One of the experimental prototypes developed, the “Object 233 IS No.1“, took advantage of the hull of the KV-13 model and had the 76.2mm ZiS-5 gun installed. The vehicle weighed 32 tonnes and had a 600 hp engine, with a maximum frontal hull armour of 120mm and a maximum of 85mm thickness in the front of the turret. It was built in May 1942 and was tested between March and April 1943 with satisfactory results.Another experimental model presented was the “Object 234 IS No.2” in which the hull of the KV-13 was maintained but the turret mounted the 122mm U-11 howitzer, designed for the KV-9 experimental tank. It was also tested between March and April 1943 with positive results.Here we have the 2 prototypes developed in the Kirov factories of Chelyabinsk in 1943. Neither of them was mass produced, although they helped the final development of new heavy tank capable of facing the new German Tiger and Panther tanks. Despite these tanks had good speed and protection, in the tests they suffered from transmission failures and poor mobility on soft ground due to their roadwheels arrangement.In the summer of 1943 the “Object 237 IS No.3” prototype was presented. It was armed with the 85mm S-18 gun installed in a new turret and with an elongated chassis with one more roadwheel. This prototype would be quite close to the model chosen later for production.In June 1943, a prototype was built with the 85mm S-31 tank gun. The S-31 was the result of joining an 85mm barrel with the base of the 76.2mm ZiS-5 tank gun. This prototype was called “Object 237“, and later IS-85.The IS-85 (Object 237), was selected to be mass produced after successful tests in August 1943. In principle it was to be armed with the 85mm gun, which had proved to be powerful enough to defeat the frontal armor of the Tiger tank, but after the Battle of Kursk results, it was decided to mount a larger caliber.This is the magnificent aspect with which the new IS-85 heavy tank came into service. The tank was definitively named IS-1 to facilitate the identification tasks. The first production units had installed the 85mm D5-T gun. The D5-T gun fired three different types of projectiles, the UO-365 HE fragmentation type, the UBR-365 armour piercing (AP) type and the UBR-365P armor-piercing sub-caliber shell.The hull of the IS-1 was the same of the KV-1S heavy tank with some modifications due to the larger size of the turret. Its size increased, adding one more roadwheel and reinforcing the suspension, which had given problems during tests at Kubinka proving ground.The career of the IS-1 was extremely short, because having 85 and 100mm guns, its combat effectiveness against German tanks was really poor. They were retired at the beggining of 1944 to be reconverted to the IS-2 model, armed with the powerful 122mm D25-T gun.One of the few Soviet units that used in combat the original IS-1 tanks was the 13th Guards Heavy Breakthrough Tank Regiment, which lost the practical totality of its 21 assigned IS-1s between February and March 1945. The subsequent analysis of these battles was what led to the transformation of all remaining IS-1 tanks to the IS-2 model.
