Large gun Me 109

[Engineman]

Sort-of interesting thread, but I feel that the Germans covered all of this at the time!
Bf 109 armament was an issue from the start but, bearing in mind it's diminutive size, it was fairly well armed, and the 2x131 + 1x108 was a hard hitter that maintained fighter performance. The engine mounted cannon on the DB 605 was fitted to the engine in a recoil mount that bolted to the engine rear gearcase. This basic arrangement proved to be
suitable to the MK 108, with considerable modification, and which was shorter than the MG 151 but not much heavier. This was important, because virtually none of the larger guns could be accommodated so easily, or at all. The ability of the DB 605 to accept greater weight and recoil loads on the basic 4 bolt mounting would also seem to be an issue.
So, looking at bomber-killing weapons, the Germans recognised that the Fw 190 was a better bet and there are many versions to do that (with Bf 109 protection from escort fighters) .
They tried large cal weapons in the Me 410 but, it fell into the trap of needing strong escort as well (something that a 37mm cannon Bf 109 would have also suffered from) and the big cannon Me 410 is reported to have suffered difficulty with aiming accuracy, no good if you miss!
The real progress might have come with the Do 335 as a bomber killer, but I suppose that the WW2 ultimate was the Me 262 with 4x108, that did have real effect.

Eng

 

[GrauGeist]

Why not go with the Fw190D-11?

It had two MG151/20 cannons (one each wing, onboard) and two MK108 cannons (one each wing, outboard).

 

[tomo pauk]

I was just surprised the Czechoslovaks and the Spaniards to rearrange the wing structure and insert the MG 151/20 mm and magazine into the wings. That they fit Hispanos left me questioning Messerschmitt's design ability... (I also had to triple check Tony's numbers, in Flying Guns WWII, that the Hispano Mk. V and MG 151/20 were basically the same weight. (OK, the Hispano fires with much more energy, but I had it in my head that the German Wonderwaffe was much lighter.)

IIRC Czechs simply went with the gondola cannons.
Spaniards indeed installed another spar, set between the legacy spar and the leading edge, so the holes for the big Hispano don't compromise the wing strength.

If somebody could have come up with a 30mm gun that fired at around 500rpm with a MV of 600-650 that might have been a very good choice. With 2-3 times the rate of fire you get a much better chance of hit/s even if each hit is only 1/2 as destructive.

Ho-155 was supposed to do 450-500 rd/min. MV of 715 m/s and weight of just 50 kg had a lot to do with it's lighter shell, 235g.
The 30mm cannon that fires the 330g shell (as the MK 108 did) at 600-650 m/s might weight 80 kg, and be incapable of being installed within the wing of the Bf 109?
The 2-3 times the rate of fire vs, say a 37mm spin off of the MK 108 or 103 will be accomplished by installing more weapons on the aircraft, not by some ww2 extra special 30mm cannonry (bar the revolver cannon).

But if I didn't mix up the pots, MG FF didn't go through the main spar, as well as, of course, the underwing gondolas.

MG FFs did go through the main spar, but the hole was small enough so wing strength didn't took the hit.

A slightly different option that achieves a similar effect might be to try to fit a 30mm MK 101 (or later MK 103) cannon pod (similar to that actually used on Hs129 - see below) on the centreline:

Can the firing line clear the prop disc?

 

[Zmauky]

Most Avia S-199s had MG 151 underhang pods, but a later smaller series had MG 131s mounted inside wing. (the picture of the model is purely for illustration - it would probably be more credible if I had put Eduard's )

 

[Shortround6]

The 2-3 times the rate of fire vs, say a 37mm spin off of the MK 108 or 103 will be accomplished by installing more weapons on the aircraft, not by some ww2 extra special 30mm cannonry (bar the revolver cannon).

I was referring to the difference in rate of fire between the 30mm and 37mm guns in general. The MK 103 upgraded to 37mm may have a bit of trouble hitting hitting 360-420rpm.
Even it did hit 360rpm that is 55% of the upper end of the cycle rate of the MK 108 gun.
The really big guns have real problems with weight and recoil.
The 55-57mm guns really show this.
The 5.5cm MK 112 went about 275kg (over 4 times the weight of the MK 108) and fired at about 320-360rpm?
The 1480g projectile with 500g of HE should take down a bomber most of the time with a single hit.
But you can install four MK 108s for nearly the same weight (more brackets and mounts) and the four MK 108s can deliver 13.2kg of shells compared to 8.88kg and 3.60kg of HE compared to 3.0kg of HE per second.

