A better thought-out '2nd gen' of German 2-engined A/C?

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wuzak said:
Jumo 211J may be better than BMW 801 at the start. (I'm not sure when the more powerful Jumo 211s became available).
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By (early?) 1943, the 211N and P (= intercooled N) are becoming available. The N was comparable to the fully-rated DB 601E, or to the Merlin 20 series running on +14 psi. The P was perhaps 5% more powerful than the N.
The best of the lot, 211R, was too late, probably not available until 1944, by what time Jumo has the superior 213 in production (although two Ta 154 prototypes were lost due the 213 problems).

wuzak said:
The inner section of the wing is what keeps the propellers from cutting the fuselage.
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Agreed. Engine needs to go further outboard on this frankenplane.
 
I think that the He 111 is a little too old a project to pour new wine into. After all, historically not much has been invested in upgrades. And Do 217 - well, with BMW 801/ Db 603 was underpowered (so the early versions of 1600-1700 hp are too weak). It would take 2000+ hp, so as a historical project Do 317 ie db 606 or jumo 222. Maybe it would be better and faster to put Jumo 211j (instead of db 601) on Do 215 and we have approx. 40% more power than Do 17.
 
Zmauky said:
For the mentioned Bf 110, one variant would be the version with BMW 132/Bramo 323. Why - well, as production of DB 601 was delayed , why not put stronger radial engines instead of the Jumo 210.
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Why not put the Jumo 211 in if you can't have the DB 601?

BMW 132 does not offer much more power than Jumo 210, while Bramo 323 is a bit better.

And would either be available before the Bf 110C entered service in 1939?
 
Historically they thought they were going to get the new Wundarplanes to replace the He 111. So yes, they did minimal upgrades to the He 111. The He 111 pre-dated the Wellington by a considerable amount. This flew 16 months after the He 111

And there was a lot of revision to most of the fuselage and tail.
The Boeing 299 flew about 1 year before the Wellington and only 4 months after the He 111

Not all planes have the same amount of "stretch" bit perhaps more could have been done to the He 111?

It might help to get a time line on the German engines. Germans could not supply Jumo 211F/Js in numbers in 1940. Had to wait for 1941. The Ju 88A-5 was built after the A-4 was approved. The A-5 used the wings/landing gear of the A-4 with the engines of the A-1 because the engines were not ready.

wuzak said:
Why not put the Jumo 211 in if you can't have the DB 601?

BMW 132 does not offer much more power than Jumo 210, while Bramo 323 is a bit better.

And would either be available before the Bf 110C entered service in 1939?
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Using the radials instead of the V-12s runs into drag problems. You don't get the performance hoped for due the drag, even though it may be a bit better than the Jumo 210.
Look at the Do 17 at sea level and compare the BMW IV V-12 to the Bramo and BMW 9 cylinder radial versions. Then compare the Do 17Z with the Bramo to the Do 215 with the Db 601. Around a 35-40mph gain in speed for the latter case.

And again, be careful of which Jumo 211s were available when if you are going to 1938/early 1939. Also lets recall that Junkers was a little late with adopting pressure cooling. Which meant larger radiators on the early engines, same power but more drag (and bit more weight), perhaps not so important for bombers but on a twin engine fighter???
 
I agree about the Jumo 211 (admittedly, radial engines look more interesting for the model - gondolas like Hs 123 in 3 early Luftwaffe colours), but if they had them in 1937, wouldn't they have installed them? Serial production of the Jumo 211 began in April 1937? And maybe not Bramo 323 but Bramo 329 (aka Sh29), a two-row 14 cylinder, with a power output of 1400hp. (although I am not sure about the dates). And it never got to production, but this is what if.
 

Probably Jumo was not able to supply any 211F/J to take part in 1940 air combat?


German 9 cyl radials were probably good for many things; being used in aircraft tailored for speed was not one of them.
For all I care, nick the Jumo 211s from Ju 87 production by 1938-40 (have the Stuka receive these radials) so more can be spared for installation on Bf 110 and/or Do 17 (215).


