Sunday 4 February 2018

Don't Leave Well Enough Alone

On March 25th, 1936, the French cavalry accepted the Automitrailleuse de Combat modèle 1935 S into service. More commonly known as the SOMUA S 35, this tank was officially classified as an armoured car, but turned out to be one of the best French tanks of the interbellum period. It combined sufficiently thick armour, powerful armament, and decent mobility. Despite its respectable characteristics, work to replace the tank began in the late 1930s. This decision appears rather logical, as the development of armoured vehicles was moving very quickly. This article will discuss the history of the SOMUA S 40, the prospective replacement for the S 35 that nearly made it into production. Alternatives to this tank designed during German occupation will also be mentioned.

Outcome of the Char G

The development of a modernized cavalry tank was, in part, triggered by French infantry command. The development of the SOMUA AC 3 “armoured car” did not go unnoticed. The infantry had an analogue of this tank, the Renault D2, but it was already unsatisfactory by 1935. As a result, requirements for a new 20 ton medium tank were developed by December 25th, 1935.

The infantry’s wishes surpassed those of the cavalry. According to specifications, the top speed of the Char moyen d’infanterie de 20 tonnes would be 50 kph, and its cruising range would reach 500 km. The infantry commanders expected these tanks to be used to arm motorized infantry divisions (DIM). Initially, the characteristics of the 20 ton tank remained at the level of the SOMUA S 35, but the traditional inflation of requirements began in 1936. The required armour thickness increased to 60 mm, same as on the Char B1 bis, and a short 75 mm gun was introduced into the front plate. This was the start of the long and sad history of the Char G tank.

The SOMUA SAu 40 at the factory. If it received an APX 4 turret and a 75 mm SA 35 gun, then it would become the 20 ton tank that the French infantry wanted.

8 companies took part in bidding for the 20 ton tank project, 6 of which had a design by early 1937. Schneider, acting through its subsidiary SOMUA, was among them. The characteristics of most projects were taken straight from the requirements. Only Renault made an original design, reducing the number of cannons to one, housed in the turret. As for SOMUA, it decided to not reinvent the wheel and took the AC 4, reworking it to meet the infantry’s demands.

Work on the project continued until February of 1938, when the infantry raised the weight limit of the Char G to 35 tons. The tank was outdated before it even reached the prototype stage. However, SOMUA quickly found a use for their 20 ton tank. It became the foundation of the CAM 2 SPG (also known under the name SOMUA SAu 40), which also nearly made it into production. This French-style StuG needed only a few more months to reach the battlefield. This vehicle deserves a separate article. Let us turn to another offshoot of the 20 ton tank project.

The initial look of the SOMUA AC 5

Discussion of further development of the cavalry tank began in late 1938. ARL, the recently nationalized tank branch of APX, took an active part in the process. Preliminary calculations showed that implementing designs developed for the 20 ton tank would increase the tank’s mass to 20.5 tons. SOMUA’s factory began working on this vehicle more intently starting in April of 1939. By then, it had been decided that no more than 450 SOMUA S 35 tanks would be produced. An improved tank would enter production in October of 1940. This tank received the factory index AC 5. The cost of the tank was calculated in August of 1939. This was the first time the name SOMUA S 40 was used. According to these calculations, the cost of one tank rose to 100,877 francs.

Experimental prototype of the SOMUA AC 5 with a model of a turret platform and turret, February 1940. The turret is a model of the ARL 2 C turret.

No radical modernization was planned so that mass production could be easily reached. Initially, the same turret was used, the APX-1 CE. The hull of the AC 5 would change much more noticeably. Its length increased, but height decreased. The front part of the hull changed the most. The sloping of the front armour increased, which meant that the tank was now better protected from the front. The only place where armour became thicker was the floor. Its thickness on the S 35 varied between 15 and 20 mm. The floor of the modernized tank was a constant 20 mm thick.

ARL 2 C turret that was to be installed on production SOMUA S 40 tanks.

