Optimal Modernization

100 mm B-34 naval gun turret at the Central Museum of the Armed Forces of the Russian Federation.

The fate of the B-34 was equally complicated. This gun was not adopted into service immediately and was reworked many times. Nevertheless, tank gun design bureaus took note of tie. There were several reasons for this. one was that the gun had a one piece round which was relatively compact and not too heavy. The gun was also more powerful than the 85 mm AA gun and roughly in the same category as the 107 mm M-60 gun. However, there was one downside. The B-34 had no armour piercing shell. For that reason, project "412-1v" of the Kirov factory dated January 1941 was declined. The B-34 was forgotten for two years and only revived in the spring of 1943, after the German Tiger tank appeared on the front lines.

On May 5th, 1943, Stalin signed GKO decree #3290ss "On restarting production of the 122 mm model 1931-1937 corps gun and production of experimental prototypes of light corps guns". This launched the development of a 100 mm corps gun with the ballistics of the B-34 naval AA gun. The most successful gun in this series was the 100 mm S-3 developed by the Central Artillery Design Bureau (TsAKB), launched into production 1.5 years later as the BS-3. Another document spoke of a 100 mm gun, GKO decree #3187ss "On improvement of anti-tank defenses" dated April 15th, 1943. Production of armour piercing ammunition began at the same time as production of the corps gun. The issue of missing rounds would be solved.

The second, improved 100 mm S-34 gun on the SU-85 chassis, January 1944.

There were several reasons for choosing a gun with the ballistics of the B-34. One was that the naval gun had a one piece round, which was not achieved with either the 107 mm M-60 towed gun or its tank variant. The B-34 also had a high muzzle velocity, 910 m/s. The muzzle velocity of the ZIS-6, the tank variant of the 107 mm M-60, was only 865 m/s. Third, the difference in weight between the armour piercing shells for the B-34 and M-60 was less than two kilograms. Finally, production of the ZIS-6 and M-60 ceased in 1941, and the tank variant was never fully developed. As a result, the 107 mm caliber fully vanished from design plans by the fall of 1943.

100 mm D-10S, side view, January 27th, 1944.

Unlike the 85 mm high power guns and 122 mm guns with ballistics of the A-19 gun, factory #9 did not initially take part in the development of a 100 mm gun. The task of developing an SPG and tank variant was given to the TsAKB. The gun received the index S-34. Work was headed by the assistant chief designer G.I. Sergeev. Like prior guns, the S-34 was developed as a system that could accept barrels of various calibers. Initially, the S-34 was a duplex. In addition to the S-34-100, there was an S-34-122 with the ballistics of the A-19 122 mm gun. A third system was added, the S-34-85 with a muzzle velocity of 1050 m/s. Eventually the S-34-100 was prioritized. The system was installed on a KV-85 tank in December of 1943. It was tested at the Gorohovets Proving Grounds from January 22nd to January 28th. 

The gun was developed as a variant for installation on a T-34 based SPG, and this is where issues began. Initial calculations showed that the mass of the SPG would rise from 30 to 32.5-33 tons, which was unacceptable. There were already complaints about the lifespan of the running gear, especially the road wheel rims, and increased mass would just make the problem worse. Second, the TsAKB's approach to gun design introduced additional difficulties. The gun breech opened to the right, not the left, meaning that the loader had to be located to the left and the driver to the right. This was not enough for the TsAKB: in their design the first two road wheels received a torsion bar suspension. It's not surprising that this design was radically criticized by the People's Commissariat of Tank Production and had to be redone.

Cutaway of the experimental SU-100 SPG, February 1944.

The TsAKB's refusal to cooperate and Grabin's unique ideas about what came first, the gun or its carriage, played into the hands of factory #9. A group headed by F.F. Petrov quickly developed its own 100 mm gun. M.Ye. Bezusov served as the chief designer. The gun was indexed D-10. Its barrel was 56 calibers long (5610 mm), just like the S-34-100, and so was the muzzle velocity (900 m/s). The D-10 had a longer recoil length than its competitor at 510-560 mm. The development was a logical progression of factory #9 design bureau's work and was compatible with many existing parts. For instance, the breech, cradle, and aiming mechanisms were taken from the D-25.