I'm not sure that any Jumo 210 or 211 engine used open-cycle cooling, ever.
 
tomo pauk said:
Probably Jumo was not able to supply any 211F/J to take part in 1940 air combat?
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Quite possibly, I don't have any good sources for dates and prototypes are not production.
edit. Sticking the standard German 9 cylinder radials on the Ju 87s of 1938-40 would have doomed them
The Ju 87 sure didn't need anymore drag and the radials offered a lot less power. They got up to 1000hp or more later but in 1939-40?
Unless you can come up with a "cheap" 14 cylinder engine you don't have enough power.
Water injection in the Bramo got it off the ground but trying to climb with (or without?) water injection to 9-12,000ft may not have been a good idea?
tomo pauk said:
I'm not sure that any Jumo 210 or 211 engine used open-cycle cooling, ever.
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The source I have is an old book by an English author and it is not reliable. Or we are confusing different things?
You may have a "sealed" system but unless you run it at several lbs/sqin/metric equivalent) over normal sea level you don't get the raised the boiling point and higher heat transfer?

Perhaps a difference between sealed low pressure cooling and sealed high pressure cooling?
 
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Some less conservative ways to have workable 2-engined A/C:
- as suggested earlier and many times: zwilling heavy fighters/recons/fighter-bombers (benefit is that most of the parts is in series production, engines don't have to be newest and fanciest)
- push-pull twins (saves a lot of engine-related drag, can use mass-produced V12 engines and still perform, no torque reaction with both engines in operation, 'natural' U/C layout is tricycle; shortcoming is pilot/crew emergency exit)
- mixed propulsion push-pull (lighter powerplant than the piston engine type at the back, jet uses very cheap fuel, no need for propeller to be produced and installed, easier emergency exit than the A/C described just above; shortcoming is the fuel load required for the jet, jets not being mature enough, no easy way to hide exhausts, that compromises NF job; jet engines not being that mature yet)
- 2-engine pusher, be it in a tandem layout or side-a-side
- similar, but in tractor configuration; both cut a lot of engine-related drag
 
The zwilling/s are more of a defensive aircraft than an offensive aircraft. The 109Z got it's extra range from filling the 2nc cockpit with fuel. Which makes for a lousy night-fighter.
If you need a bomber destroyer to handle bombers flying over Germany it may make sense, or if you need a hit and run bomber to handle things near the front lines then maybe OK.
I doubt the 109Z is going to roll much better than than P-38. It sure won't have the range. Yes you can use a lot of existing parts/tooling BUT ONLY if you don't change things a lot.
The more things you change (landing gear, wing mounted guns, fuel storage, higher gross weights in proportion to wing size) the fewer common parts you have. One of the things that doomed the Me 209 II. They made a lot of changes than helped improve the design. However with the improvements they would up with only 30-40% parts commonalty with the 109 and they decided that would not give the production head start they wanted.
There are reasons that the 110 and 210 and Fw 187 wound up as big as they were (and just about everybody else's twins than used standard sized engines). They had more capability, range/load.
tomo pauk said:
- push-pull twins (saves a lot of engine-related drag, can use mass-produced V12 engines and still perform, no torque reaction with both engines in operation, 'natural' U/C layout is tricycle; shortcoming is pilot/crew emergency exit)
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Well you do have less drag, but.

You still have double the cooling drag compared to the single engine fighter (assuming that you use engines of the same size.) You have saved frontal area, you have added a lot of fuselage length/bulk.
Better than some/most twins,
For people that want a Do 335 junior (DB601-605 engines) remember that the P-39 drive system cost about 100lbs. about 50lbs worth of drive shaft and 50lbs worth of stiffer fuselage to keep the prop in line with the engine (they allowed 1in deviation in flight).
Maybe the Germans could have built something like the Fokker using DB 601 engines.


tomo pauk said:
- 2-engine pusher, be it in a tandem layout or side-a-side
- similar, but in tractor configuration; both cut a lot of engine-related drag
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Pusher or tractor a pair of side by side engines only save a little bit of drag compared to a "normal" twin. Then you have the more complicated drive system.

This is supposed to be the engine from a Fisher P-75 with the gear box for the counter rotating propellers. However the exhaust pipes are facing the wrong way?
Now note that this engine held both cranks in the same crankcase and uses a single supercharger, it was not two separate engines, which would have taken up more room.

Tandem engines have their own problems.
You are trading aerodynamic drag for mechanical complications and CG problems. Not saying they cannot be solved, but they are going to take time.
Pretty much a last ditch effort, Again somewhat useful for short endurance missions (defensive), not so good for long endurance missions.
Props and jets have rather different speed ranges.

