Saturday, 9 March 2019

Intermediate IS

Joseph Stalin signed GKO decree #4043 "On the production of IS tanks" on September 4th, 1943. This was the end to a long process of the creation of a new Soviet heavy tank that could replace the KV-1 in production. While it was being developed, Chelyabinsk had time to master both the KV-1S and KV-85. However, such a long journey was not for nothing: the Red Army received a completely new tank, the combined characteristics of which put it among the best heavy tanks in the world at the time. The age of the IS-1 (IS-85) was not long. Very shortly it gave up its first place to its descendant, the IS-2, which became the best Soviet heavy tank of the Great Patriotic War.

A difficult beginning

The first step in putting the IS-85 into production was producing the KV-85. Both tanks used the same turrets. A number of issues that were unavoidable when putting a completely new design into production came up with the KV-85's turret and armament. It was clear that putting a whole new tank into production would be difficult. The deadlines set by decree #4043 were very lenient. The first 25 tanks were due in November of 1943, and 75 in December. The KV-85 would remain in production until November, and the SU-152 until December of 1943.

The hull of the IS-85 was similar to that of the Object 237, but with some improvements.

However, in practice these deadlines were harder to meet than it appeared. Many parts and assemblies were completely new, including the hull, which featured a large amount of cast components. Barring experimental fully cast hulls for the KV-1, this was the largest amount of cast parts used in a domestic tank to date.

The thickness of armour also increased compared to the KV-1S and KV-85. Factory #200, the producer of hulls and turrets, ran into this problem when casting turrets for the KV-85 and Object 237. You can see characteristic weld seams on some turrets that were used to repair cracks. To be safe, factory #200 began working on 5 IS-85 hulls in early October 1943, even though the quota was only two.

The turret underwent minimal changes compared to the KV-85 turret.

As a result of trials of the experimental IS tank, a number of comments and complaints were made, some of which had to be addressed before the tank went into production. For practical reasons, a number of modernizations had to be postponed. For instance, the engine. The commission considered that the V2-IS engine should be replaced with a 600 hp motor. The need for this improvement was also reflected in decree #4043. Changes to the driver's station had to be made to improve his working conditions, including raising the observation device.

The exhaust system was also to be changed. T-34 type exhause pipes were considered suboptimal, since the gases ended up in the air intakes. Elements of the transmission, cooling system, and a number of other systems also needed improvements. The IS-85 inherited a number of issues from the KV-85, especially issues with the aiming mechanisms.

Cutaway of the IS-85.

A meeting was held on September 13th where the improvements were prioritized. Instead of a horseshoe shaped copper radiator recommended by the commission, a steel finned radiator was developed. Turbocharging the V-2 engine to 600 hp was postponed to January 1st, 1944. Neither the IS-85 nor the IS-122 (IS-2) ever received such an engine, in the end.

Out of 19 changes, 2 were put into production immediately, and 12 more were to be tested until December 1st. It was decided to increase the number of spare track links to 10 and mount them on the front of the hull, but this change was not implemented until the summer of 1944. Rubber mudflaps, like the ones used on the Valentine tank, were considered for implementation, but this idea was dropped in mid-December, since the flaps were difficult to produce and there was still a shortage of rubber.

November 1943 production IS-85, serial number 31113, NIBT proving grounds, January 1944.

The replacement of half-link tracks with full tracks without a tooth was among the changes that would be introduced starting on December 1st, 1943. This eventually came to pass, but half-link tracks were still used on the IS-122, ISU-152, and ISU-122. Photos showed that tanks of one type could have several different tracks. For instance, an IS-85 produced in November 1943 tested at the NIBT proving grounds had tracks that alternated links with teeth and tracks without a tooth. At the same time, a tank tested in the summer had tracks composed entirely of links with teeth.

Work on improving the track links continued in parallel with production of the IS-85. Trials of the Object 237 #1 tank equipped with track links made from LVT steel took place between November 20th and November 30th. These tracks showed themselves poorly: 18 teeth broke over a 965 km march. Trials continued through December. Track links from LVT and 27 SGT steel were tested. Object 237 #1 was also used to test various components that would go into production.

Testers pointed out a number of shortcomings, chiefly dealing with production quality.

Preparation for mass production of the IS-85 followed the same pattern as with the KV-85. A number of blueprints still needed work, and changes to individual assemblies happened in parallel. There was a significant issue with preparing staff. ChKZ took the problem seriously. MKh-2 and MKh-3 assembly workshops were completely reorganized. An assembly line process was worked out. Keep in mind that the factory was still loaded with T-34 production. These tanks remained ChKZ's main priority. Starting with October 1943, the factory delivered 360-370 T-34s monthly. In total, 3593 T-34s were built at ChKZ in 1943. This was more than the number of heavy tanks and SPGs built in 1943 in total.

