Why no three engined heavy fighters in WW2?

[Elfnet]

Maybe it is a trivial question, but hear me out.

Before WW2, multiple countries designed/built fast, 2-engine heavy fighters (like the Italian IMAM Ro.57).

Italy, as many smaller nations, struggled because after the war broke out, they couldn't mass-produce new, high-performance aircraft engines (in 1939, the Ro.57 was the fastest Italian plane but only had two 870 HP Fiat engines), and they couldn't replace them on the Ro.57 until the Germans gave them their DB 601 engine, which still only produced 1175 HP in 1942.

For me, the solution seems simple: just put a third engine in the nose. Obviously, amongst other drawbacks, this would have reduced the range, but since Italy almost exclusively fought at short range (over the Mediterranean Sea and the coastlines) and had almost no strategic bombing capacity anyway, it didn't really matter. The same is true for the other small aircraft-producing European countries.

In fact, the builders of the Ro.57 planned a similar parallel project called IMAM Ro.67 with exactly this layout (basically a 2 engine heavy fighter with a 3rd engine in the nose). Two prototypes were greenlit by the Italian government, but after the war broke out, the company got the aformentioned German engines, so they built a boring 2 (German) engine version of the Ro.57 (IMAM Ro.58) instead and stopped the 3 engine Ro.67 project, and as far as I know, they never actually built any 3 engine heavy fighters.


My question is: why? Obviously there must have been serious downsides, but I couldn't see any fatal flaws (like the layout was well understood, not something like the coupled engines as in the German Heinkel He 177, the 3rd engine's added cost would have been offset by the significantly higher survivability, and so on).

Obviously, it would have been a wartime necessity (this is why the great powers simply built bigger engines), and I'm mainly a tank guy who doesn't know much about aircraft, but in WW2, multiple countries made two-engine tanks, like the United Kingdom (with two separate engines) and Hungary (with coupled engines), while Australia even built a three-engine tank called the AC3 "Thunderbolt" (it was only a prototype because soon after they got American Shermans).

Italy and the IMAM Ro.67 were only examples, but not a single country mass-produced any three-engine heavy fighters in WW2 (despite building other three-engine aircraft), so it is clear that this would have been a bad idea in practice, but I can't wrap my head around it (seeing the three-engined German Blohm und Voss BV 138 or the half dozen different Italian three-engine medium bombers).

 

[Nahida1645]

Germany had the tri-engined Ju 52 variant and Italy produced bombers of the same engine count.

I think tri-engined fighters would have more weight, fuel consumption rate, maintenance and even obstructing a pilot's view despite increased speed when realized in the Second World War, and could be unreliable due to the issues above, and could also trigger shortages on engines and engine parts.

 

[riggerrob]

…. Obviously, it would have been a wartime necessity (this is why the great powers simply built bigger engines), and I'm mainly a tank guy who doesn't know much about aircraft, but in WW2, multiple countries made two-engine tanks, like the United Kingdom (with two separate engines) and Hungary (with coupled engines), while Australia even built a three-engine tank called the AC3 "Thunderbolt" (it was only a prototype because soon after they got American Shermans). …..

As a “tank guy” you should know that the ultimate in complex tank engines was the Chrysler multi-bank engine installed in some M3 Lee and M4 Sherman tanks.
When Wright, Continental, Pratt & Whitney, etc. hold not meet demands for increased production of air-cooled radial engines, Chrysler took 5 stock, 6-cylinder engines and mated them to a combining gear-box.
The US Army recognized that synchronizing 30 cylinders would become a maintenance, repair and overhaul nightmare as engines aged, so - immediately after WW2 - they “gifted” all their multi-bank engines to allies. Post-war, the US Army only kept M4A2E8 Sherman’s in service through the Korean War.

 

[Dilandu]

Mostly because such plane would be rather heavy, and while fast - very hard to maneuver. It would simply not be agile enough to fight any kind of dogfight, and probably would not be able to sucseed even in hit-and-run attacks.

 

[iverson]

We dicussed the limitations of trimotors in the context of reliability once before (I can't remember which thread). But essentially, it is a matter of diminishing returns. Adding a third engine increases the power less than it increases weight and fuel consumption (as noted above). But it also adds complexity without adding redundancy. Three engines are more likely to fail than two. So if the required minimum performance can be met with an engine out, there is no need for the third. If it can't, the third engine is not much of an advantage.