Use the turbo prop line to substitute for the piston engine.
Jet engine booster packs were popular in the 1950s/early 60s because they gave a lot of temporary power (tank-off and hot-high climb) for not a lot of weight. They sucked at cruising speeds. And adding a booster pack/s to planes like a C-119/C-123 barely made a blip on their drag
 
Every A/C is a compromise
Indeed use the 109Z as a high-performance interceptor (attach a MK 103 container under the middle wing), as a fighter-bomber, while as a recon it should work with just one cockpit 'populated' - all 3 tasks are important, and something else will need to do things like 'proper' bombing or night fighting.

Shortround6 said:
You still have double the cooling drag compared to the single engine fighter (assuming that you use engines of the same size.) You have saved frontal area, you have added a lot of fuselage length/bulk.
Better than some/most twins,
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We might end up with the cooling drag comparable to a 2000 HP radial, but with 50% more power? That is a good bargain for the LW of 1942 an on.
Note that fuselage bulk of the 335 is not just due to it having two engines and a driveshaft, but it also housed a lot of fuel there, and was also with a bomb bay. Nothing of the cooling system was within the wings, leaving these free for fuel/guns/ammo.


Fokker's layout surely removes one thing to worry about and to pay for, both in coin and weight (ie. the drive shaft).
Either way, the push-pull fighter, with a wing of some 300 sq ft, should've been a much better platform for a pair of MK 101s/103s than it was the Fw 190 (even if these cannons must go in the containers/gondolas), both from structural reasons, and reasons of performance penalty.
 
Suspect that is the double Allison used in the XB-42.
 
Redoing the earlier chart a bit for just German aircraft.

Plane....................................................Ju-288......................He 111P*........................Do-217.........................Ju-88*...........................Do-215*.........................Me 210
Wing area sq ft.................................580.................................943.............................610.................................587..................................592.................................390
Wingspan...........................................61....................................74..............................62.33.................................65.6................................59................................53.66
Gross weight(lb)............................29,542............................29,760.......................33,700.............................26700............................19,400.........................23,560
power.................................................2 X 2000.......................2 X 1100....................2 X 1580.......................2 X 1340......................2 X 1100...................2 X 1350
engines.............................................Jumo 222.......................DB 601........................BMW 801..................Jumo 211H..................DB 601........................DB 601F
crew.......................................................3........................................5......................................4.....................................4.......................................4..............................2

max speed.............................................404............................247...................................320................................292...................................292............................334

max bombload...........................6600.................................4400..........................8800..................................4400................................2200............................2200
norm. load.....................................????..................................2200..........................5500..................................3300................................2200............................1100

features
dive bomb...................................Y............................................N.................................Y......................................Y..........................................N.................................Y
pressure cabin............................Y...........................................N.................................N.....................................N.........................................N...............................N
remote gun turrets...................2............................................0..................................0........................................0........................................0................................Y
power gun turret/s..................0.............................................0.................................1........................................0.........................................0................................0
manual gun stations................0...........................................3..................................4........................................3.........................................4.................................0
fixed gun sets.............................2............................................0.................................1........................................1(?)....................................1.................................4

total Guns................................6 X 7.9...............................3 X 7.9.............................6.........................................4-6................................6...................................6

The HE 111P was flying in prototype form in 1938 so it is sort of the Benchmark for the newer/faster bombers. The He 111H weigh better engines could carry more.
The JU 88 and the Do 215 are as used in 1940 with added guns. The Ju 88 was not maxed out yet. The Do-215 may have had limited growth......but.....
The Do 217 was the gen 1 1/2 that just snuck into 1940 in very limited numbers.
The Me 210 had flown in 1939 but the well known problems delayed things quite a bit, The numbers are for the A-2 dive bomber version and some of the additions, like dive brakes, slowed it down a little.
I have not shown range, which was a real problem for some of these aircraft. Trying to move a large bomb load very far was not going to happen for some of them and with the failure of the Jumo 222 powerplant, engine power in 1941-42 was a real problem. Planes can use a lot of power or a lot of wing area or combination of the two. The He 111 used the wing method. Most of the other German planes were trying to use power, and when the hoped for power didn't happen they were left a little lost.
Trying to cut wing area to keep speed up works but only at the cost of range/bombload/field performance.
Do 217 kind of shows the problem, a very good try but a max speed of 320mph in the late fall of 1940 was not good enough. Might have been great against Hurricane Is but in Late fall the Hurricane IIs were starting to show up and at least they had a chance. Spitfire Vs were going to show up about the time Do 217 bombers showed up in numbers.
That is a problem for the Ju 288 lite (BMW 801 engines) you have lost both power and critical altitude. Streamlining will only go so far.

