6. The vehicle together with ammunition, crew, and fuel must weigh 45–50 tonnes. It is desirable that this weight be met.
7. The main technical characteristics of the self-propelled howitzer are as follows:
Caliber: 152 mm
Angles of elevation: from -2° to +15–20°
Angles of traverse: +/-5° to +/-6°
Aiming rate: from 30 to 45 min. per flywheel rotation
Maximum recoil distance: no more than 850 mm
Resistance to recoiclass="underline" no more than 30–35 t
Maximum rate of fire: 8 rds/min, including relaying the gun.
Effort on flywheels: 3 kg while turning
Basic load: 30 rounds
IV. Performance requirements
8. The howitzer is served by a crew of 6: commander, driver-mechanic, gunner, loader, breechblock operator, radio operator/machine gunner.
9. The tipping parts armor must allow free access for servicing the recoil mechanisms.
10. The gunner’s fighting compartment: the seat and the location of the gun laying mechanism flywheels and sight eyepiece must support easy and fatigue-free laying of the howitzer on a target both while on the move and while stationary. The effort required to operate the firing handle must not exceed 8 kg; it would be desirable to have the firing control on the elevation mechanism handle.
11. The tank commander’s fighting position must be equipped with the KV-1S commander’s cupola.
12. The howitzer must be equipped with a folding tray/barrier for preparing shells for firing that automatically locks the firing mechanism until loading of the howitzer is complete. The design of the folding tray must facilitate the loader’s job while requiring a minimum expenditure of muscle energy.
13. The fighting compartment height at the work positions of the breechblock operator and the loader must be at least 1700 mm.
14. The ammunition storage rack must be strong and non-warping, and it must allow free and easy removal of rounds in order to achieve the highest rate of fire.
15. The placement of rounds in the ammunition storage rack must conform to the division of labor during loading between the breechblock operator and the loader: the loader prepares and places a projectile in the chamber, and the breechblock operator opens and closes the breech and inserts the case into the chamber.
16. The hand ejector, fuze wrenches, and rammer must be attached to the turret walls.
V. Requirements for the turret and its equipment
17. The rear wall of the turret must have a hatch for the crew, for loading ammunition and for ventilating the turret interior in specific types of combat.
18. The turret walls must have openings with armor plugs for firing submachine guns and supplemental vision slits for observing the surrounding environment. It is desirable to have a ball mount for a DT tank machine gun in the rear turret wall.
19. The personal weapons of crew members must be located in secure and convenient racks near their work positions.
20. The turret roof near the commander’s position must have a port for signaling with rockets and a hatch in the rear for combat using hand grenades.
21. The turret illumination must support firing the weapon and monitoring the gun.
22. Radio and telephone communications in the tank shall be accomplished using a standard tank radio and a tank intercom.
23. Turret armor: at least 90 mm on the front and 50 mm on the front and sides; it is desirable to have the side walls sloped 30° from the vertical.
24. The commander’s position must facilitate use of a map and plotting board.
25. Each crewmember must have a comfortable seat for rest and for use while on the move. The seats of the breechblock operator and loader must be capable of being folded against the walls and must not interfere with them during combat.
26. The turret interior must have places for a first-aid kit, emergency rations, and drinking water.{14}
It appeared that the project for a 152 mm SP gun based on the KV tank chassis that had been stalled for half a year and had disappeared from the development plan had finally begun to move forward. Despite the fact that the base chassis was now the KV-1S rather than the KV-1, the overall SP gun concept had not changed, and the timetable was entirely achievable. Unfortunately, matters did not proceed as planned. Representatives of the Chelyabinsk Kirov Factory did travel to Factory No. 8 and became acquainted with the design documentation for the ZIK-20, and work on the project came close to reaching a practical stage. Another factor was that construction of the model stalled, and an event that had a serious impact on the entire project occurred at the very end of October. State Defense Committee Decree No. 2457ss of October 30, 1942, split Factory No. 8 into two enterprises. The first, Factory No. 8, was required to engage in antiaircraft production. The second enterprise became Factory No. 9, responsible for howitzer artillery and tank guns. L. R. Gonor became director of the new factory; P. I. Maloletov was appointed party organizer; and F. F. Petrov was made chief of Factory No. 9’s design bureau.
The splitting of the factory affected the timeline for constructing a fullscale mockup of the superstructure, but it had no impact on the activities of Factory No. 9’s new design bureau. Back in October, before the factory was split, Petrov had led a major effort to design a number of artillery systems.
Work to fine-tune the 203 mm U-3 corps-level howitzer designed by V. N. Sidorenko had been underway at the Ural Heavy Machinery Plant since 1939. The U-3 project surfaced again in the spring of 1942: between May and June, it underwent comparison testing with its main competitor—the BL-39 203 mm corps-level howitzer designed at Factory No. 172. The test resulted in a proposal to finalize the howitzer and a recommendation to place it in service with the Red Army. However, neither the U-3 nor the BL-39 went into production. The situation with the BR-2 152 mm gun that the artillerymen so desired to mount on a vehicle based on the KV chassis was no less sad.
Petrov’s proposals were bold; their basic idea was to place the barrels from heavier systems on the carriages of the M-30 122 mm howitzer and the ML-20 152 mm gun-howitzer. He proposed six projects in all; however, only two of them were of interest. The reason is clear from the project description:
Placement of the BR-2 152 mm gun model 1935 and the U-3 203 mm howitzer on the ML-20 carriage, however, significantly increases the weight on the wheels, but there can be no doubt about the combat utility of these systems for the following reasons:
a) Not even the ML-20 152 mm gun-howitzer model 1937 or the A-19 122 mm gun model 1931/37 can currently traverse swampy terrain freely. Their bottom carriage and trails become badly bogged down. But this carriage, even with the extra load from the BR-2 gun or the U-3 howitzer, moves well over other types of ground, including plowed fields.