Sunday 15 April 2018

The Winding Road to Nowhere

The French were the first to master the production of SPGs. These vehicles missed WWI by only a few months. Enthusiasm for SPGs died down after the war ended, and France only returned to this topic in the 1930s. This article tells the story of French SPGs built on medium tank chassis, specifically the SOMUA SAu 40, which nearly made it into production.

From a support vehicle to a tank destroyer

Émile Rimailho, the head designer of Compagnie des Forges et Acieries de la Marine et d'Homecourt (FAMH), was one of the forefathers of French self propelled artillery. He was the creator of the "mobile gun mount" concept that was used in a number of vehicles that made it into mass production. They are known as Saint-Chamond, in honour of the city where the company was located.

Tanks designed and produced by the FAMH company also had this name. Their distinguishing feature was an electric transmission. Rimailho's SPGs were even more interesting. The SPG's electric motors was powered by a prime mover ahead of it, which also acted as a munitions carrier. Porsche's famous tanks with an electric transmission would not be built until more than two decades later!

A patent for Emile Rimailho's SPG dated 1921. The designer later used this concept for a light cavalry support SPG.

The end of WWI was a hit French military industry. The French army was not interested in a large order of Saint-Chamond SPGs, and the manufacturer had to hope for foreign orders. Several empires collapsed as a result of the war, and turned into a number of independent states. Local conflicts weren't something unusual, and even the difficult economic situation in the world was not an obstacle for growing arsenals.

However, a large number of surplus weapons flooded the market, which made the life of weapons manufacturers difficult. Nevertheless, several companies, including FAMH, were developing new types of armoured vehicles. These vehicles were aimed at the export market, since the French army had plenty left over from the war. The army itself was selling off its surplus, which made life difficult for domestic manufacturers.

Canon de Cavalerie automoteur de 75 Saint-Chamond Modele 1924

One of FAMH's humble successes was the convertible drive Saint-Chamond Modele 1921 Chenillette tankette. This vehicle was built to Rimailho's vision of 3-ton tankettes, which were ordered by the French army. The army's interest in these tankettes was temporary. Marshal Petain published a letter about the necessity of such vehicles in March of 1919, but the French military took no steps to obtain them until the end of the 1920s. As such, FAMH was aiming to export their vehicles. The Japanese bought one tankette, the Poles were interested, and Spain bought a whopping seven units. This was the first success of the French interbellum arms export market.

Rimailho developed a number of vehicles based on the tankette. They included the Canon de Cavalerie automoteur de 75 Saint-Chamond Modele 1924. This 11 ton SPG was meant for supporting cavalry units. The design was unconventional. The vehicle had a convertible drive and the gun was installed in the rear. In order to fire, it had to turn around. However, Rimailho designed most of his previous SPGs with this layout. This was a typical "self propelled gun mount": only the gun was armoured with a gun shield. The Canon de Cavalerie automoteur de 75 Saint-Chamond Modele 1924 remained on paper, since there were no foreign buyers, and the French army had no interest in it.

Garnier-Renault guard tank.

The French military began looking at creating SPGs in the early 1930s. This was a pivotal moment for the French tank industry. The army was torn between medium Renault D2 tanks and Char B "battle tanks". Heavy Char BB and Char d'arret tanks for the Maginot line were also discussed. The idea of a "guard tank" (char de protection) was mentioned in documents for the first time in April of 1932. According to the concept proposed by General Herr this was a special armoured vehicle armed with a 47 or 75 mm gun. Its objective was combat against enemy tanks. General Bezout, the chief inspector of tanks, refined the concept in July of 1932. According to him, this task required an SPG armed with a 75 mm gun. This kind of gun would allow it to destroy enemy tanks from a range of 800-1000 meters.

The Renault D3 was chosen as the chassis, which ended up killing the whole idea.

The requirements for a "guard tank" were composed on July 25th, 1932. The SPG in general, as well as its gun mount, was designed by APX. General Eugène François Gilbert Garnier directed the development, as he had great experience with towed and self propelled artillery. APX had no suitable chassis, so a choice had to be made among prototypes offered by other companies. The Renault VA medium tank, also known as the Renault D3, was considered most suitable. This vehicle was designed for export, and was therefore known as a "colonial tank". Since potential customers had no interest in the tank, it seemed logical to use it to make an SPG.

The Garnier-Renault had a rather good maximum gun elevation angle.

The vehicle was called Garnier-Renault, after its inventor and the chassis that it used. This was a worthy vehicle for its time. Its armour was 35-45 mm thick, which reliably protected it from anti-tank artillery that existed at the time. The SPG was armed with a 75 mm L/30 APX gun. The mount allowed it very generous elevation angles. The ammunition capacity of 160 rounds was sufficient.

However, there was one serious issue: the Renault D3 chassis. According to the requirements, the 20 ton vehicle had to reach a speed of 25 kph, but its actual speed was much less. In June 1934, when the work on the conversion was approaching completion, General Garnier was already sceptical. By 1935, he designed a new SPG. At a mass of 21 tons, it had 50 mm of armour. The mobility also improved. A 260 hp diesel engine would allow it to accelerate to 36 kph. The cruising range also increased from 140 to 400 km. A radio station and a radio operator, which were absent from the Garnier-Renault, were also added. Unfortunately, this project remained on paper.

