Luftwaffe Cannons and Machineguns topic.

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It's kind of interesting to see that in the post-WWII era aircraft guns have largely converged to 25-30mm caliber ones, the USA being somewhat of an outlier with the 20mm Vulcan but that has been replaced by a 25mm gatling gun in the F-35. Of course jets are powerful enough that they can afford to carry a high-velocity gun (typically muzzle velocity is around 1000 m/s) with a high rate of fire (gatling or revolver cannon type), but they haven't gone up from 30mm either.

In the WWII era it seems a 30mm high velocity gun entailed too much compromise (see how massive the Mk103 is, and relatively low RoF). But maybe a high velocity 25mm gun could have been feasible for mounting in a typical WWII fighter?

As for the mechanism, yeah it seems the API blowback used by the MG/FF and Mk108 was somewhat of a dead end, and not really suitable for higher velocity guns. One sort of hybrid mechanism that supposedly achieves fairly high RoF and is usable for higher velocity guns as well is "gas-unlocked blowback", where instead of the mass of the bolt preventing the backwards movement in the initial stages after firing, the bolt is locked and there is a gas mechanism that unlocks the bolt and then the rest of the action happens by blowback.
 
The German's MG213 and MK213 was highly innovative, and were 20mm and 30mm respectively.

There is a certain point where the caliber of the weapon along with it's weight, has to be worth the effort to be used in an aircraft's armament.

With the exception of the GAU-8, which had an airframe built around it, most cannon have to be able to work within the aircraft's performance profile.
 
z42 said:
In the WWII era it seems a 30mm high velocity gun entailed too much compromise (see how massive the Mk103 is, and relatively low RoF). But maybe a high velocity 25mm gun could have been feasible for mounting in a typical WWII fighter?
Click to expand...
25mm guns were probably missed opportunities for Germany, japan and Soviet Union.
Soviets have gotten close with the VJa-23 (although it's usage on fighters was meager).


API blowback was also used on the high-velocity Oerlikon S gun.
API allowed for the cartridge to be fired before it is chambered, that in combination with mass of the bolt and it's supporting members, as well as the strength of the return spring was sufficient to keep everything safe. A way to improve the RoF on the API blowback guns was to lighten the recoiling members + stiffer main spring + installation of recoil buffers; Swiss (for the Oerlikon S that became the SS2 now) and Japanese (Type 99 Mod 5) did that by the end of the war.
See here.
Germans were increasing the RoF on the MK 108 from 600 to 850 rd/min when the war was ending.
 
The MG213 must rank as the premier air to air gun, for longevity, performance and widespread application.

Eng
 


And 40 rounds seems just for outer wing cannons. The wing root guns can pack more.

Why is the wing root installed MK 108 weaponry weight so much less than for the MG 151/20 in the same location (112 to 137 kg).
Because a single MK 108 weighs 58 kg and an MG 151/20 ca. 42 kg.
 
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'Advanced primer ignition' weapons have a soft recoil, so perhaps the mounting can be made of weaker and thinner material, and thus be lighter?

Wing root MK 108 will not work anyway, since these are the API system weapons (same as the Oerlikon S, L or FF, ias well as the Ikaria MG FF) - ignition times of API are too unpredictable for a synchronized weapon.
 
Former member HoHun argumented in detail on a German forum that the low MV of the MK 108 was not that much a problem as perceived for most air combat distances where direct shooting is involved.
Like the shallower drop of high velocity guns is offset by their greater dispersion.

Not sure if he also engaged in this topic in English somewhere.

I'm amazed the MK 108 offers even A LOT more bang per weight installed as thought than the MG 151/20.
 
The problem is not trajectory, very often over emphasized by authors and internet "experts". Just tilt the guns up a bit to keep them on target at the closer ranges.
What that does not do is solve the time of flight problem. At sea level in thick air the MK 108 Ausf A ammo took 0.696 seconds to reach 300 meters. The 13mm MG 131 ammo took .49 seconds. A 300mph (484kph) airplane moves 13.4 meters per 1/10 of a second.
The MG 151/20 117g HET round needs 0.477 to cover 300 meters.
Things get a lot worse at longer ranges and better at shorter ranges. The 30mm round lost 26% of it's velocity in the first 300 meters.
Now at 20,000ft (6000 meters) in thinner air the velocity loss is not so bad.
Against slow bombers things are not so bad, against fighters? Needing to aim 4-8 plane lengths ahead makes things rather difficult. If you can get close you need less lead.

High velocity guns work a lot better at defection shooting. Which is why most of the bomber attacks were from behind or from the front. Simplified the lead problem.

But the 30mm does have a limited range problem. It is more limited firing envelope (range estimation) than actual range.
 
The D-9 packed 250 rounds per MG 151/20. How much would the Anton versions pack? As much?

And are there stats for the MG 17 weight (gun, mount, ammo) installed in the A-6?
 

The Birkigt cannon family, which includes the Hispano-Suiza HS 404 and friends, is gas-unlocked blowback.

As the cylic rate of fire of a gun goes up and up, the mechanism is perforce opening faster, so the line between conventional locked-breech operation and some sort of delayed blowback gets blurred. Even small arms like the M4 carbine and MG-42 machine gun get a measureable boost from the blowback effect even though they are primarily gas and recoil operated, respectively.
 
