Mobility Trials Part 3

This one is less of a complete report, and more a mishmash of off-roading and various associated activities I have accumulated.

CAMD RF 81-12086-97

"Figure 13. The moment before fording.

Figure 14. The moment before fording. The tractor begins tilting forward at a point before the third road wheel."

These two photographs are from trials of the Ya-13F light artillery tractor. While it is not particularly important to know at what point a tank begins tilting forward as it enters a sharp incline, a vehicle that is meant to be towing something behind it is a different matter entirely.

Towing a gun? Easy. What about towing a tank?

CAMD RF 38-11355-2350

"Towing.

Towing was done twice, after the tank travelled 432 km and 767 km. The total distance of towing was 20 km. In the first case, an IS-122 and a KV-85 were towing an IS-122. In the second, an IS-122 towed an IS-122.

In both cases, the tank was towed over a dirt road trampled by tanks, and partially on an asphalt highway covered in packed snow, at 1st and 2nd gear, with external temperature between -5 and -9 degrees.

During towing, the engine's temperature was normal. The water temperature stabilized at 65 degrees. The oil temperature stabilized at 72 degrees.

Photo #27: IS-122 and KV-85 tanks tow an IS-122 tank on a road trampled by tanks."

CAMD RF 38-11369-351

A SU-100 undergoing mobility trials, showing the perils of a gun that is too long on bumpy terrain. The end of the gun is covered to prevent mud from getting in, but the sack cannot hide the distinctive shape of the muzzle brake. This is no ordinary SU-100, but a SU-122P, a SU-100 with a D-25 gun.

The sack was there for a very good reason.

CAMD RF 38-11369-541

"Fig. 2. The 122 mm SPG D25-SU-122P hits the ground with its barrel when descending from a bump with the gun elevated at 12 degrees."

SU-122P

I briefly mentioned the SU-122P before, but not much was written about it. Here is a more detailed report from which it becomes obvious why the SU-122P didn't beat out the SU-100.

"Conclusions

Based on the trials of the 122 mm SU-122P SPG on the chassis of the 100 mm SU-100 SPG, armed with the D-25S gun, having driven it 238 km, the Red Banner Central Artillery Proving Grounds has established that:

1. The gun parts and mechanisms perform well in winter, and therefore easier, conditions. Final evaluation may only be performed on very poor terrain, and in the summer.
2. In winter conditions, the air-tightness of the return mechanism is adequate. The air pressure did not change.
3. The sight alignment is satisfactory. The zero of the sight did not shift during trials.
4. The tray-shaped cutout in the breech and sliding block are on the right, when the breech opens leftwards, make the loader's job, placed to the left of the gun, difficult and dangerous. It is necessary to turn the gun to make the tray-shaped cutout be on the opposite side. There is not enough room to put the loader on the right of the gun.
5. The fighting compartment is cramped and does not allow for comfortable work by the crew. This, combined with the aforementioned problem with the tray-shaped cutout, render the SPG a temporary measure, which must be armed with an existing gun and use an existing SPG hull with the goal of rapid improvement in firepower.
6. The calibration of the sight is inconvenient in battlefield conditions (no access to the stopper screw for the dial). The design must be changed to make the stopper screw and horizontal dial easily accessible.

Additional notes on the comfort of working in the fighting compartment

The crew of the 122 mm SPG D25-SU-122P consists of 4 crewmen: commander, gunner, loader, and mechanic-driver. A more detailed review of the fighting compartment exists as a GAKP report #0583 from February 10th, 1945.

Commander's workspace: 

The commander is placed to the right of the gun. His workspace is cramped, the commander cannot comfortably fit between the wall and the gun, especially when the gun is at maximum traverse.
The commander's seat is installed up against the wall, which does not allow to sit on it comfortably and stably. The commander slides off, especially during harsh bumps in motion. There is nowhere to brace his legs against, which does not result in a stable position. In order to remain in place, the commander is forced to place one leg on the floor and hold the other half-bent. There are no guard rails, and the commander must take great care to not fall off his seat or hit himself on the gun.
The observation devices are placed in the commander's cupola, and consist of a MK-4 device with 360 degree vision and five observation slits. These devices allow observation of the battlefield with a deadzone of 10-15 meters. In winter, the slits can be stuffed with snow, and the commander cannot see out of them.