Trials of the Garnier-Renault in 1935 ended in complete failure.

Garnier's fears were not unfounded. Trials of the Garnier-Renault in May of 1935 turned into a string of failures. The Renault D3 chassis was far from ideal, both in its technical characteristics and reliability. The APX project could have lived on, but APX's tank unit was nationalized under the name Ateliers de construction de Rueil (ARL) in 1936. The Garnier-Renault program was closed as it had no future, and another program was launched, which led to the ARL V 39. As for the concept of a "guard tank", there was another customer for it at the time of failure of the Garnier-Renault: the cavalry.

An SPG for the cavalry

General Flavigny, the commander of French cavalry, realized that cavalry needed mechanized artillery in 1931. The first step in this direction was mechanization of his units and creation of towed guns. This was a half-measure, since an SPG capable of firing directly at the enemy was necessary to escort these mechanized units. An idea was born to install a 75 mm gun on an AMC armoured car Automitrailleuse de combat. It's worth mentioning that the term AMC in the French army also included both halftracks and light tanks. This idea was first voiced in 1932, but it wasn't implemented until two years later.

Renault ACG 2 converted into a smoke tank. This chassis did not make for a good light cavalry SPG.

At the time, Renault's armoured cars were dominant. These were the AMC 34 and AMC 35 designs, the latter of which was also known as ACG 1. Requirements for an ACG 1 armed with a 75 mm gun were prepared by November 25th, 1934. Renault prepared the first materials for this project, named ACG 2, by March 15th, 1935. The vehicle received a large casemate instead of a turret, which had a 75 mm SA 35 gun slight right of center, the same gun used on the Char B1. The difference was that the gun mount, designed by APX, allowed the gun to be aimed both vertically and horizontally. Work on the ACG 2 continued even after APX became ARL and Renault's tank branch became AMX. Captain Devenne led the development at AMX.

The gun mount for the new SPG was similar to the one that later turned up on the Char B1 ter. Development reached the prototype stage, but this vehicle did not receive a gun. Later, it was converted into a smokescreen tank. The stopping of the ACG 2 programme was linked to the fact that the cavalry was disappointed in the ACG 2 and turned its sights on the SOMUA AC 4, which was accepted into service as the SOMUA S 35.

SOMUA CAM 1, a "guard tank" that was not built in metal for several reasons.

SOMUA, a subsidiary of Schneider-Creusot, also tried to take part in the "guard tank" tender. The CAM 1 project was designed in 1935, which was based on the AC 3, the future SOMUA S 35. Even Garnier's idea looked pitiful in comparison. The vehicle would weigh 18 tons, but be equipped with a 200 hp engine. This would give it a top speed of 46 kph, enough that no French infantry tank could catch up to it. The thickness of the armour was 30-40 mm, enough for the time.

The armament was even more serious, and the char de protection requirements paled in comparison. The first variant used the 75 mm Canon de 75 Mle.1922 with a muzzle velocity of 600 m/s. The second variant turned the CAM 1 into the most dangerous tank destroyer of its time. The vehicle could be equipped with the Canon CA 75 mm Mle.1933 AA gun. This gun could penetrate any tank of the time with ease from a great distance. Not a single tank that was mass produced during the first two years of WWII would stand against it. One drawback was the storage of "only" 70 shells, but in reality this would be enough for even a drawn out battle. The crew of the vehicle consisted of 5 men, same as on the APX design. The SOMUA project fell out of the race for many reasons that had nothing to do with the characteristics of the CAM 1.

SOMUA CAM 2, September 1937. The vehicle was built in this configuration.

On December 20th, 1935, infantry command composed requirements for a 20 ton tank that would replace the Renault D2. In 1936, their appetites increased. The updated requirements contained a 47 mm SA 35 gun in the turret and a 75 mm SA 35 in the hull. SOMUA also took part in this tender, but did not proceed past the presentation of a draft in 1936. After the requirements for the 20 ton Char G were changed in 1938 to make it 35 tons, all the work became pointless. However, it was the 20 ton tank project that remained the foundation for the creation of an SPG, but this time for cavalry.

Armour diagram. The gun is shown in travel position.

The order for such a vehicle, named CAM 2, was received by SOMUA in June of 1936. In many ways it seemed like a return to the "guard tank" concept, especially when it came to armament. The CAM 2 received the same gun as the Garnier-Renault. The difference was all in the gun mantlet. This gun was sufficient for fighting tanks of the period. Another advantage was that the gun was planned as the main weapon of the infantry SPG, which was being designed by ARL and BDR (Baudet-Donon-Roussel). This vehicle is better known as the ARL V 39, where V stands for the name of the ARL engineer that led the project, Captain Valla. 

This gun, as well as the ARL-BDR project, caused a number of issues for the CAM 2. The issue was that only one copy of the APX gun was available, and the Valla-BDR (the name of the ARL V 39 at the time) had priority. Both projects also shared a commander's cupola, which contained a machinegun. Only one cupola was also available, and since ARL was the designer, it's not surprising that the cavalry SPG did not receive it. Interestingly, the result was very similar to the Char B concept as envisioned by General Estienne.  