Germany needed to figure out better defensive gun set ups and accept that the high speed bomber wasn't really going to work. Yes they had accepted it over Britain from Sept 1940 on, sort of. They accepted it operationally but it doesn't seem to have sunk in to the requirements for planes under development.
If you are going to put in manual guns you need one man per gun, not an acrobat on speed leaping from gun to gun during the battle.
Also accept the fact that the MG 15 was obsolete in 1938.
Also accept the fact that remote gun stations were a bridge too far for an air force that in the summer of 1940, had yet to field a single power operated or assisted gun mount.

Yes get rid of the dive bombing requirement for the twins or relax it 45degrees or a bit less. Work like hell in a manned turret with a pair of 13mm guns.
Expand the wing on the Do 217 by about 10% ( a bit less than the K-2 version) to either shorten ground run or allow for around 3,000lbs increase in gross weight. So what if you loose 10-15mph, you can carry either more bombs or go further (or both). You should be flying at night into contested airspace anyhow.
Fix the Me 210 and use it for the fast bomber role.
The Ju 288 was trying to do too many things at once. A plane with a service ceiling of under 30,000ft does not need a pressure cabin. A 26,000lb (fuel burned off) plane should not be trying to dive bomb at steep angles using dive brakes.
 
Do 217(A or C) mentioned for 1940 is a red herring - it was too late for that year to do anything, and it was produced in very small numbers. Late 1940 means very bad weather in NW Europe anyway, even the debugged A/C were kept on the ground; BoB was over weeks ago.

Shortround6 said:
That is a problem for the Ju 288 lite (BMW 801 engines) you have lost both power and critical altitude. Streamlining will only go so far.
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You didn't lost neither power nor critical altitude, since there is no other German engine that can best the BMW 801 between 1942 and late 1943 while still being reliable (granted, the 801 paid the price in weight, drag and consumption vs. the 601 or 211 - the 'no free lunch rule' always finds the way...).

Agreed all along, but 4 MG 15s in a power-operated manned turret would've been a wholly different beast, like the .303 Browing was on Whitleys and other big RAF bombers.
 
Just trying to give the Germans the benefit of the doubt here. Like assuming they could have speeding things up a few months.
Planners (designers/production managers) would have no idea of what the weather would be like months in the Future. This also the problem of the planners (designers/production managers) If what you have is barley working in the fall of 1940 what is you opponent going to have in the spring-summer of 1941? or 1942"
You have a loss of in power and critical height from the "planned" Jumo 222 engines for the Ju 288. Yes you can fly a Ju 288 with BMW 801 engines but the speeds are all going to be off considerably and the take-off weights needed to be looked at very carefully, with about 20% less power for take-off something has to got to give. Fuel, bomb load, number of airfields it can use, truck loads of booster rockets for each squadron strength mission?
tomo pauk said:
Agreed all along, but 4 MG 15s in a power-operated manned turret would've been a wholly different beast
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A real beast gunner with four MG 15s in power turret would have been busier than a one armed wallpaper hanger with bees in his overalls.

Why the Germans never (rarely?) used the belt fed MG 17 in a defensive position I don't know. An MG 15 had about 73-74% of the ammo as the Vickers K gun and didn't fire any faster. They never seemed to put two MG 15s together? Maybe they were waiting for the MG 81 ? and it was running late?

The MG 131 seemed to trickle in rather slowly.

The 20mm MG/FF or MG/FFM was not a very good flexible air to air gun. Might have been very useful air to ground and OK in the early war as a fixed fighter gun.
 
Okay.
Do 217 will really need, at least, BMW 801 to work.
If there was more support from RLM for the DB 603 (and less DB's flirting with many other engine types - X24, W24, V16), that one might've also been an option, even if it runs at 2500 rpm instead of 2700.


I was trying to came out with a bomber that can still cruise relatively fast when bombed up with several 1000 kg bombs - a feat that was not possible with Ju 88 - and also a bit faster than the Do 217. As before noted in this thread, I'm okay with the bomber not being to lug as many tons of bombs and fuel as the Do 217 was capable for.
If/when Jumo 222 arrives - good. If the DB 603 is available in numbers and it is more reliable than it was historically - again, good. Turboed 801 is also good. Same for Jumo 213.

