Hitler was always ready to invest resources into new weapon systems if they had great destructive potential and could terrorize his enemies. The V-2 rocket and the heavy water project that would have lead to an Atomic bomb were two other such projects. In 1936, the munitions designer and builder Rheinmetall made a proposal to the German High Command for a super heavy howitzer capable of destroying the defenses of the Maginot Line defending France from German invasion. On paper this was no easy feat for the defensive line stretching along the French eastern border between Germany and Italy was a formidable one and constructed based on French experiences garnered in World War 1.
This defensive line ranged between 20 to 25 kilometers deep and was built with a complex system of strong points including 34 turrets of 75-mm gun emplacements, concrete fortifications, 19 turrets of 50-mm motors, barricades, 61 machine gun turrets providing overlapping and concentrated fire, 17 turrets of 135-mm guns and the like. Hitler knew that war was coming and his first strike would be to the West. The order was given to build the 040 before Germany had officially committed to war but Hitler went ahead and attacked early when world events seemingly gave him the advantage. Thusly, the new howitzers were not made ready in time for the German Blitzkrieg against Poland and France. As it turned out, the French Army was not as formidable as their Maginot Line and the fortifications were quickly dispatched by standard German armor and infantry. Patton was once quoted as saying "If mountains and oceans can be overcome then anything man has made can be also be overcome". How true it proved for these French defenders.
Seven Karl-Gerat howitzers were ultimately manufactured. In charge of the weapons program was General Karl Becker, a German weapons engineer and artillery general. He promoted close ties between the German military and scientists for weapons development. Due to his leadership in the development process of this particular gun, his men named the weapon after him. The Rheinmetall factory delivered the six production vehicles (the seventh was held in reserve for testing) from November 1940 to August 1941. All six of these guns saw action across the war's many fronts.
During the development phase the initial concept was that the weapon would be transported by several tracked vehicles and assembled on site. These road trials proved that the process required an extended setup time and needed more troops than was originally planned. These requirements worked against the usefulness of the Karl and led the design team to change the Karl from a towed-weapon to a self-propelled weapon in 1937. This change reduced the time needed to deliver the weapon to the target site, setup and fire the gun while using less manpower. Extensive driving and setup trials took place in 1938 and 1939 using a prototype scale model of the weapon. The road tests were carried out over different soil types to see if the weight of the super-heavy vehicle could traverse the terrain as expected. Existing tank chassis were modified for such projects: for support vehicles they chose the Panzer IV and modified twenty two Ausf. D, E and F models as such. The turrets were removed and the superstructure was retooled for each transport to carry four 3,700 lb shells and a 35-ton crane needed to load and transfer the ammunition. Based on the mission, each Karl needed two or three of these support vehicles to carry their applicable heavy munitions.
The Karl operated form an opened flat wedge breech. The mechanical loaded would push the shell into the barrel. The entire gun was affixed to a thick massive baseplate for the tremendous recoil. The muzzle velocity was measured at 6,950 meters per ton. The loading tray with the rammer was mounted onto the rear of the gun carriage. Due to the difference in diameters, the loading tray for the 60-cm shells could not be used for the 54-cm shells. A large gap was left between the carriage and the engine compartment at the front of the vehicle so that the entire carriage could recoil fluidly.
At 124 m/ton and 273,374lbs, the Karl vehicle needed a large engine. General Becker decided to used the Daimler-Benz MB 503 or the MB 507 C series engines - both were 12-cylinder liquid-cooled gasoline engines. These powerplants provided a maximum speed of 6.2 miles per hour (10 km/h). When target distances were far enough away making the 10 mph vehicle impracticable, the Karl was disassembled using a 35-ton mobile crane mounted on the support vehicles and loaded onto seven lighter four-axle trailers. The weapons chassis was loaded onto a six-axle Culemeyer lowboy trailer. The Gerat 041 introduced a new suspension system tied to 11 steel road wheels to a side.
To move the weapon long distances via the European rail network, a variant of a Schnabel car was produced. This special car allowed the whole chassis of a large vehicle to be hung between two huge pedestal-mounted swiveling arms fixed to a five-axle flat car. When it reached its destination, the weapon was detached from its supporting arms, driven to its intended firing location and set up as normal. At the firing location, the chassis was lowered to the ground to distribute the recoil forces when fired. The Karl-Gerat proved to have no problems moving over normal soil but under no circumstances was it allowed to make turns on soft soil - the reason being that soil types might allow it to throw a track (that is, have the tank tread fly off of its wheels). A separate crew arrived at the firing location before the Karl could be fired. The approach route had to be prepared ahead of time to fill in soft spots along the path and to have any ditches leveled. The firing position had to be precisely leveled because the chassis had to be backed into position to fire. The weapon could only be loaded at zero elevation and it had to be re-aimed after every shot.
Firing trials took place in June 1939 and met with the anticipated result. The destructive power of the 60-cm concrete-piercing shell made a crater up to 15-m (49-ft) wide and 5-m (16-ft) deep. However, immediate discussions ensued due to concerns of the range of the weapon. The 60-cm shell had a reported range of up to 4,720 yards (2.68 mi). This distance left the howitzer within range of smaller and more accurate enemy artillery pieces. There was also concern for the safety crew working with their many vehicles and operating in a protracted setup time before firing could commence. The crew and all applicable equipment would be susceptible to enemy fire from land or air. In short, the 040 was not your typical hit-and-run weapon.
The development section at Rheinmetall began to design a longer-range barrel to help alleviate the short-range worries and, by May of 1942, a smaller- and longer-range weapon system was ordered and ultimately delivered with a 54-cm barrel. The 041 had a range of 11,000 yards, 10,500 meters (6.25 mi). Hitler was told the weapon would be operational in March 1943 and the first three 54-cm Gerat 041s would be delivered to field units between June and August 1943. Only three 54-cm barrels were actually completed by war' end and they were mounted on Adam, Odin and Loki. In case of malfunction the smaller longer-range 040 barrel could be used on any of the other weapons with minimal conversion.
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