Gunner's workspace:

The gunner is placed to the left of the gun. The gunner's seat flips up and can be adjusted vertically and horizontally on its carrier next to the gun. The seat does not let the gunner sit on it comfortably during motion, namely, the protruding part of the guard plate forces the gunner to tilt to the left, which means he must press his shoulder and arm against the guard plate and hand trigger. The elevation mechanism is insufficiently comfortable to use, as the right hand is pressing against the guard plate and it's hard to move. The driver's seat and its back impede comfortable leg placement. Adjusting the seat horizontally is inconvenient. 

The forehead rest hits the side level when the barrel elevates.

Loader's workspace:

The loader is placed behind the gunner, to the left of the gun. During travel, the loader's seat is comfortable, placed in the rear left corner. In battle, he works while standing.
The existing configuration with the breech opening to the left, the loader's job is difficult, uncomfortable, and dangerous:

1. It's hard to place the shell on the tray and then ram it in.
2. When loading the propellant, it's hard to find the chamber.
3. When performing two-piece loading, the gunner must load the propellant in two steps:
1. Push in the propellant gently, leaving it 50-100 mm short.
2. Quickly push the propellant all the way in, during which the loader must put his entire right arm in the chamber opening and press his chest against the guard plate or the breech rear. When the breech closes, his arm is lifted to the right (further out), which quickly tires the loader.
4. The placement of the breech lock is on the right, like on the tank that carries this gun, and not on the left, where the loader is in the SPG.
   In order for proper working conditions to exist for the loader, the breech must be rotated 180 degrees so that the tray-shaped cutout is opposite of the loader.

Driver's workspace:

The driver is placed in front of the gunner. The driver's workspace does not provide him with sufficient comfort. The driver has to sit hunched over and hits his head on the gun's turning mechanism if he straightens up. When the SPG is in motion, the driver hits against the turning mechanism, specifically when the gun breech is rotated all the way to the left. The position of the horizontal aiming mechanism should be changed. 

Ammunition rack:

The ammunition rack of the D25-SU-122P contains 26 shells. The placement of shells and propellant in the rack makes their retrieval inconvenient, and should be revised. During trials, there were two cases of shells falling out of the rear ammunition rack due to weak locks.

On the comfort of the fighting compartment as a whole:

The fighting compartment is uncomfortable for the crew. The fighting compartment must be widened in order to increase the level of comfort. It is reasonable to widen the casemate, ie. make the casemate as wide as the rest of the SPG, increasing the space in the fighting compartment, which will increase crew comfort, allow for a more convenient ammunition rack, and a larger amount of shells.

Chief of the 4th Department, Engineer-Colonel Ivanov

Trials Supervisor, Engineer-Captain Vyvoldin"

CAMD RF 38-11369-541

Soviet Medium Tank Destroyers Part 2

You've read about Soviet experimental medium SPGs, but now here's some more exciting content from CAMD RF 38-11369-1, in the vein of the light SPG article.

"

Production vehicle, SPG SU-122, produced by the Ural Machine Building Factory of the NKTP

According to GOKO decrees in October-December 1942, the Uralmash factory designed and built an experimental prototype of the SU-122 SPG. The SU-122 consists of a 122 mm M-30 howitzer on the T-34 hull. 

From December 5th to December 19th of 1942, the SPG underwent trials at the Gorohovets proving grounds, over 430 km and 281 shots. The commission concluded that the SPG passed trials. GOKO decree #2559 on December 2nd, 1942, adopted the SU-122 SPG into the Red Army.

Experimental work on medium SPGs

1. On the SU-122M

The Uralmash factory, on its own initiative, developed a modernized SU-122M SPG. This vehicle was based on the SU-122 SPG trials, overseen by Major-General of the Tank Engineering Forces, comrade Ogurtsov.

