Tuesday, June 3, 2014

30x91 Mk108 ammunition

Cutaway models of ten 30x91 shells for the Rheinmetall- Borsig Mk108 aircraft gun. The Mk108 gun was developed during the early 1940's and it was the standard 30 mm cannon in fighters from 1942 untill 1945. It was used in many planes, for instance: the Me109, Me110, Me262, Me163 and the Fw190-A8. It's advantages were: light in weight (58 kg), small size (1057 mm long), easy to build in existing planes. It's disadvantages where: low muzzle velocity (±520 m/s), resulting in a very curved projectile trajectory and a short range. An enemy plane had to be approached quite close to enshure hits. However, if an enemy plane was hit, the results were spectecular; one hit of an M-shell destroyed a fighter instantaneously, 3 to 5 hits were sufficient to destroy any heavy bomber. Another disadvantage was that the cannon, either hull mounted, either mounted in a wing bay, could only take a limited amount of ammo. For the Me109 / Fw190 wing bay mounted gun an average of 55 cartridges/gun, the Me163 could take 60 rounds/gun in the hull mounted guns. With a firing rate of 660 round a minute, this was just 5 seconds of firing before all ammo was spent.
Cartridge cases are made of either brown or green painted steel. The firing cap is electricaly ignited. The main propulsion charge exists of 30 grams Np rifle powder or 29,5 grams nitrocellulose blade powder. Some shells were found to have a small silk bag of fine powder over the firing cap, functioning as an extra igniter charge, igniting the main powder charge with a bigger flame. Data for the projectiles:

Type 1- 3cm brandgranate ohne zerleger (3cm incendiary shell without self destruct element), a pressed steel body with a (typical for the mineshell) round bottom. The shellbody is filled with thermite, pressed in three degrees of tightness, the less pressed amount in top. The shell is fuzed with the Az1587. After hitting the target, the shell's nose is blown off by the initial explosion of the nosefuze, after which it starts spewing fire like a roman candle, about 1200º C hot. weight of shell : 330 ± 8 grams.

Type 2- 3cm M Geschoß 108 ausführung A mit zerleger (3cm Mineshell 108 type A with self destruct mechanism). A Mineshell filled with 85 grams of penthrite and fuzed with the ZZ1589B (Zerlegezunder /self destruct fuze 1589B -mechanical-). The shell could also be filled with 85 grams of HA41 (Hexogen Aluminium). Weight of shell : 330± 8 grams. Note that the shell has a different type of detonator, the VC70. This duplex detonator had a delay curcuit that delayed the shell for about 10 cm of flightpath, enabeling it to enter the target before exploding inside of it.

Type 3 - 3cm M geschoß ohne zerleger (3cm Mineshell without self destruct mechanism). A mineshell filled with 82 grams of Ha41 and a ring of 3 grams penthrite ,surrounding the VC70 detonator. The fuze used is the AZ1587.

Type 4- 3cm M geschoß leuchtspur, mit zeleger (3cm mineshell with tracer and self delay). A Mineshell with a tracer element , used one on five in a belt for fire corrections. The shell is filled with 73 grams HA41 and a core of 3 grams nitropenta. The fuze is an ZZ1589B, in fact a ZZ1589A with a smaller and more streamlinded outer body.

Type 5- A practice mineshell , resembling the 3cm M shell with tracer. The shell is not tarfilled to bring it to the right weight.

Type 6- 3cm panzersprengranate leuchtspur übung ohne zerleger (3cm practice APHE shell with tracer, without self destruct). Weight of shell 500± 7 grams. Fuze dummy : 3cm Bd. zerleger erzatzstuck 1592 (3cm Basefuze replacement screw 1592).

Type 7- 3cm Practice mineshell with unbalance . A tarfilled practice mineshell with a steel rod in the middle that is bend outward on top. This causes the shell to fall into unbalance , shortly after leaving the barrel. Effectively a short range practice shell. Weight 330± 8 grams.

Type 8 - 3cm M geschoß Üb ohne zerleger (3cm practice mineshell without self destruct device). A tar filled 3 cm mineshell, supposedly designed to receive a 20 mm shell fuze.

