The Samokhodnaya Ustanovka 122

Excerpt from Tanks Encyclopedia 

The SU-122 was the first major variant of the successful T-34 chassis. After the first German Sturmgeschütz assault guns were seen in operations, an order was issued in April 1942 to several design bureaus to devise plans for a prototype SPG. Many heavy guns were tested in the process. It was seen the assault gun formula amassed the advantages of a cheaper, easier produced vehicle with the use of heavier guns, albeit with the disadvantage of having to turn the entire vehicle to aim in combat. In the end, both 122 mm (4.8 in) and 152 mm (5.98 in) field howitzers were retained. Only the 122 mm (4.8 in), developed by F. F. Pietrow’s design bureau as the M-30S, was found suitable for a medium tank chassis.

Development of the SU-35

In fact, before any T-34 adaptations were produced, ten captured German Sturmgeschütz IIIs served as testbeds for the adaptation of the M-30S, under the name of SG-122. Deemed too complicated and difficult to maintain, they were phased out in favor of a proper Russian mass-built SPG. The most obvious option back then was the T-34. This choice was made by Uralmashzavod Uralsky Machine Building factory (UZTM), headed by chief engineers Kurin and Kusjin. They then developed the U-34, a long barrel 76 mm (3 in) gun armed turretless version based of the T-34 chassis.

They decided not to waste time, thus using the gun mount of the SG-122 and the hull, chassis, engine and transmission of the T-34. This led to the development of the final U-35, during the summer of 1942. After difficult trials led during the winter, the SU-35 was officially approved for production in limited numbers because of many flaws, but larger orders came after better trials and it was renamed into the SU-122.

Specifications and production

The Samokhodnaya Ustanovka 122 mm (4.8 in) or, in short, SU-122, was not at first a satisfactory model. The gun elevation was poor (initially designed for −3° and +26° with a 10° of traverse), the crew compartment was poorly ventilated and the crew was badly placed. All these defaults were corrected, as well as other minor modifications for mass-production. A slightly less sloped glacis (still 45 mm/1.77 in thick, almost 80 mm/3.15 in effective), simplified slits, rearranged fighting compartment layout, an increase in ammunition carried and a more efficient commander periscope.