MK 108 cannon
| MK 108 | |
|---|---|
| File:MK108.jpg The MK 108 machine cannon | |
| Type | autocannon |
| Place of origin | Germany |
| Service history | |
| In service | 1942-1945 |
| Wars | World War II |
| Production history | |
| Designer | Rheinmetall-Borsig |
| Designed | 1940 |
| Manufacturer | Rheinmetall-Borsig |
| Specifications | |
| Mass | 58 kg (127.9 lbs) |
| Length | 1057 mm (3 ft 6 in) |
| Cartridge | 30x90RB mm steel casing |
| Caliber | 30 mm |
| Action | blowback operation |
| Rate of fire | 650 rounds/min |
| Muzzle velocity | 540 m/s (1,770 ft/s) |
The MK 108 (German: Maschinenkanone - Machine Cannon) was an autocannon (30 mm calibre) manufactured in Germany during World War II by Rheinmetall-Borsig for use in aircraft.
History
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 bombers appearing en masse in German skies by then. Testing verified that the cannon was well-suited to this role, requiring on average just four hits with high-explosive ammunition to bring down a heavy bomber such as a B-17 Flying Fortress or B-24 Liberator and a single hit to down a fighter. In comparison, the otherwise excellent 20 mm MG 151/20 required an average of 25 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 late autumn 1943 with the Bf 110 G-2 bomber destroyers and in the Bf 109 G-6/U4.
Ammunition
The cannon used specially-developed 30x90RB 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, armour-piercing, high-explosive and incendiary. In operation, however, two major ammunition types were used: Minengeschoß ('mine-shell') high-explosive and incendiary. The Minengeschoß was made by drawn steel (the same way brass cartridges are made) instead of being cast 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. The incendiary rounds were also often fitted with a hydrostatic fuse, which detonated when it came in contact with liquid. This was to ensure that the round did not merely explode on the target aircraft's skin (which would cause little damage), but instead penetrated it and exploded when it came into contact with fuel or coolant inside the fuel tanks or radiators respectively.
The design
The cannon proved to be very effective and reliable, yet comparatively light, compact and easy to manufacture. These characteristics stem from its simple construction—80% of the weapon was made from stamped parts, and the number of moving parts was kept to a bare minimum using a simple blowback operation. However, the simple blowback operation had its disadvantages.
Otherwise, it was simple to manufacture and maintain, and its compact size and weight as well as its electrical priming made it ideal for aircraft installation. The cannon's distinctive heavy pounding sound and high rate of fire gave it the nickname "pneumatic hammer" amongst Allied aircrews, amongst whom the cannon gained a fearsome reputation due to its destructive power.
Disadvantages
Normally, gas-operated or delayed-blowback mechanisms are used in automatic weapons of rifle-calibre and larger because the chamber pressure in such weapons would be very high. Therefore, if a simple blowback system (where there is no positive lock between the bolt and barrel) is used, the bolt may recoil and open the breech while the chamber pressure is still high, causing damage to the weapon and split cases (see blowback article for more information). In the MK108, this problem was eliminated by simply reducing the muzzle velocity and shortening the barrel of the weapon to the point that, by the time the expanding gases from the fired round overcame the inertia of the heavy bolt and blew back the breech, the round had already left the weapon. This therefore allowed most of the gases to escape via the barrel, dropping the chamber pressure to a safe level. The heavy bolt then continued to move backwards into large buffer springs, which then pushed the bolt back into battery after a fresh round had been fed.
The low muzzle velocity needed for this simple operation became the MK 108's main shortcoming, with the result that its projectile trajectory dropped considerably after a comparatively short range. This made effective firing ranges short and aiming a challenge.
Operational usage
The MK 108 saw widespread use among fighters tasked with shooting down enemy bombers. Some of the aircraft deploying the MK 108 were Messerschmitt Bf 109, Messerschmitt Bf 110, Messerschmitt Me 163, Messerschmitt Me 262, Focke-Wulf Fw 190, Focke-Wulf Ta 152, Focke-Wulf Ta 154, Heinkel He 162, Heinkel He 219, Horten Ho 229 and Junkers Ju 388 .
The MK 108 was also fitted to night fighters in an unusual installation, called "Schräge Musik" (German: "Jazz Music", literally "slanting music"). In this configuration, the cannons were mounted in the fuselage, aiming upwards at an oblique angle. This allowed the night fighter to attack bombers, almost always undetected, by approaching from underneath the enemy aircraft. 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.