The first prototype SU-100 and a SU-85, February 1944.

Work on creating an SPG for the new gun went on in parallel at the Ural Heavy Machinebuilding Factory (UZTM). The same designers who designed the SU-122 and SU-85 worked on the new SPG. The work was headed by L.I. Gorlitskiy. The lead engineer on the project was N.V. Kurin. 

In many ways, the SPG called SU-100 was the result of compromises. On one hand, it was obvious that the fighting compartment needed to be wider. On the other hand, the mass restriction stood in the designers' way. Overloading the suspension was the reason why the SU-122M and other similar vehicles did not go into production. As a result, the dimensions of the fighting compartment were the same as on the SU-85. The angle of the upper front plate was also the same: 52 degrees from vertical.

Proving grounds trials, March 1944.

The SU-100 was essentially a further development of the modernized SU-85 built in late 1943. A few improvements were made, the biggest one of which was the installation of a commander's cupola. However, there were others, aside from a simple replacement of the gun. The thickness of the upper front plate and the driver's hatch increased to 75 mm. This made it much more resistant to the most commonly used anti-tank guns, chiefly the 7.5 cm Pak 40. The two-flap hatch for the panoramic sight on the roof radically changed, and a MK-IV periscopic sight was introduced on the left flap. The periscopes along the sides of the fighting compartment were removed, but an extractor fan was added to the roof. The rear plate of the casemate was no longer sloped, which opened up some space in the fighting compartment.

In addition to gunnery trials, the experimental SU-100 was put through mobility trials. In part, the engineers were interested in seeing if the protruding barrel got in the way of maneuvering.

Work on production of the experimental SU-100 prototype began in February of 1944. The NKTP set February 12th as the deadline for two prototypes, but only one was built. Technical documentation was ready by the middle of the month. The D-10S system was a priority. The SU-100 with an S-34-100 was planned no sooner than March 5th. The initially calculated mass of the SU-100 with a D-10S was 31-31.5 tons. The final mass calculations gave 31.6 tons. The ammunition capacity was reduced by a third compared to the SU-85, to 33 rounds. The overall design of the gun mount was similar, but the thickness of the gun mantlet increased to 150 mm. The front left fuel tank in the fighting compartment was removed. The suspension of the front road wheels was reinforced.

The SU-100 prototype converted to take the S-34 100 mm gun, June 1944.

The first trials of the experimental D-10S system were held on February 14th through 15th. 108 shots were made in total. Factory trials of the SPG itself only began towards the end of February. Weighing gave a mass of 30,960 kg. The vehicle travelled for 100 km with an average speed of 24.2 kph. After minor defects were corrected, the vehicle was accepted for proving grounds trials. The commission composed a list of ten improvements that would have to be made before going into production. The commission also raised the question of improving the ventilation of the fighting compartment.

The same vehicle from the sides. The commander's cupola was moved backwards.

Proving grounds trials were held at the Gorohovets ANIOP from March 9th to 27th. The gun fired 1040 shots, 517 of them supercharged. A rate of fire from 4 to 8 RPM was achieved, depending on which ammo racks were used. The vehicle also travelled for 564 km, of them 125 off-road. The vehicle also travelled and additional 300 km on highways and dirt roads over the course of gunnery trials.

Unlike the tank version of the D-10 that failed trials, the situation with the SPG variant was far more positive. Despite a number of drawbacks, the D-10S passed trials, although the commission identified 10 items to be improved. Not all of them had to do with the gun. One of the proposals suggested the replacement of the 10T-15 sight with a refracting telescopic sight. There were also complaints about the fighting compartment. The commander's station was deemed uncomfortable, the cupola had to be moved 200-250 mm backwards. A request was also made to change the seats of the commander, gunner, and loader. Hatches had to be improved, as was the travel lock and lighting. Complaints were made about fumes in the fighting compartment. Another 10 complaints were made about the chassis.