 

[Elfnet]

I don't think the Chrysler A57 is a good parallel, because in the end it was a single engine but also a very successful design with thousands produced, while something like the Hungarian 44M "Tas" was truly two engines side by side with all its problems (also, it never entered production, despite the strong incentives).

Despite the site's direction, my question isn't whether something ridiciously impractical could be made (like the Lycoming XR-7755, with 9 banks of four-cylinder engines), because it could, but a small country would never ever made something like this without the industrial might of a great power.


I don't see the reliability as a huge problem; sure, the failure rate must have increased, but if there were no other means of increasing power, the options would have been a very fast plane with less reliability or getting killed in a slow two-engine plane (that is also not extremely reliable), while the 3-engine design could have flown with two (or maybe even one) engines.

I also doubt the obstruction of the pilot's view would be a big problem because several planes did exist with this layout without problem in this period (for example, the Bellanca 28-92 "Trimotor", a race plane just before WW2). It is clearly suboptimal but would have been a wartime emergency problem.

I won't see the added weight and decreased maneuverability as big problems either. In WW2, some heavy fighters (like the 10+ ton Northrop P-61 "Black Widow") were barely more than flying bricks, while the aforesaid IMAM Ro.57 (with two engines) had the length and weight of a single-engine fighter (I mean, a 3.1 ton heavy fighter where 1/3 of that was the engines?!)


Also, in the latter half of the 1930s, Italy (as many other countries) tried the universal aircraft concept (a heavy fighter that is also a reconaissance plane and light bomber, and dive bomber), but all 3 types (Breda Ba.88, Fiat CR.25, and IMAM Ro.57) failed because they couldn't be all things at once (the Ro.57 was a fighter but too small for bombs, while the Ba.88 is more of a bomber with less manuverability).

In my layman mind, this problem could have been solved with the 3-engined Ro. 67 or something like that, because it would have provided the bigger plane with the much-needed additional power.


I brought up Italy as an example because, while most countries didn't build many trimotor military planes in the 1930s and 40s, Italy fielded several different designs in this period, thus providing not only the incentive but the needed expertise, and some of them, like the CANT Z.507 and Z.1007, were regarded as successful, high-performance types where visibility, maintenance, or cost weren't the problem (only the low reliability and other issues not related to the 3rd engine).


I understand that 3 engined planes are almost always worse than a "normal" 2 engined one, but WW2 was an extraordinary circumstance where the losing side tried to field much worse planes in desperation (like the German wooden jet Heinkel He 162 or the Japanese suicide rocket plane Yokosuka MXY-7). Compared to them, the 3-motor concept seems relatively logical.

My only guess is that the window where the 3-engine plane was already badly needed but the more powerful engines were not yet available was too short, like in Italy's case, where as soon as the German engine became available, it killed the demand because it solved the problem much better.

 

[Scott Kenny]

Maybe it is a trivial question, but hear me out.

Before WW2, multiple countries designed/built fast, 2-engine heavy fighters (like the Italian IMAM Ro.57).

Italy, as many smaller nations, struggled because after the war broke out, they couldn't mass-produce new, high-performance aircraft engines (in 1939, the Ro.57 was the fastest Italian plane but only had two 870 HP Fiat engines), and they couldn't replace them on the Ro.57 until the Germans gave them their DB 601 engine, which still only produced 1175 HP in 1942.

For me, the solution seems simple: just put a third engine in the nose. Obviously, amongst other drawbacks, this would have reduced the range, but since Italy almost exclusively fought at short range (over the Mediterranean Sea and the coastlines) and had almost no strategic bombing capacity anyway, it didn't really matter. The same is true for the other small aircraft-producing European countries.

In fact, the builders of the Ro.57 planned a similar parallel project called IMAM Ro.67 with exactly this layout (basically a 2 engine heavy fighter with a 3rd engine in the nose). Two prototypes were greenlit by the Italian government, but after the war broke out, the company got the aformentioned German engines, so they built a boring 2 (German) engine version of the Ro.57 (IMAM Ro.58) instead and stopped the 3 engine Ro.67 project, and as far as I know, they never actually built any 3 engine heavy fighters.


My question is: why?

Where do you put the guns?

 

[Fluff]

Maybe it is a trivial question, but hear me out.

Before WW2, multiple countries designed/built fast, 2-engine heavy fighters (like the Italian IMAM Ro.57).

Italy, as many smaller nations, struggled because after the war broke out, they couldn't mass-produce new, high-performance aircraft engines (in 1939, the Ro.57 was the fastest Italian plane but only had two 870 HP Fiat engines), and they couldn't replace them on the Ro.57 until the Germans gave them their DB 601 engine, which still only produced 1175 HP in 1942.

