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Even though mass production of SU-100 tank destroyers began in September of 1944, their debut in battle was delayed until January of 1945. Front line troops would no doubt have been glad to have a new methods of hunting German "beasts", but this way crews would go into battle having mastered their vehicles and built cohesion within regiments and brigades. It was three SPG brigades, the 207th, 208th, and 209th, that made up the main users of the SU-100 during the battles at Lake Balaton that began in March of 1945. These crews could now present a well balanced opinion on their vehicles.

Brand new SU-100s lined up at the Uralmash factory.

Naturally, the results of the SU-100's debut were quite interesting to Soviet tank commanders and designers, which is why units that used them in battle received the following order:
"To contribute to a report to the Directorate of Self Propelled Artillery of the Main Artillery Directorate of the Red Army on the use of the SU-100 SPGs in combat, reply with feedback on the following:

  1. Use of the SU-100 in battle. 
  2. Effectiveness of design changes made to the SU-100 compared to the SU-85 (armour, running gear, fighting compartment, etc).
  3. Comparative mobility and ability to cross obstacles of the SU-85 and SU-100.
  4. Crew comfort.
  5. Practical rate of fire.
  6. Work of the elevation and traverse mechanisms.
  7. Ammunition rack layout.
  8. Reliability of the sight mount and convenience of use.
  9. Resistance of the armour to various weapons and quality of armour.
  10. Observation of the commander, gunner, and driver.
  11. List of main defects and issues encountered during use of the SU-100.
  12. List of proposed design changes to the SU-100 that would improve its fighting or usage characteristics.
In your report, I ask you to give your conclusions regarding the relative effectiveness of the SU-100 compared to SU-85 SPGs and T-34 tanks.

Naturally, most commanders answered the first question in the most detail. Most reports were limited to just that one point and only contained a few paragraphs on their impressions of the hardware.

Artillerymen, not tankers

It was clear from the reports that the biggest issue with the SPGs was not the design, but how they were used. In defensive battles infantry cover was either weak or absent. The solution was a proposal to add more motorized infantry into SPG regiments and supply each SPG with at least one squad of submachine gunners.

The request to install at least one machine gun into the SU-100 was common. This was often included in the answer to item #12, but sometimes given as a separate answer: "The lack of machine gun in a ball mount in front of the driver... is a design flaw of the SU-100".  

In offensive battles, the unfortunate tradition of using SPGs instead of tanks was a big problem. Sometimes the SU-100 was used as a tank when there was no other choice, but in other cases the commanders did not know the field manuals and had a poor idea of an SPG's role on the battlefield. All reports included complaints and demands to "Use the SPGs according to the Field Manuals of the Armoured and Mechanized Forces of the Red Army parts 1 and 2... Use the SU-100 as SPGs to escort tanks, not as tanks." either in the opening paragraphs or the conclusions.

A column of SPGs from the 11th Guards Tank Corps. SU-100s fought in East Pomerania within the 1454th SPG Regiment.

As for the hardware, there was a wide spectrum of opinions. Commanders that received SU-100s at various times may not have had time to fully adapt to their behaviour after long and difficult service. The SU-100 was also the first vehicle for many young commanders: "No one from the HQ or battery commanders knows the SU-85, thus we cannot answer this question."

Big and heavy gun

SPG crews and tankers with a lot of experience noted the complications arising from installing a long and heavy 100 mm gun on the T-34 chassis. The gun had excellent qualities, but also caused issues:
"Compared to the T-34 tank, the SU-100's mobility decreased due to the increased weight and shift of the center of gravity forward because of the longer barrel and thicker front armour. This increased the load on the front road wheels. The barrel scrapes the ground when crossing obstacles, which can lead to the gun bursting after firing. When the vehicle turns sharply and hits the gun on terrain, it can upset the adjustment of the gun and aiming mechanism. There were cases where the elevation mechanism gear was torn off."

"A big issue of the design is the reliability of the elevation mechanism. If the gun is not locked during transit it can jam as a result of galling from vibration. It is necessary to use lubricated needle bearings."