The same vehicle from the rear. The ventilation fan was not changed, there were enough issues that the ventilation of the fighting compartment was relatively low priority.

While trials of the SU-100 with a D-10S gun continued, the S-34 did not move forward as quickly. Factory #92 produced the second and third S-34 prototype, which were sent to factory #100 on March 26th. Gunnery trials of gun #2 (18 rounds fired) showed that the crank of the semiautomatic mechanism jammed. The same defect was discovered on gun #3, which made 22 shots. On May 11th, 1944, People's Commissar of Tank Production Malyshev harshly criticized the S-34. In his words, the system designed to be installed in the IS-2 and SU-100 proved unsuitable for either. Of course, the gun was improved, but time went on and the S-34 did not excel in either the tank or SPG. Since no radical improvements were observed, UZTM was in no hurry. Guards Captain Boglevskiy, a representatived of the TsAKB, arrived at the factory on April 23rd and reported that no progress has been made since January.

Second SU-100 prototype, June 1944.

In practice, work on producing a SU-100 SPG armed with the S-34 gun began in June of 1944. The vehicle was not built from scratch. The first experimental SU-100 was converted to take the TsAKB's gun. Since the loader was located on the right, the commander's cupola was moved back. Now the loader was on the left, which meant that the ammunition racks had to be changed. Since the S-34 was bulkier than the D-10S, there was less room inside. The commission came up with even more complaints about the fighting compartment than before.

The biggest change was the addition of a second ventilation fan.

Trials of the SU-100 with the TsAKB gun were performed at the Gorohovets ANIOP from July 2nd to 7th of 1944. 877 shots were made, 511 of them supercharged. A number of complaints were made about the breech, semiautomatic mechanism, and recoil brake. The gun failed trials. There were also complaints made about the fighting compartment. It was too small for the S-34 and too difficult to fight in. It also turned out that the S-34 could not fire at moving targets. The conclusions were clear: "the installation of the S-34 gun into the SPG on the T-34 chassis cannot be recommended due to the small size of the fighting compartment". 

The commander's cupola was moved to the right in order to improve his working conditions.

The D-10S was progressing much differently. This system was considered nearly an ideal weapon for heavy tanks and medium SPGs. This conclusion was made on the basis of the theoretical penetration, which was even higher than that of the D-25. The installation of this gun into the IS-2 was seriously proposed. However, trials showed that the D-10's AP shell penetrated the front of a Panther tank from 1300-1400 meters. The D-25 could do it at 2-2.5 km. Nevertheless, the D-10S was an optimal system for a medium SPG. It had a high rate of fire and decent penetration. Now all that had to be done was to improve the gun itself and its SPG.

Another lesser known change was the thickening of the front armour of the commander's cupola to 90 mm.

The second SU-100 prototype was finished in May of 1944. The vehicle was changed during the assembly process. One of the changes was the ventilation. In addition to having two fans, they were replaced with the more powerful MV-12 type. Additional caps were welded onto the fan covers, which were replaced with a distinctive bell shape in production. The commander's cupola was shifted to the right, which resulted in a larger bulge. The cupola itself was no longer symmetrical. The armour in the front section was increased to 90 mm. The TSh-15 refracting sight was also installed, leading to a small change in the gun mantlet.

Viewed from the rear, the second prototype and early production vehicles were identical.

Trials of the second prototype took place at the Gorohovets ANIOP from June 24th to 27th. The SPG travelled 250 km, 50 of which without the travel lock engaged. The gun made 923 shots, 526 of them supercharged. As a result of the trials, the commission made 9 comments regarding improvements to the chassis and 12 regarding the sights. Nevertheless, the overall result was clear. The SU-100 passed trials and could be accepted into service after necessary improvements were made. On July 3rd, 1944, Stalin signed GKO decree #6131ss "On organization of production of the SU-100 SPG at the Uralmash factory of the People's Commissariat of Tank Production and 100 mm D-10S guns at factories #8 and #9 of the People's Commissariat of Armament."

