The Commission’s decision finally put into motion a mechanism that began inexorably ticking down to the start-up of production on the heavy SP gun. The very next day after the display, Stalin signed State Defense Committee Decree No. 2692 “On Manufacture of a Prototype 152 mm Self-Propelled Gun Based on the KV-1S Tank Chassis,” which established a deadline for building the prototype:
1. The People’s Commissar of the Tank Industry (Comrade Zaltsman), the director of the Kirov Factory (Comrade Goreglyad), the director of Factory No. 200 (Comrade Shcherbakov), and the chief designer of the People’s Commissariat of the Tank Industry (Comrade Kotin) shall manufacture a prototype of a self-propelled gun with a 152 mm gunhowitzer model 1937 (ML-20) based on the KV-1S tank chassis and provide it for proving-ground tests by January 31, 1943.
2. The People’s Commissar of Arms (Comrade Ustinov), the director of Factory No. 172 (Comrade Vykhovsky), the chief designer of Factory No. 172 (Comrade Gurenko), and the chief designer of Factory No. 9 (Comrade Petrov) shall manufacture and provide to the Kirov Factory a 152 mm gun-howitzer model 1937 (ML-20) adapted for mounting on a KV-1S tank chassis by January 23, 1943.
3. The overall development of the project, the modification of the 152-millimeter gun-howitzer model 1937 (ML-20) for mounting on the vehicle, and the allotment of mounting-part production to factories shall be done at the Kirov Factory jointly by designers of the People’s Commissariat of the Tank Industry and the People’s Commissariat of Arms, with the involvement of the following GAU engineers: Eng. Col. Komarov and Eng. Maj. Solomonov.
4. The following basic characteristics of the 152 mm SP gun based on a KV-1S tank chassis shall apply to the design:
a) The combat weight with ammunition, fuel, and crew shall not exceed 45.5 tonnes;
b) The vehicle shall have at least 20 rounds of ammunition on board;
c) The armor thickness of the fighting compartment shall be 60 mm;
d) The thickness of the frontal armor of the fighting compartment shall be 60–75 mm;
e) The height of the fighting compartment shall be at least 1700 mm
f) The rate of fire shall be 3–4 rounds per minute.
By January 6, 1943, the chief of the GAU (Comrade Yakovlev) shall issue an operational requirement to the People’s Commissariat of the Tank Industry for guidance in manufacturing the prototype.
5. The Main Artillery Directorate, the Main Directorate of the Chief of Artillery, and the Main Armored Forces Directorate shall organize and carry out proving-ground tests on the prototype at the artillery range near the city of Chelyabinsk over a 7-day period beginning the day it arrives at the test range.
6. The chief of the Directorate of Fuels and Lubricants (Comrade Kormilitsin) shall support the tests with fuel at the request of the GAU.
7. The results of the tests shall be reported to the State Defense Committee within three days after completion.
8. The People’s Commissariat of the Tank Industry (Comrade Zaltsman) and the People’s Commissariat of Arms (Comrade Ustinov) shall immediately begin production engineering for the 152 mm self-propelled gun so that mass production can begin as soon as the prototype is approved.{3}
The Chelyabinsk Kirov Factory’s deadlines for drafting the documentation and manufacturing the prototype were tight, but realistic. The SU-12 and SU-35 SP guns had been developed at a similar pace two months prior to that, and their developers had more or less met their deadlines. The Kirov Factory’s team was now facing the same task.
Meanwhile, events were proceeding precisely on schedule. On January 5, I. M. Zaltsman, People’s Commissar of the Tank Industry, issued Order No. 6ss “On Manufacturing the Prototype of the 152 mm Self-Propelled Gun Based on the KV-1S Tank Chassis.” According to the order, Chief Designer Zh. Ya. Kotin was to issue the drawings for the system by the 10th. Factory No. 200 was to supply a finished hull for assembly by January 18, and the deadline for manufacture of the KV-14 was set for January 25. Plans called for factory testing of the SP gun to be complete by the 29th and for it to be provided for proving-ground tests on February 1. The modified ML-20 gunhowitzer was to be received for installation in the KV-14 on January 23.
As stated in the decree, on January 6, 1943, GAU Artillery Committee Chairman V. I. Khokhlov (who by that time had been promoted to Lieutenant General of Artillery) sent off two copies of the operational requirement for the “152 mm self-propelled gun based on a KV-1S tank chassis.” He sent the requirement, which had been approved by GAU Chief Col. Gen. of Artillery N. D. Yakovlev, to the Kirov Factory and to People’s Commissar of the Tank Industry I. M. Zaltsman:
I. Role
1. The 152 mm self-propelled gun is intended for destroying bunkers, earth-and-timber emplacements, and other strong field fortifications by direct fire from close range during breakthroughs of enemy defenses, as well as for combating enemy artillery and shelling his rear areas.
II. Main specifications
1. The self-propelled gun is to be produced by installing the tipping parts of the 152 mm gun-howitzer model 1937 in a fixed turret that, in turn, is to be mounted on a KV-1S tank chassis.
2. The weight of a fully-fueled self-propelled gun with ammunition and crew must not exceed 45.5 tonnes.
3. The thicknesses of the fixed turret armor plates must be as follows:
a) Front: 60–75 mm;
b) Equivalent to that of the KV-1S tank on the side and rear, i.e., 60 mm;
c) Top: 20 mm
4. The basic load of the self-propelled gun must comprise at least 20 rounds.
5. The height of the fighting compartment above the floor must be at least 1700 mm.
III. Artillery system specifications
The self-propelled gun must meet the following specifications:
1. Angles of fire:
a) Elevation: from -3° to +20°
b) Traverse: 6° to each side
2. The practical rate of fire with correction of aim: 3–4 rounds per minute.
3. Constant recoiclass="underline" 850 mm (achieved by appropriate securing of the counter-rod)
4. The effort required to operate the gun laying mechanism flywheels must not exceed that of the gun-howitzer in service, namely:
a) Elevation: 8 kg;
b) Traverse: 5 kg.
5. The sights must support both direct fire from unconcealed positions and indirect fire from cover. A sight must be installed such that the panoramic sight’s objective lens extends above the turret roof. Install a headrest to facilitate the work of the gunner.
6. Loading gear: a special tray like that used in the 122 mm self-propelled gun produced by Factory No. 592.
7. Firing gear: hand- and foot-operated
8. Barrier
To protect the gun crew from recoiling parts, a special barrier must be provided that does not hinder loading.
9. Balance
The muzzle brake may be removed from the barrel in order that the effort required to operate the gun laying mechanism flywheels does not exceed that on the model 1937 gun-howitzer, provided such is supported by calculations of the strength of the recoil mechanisms. In this case, the mounting parts of the gun must be designed for greater recoil resistance.
10. The position of the traversing mechanism flywheel relative to the panoramic sight eyepiece and the gunner’s seat must be such that it facilitates the job of the gunner without removing his eye from the eyepiece. The existing traversing mechanism may be replaced with a helical mechanism.