The SU-85 was a Soviet tank destroyer based on the SU-122 assault gun, which was itself built on the chassis of the T-34 Medium Tank. During 1942 the Soviet high command had been convinced that the 7.62cm guns of the T-34 and KV tanks were powerful enough to deal with any German tank, but in January 1943 a Tiger was captured during the fighting around Leningrad. The armour on this tank was too strong to be penetrated by the 7.62cm gun at any acceptable combat ranges, and an urgent programme was put in place to develop a weapon capable of dealing with it.
The first priority was to develop a tank gun capable of penetrating the Tiger’s armour. Tests revealed that the 85mm anti-aircraft gun and 122mm A-19 corps artillery gun were both able to do this. The design bureau of General F.F. Petrov was given the job of producing a gun that could use the same shells as the 85mm gun, and be mounted in an armoured vehicle. Their work produced the D-5S 85mm gun.
The next step was to mount this gun on the chassis of the SU-122. This vehicle resembled the German Jagdpanther, with a fully armoured fighting compartment at the front of the tank, and the engine at the back. The SU-122 carried a 122mm howitzer, and did not have the correct optical gear for a direct fire anti-tank weapon. L. Gorlitskiy’s design team at Uralmash, who were responsible for mounting the gun, produced a new ball mount for the 85mm gun. The new mount was equipped with a TSh-15 telescopic sight. The new gun and mounting also required a redesigned superstructure, although the basic outline of the vehicle remained the same.
Production of the SU-85 ran from August 1943 to September 1944, when it was phased out in favour of the SU-100. It was used to equip 12-strong separate self-propelled battalions, allocated to army and front commands for use on special missions, and in medium self-propelled regiments, equipped with four batteries of four SU-85s and a T-34 command tank, and used as part of the mechanised corps. During 1944 they were also used as part of the anti-tank artillery brigades.
The SU-85 was designed to operate behind the infantry or tank force it was supporting, picking out the thicker armoured German tanks from long range. The lack of a defensive machine gun made it very vulnerable if it got too close to German infantry. It entered combat in the fighting on the Dnepr River and in the Ukraine late in 1943, and gave the Red Army a weapon capable of dealing with the newly introduced Panther Medium Tank.
After the SU-85, the SU-100 is be designed. This tankdestroyer is in principle the same design but with some modifications. A 100mm gun, wich was be larger then the 85mm gun from the T-34/85 and SU-85, heavier armor (from 45mm (SU-85) up to 75mm (SU-100)) and some details in the hull like the gun mask and the roof are different. Also the SU-100 had the same commander cupola from the T-34/85. The SU-85M was the 85mm D-5T gun, with the same TSh-15 from the SU-85, built in the SU-100 hull.