The modernization was as follows:

1. Installation of a new D-11 122 mm howitzer instead of the M-30 122 mm howitzer.
2. Increase of the fighting compartment width by 350 mm and height by 100 mm.
3. Inclusion of a telescopic sight for direct fire.
4. Improved fuel and control systems.

In June of 1943, the SU-122M underwent trials at the Ural proving grounds over 329 shots and 859 km. The experimental prototype passed trials, but was not adopted by the Red Army, as a decision was already made to produce SU-85 SPGs that used 85 mm guns with improved anti-tank capability.

Experimental prototype of the modernized SU-122M SPG designed by the Ural Machine Building Factory of the NKTP

1. Combat weight: 31.43 tons
2. Crew: 4
3. Gun: 122 mm M-30 howitzer
4. Muzzle velocity: 515 m/s
5. Horizontal traverse: 20 degrees
6. Vertical traverse: -3 degrees to 26 degrees 30 minutes
7. 122 mm shells: 40
8. Front armour thickness: 45 mm
9. Maximum speed: 55 kph
10. Average speed: 35 kph
11. Average off-road speed: 20 kph
12. Engine: V2-34
13. Maximum engine power: 450 hp
14. Fuel tank capacity: 460 L

2. On the SU-85

According to GKO decree #3289 from May 5th, 1943, the Uralmash factory developed and produced experimental SPGs in May-July of 1943.

1. SU-85-I, with a TsAKB S-18-1 85 mm gun
2. SU-85-II, with a factory #9 D5-S85 85 gun
3. SU-85-IV with a TsAKB S-18 85 mm gun

All of these SPGs were based on the SU-122 SPG. Over the period from July 25th to August 6th of 1943, all three SPGs underwent trials at the Gorohovets proving grounds over 500 km and 300 shots. A commission headed by Lieutenant-General of Artillery comrade Taranovich (today Colonel-General of Artillery). The D5-85S gun designed by factory #9 achieved superior results in trials.

Marshal of the Armoured Forces, comrade Fedorenko, Marshal of Artillery, comrade Yakovlev, and the People's Commissar of Tank Production comrade Malyshev visited the proving grounds in early August 1943, and agreed to the adoption of the SU-85 armed with a D5-85S gun, of which they informed comrade Stalin over the phone.

After this, Uralmash began mass production of SU-85 SPGs.

In September of 1943, Colonel-General of Tank Forces, comrade Biryukov visited the factory. According to an assignment from GOKO, he inspected the production lines, and occasionally offered help and coordination of factory #9 and Uralmash to aid in quantitative and qualitative improvements in production of the SU-85.

By his orders, one of the production SPGs was subjected to mobility and gunnery trials. The trials were conducted by Engineer-Lieutenant-Colonel Kovalev.

The tactical-technical characteristics of the SU-85 SPG were confirmed by GOKO decree #4436 on October 27th, 1943, provided to NKO (comrade Fedorenko) and NKTP (comrade Malyshev). With this new SPG, the Red Army received its first medium tank destroyer, capable of fighting new enemy tanks.

Experimental prototype of the SU-85 SPG designed by the Ural Machine Building Factory of the NKTP

1. Combat weight: 29.15 tons
2. Crew: 4
3. Front, side, and rear armour: 45 mm
4. Gun: 85 mm D5-S85
5. Muzzle velocity: 792 m/s
6. Horizontal traverse: 20 degrees
7. Vertical traverse: -4 to +25 degrees
8. 85 mm shells: 48
9. Maximum speed: 55 kph
10. Average speed: 35 kph
11. Average off-road speed: 20 kph
12. Engine: V2-34
13. Maximum power at 1700 RPM: 450 hp
14. Fuel: diesel
15. Fuel capacity: 460 L

3. On the SU-100 SPG

Following GOKO decree #481 from December 27th, 1943, Uralmash developed and produced two SU-100 prototypes in March-May of 1944, one with a 100 mm D-10S gun designed by factory #9, and one with a 100 mm S-34 gun, designed by TsAKB. The SU-100 was based on the SU-85.

