11. The traversing mechanism flywheel may be replaced by a naval handwheel.
12. The recoil mechanisms that extend outside the turret must be shielded by a turret thickness of 30–35 mm, and the frontal part must be shielded by 75 mm of armor. The front must have a hatch for access to the recoil mechanisms.
13. The howitzer installation must support easy loading at all allowable angles of elevation and traverse.
14. The tipping parts must be capable of being secured in the travel position from inside the vehicle to prevent the gun from moving vertically or horizontally.
15. SP gun crew: 6:
a) gun commander; b) gunner; c) loader; d) breechblock operator; e) radio operator; f) driver-mechanic
IV. Hull and vehicle requirements
1. The fixed turret that replaces the rotating turret must be strong and support easy and safe servicing of the gun and the required rate of fire.
2. The space over the tracks must be used to create a spacious fighting compartment.
3. The ammunition storage rack must be strong and support free and easy removal of round components in order to achieve the required rate of fire.
4. The placement of rounds in the fighting compartment must be consistent with the division of labor between the loader and the breechblock operator: the loader prepares and inserts a projectile into the chamber, and the breechblock operator opens and closes the breech and inserts the case.
5. The rear part of the roof and the rear of the fighting compartment must have a hatch with doors that open for entry and exit by the gun crew and for loading ammunition.
6. There must be a ventilation port with a cover that closes in the rear part of the fighting compartment for ventilation.
7. The mantlet must overlap gaps at all angles of elevation and traverse.
8. The rear walls of the turret and the front plate must have openings with armor plugs for firing the PPSh submachine guns.
9. Install a PTK panoramic sight for use by the commander to observe the battlefield and adjust fire. Provide a hatch in the roof of the firing compartment in front of the PTK for manually wiping the prism head. This opening will also be used for signaling with flags and discharging the flare gun.
10. Provide observation slits in the front and side walls of the fighting compartment as additional means of monitoring the terrain.
11. Provide each gun crew member with a seat for use while the vehicle is in motion.
12. The following communications equipment must be installed:
a) For external communication: a 9-R radio;
b) For internal communication: a TPU-3F intercom system.
13. Fighting compartment lighting must enable firing with the hatches closed.
Sight scales not illuminated by the roof light and the panoramic sight crosshairs must be illuminated separately and be powered by the vehicle electrical system.
14. The interior of the fighting compartment must have places for securing the spare parts, tools, and accessories (SPT&A) kit needed during firing; cabinets for the crew’s dry rations; and a tank for drinking water.
15. In addition to its artillery ordnance, the vehicle must be equipped with two PPSh submachine guns with 1500 rounds and 10 hand grenades, and means of securing them in the fighting compartment must be provided.
16. A special rack must be mounted on the vehicle outside the fighting compartment to carry the following items:
a) A vehicle-transportable SPT&A kit for the vehicle;
b) Entrenching tools: ax, saw, crowbar, shovels (2), pickaxe.
V. Combat specifications for the self-propelled gun
1. The self-propelled gun fires from a stationary position or from short halts. Firing on the move is allowed as an exception.
2. The self-propelled gun must be at least as accurate as the tabular accuracy of the 152 mm gun-howitzer model 1937 that is in service.
3. The self-propelled gun must be stable when fired at all elevation and traverse angles, on side slopes, and on uphill and downhill slopes.
4. The handling characteristics of the self-propelled gun, including its mobility, trafficability, etc., are determined by the KV-1S tank chassis with its weight increased to 45.5 tonnes.
VI. Additional guidance
During project development and prototype manufacture, retain as much as possible of the production facilities for the 152 mm gun-howitzer model 1937 and the KV-1S tank. Shortcuts that interfere with established production lines shall not be allowed. Only changes that are absolutely necessary shall be permitted. The howitzer recoil mechanisms shall be used without change.{4}
Work got underway at SKB-2 as soon as they received the operational requirement for the SP gun. According to reports received by GABTU, the first drawings for building the prototype were provided on January 8, and the last on the 10th. The flowchart for the parts was ready the following day. Meanwhile, work on the model was already underway. Models of the system’s mantlet were finished on the 14th, and the TV-14 model as a whole was done on the 17th. The model was approved that same day. On January 19, Factory No. 200, which was producing the hull, had the plates for the superstructure ready and began installing them on the KV-1S hull that same day. The completed hull for the prototype arrived the next morning from Factory No. 200. Assembly began that same day: alignment of the engine and the transmission was completed, and the balancing arms and torsion bars were installed.
At the same time, Factory No. 172 was preparing to manufacture the self-propelled version of the ML-20 152 mm gun-howitzer. To facilitate use of the documentation, on January 11, 1943, the self-propelled version of the gun was renamed the ML-20S (the designations ML-20-S and ML-20s were also used). The system was mounted on a frame with armor protection and a massive mantlet. The mantlet had a special opening with a cover for performing maintenance on the recoil mechanisms. A hydraulic pump was used for adding fluid to the recoil devices. The front hull plate of the KV-14 had a special recess where the pump was placed while the recoil mechanisms were being serviced. Because removal of the muzzle brake would require a great deal of work, the barrel was left unchanged.
The main modification made to the towed version of the ML-20S was the installation of the T-9 (TOD-9) telescopic sight, which had originally been developed for the KV-2 heavy tank. The T-9 was a modified KT-1 (a casemate telescopic sight) for casemate emplacement of the DOT-4 and included a prism, which gave it its characteristic “elbow.” The Hertz panoramic sight (PG-1) was retained for firing from cover. As specified by the requirement, the rollback mechanism was removed and the counter-rod was fixed in the short recoil position. Sector-type elevation and traversing mechanisms with worm gears were added. In addition, a loading tray was attached to the carriage to facilitate loading. It also served as a barrier for the loader.