Can the firing line clear the prop disc?

Thank you Tomo.
Yes, hitting your own prop with a 30-37mm shell is a good way to ruin your day
You have pointed this out a number of time over the years, the big guns had problems being synchronized. Even with electric primers.
The problem was not actuating the primer at the correct time. The problem was getting the powder charge to ignite/burn properly so the projectile exited the barrel at the correct time.

 

[Zmauky]

Hmmm small correction
.... the more I look at Eduard's model and blueprint, the less certain I am ( in my memory) that the weapon installed in the wings was an MG 131. It all looks like the barrel of the MG 151. Most of the texts on the net are copy paste, so until I check...

I should really rely more on re-reading books than memory.

 

[z42]

I was referring to the difference in rate of fire between the 30mm and 37mm guns in general. The MK 103 upgraded to 37mm may have a bit of trouble hitting hitting 360-420rpm.
Even it did hit 360rpm that is 55% of the upper end of the cycle rate of the MK 108 gun.
The really big guns have real problems with weight and recoil.
The 55-57mm guns really show this.
The 5.5cm MK 112 went about 275kg (over 4 times the weight of the MK 108) and fired at about 320-360rpm?
The 1480g projectile with 500g of HE should take down a bomber most of the time with a single hit.
But you can install four MK 108s for nearly the same weight (more brackets and mounts) and the four MK 108s can deliver 13.2kg of shells compared to 8.88kg and 3.60kg of HE compared to 3.0kg of HE per second.

The Mk 108 was indeed quite amazing for the punch it delivered for the weight. Of course it paid for it in muzzle velocity.

30mm is probably the upper limit of what makes sense, except maybe for anti-tank use. Beyond that the compromises you have to make in terms of weight, RoF, muzzle velocity and recoil just become too big.

Another thing is that a single 30mm hit was usually all that it took to shoot down a fighter. So anything beyond that is just overkill, and it's better to focus on increasing RoF (including using multiple guns) and muzzle velocity to increase the chance of hitting that fighter. Yes, this thread is about shooting down heavy bombers, but even if this is the intention a fighter designed to shoot down bombers might well find itself in a position of having to deal with enemy fighters. So I don't think we can entirely discount that use case, either.

If I were the Germans, my dream fighter gun would be something like the Mk 108 except with higher muzzle velocity. Something like 700-750 m/s ought to be good enough. Can one design such a thing without a huge weight (and RoF?) penalty compared to the Mk 108? Assuming the same shells, at 750 m/s the muzzle energy would be almost double the muzzle energy at the 540 m/s of the Mk 108. For 700 m/s it drops to a factor of 1.68. Oh, and try to make it a bit slimmer so it could be installed in a wing?

EDIT: Or if we take the muzzle energy of the Mk 108 as a rough upper limit, a 330g shell at 540 m/s means a muzzle energy of 48kJ. Taking, say, the post-war widely used NATO 25x137 caliber firing a 223g shell, if we take that shell and fire it at 700 m/s we get 55kJ, so in the same ballpark.

Thank you Tomo.
Yes, hitting your own prop with a 30-37mm shell is a good way to ruin your day
You have pointed this out a number of time over the years, the big guns had problems being synchronized. Even with electric primers.
The problem was not actuating the primer at the correct time. The problem was getting the powder charge to ignite/burn properly so the projectile exited the barrel at the correct time.

I recall reading that there were a number of occasions where German fighters shot away their props due to malfunctioning or misadjusted synchronizing gear. Though with a 30-37mm shell it might do a lot more damage than just destroying a prop blade, the blast and shrapnel might destroy the engine and set it on fire?

I've come around to the opinion that the Allied approach of putting the guns in the wings outside the prop arc was the better choice. Yes, it meant convergence issues, but it got rid of the weight, reduced RoF, and additional rather nasty failure modes of synchronized guns.

 

[tomo pauk]

I was referring to the difference in rate of fire between the 30mm and 37mm guns in general. The MK 103 upgraded to 37mm may have a bit of trouble hitting hitting 360-420rpm.
Even it did hit 360rpm that is 55% of the upper end of the cycle rate of the MK 108 gun.

Benefit is that every hit is about 50% more powerful than what the 30mm can do, and with greater MV (650-700) the hit probability is also much increased. Germans reckoned that MK 108 have had 1/2 of hit probability of the MK 103, so the in-between MV might still get us to achieve about 50% greater hit probability than the MK 108. Multiply the increased hit % with the increased damage of the shell, and there is a math that favors a big gun in the nose of the Bf 109.

