MK 108 cannon - WikiHQ

The MK 108 (German: Maschinenkanone—"machine cannon") is a 30 mm caliber autocannon manufactured in Germany during World War II by Rheinmetall‑Borsig for use in aircraft.

Development

The weapon was developed as a private venture by the company in 1940 and was submitted to the Reichsluftfahrtministerium (RLM—Reich Aviation Ministry) in response to a 1942 requirement for a heavy aircraft weapon for use against the Allied heavy bombers appearing over German-controlled regions by then. Testing verified that the autocannon was well suited to this role, requiring on average just four hits with its 85g RDX-load (in a 330g shell) and a resulting strongly brisant high-explosive ammunition, to bring down a heavy bomber such as a B-17 Flying Fortress or B-24 Liberator, and just a single "shattering" hit to down a fighter. In comparison, the otherwise excellent 20 mm MG 151/20 (18g of HE in a 92g shell) required about 15 to 20 hits to down a B-17.

The MK 108 was quickly ordered into production and was installed in a variety of Luftwaffe fighter aircraft. It saw first operational service in summer 1943 with the Bf 110G-2 bomber destroyers and in the Bf 109G-6/U4.

Design details

Ammunition

thumb|30x90RB ammunition, as used in the MK 108 Machine Cannon

thumb|Side view, MK 108

The cannon used specially developed 30×90RB mm ammunition—30 mm calibre, 90 mm case length, rebated/reduced rim. Unlike most other weapon rounds, which used traditional brass for the case, the MK 108's ammunition used steel cases. Several types of ammunition were developed, including practice, armor-piercing, high-explosive and incendiary. In operation, however, two major ammunition types were used: mine shell and high-explosive incendiary. The mine shell was made by drawn steel (the same way brass cartridge-cases are made) instead of being forged and machined as was the usual practice for cannon shells. This resulted in a shell with a thin but strong wall, which hence had a much larger cavity in which to pack a much larger explosive or incendiary charge than was otherwise possible.

Construction

The cannon proved to be relatively light, effective, reliable, compact and easy to manufacture due to its simple construction—80% of the weapon was made from stamped parts, and the number of moving parts was kept to a minimum by using advanced primer ignition blowback (APIB) operation. The simplicity of construction allowed inexperienced workers to make the parts (mostly women). Also a cartridge case with a rebated rim is used, so that the extractor claw can hook over the rim and still fit within the chamber.

Primer ignition is timed so that the bolt is still moving forward when the propellant is ignited. The expanding gases from the fired round stop the forward motion of the bolt, then reverse its motion. The key characteristic of the APIB system is that, because the resistance due to the weight of the bolt and the mainspring are supplemented by the bolt's considerable forward momentum, the propellant gases are contained in the barrel for critical microseconds, and the projectile will have had time to leave the muzzle, allowing the gas to escape forward and reducing the chamber pressure to a safe level before the bolt and cartridge case emerge from the opposite end. compared to the approx. 720-790m/s but a fire rate of 600-850 rounds per minute depending on variant.

Another significant feature is that, in the APIB system the cycle starts with the bolt open, which prevents an autocannon using the APIB principle from being accurately synchronized with an aircraft's propeller. Messerschmitt Bf 110, Messerschmitt Me 163, Messerschmitt Me 262, Focke-Wulf Fw 190, Focke-Wulf Ta 152, Heinkel He 162, Heinkel He 219, and Junkers Ju 388.

The MK 108 was also fitted to night fighters in an unusual installation, called "Schräge Musik" (literally "awkward music" or "slanted music"). In this configuration, the cannons were mounted in the fuselage, aiming upwards and slightly forwards at an oblique (18 to 30 degree) angle, depending on fitment and aircraft. This allowed the night fighter to attack bombers, often undetected, by approaching from underneath the enemy aircraft - many British heavy bombers had neither weapons on the ventral fuselage nor windows for vision. This installation was so effective that discovery and news of its adoption was much slower than usual in reaching British night-bombing forces, as there were rarely any survivors from the attacks to report the new threat. This system was fitted to some versions of the He 219 Uhu, late-model Bf 110 night fighters, Junkers Ju 88 & 388 and the Dornier Do 217N model. It was also fitted more rarely to the (prototype) Focke-Wulf Ta 154 and Fw 189 along with the planned, two-seat Me 262B-2 jet night-fighter. In the latter case this produced a jet fighter with six MK108 cannons - with the fitment of the projected mass-produced, mid-VHF band FuG 218 radar.

The MK 108 mechanism was scaled up in the MK 112 cannon, using a 55×175RB cartridge. The MK 112 was intended to be fitted in pairs in the nose of Me 262 fighters, with 25 rounds per gun, and also in the nose of later models of the Arado Ar 234 for night fighter duty. Underwing mounts for the twin-engined, high-speed Dornier Do 335 heavy fighter were in development as well. The gun was not finished in time to see operations in World War II. The US captured some of these prototypes and knowledge gleaned from them was incorporated into the experimental US 57 mm T78 autocannon, but this did not see production either.

References

External links

MK 108 ammunition types
World War 2 Fighter gun effectiveness comparison
factsheet from the National Museum of the United States Air Force, Dayton, Ohio
German language page on the MK 108 autocannon