KV-13

Other members of the design team were K. I. Kuzmin (hull), N. M. Sinev (turret), S. V. Mitskevich (chassis) and G. N. Moskvin (general assembly). KV-13 was designed as a universal tank — medium tank weight and heavy tank protection. This project was distinguished by its extensive use of cast armour. Casting was used not only for the turret, but also for the main elements of its hull — the glacis, turret ring and rear. This reduced the usable internal space, but increased the effective armour protection whilst reducing the amount of armour needed for its manufacture. The last point was especially important in light of the State Defense Committee's decree of 1942 to minimise the consumption of armour materials.

The first test unit was designed and produced in an extremely short period of time and in May of 1942 it was delivered to the factory testing facilities. The tank weighed 31.7 tonnes and was armed with a 76.2 mm ZiS-5 gun and a coaxial DT TMG. The maximum front armour thickness of its hull was 120 mm, that of its turret — 85 mm. Its V-2K 600 h.p. engine allowed it to reach speeds of up to 55 km/h. Elements of the T-34's chassis, including tracks, were used, whilst the road wheels were taken from the KV. The KV-13 had an improved (U-shaped) radiator similar to the one previously used on the Kirov Factory variant of the T-50 tank. This allowed for a more efficient engine block configuration and also increased the efficiency of its air intake.

Several flaws surfaced during the testing of the first experimental unit of the KV-13: poor acceleration due to transmission problems, tracks and rollers easily damaged, tracks being thrown while making turns, etc. In July 1942, in the middle of its testing, the head designer N. V. Tseits died and N. F. Shamshurin was appointed in his place. On his initiative the KV-13 received the transmission developed by F. A. Marishkin for the KV-1S, as well as some other parts of its chassis. However, even after these improvements, the tank did not pass its tests and the military quickly lost interest in it. Despite these early failures, in December of 1942 the assembly of two new variants of the KV-13 began at the Experimental Tank Factory. The new vehicles shared only the hull, torsion bar suspension, and chassis from the first version. The turrets and many other elements were completely new designs. The transmission used was significant in its use of a planetary 2-step travesing gear designed by A. I. Blagonravov. The cooling system was improved, while the track links were lightened by making every other link flat (the so-called «Chelyabinsk tracks»).

The appearance of the German heavy Tigers on the Eastern Front played a direct and decisive role in increasing the speed of development of these new models. Kirov factory in Chelyabinsk and factory No.100 (the new name of the Experimental Tank Factory) of of NKTP were to manufacture two experimental tanks of the new «Josef Stalin» (IS) class, based on the two latest models of the KV-13, and prepare them for testing. The model armed with a 76.2 mm gun was designated IS-1 (Object 233). The second model, armed with a 122 mm U-11 tank howitzer was designated JS-2 (Object 234).