The really big guns have real problems with weight and recoil.

I can't remember me advocating for really big guns. To the contrary, my suggestion is to go modest wrt. size, weight and power with the 37mm weapon.

The 55-57mm guns really show this.
The 5.5cm MK 112 went about 275kg (over 4 times the weight of the MK 108) and fired at about 320-360rpm?
The 1480g projectile with 500g of HE should take down a bomber most of the time with a single hit.
But you can install four MK 108s for nearly the same weight (more brackets and mounts) and the four MK 108s can deliver 13.2kg of shells compared to 8.88kg and 3.60kg of HE compared to 3.0kg of HE per second.

Chances that our valiant Bf 109 carries either the MK 112 or 4x MK 108 in the real battle is slim to none, and Slim has already left the city

 

[Zmauky]

Flogging a dead horse (And listening to the Sex Pistols album)
Maybe one of the big guns could be the Breda 37/54, according to the pictures / blueprints, it looks like the part with the bolt is shorter than the Flak 36, so maybe it can be pushed with an acceptable displacement of the cabin like in the Yaks . It is automatic and is even installed in the plane, albeit a twin-engine CANSA FC.20

 

[Jimbob]

From my collection.

 

[Shortround6]

Maybe one of the big guns could be the Breda 37/54, according to the pictures / blueprints, it looks like the part with the bolt is shorter than the Flak 36

It wasn't a bad AA gun, it wasn't the best but it was well ahead of the German Navy, the French and the Japanese.
Doesn't mean it was a good aircraft gun though.
As soon as you start changing things you need to do a lot more testing. Moving the gas port and cylinder back changes the timing of when the breech block starts to open up and speed at which it opens up. You can play games with the size of the gas port and venting and with springs but that requires a lot of test firing.
gun feeds from a 6 round magazine in the top (or side) which while workable (just) for twin engine plane with a crewman who could replace the magazine (or top it up?) in a single engine fighter you are really stuck, unless the pilot flies bare foot and has really talented toes.
Yes you can design a new feed mechanism but that also takes time and testing.
The single mounting shown used a spring equilibrator which allowed the gun to recoil 10cm which helped dampen the recoil bit is not included in the weight of the gun. It also means that the magazine recoiled with the gun and that is a real complication for fitting significantly larger magazines.
The twin mounts were rather well noted for having severe vibration which they transmitted to the ships structure. Bolting the gun to the engine does not seem to be a good solution.

The MK 108 had a rather short effective range but the idea that a plane could "stand back" and lob shells from out of the range of the US .50 cal guns didn't work out so well.
While the US .50 was not a supergun it had a longer effective range than many 9but not all) other 20mm and smaller Aircraft guns. Trouble is that trying to lob shells in with 2 seconds or more of flight time meant you needed an aiming system more advanced than anybody had it service in WW II. People could get occasional hits at long range. But not enough for it to be a viable alternative. A lot of people tried.

This was for intercepting bombers, not defending against fighters.

 

[Engineman]

The MK 108 had a rather short effective range but the idea that a plane could "stand back" and lob shells from out of the range of the US .50 cal guns didn't work out so well.
While the US .50 was not a supergun it had a longer effective range than many 9but not all) other 20mm and smaller Aircraft guns. Trouble is that trying to lob shells in with 2 seconds or more of flight time meant you needed an aiming system more advanced than anybody had it service in WW II. People could get occasional hits at long range. But not enough for it to be a viable alternative. A lot of people tried.

Yes, the MK 108 did still need a fairly close range for accurate hits, but this was not a big problem for fighter v fighter with no defensive fire. However, against bomber formations, the risk from return fire was great to propeller fighters. Head-on was a good strategy but, just one MK 108 would not get many rounds off per accurate pass and even a Bf 109 G took a long time to set-up head-on attacks and the escort fighters got time to intervene.
As I pointed out, the stand-off Me 410 was inaccurate at long air to air ranges, this is usual for long ranges against moving targets with bullets, even the best modern computer gunsight cannot allow for target manoeuvre and with longer bullet flight time comes less accuracy, even against B17.
The Me 262 with 4x MK 108 had the aircraft speed to be effective and less vulnerable, the WoF from the 262 gave 11 rounds in a 1/4sec burst, head on, a killer if accurately flown.