For me, the solution seems simple: just put a third engine in the nose. Obviously, amongst other drawbacks, this would have reduced the range, but since Italy almost exclusively fought at short range (over the Mediterranean Sea and the coastlines) and had almost no strategic bombing capacity anyway, it didn't really matter. The same is true for the other small aircraft-producing European countries.

In fact, the builders of the Ro.57 planned a similar parallel project called IMAM Ro.67 with exactly this layout (basically a 2 engine heavy fighter with a 3rd engine in the nose). Two prototypes were greenlit by the Italian government, but after the war broke out, the company got the aformentioned German engines, so they built a boring 2 (German) engine version of the Ro.57 (IMAM Ro.58) instead and stopped the 3 engine Ro.67 project, and as far as I know, they never actually built any 3 engine heavy fighters.


My question is: why?

Where do you put the guns?

That’s the right question.

Also why you can have a trimotor bomber, but not a fighter.

 

[Aubi]

Yeah, guns. Remember that one of the advantages of twin-engined fighters was that they could put the armament in the nose, and that it could be heavier than most guns mounted in wings. And tightly grouped for better effect.

 

[Scott Kenny]

Yeah, guns. Remember that one of the advantages of twin-engined fighters was that they could put the armament in the nose, and that it could be heavier than most guns mounted in wings. And tightly grouped for better effect.
(Image deleted for space)

And more importantly, guns in the nose you don't have to set to converge, so they can hit a lot further away.

 

[Silencer1]

First disadvantage IMHO of the 3-engined aircraft (of classical layout) was the limited forward field of view. Air-cooled engine became even more larger obstacle for the pilot' visibility. And, in the perspective, such layout also prevents to use a nice for radar antennae installation.

One more thing: the list of available aircraft engines, according to their power, has been tended to the be high as possible. I mean, that aircraft designers could easily use, say, "Merlin" to the single-engine "Spitfire' or twin-engine "Mosquito". Although, what aircraft needs 3 "Merlins"? Add one more "Merlin" - and you produce heavy bomber. Or subtract one engine - and you produce heavy fighter.
I only could remember medium-power "Peregrine" - and still, no one decide to use three of them to mount on combat aircraft.
So, my opinion, is that aircraft designers tries it best to build their high-performance aircraft around high-performance engines, but not try to design aircraft around three engines.

 

[pathology_doc]

Maybe it is a trivial question, but hear me out.

Before WW2, multiple countries designed/built fast, 2-engine heavy fighters (like the Italian IMAM Ro.57).

Italy, as many smaller nations, struggled because after the war broke out, they couldn't mass-produce new, high-performance aircraft engines (in 1939, the Ro.57 was the fastest Italian plane but only had two 870 HP Fiat engines), and they couldn't replace them on the Ro.57 until the Germans gave them their DB 601 engine, which still only produced 1175 HP in 1942.

For me, the solution seems simple: just put a third engine in the nose. Obviously, amongst other drawbacks, this would have reduced the range, but since Italy almost exclusively fought at short range (over the Mediterranean Sea and the coastlines) and had almost no strategic bombing capacity anyway, it didn't really matter. The same is true for the other small aircraft-producing European countries.

In fact, the builders of the Ro.57 planned a similar parallel project called IMAM Ro.67 with exactly this layout (basically a 2 engine heavy fighter with a 3rd engine in the nose). Two prototypes were greenlit by the Italian government, but after the war broke out, the company got the aformentioned German engines, so they built a boring 2 (German) engine version of the Ro.57 (IMAM Ro.58) instead and stopped the 3 engine Ro.67 project, and as far as I know, they never actually built any 3 engine heavy fighters.


My question is: why?

Where do you put the guns?

That’s the right question.

Also why you can have a trimotor bomber, but not a fighter.

Even the trimotor bombers have issues when it comes to forward defence. You can only fit a limited amount of it, and there are arcs which are forbidden to those guns.

 

[GTX]

Weight + cost vs performance

 

[Elfnet]

Where to put the guns:

According to this forum thread, the tri-motor IMAM Ro.67 would have three 20 mm cannons, each firing through the propeller hub of their German DB 601 engines (the user "Flitzer"s illustration is also based on this):

Italian projects of WWII

This would have been an insanely powerful armament in 1939 (early BF 110s had only 4 machine guns; half of them were switched to 20 mm cannons later). The only stronger I could think off the top of my head was the British Westland Whirlind (with four 20 mm cannons) but only in 1940.