The realities of the Great Patriotic War were such that medium SPGs did not last for long in production. The situation with the SU-100 was the opposite. To get ahead of ourselves for a moment, this SPG turned out to last longer in production than any other Soviet vehicle developed during the war. However, there was a problem with production of the D-10S. The GKO decree gave realistic deadlines for putting it into production. Only 40 vehicles were expected in September of 1944, 90 in October, 150 in November, and in December UZTM would fully transition to the SU-100, giving 210 vehicles. A hybrid known as the SU-85M would be produced from September to November. This was a SU-100 hull with the armament of the SU-85A. Thanks to this flexible plan, the quota was fulfilled in the fall, and even overfulfilled in December. 220 vehicles were delivered, 10 more than required. The September production vehicles cost 185,000 rubles. By October, the price dropped to 176,000 rubles.

Production SU-100s at the delivery courtyard, spring 1945.

Production vehicles were similar to the second prototype, but with some changes. Road wheel rims without cooling openings were used. These were introduced on the SU-85 and showed higher lifespans. Two smoke bombs were attached to the upper rear plate. A cap to hold the track lock was introduced on the roof of the hull, to the right of the panoramic sight hatch.

Another introduction was made to the gun mantlet, characteristic grooves that improved the installation of the gun. A TSh-19 sight was installed instead of the TSh-15. This was the cause of some production issues in the fall of 1944. 8 cases of field of view obstruction and fouling of the sight frame were recorded in October of 1944. The frame was trimmed and the defect was not observed in November. The bump stop for the suspension arms was also slightly tilted.

The roof of a typical SU-100 produced between September 1944 and January 1945.

Initially, the biggest cause of defects on the SU-100 was the gun mount. The frame rim was the most frequent cause of defects (17 instances). Like with the sight defects, these were usually corrected immediately (the rims were welded). There were also issues with the mantlet and aiming mechanisms. There were frequent complaints about the elevation mechanism, the problem was serious enough that a technical meeting was called to resolve the issue. Analogous problems remained, which caused a redesign of the D-10S (work began in December of 1944).

A reinforced road wheel designed in early 1945.

A much biggest list of complaints arrived at the UZTM in late 1944, when the vehicles were issued to troops. Complaints about the TSh-19 sight were frequent, cracks were noted in the casemates. The biggest issue was with the road wheels, specifically the front ones. Even though the suspension was reinforced and the design was improved, the front two road wheels still exhibited heavier wear. Not only did the rims fall apart, but cracks were noticed in the disks and welds. This defect was unavoidable. Similar problems were noted in the German Panzer IV/70 tank destroyer. Improved armament and front armour resulted in an overloaded front suspension. As a result, the UZTM had to both deliver new road wheels and develop a reinforced front road wheel and suspension arm.

February-April 1945 production vehicle. The casemate hatch now only had one flap.

Issues with cracks were resolved by changing the design of the casemate. According to documents, the first such change was introduced in April of 1945. Instead of interlocking plates, a butt weld was used to attach the side armour. The second casemate hatch flap was cancelled in February of 1945, although most February production SPGs still had the old hatches. These vehicles are often called the second production series, but SU-100 production was not split into series. Around this time, the toolbox on the left fender was moved backwards. The new hulls were put into production in March of 1945. The "R" marking (equivalent thickness, same 75 mm armour as the front plate) vanished from the driver's hatch. 

Improvements to the SU-100 continued in April. In part, the tow cables were improved, and the 9-RS radio replaced the 9-RM. The factory also transitioned to a factory #112 style engine foundation. Changes introduced into the hull design improved their quality. For instance, 5.6% of the hulls inspected in February of 1945 had defects, and 3.2% in March. However, there were also gearbox defects. Measures taken to combat these issues had little effect on the production volume. The factory delivered 210 SPGs in January, 215 in February, 211 in March, 214 in April. By May 1st, 1945, 1350 SU-100 SPGs had been delivered.