Eng

 

[Engineman]

Most Avia S-199s had MG 151 underhang pods, but a later smaller series had MG 131s mounted inside wing. (the picture of the model is purely for illustration - it would probably be more credible if I had put Eduard's )
View attachment 794107

S-199 mod. 17/7.9N was built-in wing MG17 gun mod, not 131 in-wing.

Eng

 

[Zmauky]

S-199 mod. 17/7.9N was built-in wing MG17 gun mod, not 131 in-wing.

Eng

Thanks for the information, I'm missing literature specific about S-199, those about Messerschmitts only mention the usual 2x13.1 + 2x20 mm

 

[hobbes154]

Just being boring, the historically used (though not together) engine mounted MK108 + two MG FF in the wings has more firepower (according to Anthony Williams) than 4 Hispano IIs. Very few single engine WWII planes had more firepower than that (late Tempest w/ 4 Hispano V or Fw 190 bomber destroyers w/ 2 MK108). Two MG 151/20 instead of FF would give more firepower but worse ballistic matching and weight/aerodynamics.

Hard to carry more on a 109 without degrading performance and hard to find more weight efficient weapons from that period (last column of table 2).

30mm is probably the upper limit of what makes sense, except maybe for anti-tank use. Beyond that the compromises you have to make in terms of weight, RoF, muzzle velocity and recoil just become too big.

Agreed, aircraft gun calibers above 30mm haven't lasted even for dedicated anti-tank aircraft (A-10).

If I were the Germans, my dream fighter gun would be something like the Mk 108 except with higher muzzle velocity. Something like 700-750 m/s ought to be good enough. Can one design such a thing without a huge weight (and RoF?) penalty compared to the Mk 108?

Depends on your threshold for "huge" but I'm going to say probably not with the technology of the time.

The MK 108 is already by far the most weight efficient gun in the linked article. If you stick with API blowback "high rate of fire and high muzzle velocity tend to be mutually exclusive". If you go gas operated then you're talking basically a scaled down MK 103? Let's say it's scaled down roughly how the Oerlikon FFL was from the FF (since those are about the relative muzzle velocities you want), you could save about 30% of the weight, that's still about 100kg vs 60kg for the MK 108? And unless you raise the RoF probably less powerful than the 108.

Sounds like you basically want an ADEN/DEFA type round but even with next gen tech those guns definitely had a huge weight penalty!

 

[z42]

Depends on your threshold for "huge" but I'm going to say probably not with the technology of the time.

The MK 108 is already by far the most weight efficient gun in the linked article. If you stick with API blowback "high rate of fire and high muzzle velocity tend to be mutually exclusive". If you go gas operated then you're talking basically a scaled down MK 103? Let's say it's scaled down roughly how the Oerlikon FFL was from the FF (since those are about the relative muzzle velocities you want), you could save about 30% of the weight, that's still about 100kg vs 60kg for the MK 108? And unless you raise the RoF probably less powerful than the 108.

Sounds like you basically want an ADEN/DEFA type round but even with next gen tech those guns definitely had a huge weight penalty!

You're probably right. The Mk 108 achieved its high punch/weight due to firing big shells at decent RoF and the low muzzle velocity kept the weight down. So by increasing the muzzle velocity it's pretty inevitable that weight increases.

Though if you had something like a 100 kg gun you suggest above, maybe you can get rid of the Mg 131's. That would save about 30kg, plus the mounting and synchronization gear, as well as reducing drag. And the Mg ammunition weight too, though you likely want to carry more cannon shells instead. And those shells would be heavier than the Mk 108 shells too.

As for the ADEN/DEFA, those were about 90kg, had pretty decent muzzle velocity, and amazing RoF. A big step up in punch/weight even compared to the Mk 108. In retrospect, the Mk 213 line of development would have been more fruitful than the various 50+mm aircraft cannons they were working on.

 

[HP52]

We all know about the Yak-9T and K where the cockpit and pilot were moved back approx. 0.4 m to install the 37/45 mm gun.


And the Japanese example would be the Ki-61-II where the engine was moved forward so that they could replace the 12.7mm with 20mm guns.


Ultimately, it wasn't (too big) problem for the Czechoslovaks and the Spaniards to rearrange the wing structure and insert the MG 13.1 / 20 mm Hispano into the wings themselves (not the underhang nacelles).


Would a heavier and much more specialized Me 109 with, say, 1x37 mm in the fuselage and 2x MG 131 in the wings or 5x20 mm (3x fuselage + 2x wings) be a viable bomber destroyer in 1942-1944?
And would such a thing be constructively feasible at all?

