MK 108 cannon

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 RDX-loaded, and resulting strongly brisant high-explosive ammunition to bring down a heavy bomber such as a B-17 Flying Fortress or B-24 Liberator and a single "shattering" 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 110G-2 bomber destroyers and in the Bf 109G-6/U4.

Ammunition

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: Minengeschoß ("mine-shell") and high-explosive incendiary. The Minengeschoß was made by drawn steel (the same way brass cartridges 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 MK 108 was optimized for a high rate of fire at the expense of ballistic performance.

It was easy to maintain, and its compact size, low weight and 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, who feared 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 is 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). To avoid this, simple blowback guns have to use low-powered cartridges or a very heavy bolt.

In an Advanced Primer Ignition Blowback design such as the MK 108, this problem is prevented by using a longer chamber, which allows the cartridge to enter into the chamber a small distance past the breech face. When the trigger is pressed, the bolt is moved forward by expansion of the buffer spring. It chambers the cartridge, but instead of coming to rest against the breech face, it follows the case a small distance into the chamber. 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, expelling it from the chamber, along with the spent cartridge case, which, as the bolt moves rearward, strikes the ejector, throwing it from the mechanism. The heavy bolt continues rearward, compressing the buffer spring, which then expands, pushing the bolt forward again, picking up a fresh cartridge from the magazine (or belt in a belt-fed weapon) and chambering it. This cycle of events continues until the trigger is released. Before this sequence of events occurs, the projectile has already left the muzzle, allowing the propellant gas to escape, reducing the chamber pressure to a safe level.

The APIB design permits a higher rate of fire and a lighter bolt than simple blowback operation, but the length and speed of travel of the bolt within the chamber are limited by the stresses placed on the case by the sliding motion, which takes place under high gas pressure. To keep these within limits, the bolt needs to be very heavy or the barrel short. The designers of the MK 108 opted for a short barrel. In the APIB system, the cycle starts with the bolt open (See: APIB design mandates a firing cycle starting with an open bolt), which prevents an autocannon using the APIB principle from being accurately synchronized with an aircraft's propeller. (See: firing through a spinning aircraft propeller's blade arc).

Operational usage

The MK 108 saw widespread use among fighters tasked with shooting down enemy bombers. Some of the aircraft deploying, or intended to be armed, with 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 (Colloquialism): "Jazz", 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 & 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 this would have made an effective night interceptor — albeit with the drag-producing Hirschgeweih "stag's antlers" eight-dipole transceiving antenna array as actually fitted to some Me 262 B-1a/U1 night fighters.

Related designs

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. Only 15 prototypes were built; of these 10 were delivered for tests and 5 were kept at the factory for improvements based on expected feedback. Of the ten guns delivered for tests, seven were of an early model, weighing 300 kg (660 lb), and three were lighter at 275 kg (606 lb) — these were both significantly lighter than the slightly "smaller" 50-mm calibre, 21-round armed Bordkanone-series BK 5 cannon, itself weighing some 540 kg (1,190 lb). The MK 112's projectile was supposed to weigh 1.5 kg (3.3 lb); of these 420 g (15 oz) were reserved for the explosive. 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.