This disassembly can be done in TsAKB shops.
Please provide information about the test program and the procedure for performing such tests as you deem necessary.
I further report that the S-51 development project of TsAKB of the People’s Commissariat of Arms also examined installation of the BR-2 152 mm gun on the same chassis.
Under our agreement with you, we anticipate receiving the BR-2 system in TsAKB in order to manufacture the prototype vehicle for this gun. TsAKB has a KV tank for building the vehicle.{10}
The GAU’s Artillery Committee approved the test program for the S-51 on March 2. The SP gun had arrived at the Gorokhovets Artillery Proving Ground prior to that—on February 26. The technical documentation came two days later. The problem with the lack of hoisting equipment at the proving ground was solved by breaking down and measuring the howitzer at Factory No. 112. However, the system could not be disassembled as completely as required. Before the tests were performed, the S-51 traveled a distance of 150 kilometers, 120 kilometers of it traveling from the proving ground to Factory No. 112 and back.
Testing of the SP gun began on March 16, 1943. Because the proving ground lacked high-explosive shells for the B-4, the test for firing accuracy was performed using concrete-piercing projectiles, thus shortening the proving-ground tests. On the 16th, 18 rounds were fired. During the entire course of the firing test, which lasted until March 24, 209 rounds were fired, 135 of them supercharged.
During firing at an angle of 30°, it was found that the S-51 moved backwards 1000–1300 mm, and it had a counterrecoil of 200 mm. When fired, the SP gun’s bow lifted 250 to 450 mm, and its stern lowered by 200–300 mm. At the same time, it was observed that the right side jumped by 100 mm more than the left. When fired at an angle of 45°, the vehicle moved backwards 750–1000 mm; it had a counterrecoil of 330 mm; its bow lifted 100–180 mm; and its stern lowered by 120–140 mm. Firing at an angle of 57° revealed a backward movement of 400–750 mm, a counterrecoil of 300 mm, an upward movement at the bow by 60–150 mm, and a downward movement at the stern of 50–100 mm. Thus, the S-51’s stability was considered unsatisfactory. The sight was also highly unstable.
Firing for durability (with 100 supercharged rounds), no damage was found on inspection. The rate-of-fire tests with a seven-man crew yielded an average figure of one round every 1.5 minutes.
The road tests took place between March 25 and April 4. The SP gun was tested under different road conditions; the snow depth reached 200 mm. Overall, the S-51 travel 524 kilometers, 16 of them on cobblestones and 40 off road. Most of the on-road tests took place on asphalt highway, as required by the trips to Factory No. 112. A maximum speed of 32 km/h was reached in the on-road tests, and 22 km/h off the road. No special tests were carried out because of various running-gear failures. At 240 kilometers, the right-side steering clutch and brake band went out, and the right steering clutch broke down again at 360 kilometers. Problems occurred with the tracks several times; bolts on the track roller cap broke. However, there were no problems with the engine or torsion bars, again confirming the durability of the running gear after the firing test program.
The tests concluded on April 6, and the proving-ground commission issued the following finding on the 9th:
1. The S-51 self-propelled howitzer successfully passed most tests because:
a) The operation and durability of the gun’s assemblies and mechanisms are satisfactory.
b) The durability of the gun mounting parts is satisfactory.
c) The operation and durability of the vehicle assemblies and mechanisms are satisfactory, except for the transmission group, which had a number of defects during testing caused by its overall poor technical condition prior to the tests and wear to parts.
2. Shortcomings of the S-51 self-propelled howitzer during firing are as follows:
a) A large movement backwards and instability of aim, especially at low elevation angles, due to the vehicle’s instability during firing.
b) A large lateral spread of rounds caused by sideways movement of the gun during firing.
These flaws can be eliminated by placing trails on the vehicle’s rear as was done on the SU-14 203 mm self-propelled gun designed and manufactured by the Kirov Factory, which underwent proving-ground tests in 1936.
Considering that the howitzer will primarily be fired at high angles of elevation during which backward movement and instability of aim are less significant, the placement of wooden beams constructed from locally improvised materials can be recommended to eliminate the vehicle’s backward movement on the rare occasions when the gun is fired at low angles of elevation.
3. The Gorokhovets Artillery Proving Ground believes that the S-51 203 mm self-propelled howitzer is more maneuverable and exhibits better firepower and lethality than the B-4 203 mm howitzer, and it can be recommended for adoption by the Red Army when the flaws identified during testing and noted in this report are corrected.{11}
Based on the test results, Grabin wrote Beria to express his thoughts about putting the S-51 into production. The TsAKB’s chief felt that a total of 20–30 SP guns of that type should be manufactured. The Self-Propelled Artillery Office had a negative reaction to that idea. They felt that it did not make sense to use the KV-1S chassis to manufacture a heavy SP gun with a light gun shield. In addition, the S-51’s combat weight was 50 tonnes, and the tankers still remembered the way things were in 1942 when KV-1 tanks weighing that much were dropping their engines and transmissions. A decision was made to conduct additional tests on the SP gun, but they did not take place because the chassis required major overhaul after the road trips that spring.
The S-51 program temporarily ground to a halt. GAU Artillery Committee chairman Lieut. Gen. Khokhlov attempted to get things moving again by sending Yakovlev a letter on July 22 expressing his thoughts about how the SP gun would be used in battle:
Because our troops will soon be crossing the border between the Soviet Union and Eastern Prussia, the Red Army’s artillery will acquire the mission of engaging the enemy’s permanent fortifications.
According to information from the Intelligence Directorate of the Red Army’s General Staff, the concrete in the fortifications in that region is between 1.4 and 2 meters in thickness.
The most suitable system for use against concrete of that thickness is the B-4 203 mm howitzer, which is capable of penetrating modern concrete of the following thicknesses:
At a range of 1000 m: 1.4 m;
At a range of 2000 m: 1.3 m;
At a range of 3000 m: 1.2 m.
Experience from 1939 through 1944 has demonstrated that this howitzer is most effective against concrete bunkers when fired from short range.
The TsAKB of the People’s Commissariat of Arms has developed and manufactured a prototype S-51 203 mm auxiliary-powered howitzer with the B-4’s traversing mechanism mounted on a KV tank chassis.
The prototype has been range tested with satisfactory results, except for stability during firing: a 1.5-m backward movement by the howitzer, and a 450 mm upward hop at the bow.