The KV-85 was a stopgap solution to the problem of replacing the ageing KV-1 heavy tank. A new turret, intended for the IS-85 (later redesignated the IS-1) and armed with the same 85mm (3.34in) gun as the T-34/85, was mounted on the KV-1-S chassis.
Concurrent to the development of the new self-propelled gun series in 1943, Soviet design teams completed longer-term projects for a new generation of heavy tanks to replace the ageing KV-l types. By the start of 1943, 21 heavy tank designs had been created, but at one point, work was threatened when Josef Stalin decided to cancel heavy tank production after scathing reports about the poor mobility and armament of the KV-l from experienced commanders such as General P. A. Rotmistrov. Fortunately Stalin relented under the combined impact of lobbying from the NKTP, and the more immediate need to counter new German medium and heavy tanks.
The need to hasten the deployment of heavy tanks to counter the German threat led Lieutenant-Colonel Kotin to divide his TsKB-2 bureau to form two teams, each with their own design specifications. One team was instructed to undertake a modernization of the KV beyond the KV-l-S, which was being introduced in mid-1943 as a stop-gap for the new heavy tank. This project was eventually designated the KV-85.
The vehicle was essentially a reworked KV-l-S hull, which increased frontal armour from 82mm (3.2in) to 110mm (4. 3in) .The second team, headed by N. V. Tseits (recently released from a forced labour camp), began work on the KV-13 which had a heavily re-designed hull and chassis. This vehicle, designated IS-85 (IS - losef Stalin), was superior in mobility and had better all-round armour protection than the KV-85.
Technical delays in completing the IS-85, compounded by urgent requests from the front for tanks with more powerful armament, led to 148 KV-85s being produced as a temporary expedient in the autumn of 1943 by placing the IS-85 turret mounting a 85mm (3.34in) D-5T gun on the KV-85's hull. The larger turret mounting of the KV-85 increased the size of the under turret box which could only be accommodated by modification of the hull's width. The fifth crew member was also lost because of the demands for ammunition racks containing 70 rounds each, as well as because of the size of the gun's breech.
The IS-85 was based on the KV-13 prototype. Although the development process received a significant increase in resources in 1943, the vehicle was in fact the fruition of a much longer process of evolution than the hasty KV-85 design. The first KV-13 prototype was tested by the Experimental Tank Factory at Chelyabinsk as early as May 1942. One of the most notable features was the extensive use of casting in the manufacture of the turret and large sections of the hull. The initial trials revealed flaws in the vehicle's transmission, as well as a tendency to damage and throw tracks.
Despite remedying many of these defects through adopting parts of the KV-l-S chassis and transmission, further work was slowed by the previously noted reticence of Stalin and many military personnel about heavy tanks. Even so, the drive and self-belief that had characterized Kotin's earlier work on heavy tank design saw him instruct the design team to re-develop the internal workings of the experimental vehicle. This was accomplished by the start of 1943 with only the hull, torsion bar suspension and chassis of the first prototype retained. Within a few months, Kotin's initiative was justified when the demand for new heavy tanks was realized by the GKO. Whilst the KV-85 was rushed into service, work continued on the IS-85, which was subsequently renamed IS-I. In August the completed model was demonstrated to Stalin to great approval, and production was authorized.
The design of both KV-85 and IS-1 were strongly influenced by the need to mount a more powerful gun than the standard issue 76.2mm (3in) gun, which was relatively ineffective against the Tiger and Panther. Test-firing of various gun calibres against a captured Tiger at the Kubinka testing grounds showed that the 85mm (3.34in) 52-K model 1939 AA gun could penetrate the vehicle's 100mm (3.93in) frontal armour at up to 1000m (9144yd), although the low quality of Soviet telescopic sights meant that in practice, these long-distance ranges were rarely achieved by the Soviet crews with any real consistency.
Under orders from the GKO, the Central Artillery Design Bureau tested the S-31 and D-5T gun variants. The latter proved the more effective weapon and was accepted for production. The installation of an 85mm (3.34in) gun affected the construction of the turret designed for the KV-85 and IS-1, requiring an increase in size to avoid greatly degrading the crew's workspace and the vehicle's combat effectiveness. This increased the hull size and weight of both of these heavy tanks.
KV-85 AND IS-1 IN COMBAT
The combat performance of the KV-85 and IS-1 series was chequered. In one engagement in the Ukraine in November 1943 the 34th Guards Heavy Tank Breakthrough Regiment was repulsed with the loss of one-third of its 20 KV-85s by fire from Panzer IVs and Marder II self-propelled guns. Naturally, tactical factors could influence the level of loss, and it is worth noting that on the following day, a German counterattack was beaten off with no Soviet casualties.
However, after-action reports collected by the GBTU highlighted the need for better protection and a larger gun in order to engage German tanks at longer ranges. Similar comments were recorded about the IS-1 after it entered service in September 1943. A major problem was the need for greater protection against long-range enemy fire. Whilst enemy rounds at long ranges did not always penetrate the IS-1’s armour, their impact created splintering inside the turret, and wounded the crew.
That said, as the engagement of 4 March 1944 at Staro-Konstantinov, which involved 1st Guards Heavy Breakthrough Regiment, would prove, the IS-1 was more than capable of standing up to the might of the Tiger; it was the gun that was the real issue with both of the new heavy tanks. The production of the KV-85 was therefore terminated at the end of 1943, and following that, the IS-1 was re-armed in early 1944.