BUT
I'm highly doubtful about this claim (and thus arrangement) since the Ro.67 was terminated in early 1940, before the Germans gave their V12 engine to Italy. Without the DB 601, Italy would have used radial engines (most likely the Fiat A.74), so they couldn't have used the hub-guns.


Because their real IMAM Ro.57 used two 12.7 heavy machine guns (despite being a heavy fighter), I presume a tri-motor heavy fighter would also used these weapons. It might look abysmally weak, but remember, Italy basically stopped aircraft development in 1943, so the design phase would have been done between 1936-42, and the two heavy machine gun armament was the weapon of choice on basically every single Italian plane at the beginning of the war.

The Ro.57 (with 2 engines) had the machine guns in the empty nose, but since these guns could be synchronized (they were used on single-engine fighters), I think they would have put two 12.7 mm heavy machine guns on the top of the central engine.
Also, later the IMAM Ro.58 (Ro.57 with two DB 601 engines) had belly-mounted cannons, so they could have +2 heavy machine guns there. Also-also, it might have looked silly, but they could have simply added syncronized machine guns to the wing-mounted engines, for a total of up to 12 or so heavy machine guns. The Americans would have been jealous.

Other possibilities include cannons under the wings; I don't know of a single Italian fighter in this timeframe that used this arrangement, but it wouldn't have been a hard task (as other countries used this).

The third option is inside the wings. While the aforesaid Ro.57 had thin wings, the Ro.67 was a completely different design, and the Series III of the Macchi C.205 (a single-engine fighter) had wing-mounted 20 mm cannons, so it wouldn't have been unheard of in Italy.

I think the most likely arrangement would have been two 12.7 mm Breda-SAFAT heavy machine guns on top of the central engine.
The 3rd engine's main advantage would have been high top speed and possibly a higher bomb load, not more firepower.


I don't think the added cost would have been a big deal since Italy built so many types of three-engine planes. As the example with the Hungarian tank's two engines, it was a horrible idea, and they didn't want to use it, but since no alternatives were available and the vehicle was badly needed, they improvised.

Also, the Italians weren't afraid of experimenting, even before 1943: they built planes like the mid-engine Piaggio P.119 or the motorjet Caproni Campini N.1., so I don't think putting cannons on a tri-motor heavy fighter would have been impossible.

 

[Scott Kenny]

There are some specific limits on which weapons can be synchronized to fire through a propeller arc. The weapon needs to fire from a closed bolt, like the Browning MGs. This allows the synchro gear to prevent the gun from firing if a propeller blade would be in front of the muzzle.

 

[Arjen]

An impractical suggestion: pusher propellers. Will allow lots of forward firing guns.
Face it. A trimotor fighter, propeller driven, big and clumsy, is as useful as an ashtray on a motorbike.

 

[Nicknick]

Most is allready said and I agree to the point that having centrally mounted weapons without any limitations by the prop is a hughe advantage of a twin engine design. It might have been usefull to use a third engine in the fuselage to drive the supercharger for the outer engines, this solution gives less redundancy, but is very efficient when it comes to variable supercharger speed (the Russians had such designs for bombers). Integrating the cahrge air coolers in the fuselage could also be more aerodynamically efficient that putting it in the engine nacells.

 

[Scott Kenny]

Most is allready said and I agree to the point that having centrally mounted weapons without any limitations by the prop is a hughe advantage of a twin engine design. It might have been usefull to use a third engine in the fuselage to drive the supercharger for the outer engines, this solution gives less redundancy, but is very efficient when it comes to variable supercharger speed (the Russians had such designs for bombers). Integrating the cahrge air coolers in the fuselage could also be more aerodynamically efficient that putting it in the engine nacells.

IIRC the Germans used a third (or 5th?) engine for some of their specialized high altitude aircraft. And technically you could put an intercooler in the wing between the extra engine's supercharger and your propulsion engines.

 

[Arjen]

I would not describe any of the WW2 aircraft with a separate, dedicated engine for supercharging as fighters.

 

[Nicknick]

@Arjen You might have realized, that we are talking about fighters which might have been, since we are lacking three engine fighters in the real world...

 

[Arjen]

I had realized that, thank you for your concern. In my view, a WW2 aircraft with a separate, dedicated engine for supercharging is marginally suitable as a bomber or a reconnaissance aircraft, wholly unsuitable as a fighter. The Pe-8 dropped the concept entirely after two prototypes, the Do 217P never made it past pre-production, Hs 130s were likewise built in single-digit numbers, and had the SNCAC NC.150 reached production, it was planned to have supercharged engines.

since we are lacking three engine fighters in the real world...