In addition to an altered hull, the AC 5 had a number of less visible upgraded components. For starters, a 13.75 L engine was planned for the tank. At 2200 RPM it developed 230 hp, and would result in a top speed of 45 kph. The observation devices were altered along with the hull. The suspension was reinforced to deal with the increase in mass, and the idler was moved forward to improve off-road mobility.

Experimental prototype of the ARL 2 C turret. For the most part, it was assembled by welding.

The turret was another difference. According to initial plans, the first 50 SOMUA S 40 tanks (##451-500) would use APX 1 CE turrets, just like the SOMUA S 35. However, the prototype already had not only a new turret platform, but a model ARL 2 C turret. Its development began in January of 1939. Unlike the APX 1 CE, it was assembled largely by welding. Only the commander’s cupola was cast. Rolled armour had better toughness at the same thickness. Unlike the APX 1 CE, which could be penetrated by the German 3.7 cm Pak from 200-300 meters, the ARL 2 C held at nearly point blank range.

Since the rotating commander’s cupola was far from ideal, it was discarded. It turned out that making an enlarged immobile cupola was much easier.

The view from the front shows how much larger the cupola became.

The first contact for the production of 50 AC 5, #98 302 D/P, was signed on September 21st, 1939. The start of the Second World War forced French command to delay the production of improved S 35s. According to corrected plans, tank #451 would be completed in June of 1940. The first batch would not remain in production for long. The French planned to produce 30 SOMUA tanks per month starting in May of 1940. In reality, only 22 S 35s was built in May. However, 22 tanks were built in June before the armistice, so a production volume of 30 tanks per month was feasible. The next contract, #130 PR/PX, required the production of 324 tanks (##501-824). Cail-Denain would aid Creusot, the main subcontractor, in the production of hulls.

The fourth SOMUA S 40 hull, May 1940. The tank was one month away from production.

Plans for mass production of the SOMUA S 40 did not remain on paper. Creusot produced the first hull in January of 1940. The order for SOMUA S 35 hulls meant that the next two hulls were only finished in April, and five more in May. SOMUA corrected the production plans: the first 14 finished tanks were expected in July, 27 in August, and 32 in September. The first S 40 tanks with the ARL 2 C turret would enter production in September of 1940. Alas, the result of the May-June 1940 campaign mean that the production did not continue past a few half-finished chassis.

The Christie alternative

The AC 5 was far from the only prospective French cavalry tank by the start of 1940. The AMX design bureau was also working on this subject, but they did not progress past drafts. Still, their ideas were rather interesting, especially its American roots. It is widely known that John Walter Christie worked closely with the British, especially William Morris, the founder of Morris Motors Limited. The result of this relationship was the Cruiser Tank Mk.III: not the greatest vehicle, but a major milestone for British tank building. Real progress in cruiser tank design began with this vehicle. It was based on the chassis of the Christie Convertible Medium Tank M1931, which Morris bought and indexed A13E1.

The Christie Airborne Tank M1937 was also tested in Farnborough, but the British rejected it. Nevertheless, the suspension of the Cruiser Mk.III came from this tank. The fact that Christie tried to set up a connection with the French is less known. He managed to sell the patents for his suspension. The tank was also shown off at a demonstration in Vensen.

Unlike other tanks by the American inventor, the M1937 was unique in that it was a purely tracked vehicle.

The Christie Airborne Tank M1937, which was demonstrated in France in 1938.

The French took little interest in Christie’s tank in the spring of 1938. The situation changed in April of 1939, when the French began closely cooperating with the British in the field of tank design. They saw the Cruiser Tank Mk.III and were stunned by the maneuverability of the British tanks. At the same time, the tank resulted in mixed feelings, specifically the bulletproof armour. The British cruiser was also very expensive, even by French measures. The Liberty aircraft engine, not especially known for reliability, also did not impress the French engineers. However, the idea of a cruiser tank was deemed interesting. A similar vehicle was designed.

