Christened ‘Münchhausen’ after the fabled Baron of that name – usually pictured riding a cannonball, the Rheinmelall-Borsig firm had begun design of the Gerät 104 as early as 1939, intended by the RLM to be carried by an aircraft for use against fortified bunkers and naval targets. The RLM had specified that a shell of 700kg (1,543lb) weight be capable of penetrating deck armour up to an angle of 60°. The recoil problem of firing a large-calibre heavy shell was overcome by propelling a counterweight of equal mass in the opposite direction simultaneously along the barrel. As shown in the illustration, this recoilless weapon was to have been mounted beneath the Dornier Do 217. Due to the weight of the shell and the counterweight, reloading in flight was not possible. Initial velocity to the shell was imparted by a black powder charge contained in the counterweight, which was propelled out into the air-stream. A ground static test-firing with a Do 217 mounted on a movable trolley was carried out in 1941, but although the weapon functioned as planned, the aircraft suffered heavy damage to the rear fuselage and elevators from the exhaust gases. Improvements in the form of additional protection for the skinning and side openings for the gas exhaust showed no acceptable results.
The RLM had ordered three examples of the Gerät 104 in 1939, intended to be retracted hydraulically into the ventral fuselage of the Do 217 and Ju 288G. In all, 14 rounds were fired, but the installation of an additional 110kg (242lb) of extra strengthening to the Do 217 air-frame was never carried out before the weapon was cancelled in 1941. The recoilless Gerät 104, however, did serve as a useful forerunner for smaller and lighter weapons working on the same principle – the Sondergeräte (special devices) SG 113 ‘Förslersonde’, SG 116 ‘Zellendusche’, SG 117 ‘Rohrblock’, SG 118 ‘Rohrblock’ and the SG 119 ‘Rohrbattserie’.
