First Try at a Medium Tank
To a casual observer, German tank industry was like a jack in a box. 15 years after the end of WWI it suddenly sprang to life and began to crank out world class designs. However, there is no such thing as miracles. In addition to German specialists like Joseph Vollmer and Otto Merker who worked abroad, Germany had its domestic tank program. It was conducted in secret and birthed some very unconventional designs. One of them was the Grosstraktor, a medium tank that became the culmination of German tank building of the 1920s. It was the first step on the way to the PzIV, the most numerous German tank of WWII.
High priority medium tank
The end of WWI triggered a collapse of German tank production. The Treaty of Versailles prohibited Germany from developing of producing tanks or other armoured vehicles. German military command also put in their effort to defeat their own tank designers. It’s hard to describe the condition of German tank building towards the end of the war in any other way.
A model of the Armeewagen 20 designed by Krupp. The hull and turret are the initial versions.
Joseph Vollmer’s attempts to restore normalcy were partially successful. Production of the LK-II light tank went slowly and with great resistance from the German military. Some of them continued to dream about steel monsters like the 120 ton K-Wagen. One can confidently say that German generals plainly revealed their own incompetence and amateurishness. Such “successes” meant that work had to start from screatch.
Before working on a new tank, the Germans studied foreign developments carefully. It became clear that German tank design was far behind, especially when it came to light and medium tanks. While the Germans tried to adapt British developments, the British themselves considered those tanks obsolete.
The idea that the “pin cushion of guns” was a dead end was first arrived at by the French. As a result, they produced the FCM 2C, the best tank of WWI, but one that came too late for it. The tank was a stepping stone, and one that the Germans paid close attention to. The concept of a tank with a cannon turret and a machine gun turret was adopted for the German tank. However, it would not be a heavy tank. It would have been too expensive, and the lesson from the A7V that was built in a batch of only 20 was well learned. The Germans initially did not have plans for a light tank either. They were not the only ones to realize that the medium category was optimal: most successes of British tank building in the 1920s were in this space. Some requirements for a German medium tank clearly had British roots.
A redesigned Kruppturret. Daimler-Benz and Rheinmetall tanks used the old style of turret.
The first requirements for a medium tank named Armeewagen 20 were prepared by May of 1925. According to them, the tank would weigh 16 tons, have a top speed of 40 kph, and a cruising range of 100 km. The tank would be 6 m long, 2.6 m wide, and have a height of 2.35 meters. The 14 mm thick armour would protect it from rifle bullets. The crew would consist of 6 men, including a radio operator. The armament included two machine guns and one 75 mm cannon.
There was also an interesting idea of making the hull watertight. That’s right, the German tank was planned as an amphibious vehicle from the very beginning! This requirement did not come out of nowhere. The Germans carefully followed the development of the Medium Tank Mk.D. This was the first amphibious tank, albeit it did not float very well. The Germans also knew that similar developments were underway in the US, not including Christie’s amphibious tanks.
20 caliber long gun that was used on experimental German tanks.
Work on the Armeewagen 20 (A.W.20) began within Krupp’s conglomerate. They were led by Erich Muller, the chief armaments designer. Muller is known as the creator of a number of weapons systems. At this time, the idea of assigning an artillery designer to work on tanks was normal practice. Along with him, a team headed by Georg Hagelloch and Erich Wölfert worked on the A.W.20. They already had experience working on the Kleiner Sturmwagen, a competitor of the LK-II.
In addition to designing the overall concept, Krupp was responsible for creating a 75 mm gun for it. The 75 mm L/20 mountain gun was taken as the starting point. It was installed in a two-man turret along with a coaxial machine gun. The turret was requested to be as small as possible.
The decision to use a short barrelled weapon was no accident. Sure, it had inferior muzzle velocity to the French 75 Mle.1897, but it was much more compact, and would result in a smaller turret. This was important for a tank weighing 15-16 tons. The HE power of the weapons was about the same. The anti-tank capabilities were also satisfactory, as hardly any tank had more than 20-25 mm of armour in the 1920s.
Three way race
Krupp presented the first designs of the A.W.20 on March 1st, 1927. The work was already a competition. Remembering that a monopoly on tanks in WWI led to poor results, the 6th Department of the Weapons Directorate offered work on the project to two more companies: Daimler-Benz and Rheinmetall. That is about the time that Heinrich Kniepkamp, later a key figure in German tank development, began working in the 6th Department.
Planetary gearbox used on the Daimler-Benz tank.
Even tough Krupp now had competition, the center for development was still in Essen. Specifications for the turret and armament were ready on March 19th. The full weight of the turret and gun would be 1660 kg, of which 550 kg was reserved for the gun. The gun mount had impressive elevation range: from -12 to +60 degrees. A special bulge in the turret allowed for increased depression. An electric drive could turn the turret completely around in 30-75 seconds. Without the drive, a sull traverse took 120 seconds. Large hatches were placed in the sides of the turret, a layout that would become common with German tanks.