Several changes were made to the old SPG to increase its combat ability:

1. As requested by USA GBTU, the front armour was increased. Instead of the 45 mm armour of the SU-85, the armour of the SU-100 was 75 mm, at the same angle.
2. Robustness of the front road wheels was increased.
3. Ventilation was improved.
4. Gearbox was improved.

Other changes were made, 15 in total. The SU-100 prototypes were tested twice. First trials of the SU-100 with the D-10S gun were carried out by a commission headed by Colonel Rupyshev at the Gorohovets proving grounds from March 9th to March 27th, 1944. The trials lasted 564 km and 1040 shots. The SPG passed these trials. However, the commission requested that some design defects be corrected before entering production.

Secondary trials were performed by the same commission at the Gorohovets proving grounds from June 24th to July 6th, 1944. 

The SU-100 prototype with the S-34 gun underwent trials. Trials were done in comparison with the SU-100 with the D-10S SPG. The SU-100 with the S-34 gun was trialled over 877 shots and 250 km, and did not pass trials. Work on this SPG concluded, as the SU-100 with the D-10S gun was more promising.

The D-10S prototype, after being corrected after March trials, was tested simultaneously. The SPG passed trials over 923 shots and 250 km, and was recommended for adoption by the Red Army.

Comrades Colonel Rypushev and Engineer-Majors Konev and Volgushev performed exemplary work during design and trials of the SPG.

GOKO decree #6131 from July 3rd, 1944, accepted the SU-100 for mass production and use by the Red Army.

Experimental prototype of the SU-100 SPG designed by the Ural Machine Building Factory of the NKTP

1. Combat weight: 31.6 tons
2. Crew: 4
3. Front armour: 45-75 mm
4. Side and rear armour: 45 mm
5. Gun: 100 mm D-10S
6. Muzzle velocity: 900 m/s
7. Horizontal traverse: 16 degrees
8. Vertical traverse: -3 to +20 degrees
9. 100 mm shells: 33
10. Maximum speed: 50 kph
11. Average speed: 30 kph
12. Average off-road speed: 18-20 kph
13. Engine: V2-34
14. Maximum power at 1700 RPM: 450 hp
15. Fuel: diesel
16. Fuel capacity: 460 L

4. On the SU-122P SPG

In the end of 1943, Uralmash, on its own initiative with approval from USA GABTU, began work on equipping the SU-85 with a 122 mm gun. This work was conducted with the goal of exploring the viability of 122 mm guns on medium SPGs.

In June of 1944, a prototype was produced, indexed SU-122P. This prototype differed from the SU-100 experimental prototype only in armament. The SU-122P SPG was armed with a 122 mm D-25S gun.

In November 1944-February 1945, the SU-122P prototype was tested on the NIBT proving grounds over 1000 km, and fired 258 shots at the Leningrad proving grounds.

The pause between the prototype being ready and trials beginning can be explained by Uralmash being occupied with work on the SU-100, and did not submit the SU-122P for proving grounds trials.

Mobility and gunnery trials showed that armament of medium SPGs with a 122 mm D-25S gun is possible.

USA GBTU decided that an SPG with a rear fighting compartment be developed for this gun, as placing a 122 mm gun in a SU-85 (SU-100) SPG does not result in a comfortable placement of crew and ammunition. Work on the SU-122P ceased.

Experimental  prototype of the SU-122P SPG designed by the Ural Machine Building Factory of the NKTP

1. Combat weight: 31.5-32 tons
2. Crew: 4
3. Front armour: 45-75 mm
4. Side and rear armour: 45 mm
5. Gun: 122 mm D-25S
6. Muzzle velocity: 781 m/s
7. Horizontal traverse: 16 degrees
8. Vertical traverse: -3 to +17 degrees
9. 100 mm shells: 26
10. Maximum speed: 50 kph
11. Average speed: 30 kph
12. Average off-road speed: 18-20 kph
13. Engine: V2-34
14. Maximum power at 1700 RPM: 450 hp
15. Fuel: diesel
16. Fuel capacity: 400 L

5. On the SU-85PM SPG

The experimental SU-85PM SPG was produced at Uralmash on its own initiative in August-December of 1943. This SPG was composed of a production SU-85 equipped with an increased power D5-S85PM gun, designed by factory #9. Due to a longer barrel and increased density of propellant, the muzzle velocity increased to 900 m/s, as opposed to 800 m/s of the stock gun.