Type 9- 3cm M geschoß Üb ohne zerleger (3cm practice mine shell without self destruct device). A tar filled practice mineshell, supposedly threaded to receive the ZZ1589B, however only found as an practice shell.

Type 10 - 3cm sprengranate übung ohne zerleger (3cm high practice high explosice shell without self destruct). A machined steel body with a pressed steel windshield.

It must be noted that in most M shells, an aluminium ring can be observed i.w.o the driving band on the inside of the body.This ring is placed to prevent the thin walled shell body from being crushed while the drivingband is pressed into the rifling grooves of the gunbarrel.

The Mk 108 gun was nicknamed "pneumatic hammer" by allied aircrews , referring to the distinct sound of this "tool"

Left , the AZ 1587 (Anchlagzünder 1587 / Impact fuze 1587) a non self destruct impact fuze. Functioning of the fuze. An aluminium firing pin housing, containing the firing pin (see right side above the shell), is held out of line of the detonator. To enable the firing pin to pierce the detonator, it has to move 2mm to the center. A ball on the bottom of the fuze also prevents the firing pin to move to the middle. The ball on its turn cannot move upward, held in place by two centrifugal pins (one at 180º each) which are pushed inward by a brass coiled spring. The ball lies in the right one of two equaly sized holes that are drilled slightly overlapping. The right hole, holding the ball however is drilled 2º outward toward the top. After firing, the pins are thrown outward by the centrifugal force, the ball however is held back until acceleration decreaces. The ball than starts "climbing up the wall" of the 2º drilled hole due to the centrifugal force, towards the top of the fuze, where it falls in the recess of the firing pin housing. The weight of the ball, swung outward by the centrifugal force will now slide the firing pin housing outward (in this picture that would be to the right), placing the firing pin over the detonator and under the hammer pin above it. At impact the hammer pin forces the firing pin into the detonator.

In the middle, the ZZ1589A (Zerlegezünder 1589A / Self destruct Fuze 1589A). Functioning of the fuze: The firing pin is placed in a housing with six holes drilled in a radial pattern in the body. These hold a small ball each. Around the body,a grooved steel ring is placed, The inner groove machined in diameter to receive just under half the diameter of the balls. A strong spring on top of the housing wants to press the firing pin into the detonator, but is prevented from this by a brass coiled roll. After firing , the centrifugal force forces the balls into the groove, the brass foiled roll starts to unwind, forming a hole large enough to let the firing pin and the housing pass. The balls, forced into the grooved steel ring now "lock" the firing pin housing in place. If a target is hit, the firing pin will simply be hammered down into the detonator, exploding the shell. If no target is hit, a moment will come where speed and spin will decreace. At the moment that the downward pushing spring overcomes the locking force generated by the balls in the groove, the balls will be pushed back in the housing and the firing pin will be driven into the detonator. 

Right, the ZZ1589B Internal parts and working as the ZZ1589A , differs only in a smaller fuze body.

Type 11- 3cm M geschoß Üb ohne zerleger (3cm practice mine shell without self destruct device). A practice mineshell with a blind nose piece resembling the ZZ1589B. The filler is made from a mixture of sand and tar. This mixture is locked up by a 1,5mm thick steel plate, locked up by the nose piece.

Type 12- 3cm M geschoß leuchtspur, ohne zeleger (3cm mineshell with tracer, without self destruct). A Mineshell with a tracer element , used one on five in a belt for fire corrections. The shell is filled with 73 grams HA41 and a core of 3 grams nitropenta. The fuze used on this projectile is the AZ1504, which is also used on the 2 cm Spr.Gr.
In functioning it resembles the AZ1587.

Type 13- 3cm SprGr. L'spur o. Zerl. (Sprengranate leuchtspur ohne zeleger / 3cm High explosive shell with tracer, without self destruct). A thick walled high explosive projectile with a drilled tracer element. The shell is filled with Trotyl and has a core of 3 grams Nitropenta, packed in a cartboard tube. The fuze used on this projectile is the AZ1504, which is also used on the 2 cm Spr.Gr. In functioning it resembles the AZ1587.

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