The situation only got better in the spring of 1943 when production of T-34 tanks at ChKZ ended.

The production IS-85 turret was the same as on the KV-85.

Despite all issues, the IS-85s were ready by the end of October of 1943, ahead of schedule. This was possible in many ways due to two backup sets of hull and turret armour. Externally, these tanks were similar to the Object 237, but it was easy to tell them apart. The production tank has the driver's observation device raised higher and new exhaust pipes: their design was simplified and they received special grooves that directed the gases to the side, so they wouldn't go into the air intakes. The upper rear plate was also simplified.

The driver's observation device allows us to tell this tank apart from the Object 237.

Thanks to a backlog of hulls, the first tank due in November was delivered on November 5th. However, an expected dip in production numbers took place right after. Only 3 tanks were delivered by November 16th, 7 by the 20th, 14 by the 25th, and 24 by the 30th. The last "November production" tank was delivered on the morning of December 1st.

The tanks were built in a very tense atmosphere. According to a report from ChKZ for 1943, most workers knew nothing of the IS-85 by the time it started production. The first 10 tanks were built with assistance from those who participated in the assembly of the experimental Object 237. Thanks to this, the first 10 tanks were of satisfactory quality. However, when mass production began the quality dropped noticeably. Many issues had to do with insufficient technical documentation. 200 mistakes were found in November alone. As a result, out of the 25 tanks that were delivered only 14 passed QA. The rest had to be worked on some more.

The first marches revealed a serious design defect: the right track hit the rim of the first suspension arm, which deformed the tracks. Issues with separators of ball bearings in the planetary turning mechanism also came up. This defect was discovered in late November. As a result, the QA process had to be stopped temporarily for the ball bearings to be changed.

The new exhaust pipes can be seen from above.

There were also plenty of issues with hull production. Only one hull and turret were delivered in the first third of November, 22 hulls and 13 turrets were ready by the 20th, and 58 sets in total were built in November. They were built from 42s type steel. On NKTP orders, 5 more hulls and turrets built from highly hardened 52s steel were started. 2 were delivered in November. 10 more were planned for December.

Many issues were revealed during the casting process. The amount of assembly and welding work needed was twice as much as on the KV-1S. There was a deficit of oxygen for welding. A second change in heavy tank production in the span of one year did not help matters, as the factory had to build 3 types of hulls simultaneously, and 4 types starting in October.

The D-5T gun did not entirely meet the requirements of modern war by the time the IS-85 went into production.

All things considered, the launch of the IS-85 was not as full of drama as the launch of the KV-85. This is especially notable considering the fact that the KV-85 was merely a modernization of the KV-1S, while the IS-85 was a brand new vehicle.

There was additional pressure on ChKZ and factory #200 due to the fact that the IS-85 was only an intermediate production. Decree #4043 stated that the IS-85 would soon be replaced with a brand new tank. This was the IS-122, later IS-2, accepted into service on October 31st, 1943. This was the same IS-85, but with a more powerful 122 mm D-25T gun and minor changes to the turret. The acceptance of the IS-122 meant that the IS-85 program was reduced in scope. Instead of 75 tanks, only 40 would be built, but this was not a reprieve for the factory. Instead of the 35 cancelled IS-85 tanks, 35 IS-122s had to be built in December.

IS-85 on a snowy winter road.

Factory #200 planned to produce 90 IS-85 hull and turret sets in December of 1943, but this plan was also corrected. Now 40 of the turrets had to be built to accommodate the D-25T gun. In total, factory #200 built 163 sets of hulls and turrets for IS tanks, of which 123 were for IS-85s.

The factory had new problems come December. A cavity was found inside of the large and complex front hull part, with a porous area in the bottom. A change to the production process solved this problem only partially. As a result, the management of workshop #1 lost its jobs. This defect was not the only cause. It was revealed that acceptance of hulls and turrets was done without finalized technical requirements until early 1944.

Visibility diagram from the fighting and driver's compartments.

Due to a large amount of existing hulls and turrets, the IS-85 remained in production for a little longer. ChKZ built 40 IS-85 and 35 IS-122 tanks in December, and 40 more IS-85s in January. In total, 107 IS-85 tanks were built. Due to a small production run, the IS-85 was not changed significantly. In addition to the alterations to tracks, a new turret traverse mechanism was introduced in January 1944. Around this time IS tanks received new TSh-15 sights, but it's likely that the IS-85 was still equipped with the earlier 10T-15 sight.

The quality increased through December and January, but there were still some complaints until the very end. The acceptance criteria were still not finalized up to the end of production. A number of required changes remained on paper.