Two precise holes in the side of a King Tiger tank, the result of a SU-100's work at Lake Balaton in March of 1945.

"The SU-100 had no defects with the exception of jamming of the elevation mechanism."

"The weakest part of the SU-100 is the elevation and traverse mechanisms as well as the road wheels (specifically on the right side)." 

"When firing, the SU-100's breech often does not fully close. The reason for this is the weak spring of the semiautomatic mechanism. It is necessary to improve the spring by thickening it."

Another big issue was that the gun was too big for the T-34's chassis. The designers' efforts could not change the fact that the vehicle was very crowded.

"The crew's work is made more difficult. 1) The size of the fighting compartment is reduced due to the larger gun. 2) The gun sight is shifted to the right, it is hard for the gunner to aim and it's nearly impossible to correct fire with the hatch closed. 3) The cramped fighting compartment constricts the loader's work and reduces the rate of fire."

"The only drawback of the SU-100 is the fact that when the gun turns to the left, the commander is pressed against the right side due to the shifted cradle. The breech is very close to the rear bulkhead and complicates the loader's work, which impacts the rate of fire (especially with HE shells, as they are longer).

SU-100s in Prague, spring-summer of 1945.

As for the rate of fire, some crews claimed that it was as low as 2 RPM, but as they got more practice the value went up:
"With a well practiced crew, the rate of fire is 4-5 RPM."

"The average practical rate of fire of the SU-100 is 4-5 RPM. Using ammunition stored under the gun is more difficult."

The value of 4-5 RPM likely applies to AP shells, which were the SU-100's main type of ammunition. Units that faced few tanks and ended up supporting infantry exhibited a lower rate of fire. As stated above, this was due to the length of the HE shell and more complicated aiming procedure.

More armour, more fuel

As for armour protection, there were no miracles here. The armour of a T-34 tank, even if reinforced slightly, was not enough against enemy guns by 1945. No one expected the armour to take a hit from an 88 at any range, but there were issues with more than just AP shells:

"The quality of the armour is satisfactory, but the quality of the weld seams is unsatisfactory. When the hulls are hit the seams burst."

"The reliability of the armour withstands all hits of HE shells up to 105 mm at the range of a kilometer or greater."

Finally, those SPG regiments that used the SU-100 from Budapest to Vienna, often in long raids alongside the tanks, complained about the reduced fuel capacity. This was also a necessary evil, but the result was that it was hard to use the SU-100 alongside the T-34.

 "The main defects and issues of the SU-100 include:

  1. Sometimes the elevation mechanism gears break when firing.
  2. The oil pipe that leads from the oil filter to the stop valve breaks from vibration due to the thinness of the pipe, excess length, and brittle metal.
  3. The bearings of the front road wheels break and their tires separate due to the shift of the center of mass forward and increased load.
  4. The hull weld seams are insufficiently robust.
  5. The lack of machine gun makes fighting enemy infantry difficult. The use of the SU-100 as a tank is ill-advised.
  6. The rear casemate plate is thin.
  7. The narrow traverse angle makes it hard to aim, especially at moving targets." 
SU-100 SPGs preparing to enter Red Square during the Victory Parade held on June 24th, 1945. There were hundreds of these vehicles in service by the summer of 1945.

"One design drawback is that it is impossible to remove the front right fuel tank if it is damaged without removing the gun, which is difficult due to the gun's weight. The reverse gear bearings are weak and don't hold up when the SPG is reversing up a slope or towing another vehicle backwards."

"Conclusions: the SU-100 is a superior design to the SU-85 both in firepower and performance, but it falls short of a tank. It is necessary to make several changes: a) reinforce the front road wheel bearings, as reinforcing the spring alone did not give the required effect. b) it is necessary to install a more robust hose for starting in the winter. c) increase the fuel capacity. d) reinforce the hatch springs. e) give the commander a machine gun and increase the gun traverse angle."

Despite all the drawbacks listed in the reports, they confirmed the SU-100's effectiveness as an anti-tank weapon:

"The SU-100 is the most effective means of fighting enemy tanks out of all known SPGs."

Original article by Andrey Ulanov.


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