Production of the SU-100, May-June 1945. The second casemate hatch flap and bracing on the side plates are missing.

The end of the Great Patriotic War coincided with a new batch of changes to the SU-100, primarily the hull. A beamless front section was introduced as of May 1st. The bracing connecting the side armour was removed. A part of May production vehicles still had the old hulls, which were used in production until they were used up. The thickness of the lower front plate was increased to 60 mm. The hatch hingest were rationalized. May had the most visible changes to the design of the SU-100. The issue of disintegrating road wheels was also solved in May. The D-10SK gun was also put into production.

Another visible change was made in June. A toolbox on the right side of the casemate was added. Other changes in the summer of 1945 were not as major. For instance, an attachment for an unditching device was added in July of 1945.

Assembly of the last SU-100 at the UZTM factory.

Production continued at the same rate in the spring-summer of 1945. 210 SU-100s were delivered in May, the same in June, 200 in July through August. The end of the Second World War reduced production volumes. 165 were produced in September, 160 in October, 140 in November, 150 in December. Improvements at this stage were minor and ceased in December. UZTM knew in November that SU-100 production would soon stop. The factory had to shift to civilian designs. This had an impact on the attitude towards SU-100 production. The last SU-100s were delivered in 1946: 50 in January, 100 in February, 102 in March. The overall production run numbered 3037 vehicles. The SU-100 became the most numerous Soviet medium SPG.

Factory #174 SU-100, Central Armed Forces Museum.

This was a pause in SU-100 production, but not the end. Production of the T-34-85 at factory #174 in Omsk ended in 1946. The factory was supposed to begin T-54 production, but the vehicle still needed improvements, which resulted in a delay. It was decided to begin production of the SU-100 to keep the factory busy. The quota was not large, only 200 vehicles. The GBTU considered the SU-100 obsolete and its only real purpose was to subsidize tank building in Omsk. Factory #174 delivered 194 SPGs in 194 and 10 more in 1948, after which T-54 production began.

Due to the small production run, the SU-100 was more expensive and cost 303,900 rubles per unit. The Omsk vehicles were visibly different from the ones built in Sverdlovsk. They had different tow hooks, hubcaps, commander's cupolas, fenders, and other components. The upper rear plate hinges were rationalized. Many elements of Omsk vehicles were inherited by the SD-100. 

The organization of SPG regiments (SAP) equipped with the SU-100 was identical to those equipped with the SU-85. TO&E #010/462 authorized 21 vehicles (four batteries of 5 each plus a commander's vehicle). Since production began in September of 1944 and deliveries began closer to November, the SU-100 did not have as much of an impact as the SU-85. The SU-100 only saw actual combat in early 1945. Discovery of defects also had an effect on the SU-100's combat debut, chiefly the excessive wear on the front road wheels. There were also complaints pertaining to the various components of the gun mount and cracks that appeared near the joints between the two side plates.

Illustration of the cracks from a report by the 1st Guards Tank Corps. This defect was a common one and resulted in a revision to the design of the casemate.

The first users of the SU-100 were the SAPs of the 1st Guards Tank Corps. This happened in January of 1945, during fighting in Hungary. By January 5th, the corps included the 1453rd, 1821st, and 382nd Guards SAPs with the new SPGs. The debut took place on January 11th, when the Germans counterattacked with up to 100 tanks, as reported by the corps. The 1453rd and 1821st SAPs destroyed 20 tanks. In subsequent fighting on approach to Budapest the SU-100 showed themselves well, but the 1st corps reported a number of drawbacks (chiefly regarding the cracks in the hull and issues with the front road wheels). There were also complaints about the TSh-19 sights. A number of vehicles in use dated back to September-October production when the issues with the sight were not fully resolved.

Defensive fighting revealed the need for a machine gun. There were different opinions about this. A number of crews wanted a coaxial gun, like on tanks, others wanted it on a pintle mount. The need for a machine gun was not debated. On January 8th the 382nd Guards SAP lost 10 SPGs as a result of an enemy infantry attack when it turned out that they had nothing to oppose it. Half of the regiment's vehicles were lost without even engaging enemy AFVs.