SOMUA CAM 2 at the factory, 1938. It does not have a cannon or a cupola.

The Canon de 75 Automoteur SOMUA project was finally ready by September of 1937. The chassis was similar to that of the SOMUA S 35, but with some changes. A bulge was added to the the front of the hull, necessary for the gun mount. The width of the vehicle increased to 2.51 meters. The track links were reinforced. The turret platform was seriously changed, as was the engine deck. This was necessary as the crew of the CAM 2 consisted of 5 men. It seemed rather odd that the 75 mm gun had two sights, but only one aiming mechanism. The crew was positioned very well, and they were not too cramped in the tank. The cupola included a rangefinder, which meant that in battle it had to be turned backwards.

Another interesting feature of the SPG was that the gun could be retracted during travel to reduce the barrel overhang. The Swedes later used a similar solution on the pvkv m/43. The mass of the vehicle was 21.6 tons. Since only one prototype of the cannon and cupola existed, and ARL was not to keen on handing them over to their competitors, the first prototype of the CAM 2 was assembled without them on December 25th, 1937.

The resemblance to the SOMUA S 35 is clear.

The Valla-BDR retained its influence on the development of SPGs for the cavalry. A second gun was later built for the two SPG prototypes, but only the first one was functional. Work on a second cupola also dragged on. Meanwhile, the French military demanded that both vehicles be trialled simultaneously. The prototype of the Valla-BDR was ready by the end of June of 1938, and joint trials began later: in February of 1939.

The need to fit five men influenced the shape of the turret platform and the engine deck.

The CAM 2 travelled 229 km in trials at Bourget, achieving a top speed of 35.25 kph. Trials continued until August 20th, 1939. Overall, the trials were a success, but issues were found. Like many SPGs with a forward gun, the CAM 2 suffered from overloaded road wheels. However, there was no time to introduce changes. A war broke out. On October 15th, 1939, the CAM 2 was accepted into service as the SOMUA SAu 40.

A fully equipped SAu 40.

The first order was for a batch of 36 vehicles. 12 of them would be commanders' vehicles, without a cannon. They would be assigned to tank destroyer units in light mechanized divisions  (Division Légère Mécanique, DLM). The vehicles would be split up into 12 groups of 3 vehicles each. The first two SOMUA SAu 40 were expected in October of 1940, 6 more in November and December, and 8 vehicles per month from then on. The order was split up between Creusot and Cail-Denain, just as the order for SOMUA S 40 tanks.

The 47 mm gun would have been installed in production vehicles.

The situation changed by April 25th, 1940. Having evaluated the 30 caliber cannon, the military decided to change the armament. A replacement was quickly found: the Canon de 47 mm SA Mle. 1937 anti-tank gun. This cannon, developed by AMX, had impressive penetration, and more importantly, was already in production. On May 9th, 1940, the contract for the SOMUA SAu 40 was changed. The number of SPGs to be built grew to 72. Instead of 75 mm guns, they would be armed with the 47 mm SA mle.1937. Research by François Vauvilliers indicates that the gun mount would be similar to the one used on the prospective AMX medium tank.

The end of its fighting career: the vehicle was taken by the Germans. Périgueux, Aquitaine, June 1940.

All of these changes remained on paper. The rapidly developing situation at the front meant that production never really began. The only prototype was sent into battle, as it thankfully was built from real armoured steel. The single working cannon was moved to it from the ARL V 39. The vehicle did not have time to participate in combat: its unit did not fight, and the SPG fell into German hands after the armistice was signed. This was the end of its story.

Canon Automoteur de 16 livres sur chassis SOMUA, the last attempt to make a tank destroyer out of the SOMUA S 35.

No work was done on prospective SPGs during the German occupation. ARL workers tried to create a modernized tank, the SARL 42. Work did not progress past paper, and ended completely in 1942. Work on French tanks resumed after the liberation of Paris in August of 1944, but the SOMUA S 35 chassis was no longer used. Even the SARL 42 was already obsolete by then. However, the SOMUA S 35 returned as an SPG chassis some time later.

Theoretically, the 17-pounder could fit.

AMX prepared a number of designs of SPGs made from obsolete tanks. The Canon Automoteur de 16 livres sur chassis SOMUA was one of them. It was an attempt at making a tank destroyer with a 17-pounder gun out of the SOMUA S 35. Calculations showed that the fighting compartment could fit not only the gun, but also 86 rounds. The mass of the SPG was 21,658 kg, or about the same as the SOMUA S 40. The project did not continue, as there were very few tanks available for conversion. In addition, the French army had enough GMC M10s, which were much more effective on the battlefield.


  1. Oh this is an interesting article. I've run into mentions of these things in several places over the years (not the least being WoT) but never seen any real background details until now.

  2. I always wondered: the SAu 40: did it have a machine gun in the turret ?

    1. That's clearly part of the design, yes. If you look at the photo of the captured prototype that even has the armoured "trough" to protect the barrel in place.