The SPG was trialled in January-February of 1944 at the Gorohovets proving grounds, firing 755 shots. The SPG passed trials, but due to the installation of a D-10S gun, which was significantly more powerful than the D5-S85PM gun, the need for the latter disappeared.

Experimental prototype of the SU-85PM SPG designed by the Ural Machine Building Factory

1. Combat weight: 29.5 tons
2. Crew: 4
3. Front, side, and rear armour: 45 mm
4. Gun: 85 mm D5-S85PM
5. Muzzle velocity: 900 m/s
6. Horizontal traverse: 20 degrees
7. Vertical traverse: -3 degrees 20 minutes to +26 degrees
8. 85 mm shells: 48
9. Maximum speed: 55 kph
10. Average speed: 30 kph
11. Average off-road speed: 18 kph
12. Engine: V2-34
13. Maximum power at 1700 RPM: 450 hp
14. Fuel: diesel
15. Fuel capacity: 460 L

6. On the SU-101 and SU-102 SPGs

Following the order of GBTU, Uralmash designed two experimental SPGs with rear fighting compartments in March of 1945. The prototypes differed in armament.

• The SU-101 SPG was equipped with the D-10S 100 mm gun.
• The SU-102 SPG was equipped with the D-25S 122 mm gun.

At the end of April 1945, Uralmash produced the first SU-101 SPG. After 76 km of trials, design defects became apparent. The driver's compartment rapidly heated to 50-60 degrees, making it impossible for the driver to work. Other defects were also discovered.

Gunnery trials showed weakness of attachment of armoured plates, as well as insufficient robustness of some plates.

Currently, Uralmash is improving the design of these SPGs. All changes are being made to the second prototype with the D-25S gun. Factory trials are scheduled for the end of June of 1945.

The new SU-102 prototype differs from the SU-100 in the following way:

1. New engine and new layout of components.
2. Increased armour of the fighting compartment and hull.
3. Increased power of the gun and machineguns.
4. Improved performance over horizontal obstacles (trenches) and movement on forest roads due to gun position.

The production of these experimental SPGs is a step forward in the development of medium SPGs, but the SU-101 and SU-102 no longer meet modern requirements for armament, armour, and transmission design.

The new medium SPG prototypes, in addition to removal of currently known defects, require subsequent improvement in all main design criteria.

Experimental medium SPG with rear fighting compartment.

Tactical-technical characteristics of the SU-101 and SU-102 SPGs

1. Combat weight: 33-33.5 tons
2. Crew: 4
3. Ground pressure: 0.84-0.85 kg/cm^2
4. Length with gun: 6850-7000 mm
5. Width: 3070 mm
6. Height: 2200 mm
7. Clearance: 430 mm
8. Front hull armour: 90 mm
9. Front fighting compartment armour: 120 mm
10. Side fighting compartment armour: 90-75 mm
11. Rear armour: 40 mm
12. Gun: 100 mm D-10S or 122 mm D-25S
13. Muzzle velocity: 900 (D-10S) or 781 (D-25S) m/s
14. Horizontal traverse: 20 degrees
15. Vertical traverse: -4 to +20 degrees
16. Sights: TSh-19 or TSh-17
17. 122 mm shells: 28
18. 100 mm shells: 36
19. Maximum speed: ~50 kph
20. Range (highway): 250-300 km
21. Range (off-road): 150-200 km
22. Engine: V2-44
23. Maximum power: 500 hp
24. Fuel: diesel
25. Fuel capacity: 440 L"