Remember it was a very small aeroplane. I have only seen one - twice. The first time was at 5am on a winter's Australian morning while riding a BSA 500 toward the Georges River near Bankstown airport. The fog was getting pretty thick close to the river at 5.30 am and a low loader truck was approaching headlights on - There was an Bf 109 on a truck. Hardly worth mentioning there was a Mustang behind the Messerschmitt. Both were arrested on the wharf an hour or so later. The Bf108 also points to the space issue
Hampdemon

 

[Engineman]

You're probably right. The Mk 108 achieved its high punch/weight due to firing big shells at decent RoF and the low muzzle velocity kept the weight down. So by increasing the muzzle velocity it's pretty inevitable that weight increases.

Though if you had something like a 100 kg gun you suggest above, maybe you can get rid of the Mg 131's. That would save about 30kg, plus the mounting and synchronization gear, as well as reducing drag. And the Mg ammunition weight too, though you likely want to carry more cannon shells instead. And those shells would be heavier than the Mk 108 shells too.

As for the ADEN/DEFA, those were about 90kg, had pretty decent muzzle velocity, and amazing RoF. A big step up in punch/weight even compared to the Mk 108. In retrospect, the Mk 213 line of development would have been more fruitful than the various 50+mm aircraft cannons they were working on.

The MK 213 was the best development, fortunately just too late! The initial ADEN copy was similar, with improved steels etc and was a great gun for many air forces, up to this day!
A good installation of a reliable MK 213 as engine cannon on the Ta 152 would have been effective. However, Twin or Quadruple MK 213/ADEN in the nose/fuselage of a jet was the way forward in the late 40's till the early 60's, with further developments to today.

Eng

 

[tomo pauk]

Though if you had something like a 100 kg gun you suggest above, maybe you can get rid of the Mg 131's. That would save about 30kg, plus the mounting and synchronization gear, as well as reducing drag. And the Mg ammunition weight too, though you likely want to carry more cannon shells instead. And those shells would be heavier than the Mk 108 shells too.

(my bold)

A ~100 kg (presumably 30m gun) would've been probably the best match for the Bf 109 in the bomber-destroyer role. In case it is a 'MK 108 magnum' (ie. an API gun) the peak recoil would've still been mild. Probably worth for 500+ rd/min with the 330 g Mine shell fired at 700 m/s? Hit probability over the MK 108 would've received a major improvement - good against the bombers bristling with HMGs, as well as against the fighters.
In case it is a 'baby MK 103', it might be possible to install two as wing root guns on the Fw 190. Again, remove the cowl MGs so the weight and drag increase is (partially here) balanced out.

(bonus - devise a shorter, 250-270 g Mine shell, hopefully doing 780-800 m/s, in order for the gun to be useful as a Flak piece)

 

[don4331]

(my bold)

A ~100 kg (presumably 30m gun) would've been probably the best match for the Bf 109 in the bomber-destroyer role. In case it is a 'MK 108 magnum' (ie. an API gun) the peak recoil would've still been mild. Probably worth for 500+ rd/min with the 330 g Mine shell fired at 700 m/s? Hit probability over the MK 108 would've received a major improvement - good against the bombers bristling with HMGs, as well as against the fighters.
In case it is a 'baby MK 103', it might be possible to install two as wing root guns on the Fw 190. Again, remove the cowl MGs so the weight and drag increase is (partially here) balanced out.

(bonus - devise a shorter, 250-270 g Mine shell, hopefully doing 780-800 m/s, in order for the gun to be useful as a Flak piece)

The issue with 30mm guns in wing roots is the combination of the variability of the action and the variability of the power burn.

With higher rate of fire guns, both wing root guns are guaranteed ready when the 'signal' is given to fire at reasonable of fire for synchronized guns. With both guns firing, the aircraft remains a steady gun platform..​

But when you get into larger individual weapons, you have to slow the rate of fire way down to ensure both guns are charged - which slows firing rate way down.​

​

The other challenge is not shooting your own propeller off. Unfortunately, with the amount of power being burned in 30mm casing, and the difference in burn times between cold and hot temperatures, it becomes more than the time difference between propeller blades coming into line of fire. More of issue with MK 103 as it has more powder, but definitely concern. For some reason, pilots have real concerns about their propellers being shot off....​

​

p.s. Synchronization gear for the electrically primed German guns is pretty light - more/less just a set of points in a distributor; just a couple kg. They only close the circuit when safe to fire.