SU-100 on approach to Berlin, April 30th 1945.

The SU-100 was used intensively in the fighting for Hungary. These vehicles were used to defend against the German counteroffensive at Lake Balaton. One of the defending units was the 912th SAP from the 207th SPG Brigade (former 1st Order of the Red Banner Tank Brigade). The regiment went into battle on March 8th, 1945. On March 11th it took up defensive positions near Kisvelence. The German tanks attacked on the next day. The Germans lost 9 tanks and up to 70 soldiers killed, the 912th SAP lost 2 SU-100s burned up and 2 knocked out. 2 SPG crewmen were killed, 17 wounded. These kinds of losses shocked the Germans, and they did not attack the positions of the 912th SAP until March 18th.

On March 20th the regiment attacked in support of the 34th Rifle Division. In battle with 14 German tanks the SPG gunners destroyed 7 tanks (one Panther) and knocked out two Panthers. Over the course of the March 20th-22nd offensive the regiment destroyed 7 tanks and knocked out 2, having lost 5 SU-100 burned up and one knocked out (5 SPG crewmen were killed, 26 wounded). Battles for the Raba canal and Sopron city followed. From March 23rd to 30th the SPGs destroyed 6 more tanks and SPGs, four APCs, 14 AT guns, and up to a battalion of infantry. Their losses totalled 1 burned up SU-100, 3 knocked out by mines, and one burned BA-10 armoured car. 6 SPG crewmen were killed, 9 were wounded.

The 912th SAP remained in action until April 15th. Over a month of fighting 15 SU-100 were burned and 1 SU-100 was knocked out. 21 SPG crewmen were killed and 87 wounded. Five SU-100s arrived as reinforcements. The remaining 10 vehicles were transferred to the 208th SPG Brigade. The 912th SAP reported 39 destroyed enemy tanks and SPGs, 30 APCs, and 55 guns. 20 enemy tanks were captured, as well as up to 40 tractors, 17 guns, and up to 500 soldiers, including Hungarians.

SU-100 in Berlin.

Considering that units that used the SU-85 were actively transferred over to the SU-100, these vehicles often appeared in newsreels by April of 1945. Their input into the war was significant. The SU-100 played a big role in fighting in Germany, Austria, Czechoslovakia, and were actively used in Berlin and Vienna. Despite some issues, the SU-100 was the best Allied medium SPG. Thanks to significant reinforcement of the front armour, the SU-100 was very resistant to German artillery, especially the 75 mm caliber. The closest analogue to the SU-100 was the German Panzer IV/70 tank destroyer, but it had significantly poorer mobility. The armament of the SU-100 was also more powerful, allowing it to destroy Panthers at a range of over a kilometer. The only tank that the SU-100 could not destroy frontally was the King Tiger, but fewer than 500 of them were built. There were more powerful SPGs for fighting this rare beast.

Victory parade, Red Square, June 24th, 1945.

Large sale production and impressive characteristics gave the SU-100 a long life. Even though the GBTU dreamed of a successor back in 1945, this was the last Soviet medium SPG built in such amounts. The SU-101 (Uralmash-1) was never put into mass production, and the SU-122-54 designed in the 50s was produced in a small batch. It turned out that the D-10S was sufficient for most tasks. The SU-100 served in the Soviet army for a long time and was modernized several times. In its later years the SU-100 was used as a training vehicle, but officially it was only removed from service in 1997 (alongside the T-34-85).

SU-100 in Yemen. These vehicles are used as mobile artillery.

The SU-100 was sent in large amounts to Poland, Czechoslovakia (where it was also produced under license as the SD-100), Bulgaria, Yugoslavia, DDR, Romania, Albania, Cuba, China, and other nations. This vehicle took part in many local conflicts. Some SU-100s fight to this day (for instance, in Yemen). It's too early to conclude this vehicle's career.