Leichttraktor (VK-31)
The Leichttraktor (VK-31) was a German experimental tank, which, as part of its cover, was described as a light tractor, which translated in to German was “Leichttraktor”. Work started on the VK-31 two years after the heavy Grosstraktor. A request for tender for a tracked combat vehicle up to 12 tons in weight was released by the Reichswehr on March 28, 1928. Prototypes were to be constructed by 1930 and cost less than 50 000 Reichsmarks. The winner would receive an order for at least 17 tanks.
The request was delivered to Daimler-Benz, Krupp, and Rheinmetall-Borsig in May of 1928. By that time, the weight requirement had been restricted to 7.5 tons. Additional requirements included 14 mm of front and side armour and a crew of 4. The armament of the VK-31 was to be a 37 mm KwKL/45 main gun and a 7.92 mm Dreyse machine gun. The crew of the VK 31 in both firms designs consisted of four people: the driver, commander, radio operator and loader.
The tank also had to have a radio to provide voice communication with a range of 2-3 km and a morse capability of up to 7 km. Chemical protection was also required, in case of a gas attack. Maximum highway speed was to be 25-30 kph (40 kph by some sources) and 20 kph off-road. A range of 150 km or 6 hours of continuous movement was specified. The tank also had to be able to ford water 0.6 m deep, a 1.5 m wide ditch and climb a 60% (31°) slope for up to 1 km at a minimum speed of 3 kmh. To improve cross-country performance, ground pressure should not exceed 0.5 kg/sq.m. The specification was approved on 19 July 1928.
Daimler-Benz declined to participate, leaving Krupp and Rheinmetall-Borsig in the competition. Both companies co-operated to develop a “Kleintraktor” (renamed “Leichttraktor”, or light tractor) together, and their results were consequently very similar, with the engine at the front and a turret at the rear. Not having much experience with tank suspensions, Rheinmetall engineers used the tracks from an existing tractor. Each side had 12 road wheels, two wheels per bogey, one tension idler, and two support idlers, a front directing, and a rear leading wheel. In order to protect this system, a side screen was retained, with three openings to clean the mud out of the suspension. The hull was a combination of welded and bolted, with armoured steel 4-10 mm thick. The layout was inherited from a tractor. The front, which had the transmission, also housed a Daimler-Benz M36 engine, 36 hp in power. The middle part had the control compartment (the driver sat on the left side). The driver had a small rectangular turret with vision ports above his head.
Krupp engineers did not trust tractors and built their own chassis. Krupp’s prototype vehicle was slightly longer and taller, which allowed two hatches on the side for entrance and evacuation. The Krupp tank had 6 small diameter road wheels per side, with a vertical spring suspension, grouped in two groups of three, with the front-most and rear-most wheels slightly larger than the others. There were also two idlers, a front directing, and a rear leading wheel. Rheinmetall was responsible for both turrets. All VK 31s had radio stations. Four prototypes with different armament, crew, weight, and suspension features were manufactured (numbers 37 & 38 by Krupp, and numbers 39 and 40 by Rheinmetall.
The four prototypes were shipped to the Soviet-German joint testing ground and tank training ground (known to the Germans as Panzertruppenschule Kama) near Kazan. The facility had been established under the terms of the Treaty of Rapallo – agreed to between the USSR and Germany in 1922 under conditions of high secrecy and security. The testing facility was used from 1926 throught to 1933 and was codenamed “Kama” (from a combination of the words Kazan and the surname of Reichswehr Oberstleutenant Malbrandt, who had been assigned to select the location for testing. The facility was consequently codenamed KaMa – for Kazan-Malbrandt). All four Leichttraktor (VK-31) prototypes were shipped to Kama where the Special Technical Commission (TEKO) created to exchange experience in the area of tank design and construction, began its work.
By 1933, each prototype had traveled between 1,660 and 1,865 km. Soviet specialists agreed that these vehicles are not of any interest to the RKKA. However, Soviet engineers were interested in the suspension and the turret layout (particularly the coaxial machine gun), as well as the tanks’ radios. German specialists were not impressed by the technical and tactical data either. Installing the engine in front of the tank to some extent saved the crew from death or injury in combat, but the thickness of the armor was obviously not enough, and could not keep out shells and larger caliber bullets. Further modernization and up-armouring was not possible without increasing weight, and reducing the already unimpressive mobility of the tanks. In addition, because of the shortcomings of the cooling system, the enginese frequently overheated and the durability the tracks was extremely low.
All four Leichttraktors were shipped back to Germany in 1933, when KaMa was closed down. In 1934, the tanks were moved to Munster for the winter. From 1935 to the start of WWII, they were used as training vehicles in Pultos. One source mentions that Rheinmetall won a production order of 289 vehicles around 1928 but that the order was subsequently cancelled as better lighter tank designs (the Kleintraktor, which would go on to be rather more well known as the Panzerkampfwagen I) came along; however, there is no source for this statement that 289 were ordered mentioned.
In 1933 another prototype light tank was built which could be classified as a self-propelled gun. Instead of a turret, a fixed PaK L / 45 caliber 37 mm (antitank variant) gun was installed with vertical guidance adjustment from -10 ° to + 30 °. The crew was reduced to three. It has been mentioned that this modification was a Rheinmetall prototype, but judging from the chassis it was a modification of a Krupp prototype. Unfortunately, the details of the tests of this machine are unknown.