Intermediate result

Correction of defects on tanks assembled in November and December led to a delay in shipping the tanks out. The first large delivery of these tanks was made on December 23rd, 1943, to the 58th Guards Tank Regiment. 21 tanks were sent there. The same number went to Tula to equip the 8th regiment, and 17 more to the 13th regiment, also refilling in Tula. 61 tanks were issued in December.

The 13th Guards Tank Regiment received its missing 4 tanks in January. 21 tanks were sent to the 1st Guards Heavy Regiment. 15 more tanks were sent to Tula on January 27th. 4 regiments were fully equipped with IS-85 tanks, and at least one partially.

An IS-1 at the NIBT proving grounds, summer of 1944. As you can see, this tank is rather beaten up.

The structure of Guards Tank Regiments armed with the IS-85 tank was the same as the structure of other similar units. The regiment consisted of 4 companies of 5 tanks each and a commander's tank. In February of 1944, before the IS-85 saw combat, TO&E 010/460 was passed, renaming Guards Tank Regiments armed with heavy tanks to Guards Heavy Tank Regiments. In addition to 21 heavy tanks, each regiment contained 3 British Universal Carriers and 1 BA-64B armoured car for reconnaisance. Around this time the IS-85 was renamed to IS-1. However, there was also confusion, as these tanks were also called KV-85 in documents. The IS-2 received the same treatment. An unprepared researcher can be shocked by reading about orders to train KV-2 crews in the summer of 1944. The index KV-122 also applied to the IS-2.

All of this tank's tracks had teeth.

Since the tank was very new, the brass needed accurate information on the combat and driving experience with the IS-85. Engineer-Major A.I. Shamin, senior assistant to the Chief of the Experimental Department at the NIBT proving grounds, was attached to one of the regiments equipped with new tanks. This was not the first trip of this type, and the Major had no shortage of experience in this role. He was the author of the field repair instructions for KV-1 tanks. Shamin arrived at the 13th Guards Heavy Tank Regiment on February 5th, 1944.

By February 5th the tanks had already received their crews. Shamin read a lecture to notify the tankers of the differences between IS-85 and KV-1S tanks, as a number of the crews had experience with them, as well as the peculiarities of the new tanks. Exercises were also held with the regiment's repair crews. Repair master Moskalenko from ChKZ was also attached to the regiment. His presence was fortunate, as the tanks suffered from predictable growing pains. The gearboxes on two tanks had to be replaced.

The upper rear plates of the Object 237 and the production tank were very different.

The 13th Guards Heavy Tank Regiment went to the front on February 10th. On the 15th, the train with the tanks arrived at Fastov, where they were fuelled up. The tanks were offloaded at Belaya Tserkov station. By 7 am, the tanks were concentrated at the south outskirts of Dzhurdhzhentsy village. Not all of the tanks gathered here: 5 were late due to technical difficulties. Three had their engines stall from air entering the fuel system and two more had other engine issues. Even though these problems were not critical, the regiment was not fully equipped for its first battle.

The combat debut of the IS-85 took place on February 19th. 1st company (5 tanks) supported the attack of the 109th Tank Brigade towards Lisyanka. This debut was not great: Panthers in ambush allowed the IS-85 to close to 600-800 meters and opened fire. Two heavy tanks burned up, three more were disabled. The failure of the 1st company was due to the way the tanks were used. Instead of sending heavy tanks in the second echelon, they were used as a battering ram at the very front.

Map of the IS-85's first battle, February 19th, 1944.

The IS-85 was built with the expectation of taking frontal fire from the 88 mm KwK 36 gum used by the Tiger tank. The Panther turned out to be a more dangerous enemy. These tanks were faster, had better frontal armour, and were armed with the 75 mm KwK 42 gun that had higher penetration. The gun could penetrate the front of an IS-85 tank from 600-900 meters.

A number of penetrations were also due to the design of the hull and turret. Two tanks were hit into the driver's hatch, one of which had it knocked out by a ricocheting shell. In two more cases, the hull was penetrated in its right side. Shamin criticised the shape of the front hull in his notes.

A diagram of hits on an IS-85 tank from Panthers during the fighting for Lisyanka. #2 marks the shell that ricocheted and knocked out the driver's hatch.

A pause until March 2nd followed the unfortunate debut. Subsequent battles showed that the tanks were well protected against Tiger fire. 88 mm Flak 18 guns opened fire from an ambush from 600-800 meters. One of the tanks received 6 hits, but only one penetrated, the one that landed on the right side of the front hull. However, a crack formed on the side after one of the hits.

One tank also suffered a penetration of the lower front place from the exotic 2,8 cm s.Pz.B. 41.

5 running tanks remained in the 13th Guards Heavy Tank Regiment after the fighting for Uman. Two more tanks broke down during the march to Yampol. The remainder made it to Beltsy, where they also stopped due to technical issues.