The tank was also supposed to have a machine gun turret in the rear with a high angle of gun elevation. This machine gun doubled as an AA weapon. A wooden model of the A.W.20 was prepared by March 25th. Its demonstration was met with final approval. On March 30th Krupp received a contract for the development and construction of two tanks.
Diagram of the engine and transmission of the Gr.Tr.DB. As you can see, Ferdinand Porsche’s team did not look for an easy solution.
The competition began even before any prototypes were built. Krupp was the second company that received a contract. The first, Daimler-Benz, received a contract on March 26th. Ferdinand Porsche was in charge of developing Daimler-Benz’s entry. Development was also directed by Rudolf Mertz. Since Krupp was already designing a turret, Daimler-Benz was only going to build a chassis.
A contract for two tanks was later given to Rheinmetall. Work in Dusseldorf was headed by Heinrich Romberg, Rheinmetall’s chief designer of artillery and military vehicles. Unlike Muller, who was an artilleryman, Romberg had direct experience with tanks. Arthur Gaunitz and Georg Engel were also involved with the chassis. In addition to the development of a tank, Rheinmetall also received the contract for building them.
Assembly of the Daimler-Benz tank in Unterluss. The running gear design can be seen.
Daimler-Benz did not end up in this race by accident. It was at Daimler-Motoren-Gesellschaft that A7V tanks were built at in WWI. Later, the factory was given the name Daimler-Benz Werk 40. Even though the chassis was being built according to the same specifications as Krupp’s, it was radically different from its competitor. The internals were different. The Daimler-Benz IV b. 6-cylinder linear aircraft engine was chosen. This 31.2 L 260 hp motor was developed during WWI for use in heavy aircraft. The engine was the heaviest of all proposed, 604 kg. A secondary motor was needed to start it.
A bulky gearbox with a preselector giving 6 speeds forward and 2 back was installed to the left of the engine. The gearbox and engine were connected with a geared drive. There was no clutch in this system. The Daimler-Benz A.W.20 used a planetary turning mechanism. Gears were switched with a special lever on the steering wheel. The gear shifter and turning mechanism operated with hydraulic assists that ran on oil.
Trials showed that the suspension was too weak.
As you can see, the drivetrain was far from simple. The final drivers were also rather complex. They consisted of gears that connected to the drive sprockets externally. There was also a drive to two propellers that could be installed on the rear of the tank. The two propellers could turn to steer the tank in water.
The hull met the military’s requirements. Since the tank was supposed to be amphibious, the lower front plate was raise high up. This was also a good solution when crossing obstacles. The driver, radio operator, and commander, who also operated the forward machine gun, sat in the front. A fighting compartment with a two man turret was behind it. There was also a one man machine gun turret in the back. Its crewman was not in the greatest conditions, as he was flanked by transmission elements.
The competed tank before leaving the factory. In reality, these tanks went through trials without turrets. Note the high elevation angle of the rear machine gun: it was supposed to double as an AA gun.
Unlike the drive train, the suspension was not very complicated. The drive sprocket and idler connected with the tracks by means of a pin-gear drive. They were covered by spaced armour from the outside. Each side had 16 small road wheels and 3 return rollers. The road wheels were grouped into bogeys with a leaf spring suspension. The 4 front wheels were installed at an angle to help climb obstacles. The suspension elements were covered with removable armour. The suspension was simple and easy to service. This tank used stamped tracks.
This photo was taken after trials in the USSR.
Krupp’s chassis also used an aircraft engine: the 250 hp BMV Va. This 6-cylinder engine was smaller (22.9 L) and lighter (317 kg). This tank also had a complicated drive train. The massive 6-speed gearbox had a pneumatic drive. Unlike the Daimler-Benz design, it was placed to the right of the engine and was linked to it via a multi-disk clutch. The turning mechanism was also planetary, but it used hand operated band brakes.
Diagram of the Krupp drivetrain.
The running gear was more complicated. Unlike the Daimler-Benz design, the drive sprocket used a lantern gear scheme. The track links were made from compressed rubber.
The suspension was also complex. Of the 17 road wheels, 12 were coupled into a strange system: bogies of two with coil springs. Three bogeys were connected in one assembly. Three additional road wheels were placed in the front to soften blows from obstacles. Two of them were also gathered into one bogey. One such wheel was also installed in the rear. Finally, a pair of rigidly mounted wheels was placed before the drive sprocket. The layout was functional, if bulky. The suspension was covered with bulletproof spaced armour.
The diagram shows where the propellers were supposed to be installed. In reality these tanks were never tested in water.
Krupp designers changed both the gun and gun mount. A new mount allowed the turret roof bulge to be removed. The machine gun turret was also altered. Interestingly enough, only Krupp’s tanks received the new turret. Daimler-Benz and Rheinmetall kept using the old designs for both turrets.
