S-51

All info translated from Russian. I am unsure that the information is valid excepting that the Chassis was a KV-1S.

• KV-1S (Object 238) High speed variant of the KV-1 with lighter armour and new transmission. Production model.

Wording of the below sentence is as published and translated from russian (It seems to imply that they almost made 2 prototypes (samples) in 1943.

"Unlike its predecessor, the KV-1s could not boast a large number of experimental machines created on its basis. Almost made it2 samplesin 1943:with 152-mm howitzer C-41 in standard tower and self-propelled installation of 203-mm howitzer B-4 on the chassis of the KV-1S."

https://www.battlefield.ru/ru/tank-development/28-heavy-tanks/47-kv1s.html?start=2

need verification of history info

This is fully use google traslate with some edits https://www.aviarmor.net/TWW2/tanks/ussr/s51.htm

On end of 1943, the GAU returned to reestablish of mobile heavy duty self-propelled artillery systems, ten years earlier was supposed to arm with 203-mm howitzer model 1931, which planned to resume production at the factory "Bolshevik" in 1944.If it succeeded WPRA would get a mobile devastating weapon. By order of the Commissar of weapons Ustinov in November 1943 was announced a competition to develop a self-propelled guns, which received a semi-official title of "Knight".

Within weeks some preliminary draft presented from plant № 100 NKTP, CB Uralmash and TsAKB. The first of them created under the leadership of J. J. Kotin, a self-propelled gun carriage with trailer which in some ways it resembled the French GPF 194, only with more powerful gun. Draft CB UZTM had two options: 203-mm howitzer B-4 on the chassis of the KV-1S (modernized draft SAC U-19) or two 152-mm howitzers on two SU-122 chassis. Before using it's gun, it was supposed to connect both chassis, while preparing to fire was about 30-40 minutes, versus 20 minutes in the on Kotin design. It is clear that the development of plant number 100 and UZTM don't give adequate fire support on it's technological complexity. In order to build a single prototype was only TsAKB who submitted the draft SAU under the symbol S-51. As a basis has been chosen chassis of the KV-1S, but quickly found out that it has insufficient length of the support surface and needs adjusment, a modified version with 7 or 8 rollers. However, NKTP relented to change it's production because the amount of modification required is relatively large, and the number of issued ACS is unlikely to be exceeded several tens of units. In this regard, decided to establish a howitzer on the chassis of the KV-1s, which was not the best solution. The chassis almost did not undergo changes except the engine. Since the tank turret was dismantled, and mounted a 203-mm howitzer B-4 on the open gun-carriage. Because the weight of the 203mm howitzer was slightly heavier than weight of a fully loaded turret, full combat load weight was almost 50 tons. Thus, it was not expected to have good mobility.

The first prototype of the S-51 entered the factory test in February 1944, conducted under the reduced program. However, interest of high powered artillery was so great that it was still not finished was transferred to ANIOP. At here revealed all of the major disadvantages of the S-51. Because of the high line of fire when firing self-propelled strongly swayed by inertia and went back to the lateral displacement and the elevation angle was relatively small, the impact was so strong that the crew could not hold on their seats. The high knocking recoil, great (big ?) dispersion of projectiles (required installing opener [should be rammer ?]) and the extreme discomfort for the crew. In addition, the chassis of the KV-1S is clearly no good to install it on guns of this caliber. Comparing all the data obtained in the COD felt that the S-51 is sent to production, but do not succeed. First of all, the issue of the KV-1S was completed in December 1942 - that is, get in the required amount of gear for heavy self-propelled guns was only possible after the alteration of serial machines. Another important reason was the lack of howitzer B-4, which not on production scale.

In return, engineers and design bureaus TsAKB plant number 100 had developed a version of SPG on the chassis of heavy tank, with the installation of a 152-mm howitzer BL-2. A prototype self-propelled guns, named S-59, was made in early 1944 on May-June, she underwent range test. There is some discrepancy in the data on the type of used gear. According to factory records S-59 received the chassis and body from the IP-85, but according to a report on tests self-propelled guns had "improved chassis of a heavy tank KV. By linking self-propelled gun in line with an earlier S-51, only against the advice of folding openers, diminishing returns at a shot, have not been established. In fact, TsAKB "occurred on the same rake, although it can be assumed that the calculation was made on a smaller caliber guns. However, when firing full charge at S-59 began a similar problem as that of S-51, so the mass production of the issue was not raised. As a result, this direction in the development of ACS singular power has been closed.

Nevertheless, the projects of heavy self-propelled guns continued to appear until the end of the war. For example, July 29, 1944 Head of the Leningrad branch TsAKB Igor Ivanov presented the technical management of the IEC preliminary draft of ACS, which was supposed to use the chassis paired T-34 tanks and 210-mm guns BR-17 or 203-mm howitzer BL-18 . From a year earlier draft of the proposed CB UZTM this ACS differed by more simple design. Development TsAKB aroused some interest in leadership of the People's Commissariat of arms, but the timing of the project they were put little real - a prototype self-propelled guns required to submit not later than 30 September of that year, and up to the challenge of Leningrad failed.

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