Draft of the AMX 40 cavalry tank, March 1940.

Atelier de Construction d'Issy-les-Moulineaux (AMX)  began working on the new tank in early 1940. Joseph Molinie, the future chief designer of AMX and the creator of the most known French post-war tanks, was in charge of the project. The task was a difficult one, and unconventional solutions were required. The AMX design bureau completed blueprint #0-387 on March 4th, 1940. It depicted a tank called Char cavalerie AMX 40. Molinie’s group came up with a very original design, barring the Christie suspension. This was a tank with a layout that was closer to the Renault R 35 or Renault AMC 35 rather than the SOMUA S 35. The two-man turret of the latter served as a starting point for the AMX 40’s turret.

In order to meet the requirements (which were made even steeper: the front armour was thickened to 60 mm), the tank’s layout had to be as dense as possible. Perhaps too dense. The AMX 40 had narrow tracks (a tradition for Christie tanks), which could have resulted in issues with off-road mobility. On the other hand, the power to weight ratio of 10 hp/ton wasn’t bad by French measures, and would result in equal mobility to the SOMUA S 35. The tank’s top speed was estimated at 45-50 kph. Since the tank was planned to be purely tracked, no additional gears or chains were used.

A reconstruction of the AMX 40 exterior by Vsevolo Martynenko.

Like the requirements stated, the tank was armed with a 47 mm SA 35 gun and a coaxial 7.5 mm MAC Mle.1931 machine-gun, as well as a second machine-gun on an AA pintle mount behind the turret bustle. 176 shells were stored along the perimeter of the turret. The diameter of the turret ring was less than a meter, and the turret was 1.26 m wide overall. It was very cramped.

A hatch was included in the rear of the turret, but one would have to climb around the gun to use it. The driver also had to perform feats of agility, as his hatch could only be used if the turret was turned away.

The AMX 40 should be treated as a thought experiment. Due to a number of obvious issues, it is unlikely that it would have been built in metal. There was no discussion of building a prototype, and work ended in June of 1940 for obvious reasons.

Paperwork under German occupation

The partial occupation of France in the summer of 1940 did not mean that French tank design stood still. According to the agreement with the Germans, the Vichy government had no right to develop or produce armoured vehicles. Nevertheless, work on prospective projects continued in secret. Primarily, it focused on modernization of existing vehicles. For example, a turret for the Panhard 178 armoured car with a 47 mm SA 35 gun was designed, and even produced in small numbers. Lorraine designed a 4-wheeled version of the Lorraine 37L tractor in the guise of a tractor, known as the Lorraine 37/44.

In other words, French tank building was alive, albeit barely. Later, this work became the foundation for the rebirth of French tank development, which began immediately after the country was freed from occupation. The same people who headed modernization before the war were at the helm once more.

A design for a two-man turret for the SOMUA S 35, developed by FCM engineers. Summer 1942.

Forges et Chantiers de la Méditerranée (FCM). Also found itself outside of German control. Aside from production of the Char B, this company developed many other things, including turrets. A modernization of the SOMUA S 35 was developed here in 1942. FCM decided to put an end to one of the greatest drawbacks of French tanks: the fact that they fit only one crewman. A new tank turret was designed in 1942, a largely welded design. The turret ring diameter was increased to 1435 mm, and a turret basket was introduced. A commander’s cupola with a hatch was another modification. If necessary, the hatch could be used to hold an AA machine-gun pintle mount. The main armament remained the same 47 mm SA 35 gun, but the machine-gun was swapped out for a faster firing MAC 1934 Mle.39.

A modernized turret with three crewmen. A more powerful gun was also planned.

An even more radical modernization was planned within the same project. A three-man turret would be installed on that same turret ring. The commander could finally stick to his own duties instead of playing a one-man band. The commander’s cupola was moved in the three-man variant, opening up space for the loader. Another notable difference was that the more powerful SA 37 gun was planned instead of the SA 35. It was still obsolete in 1942, but allowed the tank to fight against tanks with 50 mm of armour.