Doh
Yes, indeed, MG 17 is the one for the 4-gun turrets, or even to be used in twin mounts.

Shortround6 said:
The MG 131 seemed to trickle in rather slowly.

The 20mm MG/FF or MG/FFM was not a very good flexible air to air gun. Might have been very useful air to ground and OK in the early war as a fixed fighter gun.
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Methinks that there was much more mileage in the MG FF and FFM designs, than the Germans took advantage from. Belt feed (don't wait until 1943-ish), big drums (yes, too clumsy for a flexible installation), greater RoF, the FFM necked down to 15mm (so it can do 800+ m/s for a better hit probability) for defensive installations (but still better suited for power-operated turrets)...
Ammo for the 'old' MG FF ammo was with a greater propellant weight, so pairing it with a short, ~100 gram HE shell might've seen a ~750 m/s MV - again for the better hit probability - not just for bombers, but also for fighters, already before ww2. Or indeed neck that one down to 15mm, for close to 850 m/s?
Surely beats the 13mm, and can fit in the places where MG 151 cannot.
 
A lot of the flexible 20mms used 30 or even 15 round magazines. Due to the size of breech end of the weapon they had restricted traverse/elevation.
With the size of the housing/yoke, sight and sometimes flash hider this thing was a beast for the crewman to fight the Slipstream. Note the posed photo has no magazine fitted.
tomo pauk said:
Ammo for the 'old' MG FF ammo was with a greater propellant weight, so pairing it with a short, ~100 gram HE shell might've seen a ~750 m/s MV
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Well, when they paired it with 92 gram projectile they got 700m/s so 750m/s may be a bit optimistic?
tomo pauk said:
Or indeed neck that one down to 15mm, for close to 850 m/s?
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I think you are expected more from the MG/FF than it can deliver. The MG 151 had several things going for it. The cartridge used a higher chamber pressure than the MG/FF and it used a longer barrel. MG 151 15mm used longer barrel than the MG 151 20mm gun.
The MG 151/20 generated about 30% more muzzle energy than the MG/FFM so expecting modified MG/FFs (or Ms ) to come close is going to take a lot of work.
The MG/FF used pure blowback (or used the weight of the forward moving parts to help) so that the weight/s and spring pressure had to be carefully balanced. Also the barrel length (part of the weight) as the projectile had to clear the barrel before the breech block/cartridge case moved back to far. Much like the MK 108.
If you want more velocity you are going to pay for it with more weight, slower rate of fire, longer barrel (also more weight) or some combination.
The "locked" breech guns had a little more room for 'improvements'. The bolts often moved at two speeds. The initial movement with barrel and bolt locked together while the pressure dropped and then when unlocked the bolt alone could move faster to finish extraction/ejection and reloading.

And then you have the actual ammunition/projectile problems. The 20mm mine shell was only slightly better range than the 30mm MK 108 as far as effective range went. The 20mm mine shell started out faster but it lost velocity faster.
2 things affect the external ballistics, sectional density which is the weight per unit of frontal area(cross sectional area) and the shape of the projectile. You can juggle the two a bit but an 92 gram 20mm projectile is always going to have problems compared to a 130 gram projectile. There is only so much a pointy nose can do

Going to small arms for an extreme example.

The Hornet gives about 2650-2700fps with a 45 grain bullet. The Swift gives about 4100 with at 48 grain bullet.
about a 50% increase in velocity. However the Swift needs 3.5-4 times the propellent to do it. It also needs different propellent.
Yes you can juggle bullet weights and velocity, but that only goes so far and short/light projectiles slow down quicker.

Improvements were made to the Oerlikon system/family but it sometimes took advances in propellent or metallurgy to do it.
 
Shortround6 said:
Well, when they paired it with 92 gram projectile they got 700m/s so 750m/s may be a bit optimistic?
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It might be for the FFM, but might be possible for the FF. Former achieved 585 m/s when firing a 115g HE shell, while later achieved 600 ms when firing a 135 g HE shell - IOW, there was more oomph in the FF ammo vs. what FFM used. Ammo for the two was not interchangeable, even if outward looked the same.


The FF/15 will still have a deficit of 20% wrt. muzzle energy.