There is a lot of "stuff" going on in a gun as it fires. A lot of it is going on in milliseconds. A 600rpm gun is firing 10 shells per second so you have 100 milliseconds to cycle the gun.
The longer the bolt travel the higher the speed of reciprocating parts. The larger the gun (caliber) the more weight in the bolt/recoiling mass so the more force involved.
The barrel time (time from primer initiation to projectile exit) is a lot shorter than the cycle time and the pressure curve is spaced through the barrel time. Primer ignition starts the rise in pressure which peaks several inches down the barrel as the projectile travels. Peak pressure can range from high 20,000 psi to just under 50,000psi for most guns. For most guns the pressure in barrel when the projectile exits is around (?) 5,000-10,000psi. A lot of guns start to unlock the breech (or bolt starts to move) while the projectile is still in the barrel.
Since the cartridge case is the 'seal' for the back of the barrel it can't move too much and be unsupported by the chamber walls, depending on pressure in the barrel as the cartridge case moves back (and the thickness and quality of the case/materials) you can get case ruptures and high pressure gas leaking in the action and wrecking the gun.
The German MK 108 worked but it was walking a tight rope. The very short barrel allowed for the projectile to exit the barrel very quickly and allow the pressure to drop. On the MK 108 and all blowback guns the bolt and cartridge are already moving backwards as the projectile starts to move forward. The much heavier bolt/assembley just accelerated much more slower but lets go back to the milliseconds. You need roughly 50 milliseconds for the forward travel of the bolt so you only have about 50 milliseconds from the primer starting to ignite the power (bolt is still moving forward but slowing down) the forward travel to stop, the bolt to start to accelerate reward, and travel the distance to the recoil spring/buffer at the rear. Actual barrel time is much less than 50 milliseconds.
The trick with blowback guns is finding that spot where the gun opens up quickly for high firing rate but not so quick that the fired case ruptures leaking high pressure gas into the action. Now even slowing a gun down by 10% (540rpm) can buy time for longer barrel time which allows for higher velocity.
Locked breech or semi locked breech guns use lighter recoiling parts for faster bolt travel which allows a few extra milliseconds for the barrel time.
There area few other things that the blowback guns did. Like use a long chamber to support the case walls as the bolt/cartridge case move back while still under pressure.
But this increases bolt travel cutting into rate of fire.
By the late 1930s gun designers had around 30-40 years to figure some of this out. A number of gas operated guns had gas ports moved to change the unlock point. The Lewis gun kept the same port location but changed the op-rod or bolt. There was a certain amount of travel on the op rod after the gas pressure started to move the rod before the rod started to turn the bolt and unlock the action. The later guns allowed for just a fraction of inch more travel before the bolt started to turn. This allowed for less pressure to be in chamber as everything unlocked and made the ejection a bit less violent and reduced the chances of case rupture.
Metallurgy got a lot better from the 1890s and WW II and metallurgy has gotten a lot better since. The uniformity of cartridge case brass and substitutes (aluminum and steel) has gotten a lot, lot better.
 
Slight correction to your otherwise excellent summary, in the MK 108 (and all Becker-based autocannons like the USN's defensive Oerlikons or the Type 99 wing cannons in a Zero), the bolt is moving forwards when ignition starts at full speed. That's (part) of why they all use rebated rim bases (that's what the RB in 30x90mmRB stands for, not rhythm and blues). That way, the impulse of the round being fired doesn't just have to overcome the static inertia of the bolt as it is sitting still, it has to reverse the direction of the bolt.

This clever feature roughly cuts bolt weight in half, but it is also the reason why all autocannons based on the Becker patent have firing timing that is too inconsistent for synchronization systems to fire through propeller arcs.
 
Engineman said:
The MG213 must rank as the premier air to air gun, for longevity, performance and widespread application.
Click to expand...

One of the what-if scenarios I've pondered is the difference it would have made if both the MG 213 had made it to production and the R4M rocket was available sooner.

With the MG 213 as a motorkanone on the Bf 109 you've not only practically doubled the rate of fire of the MG 151/20 but are also using a more powerful round with a shorter time of flight, increasing hit probability. Even if they stuck with the MG 151/20's 20×82mm round, it would be a considerable increase in firepower while keeping the weight on the centerline, and with the reliability of a power-driven gun I would go as far as saying you could eliminate the 13mm machine guns too. If for example the 2 x MG FF cannon armament on the E model was considered sufficient for combat with fighters, then a single MG 213 in 20x82mm would have offered a higher rate of fire and significantly increased muzzle velocity. This would give you adequate firepower without compromising maneuverability and with only a single barrel on the centerline there are no concerns about harmonization and compensating for drop from different calibers.

By using large and heavily armed bombers, the USAAF essentially forced the Luftwaffe to create a separate class of fighter, the bomber destroyer that was weighed down with heavy cannon and extra armor to the point that it was no longer able to hold its own against escorting fighters and required its own escort. I believe that had the focus been on rockets rather than heavier cannon like the MK 108 early there would have been a different dynamic at play.

The Bf 109 G for example was frequently up-armed with a pair of MG 151/20 cannon that added 300 lbs without the ammunition and almost 500 lbs when loaded, as well as increasing drag, and having that weight on the wings reduced maneuverability. 24 x R4M rockets by comparison weighed 200 lbs and could be fired from a relatively light and low drag wooden rack, that could also conceivably be jettisoned. This gives us a scenario where regular fighters could be carrying enough firepower to make a couple of effective passes at a bomber formation from far enough to not requite extra armor and once that is done, free from the rocket weight and drag they could still be agile enough to dogfight with the Allied fighters escorting the bombers. The single MG 213 armament would also be adequate for dispatching stragglers from bomber formations broken up by rocket salvos.
 
GrauGeist said:
The problem with the R4M (and other grenate systems), was a drag penalty, which affected performance.
Click to expand...

That's true, but any heavy armament installed for use against four engined bombers was going to result in increased drag, my point was that at least with rockets that is eliminated once the munitions are expended, unlike cannon.
 
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