T-34 and IS-1 tanks from the 3rd Guards Tank Corps enter Uman, March 1944.

Two tanks from the 13th Guards Heavy Tank Regiment wee total writeoffs. The remainder were either only knocked out or suffered breakdowns. One tank fell into the river while crossing a bridge near Polkovnichye. The crew died, but the tank was later pulled out. Shamin remained with the regiment until May 2nd, 1944, after which he returned to the NIBT proving grounds. He received an Order of the Red Star for his actions. The Engineer-Major composed a detailed report, and the conclusion was far from positive.
"1. The IS-85 heavy tank does not have sufficient advantages in combat with enemy heavy tanks. The combat qualities of the IS-85 are lowered by a number of design and production flaws:
  1. The armament of the IS-85 is not appropriate for the enemy's heavy tanks.
  2. The front of the hull and turret are insufficiently protected.
  3. The engine cannot be started quickly. The electro-inertial started is unreliable.
  4. The gearbox, running gear, and final drive bearing are insufficiently reliable.
  5. The quality of welding seams is insufficient.
  6. The cruising range over dirt roads or off-road is short.
  7. The engine heater is unreliable.
  8. The tank does not carry enough fuel.
2. To improve the IS-85 heavy tank, it is necessary to:
  1.  Install a more powerful gun than the D5-T-85.
  2.  Improve the front armour of the hull and turret by using a steeper slope and higher quality armour.
  3.  Increase the reliability of mechanisms and components of the tank by means of design and production changes.
  4.  Improve crew training."
This report came late. On March 10th, 1944, less than a month after the first IS-1was used in combat, the IS-2 made its debut. It turned out to be much more successful. Nevertheless, Shamin's report had an influence on the development of the IS tank family. The penetrations of the front armour indicated that it needed to be changed. Other NIBT proving grounds specialists wrote about this before Shamin returned from his trip.

Effect of fire from the 88 mm Flak 18.

The peak of the IS-85's combat career was in the winter and spring of 1944. The 8th Guards Heavy Tank Regiment was the neighbour of the 13th regiment, both subordinate to the 3rd Tank Corps. The regiment arrived on the front at around the same time, February 19th. 5 tanks remained in the regiment by March 5th. Like with the 13th regiment, a number of losses were due to technical reasons. The 58th and 1st Guards Heavy Tank Regiments fought within the 1st Guards Army in March of 1944.

An order was given on March 24th to pull out the regiments to be reformed. All surviving IS-85s were pooled into the 1st Guards Heavy Tank Regiment. Almost all regiments that were armed with the IS-1 were given IS-2 tanks. The IS-1 resurfaced as a commander's tank, for instance in the 71st Guards Heavy Tank Regiment.

The only regiment to retain IS-1 tanks for a long time was the 1st regiment. By April 2nd it still had 5 tanks of this type. As of May, the regiment had a rather mixed composition: 18 IS-1, 4 IS-2, 5 SU-76, 5 ISU-152, and even a T-70. Of those, 15 IS-1s needed major repairs, and were soon pulled out. In July 1944, at the start of the Lvov-Sandomierz offensive, the regiment had 12 IS and ISU vehicles, 3 T-34s, and 5 SU-76es. The regiment remained heterogeneous: on August 6th it contained 6 IS-1, 1 IS-2, 1 KV-85, and 1 ISU-152 running. One KV-1S, one IS-1, one IS-2, and two ISU-152s were in need of medium repairs.

The regiment took part in the East Carpathian Operation in this form. This was the last massed use of IS-1 tanks. In addition to the 1st Guards Heavy Tank Regiment, these tanks were used here by the 12th Guards Tank Brigade, which was refilled with 11 IS-1, 4 IS-2, and 30 T-34 tanks by October 18th, 1944.

Knocked out IS-85s, Slovakia, fall 1944. It's possible that these were tanks from the 12th Guards Tank Brigade.

Few IS-1s survived the war, but one of them managed to take part in a parade in the late 1940s. Not a single tank of this type survives to this day. It's unlikely that the tankers took to it kindly, as it was accompanied by too many technical issues. It also did not have as good a gun as the IS-2. However, the IS-1 was an important milestone in domestic tank building and the last step towards the IS-2, which became one of the best tanks of WWII.


  1. I have lost the reference but I thought the debut if the IS was in the battle of Korsun–Cherkassy, ut looking at the information displayed here it looks my information may be incorrect. Have you ever readed anything about the use of the IS in that battle? Thanks

    1. Take a look to this document, KV85 was fighting in November 26th 1943

      Here you have a translation to Spanish

    2. Th village of Lysyanka is indeed part of the Korsun area.

    3. That's right Unknown, thank you. Were can I get more information about the IS operations in that battle?