Shortround6 said:
If you want more velocity you are going to pay for it with more weight, slower rate of fire, longer barrel (also more weight) or some combination.
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I am already paying it with much lower shell weight. Long barrel means a gain of a few lbs, indeed.

Shortround6 said:
Improvements were made to the Oerlikon system/family but it sometimes took advances in propellent or metallurgy to do it.
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Sometimes it was a thing of 'why we didn't think of this before?'.
Like the belt-feed system - it took Japanese and Germans until 1943 to do it; compare the Fench/British do it more than a year earlier with Hispano despite the Hispano being a newer gun. Lighter reciprocating parts were a thing known to improve the RoF well before the ww2 (it was done with Swiss on the Oerlikons by mid-1930s, and again by them and Japanese by the end of the ww2); lighter parts = lighter guns.
Lighter shells = greater MV (yes, you are sacrificing the hit power, but we're still head and shoulders above the HE shells on the HMGs and 15mm cannons - not too shabby for a cannon that weights as a lot of HMGs).
 
Not really. The operating system depended on recoil energy with changes in different proportions than muzzle energy, more later.
tomo pauk said:
The FF/15 will still have a deficit of 20% wrt. muzzle energy.
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See below.
tomo pauk said:
I am already paying it with much lower shell weight. Long barrel means a gain of a few lbs, indeed.
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Needs more development, see below.
The Germans solved the belt feed problem by using electric motors on the MG/FFM. The British Hispano already had an abundance of recoil energy as seen by the muzzle brakes on the drum feed guns. Take the muzzle brake off and used the recoiling parts to cock the springs in the belt feed system soaking up some of the recoil.

You can compare guns/ammo using similar weigh projectiles and varying the speed. However when trying to figure out recoil (or the energy in a recoil system) things get trickier.
Momentum is much more important than kinetic energy although we can figure that out if we know enough factors.
Guns come back (recoil) with the same force that projectile goes forward with. However that is measured in momentum.
Momentum is simply weight times speed.
Kinetic energy of the projectile (or the recoiling gun/parts) is weight X speed X speed.
If we want to get tricky we can figure in the weight of the propellent/gases X the escape speed of the gases to add into the projectile weight and speed to figure out the total momentum of the forward 'stuff' and then we can, by knowing the weight/mass of the moving parts figure out the speed of the recoil parts. There is going to be some excess force absorbed by mounting structure (or the shooters hand/shoulder).
Going back to small arms for a minute. The above mentioned 220 swift was rather close in power to the old 45-70 black powder military round.

Photo is is of an 7.62 X 51 NATO.
The 45-70 came in a wide variety of loads but a popular one was a 405 gain bullet at just under 1400fps. it's bullet was almost 8.5 times heavier than the 220 Swift and it had close to the same kinetic energy. However it had about 2.9 times the momentum and when fired out of the same weight rifle it had about 2.9 times the recoil (adjust a small bit for the difference in powder). Extreme example.

In the MG/FF-M you need a certain amount of momentum (force) to make the gun work, to little and the gun jams, fails to get enough travel on the moving parts.
Too much force and the gun breaks. The lighter, higher velocity ammo had about 80-84% of the recoil of the older heavy projectile ammo.
To keep the gun working you need to some of the speeds of the reciprocated parts the same. I called it a blow back earlier but the more correct name is API blowback and that requires a lot more things to balanced than a regular blowback in order to work. The firing pin is hitting the primer before the round is fully seated in the chamber and while the barrel is still moving forward.

If you want a longer barrel for more velocity that changes the weight of recoiling parts and the spring strength needs to be changed. You need a certain relationship between the weight and the spring because the force of the round going off has to stop the forward momentum , reverse it and accelerate to the rear and then slowdown stop at the rear (buffers help) so the bolt changes speed. At different points in time (travel) the weight has more or less influence than the spring.

The MG 151/20 ammo had almost 40% more room for propellent than the MG/FF ammo.

The fatter case adds quite a bit since the power space is below the projectile and forward of the case head, (ahead of the rim area)
I would also note that the 20 X 82 mm with 115G shell has 1-2% more momentum than the 134g 20 X 80 mm round.
The 20 X 82 MG 151 used short recoil and was less sensitive to recoil variation although many guns do have a band of performance that they work well in (don't jam) and is expanded at peril

The MG 151/15mm was already operating below the the 20mm MG/FFM in terms of recoil impulse momentum. Not a lot but it seems like a lot of effort for not much gain.
 
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