staraya Russa region, Novgorod oblast. It has those very same road wheels based on the KV-1S design. The tank also has links with "diagonal" sides. The very first KV-1S road wheel design survived to this day too. It can be seen on KV-1 #43666, displayed at the Central Museum of the Armed Forces of the Russian Federation. 


  1. What's to recommend this tank over a T-34? A roomier turret?

    1. Three man turret, commander's cupola, better gearbox. You can make some argument for better all-round armour, I suppose. But you're right, there weren't many things that the KV-1S could do that the T-34 could not.

    2. Armour is better only on the turret. Thinning out the armour from 75 to 60 mm made it vulnerable to PaK 38 even from the front, while before it was almost invulnerable from the front and side. In some sections it is more vulnerable than T-34. Cupola has no hatch. Surprisingly in some/many aspects from mobility, KV-1S is better. Mainly in off road performance thanks to better gearbox and totaly superior tracks. But this also pay for his succesor.