This work was destined straight for the archive. FCM had little production capability. After the occupation of the rest of France in 1942, work ceased for nearly two years.

SARL 42, an even more radical modernization of the SOMUA S 35.

An even more radical modernization of the SOMUA S 35 was developed on German occupied territory. It was headed by Maurice Lavirotte, a key figure in the Char B program and the lead designer at ARL. He had a direct connection with the SOMUA S 40 program. It’s not surprising that he didn’t stop at just the turret. The project, titled SARL 42 (SOMUA-ARL 42) consisted of a serious modernization of the whole tank. The hull, initially designed at ARL, was radically changed. The front received greater slopes, the radio operator’s station was deleted, and the hull’s shape became simpler.

Another change was the turret ring. The diameter of the ring was now a hair over 1500 mm, and the diameter of the turret race was 1580 mm. As for the chassis and engine, it was left over from the SOMUA S 40. The mass of the SARL 42 would be 22 tons.

The hull of the SARL 42. Only the driver remained in the driver’s compartment.

The development of the SARL 42 was done by two groups. One of them worked in Cossad, outside of German occupied territory. The second group, which was developing the turret, was headed by engineer Devenn, who designed turrets for a number of tanks alongside Lavirotte before the war. The SARL 42 turret was rather unusual. It did not fully use the turret ring, especially in the front and the sides. Devenn’s team tried to give it the smallest silhouette possible. The design of the turret suggests that it would use as much welding as possible, which was highly atypical for French prewar tanks. The front of the turret held the gunner and loader (who doubled as the radio operator). The commander was placed in a cabin, which was pulled far back. It also contained a rangefinder.

The third group, headed by engineer Lafarge, was working on the gun. Two options were designed: a 75 mm L/32 gun, and a much more powerful gun of the same caliber, with AA ballistics. Typically the length of the gun is stated as 44 calibers, but that is incorrect. In practice, the length of the barrel was 4000 mm (53.3 calibers). The Schneider Canon CA 75 mm Mle.39 was used as the starting point.

Canon CA 75 mm Mle.39. This gun was taken as the starting point when designing the SARL 42.

These respectable characteristics would have allowed the SARL 42 to combat most WWII era tanks. The issue was that Lavirotte’s group didn’t even have the ability to assemble a prototype. All work stopped after the Germans occupied the remaining territory of France in November of 1942.

Nevertheless, the development of the SARL 42 was not a waste of time. The people who designed it became the backbone of the team that designed the first French post-war tank, the ARL 44. The plan was to arm it with the 75 mm SA 44 gun, effectively the same gun that Lafarge designed for the SARL 42. The tank itself was obsolete, and vanished into the archives.


  1. Is the ending saying that the ARL 44 was obsolete, or that the SARL 42 was obsolete?

    The whole history of French armor is fascinating. The what might have been are a rich source speculation. And lots of fun :-)

    1. The SARL 42 was obsolete. The ARL 44 wasn't great either, but it wasn't a bad stepping stone for rebuilding the French tank industry. Or it would have been if the French military could ever settle on their requirements.

  2. From 1938, instead of wasting effort on the unnecessary AMX 38 infantry support tank, AMX could & should have started development of something very much like the AMX 40 for the Cavalry. The worst element of the design was the tiny two man turret, which would have been unbearably cramped & very difficult to fight in. The most practical solution would be to reduce the total crew size to two, & use the already existing one-man APX-4 turret; possibly simplified by replacing the rotating cupola with a fixed alternative. Turrets could be made available by phasing out Char-B production & switching APX-1 CE production to APX-4. As Infantry requirements would have starved the DLMs by ordering as many S-40s as SOMUA could Possibly build, such an "alternative history" version of the AMX-40 could have provided a reasonably cost-effective choice to replace potential S-35 losses. If enough could be manufactured, they could also begin replacing the Cavalry's H-35s.