With good reason.

 

[Justo Miranda]

Hi

 

[Nicknick]

So you need to accept, that we talking about fictional fighter concepts. Instead of just saying that a third engine for superchargers would have been worse for fighters than for bombers, you should give reasons for it. A third engine in the fuselage is more disturbing in a bomber (bomb bay!) than in an otherwise twin engine fighter like the (not so successful) BF110.

Twin engine fighter are pretty lame around the longitude axis, this is the main reason why the concept failed in most cases, and a third engine wouldn't have change much about it. It might have been an option for high altitude fighters, which don't require a lot agility but a much supercharging to shot down high flying bombers.

The only successful twin engine fighter in WW2 which comes into my mind was the P-38, which was designed with turbocharging in mind. The twin boom design was chosen to enable enough room for the Turbo supercharging system, so this was never a candidate for third engine driving the supercharger. This was done for a long range and enabled also a lot of firepower, but it only worked with new tactics (Thatch weave).

 

[Justo Miranda]

So you need to accept, that we talking about fictional fighter concepts. Instead of just saying that a third engine for superchargers would have been worse for fighters than for bombers, you should give reasons for it. A third engine in the fuselage is more disturbing in a bomber (bomb bay!) than in an otherwise twin engine fighter like the (not so successful) BF110.

Twin engine fighter are pretty lame around the longitude axis, this is the main reason why the concept failed in most cases, and a third engine wouldn't have change much about it. It might have been an option for high altitude fighters, which don't require a lot agility but a much supercharging to shot down high flying bombers.

The only successful twin engine fighter in WW2 which comes into my mind was the P-38, which was designed with turbocharging in mind. The twin boom design was chosen to enable enough room for the Turbo supercharging system, so this was never a candidate for third engine driving the supercharger. This was done for a long range and enabled also a lot of firepower, but it only worked with new tactics (Thatch weave).

Black Widow???

 

[Justo Miranda]

Me.262 Heimatschützer I

 

[Nicknick]

Well, yes, but the black widow was a night fighter, in that role, also the BF 110 had some success. Night fighters don't rely on manoeuvrability so much, but more on fire power and radar systems, so they differ very much from conventional fighters.

 

[Arjen]

Desirable characteristics for a WW2 fighter:
- excess power (which can be achieved by brute force (P-47) or lightweight construction (A6M))
- agility (can be compromised by brute force approach)
- guns sufficient to deal with bombers (can be compromised by lightweight construction)
Single engine fighters rule. A6M, Spitfire, Fw 190, Mustang, La-7, G.55. There are niche opportunities for long-range and heavy fighters: P-38, Mosquito FB. Twin-engine night-fighters: Mosquito NF, He 219, P-61.
None of the above, with the P-61 a possible exception, have space for a separate supercharger engine AND fuel. Show me a WW2 aircraft that has space for a separate supercharger engine AND fuel, and it will probably be a bomber with a big fuselage. Or a night-fighter adaptation of one of the German bombers.
All four WW2 aircraft types - all bombers or reconaissance aircraft - that had a separate supercharger, had it for high-altitude operations. None reached production with a separate supercharger engine in place: better options were available for high-altitude flight. Mechanical superchargers, turbochargers.
Lugging that iron lump along in the fuselage will be the death of any WW2 fighter.

 

[Nicknick]

Read my text and you will find out, that I mentioned the P-38 as the only successful multi engine fighter. All the other examples were night fighters, because, as I said before (thanks for repeating it…), they lack maneuverability…

I also mentioned the possibility to use such a configuration for high altitude planes, despite I’m aware, that turbochargers are the better option, but those weren’t ready for production until the late war (at least for the Axis).

The early turbochargers needed terrible lot of space, that’s why the P-47 was such a big fat plane and why the P-38 had two booms which were filled with turbocharging stuff. Despite the thermodynamic benefits, even high-altitude fighter like the Ta152were equipped with mechanical charging systems. A separate engine driving the supercharger would have eliminated or at least simplified the gear system and enabled a smooth variable speed supercharger without the losses of throttling or an hydraulic clutch.

Please show me were exactly the BF 110 stored its fuel in the fuselage and why no charging system could have been integrated inside.

 

[Desertfox]

Technically the XP-58 Chain Lightning was a four-engined! fighter. Your best bet for a three engine fight could be a P-38 derivative with a third pusher engine behind the cockpit, or sticking another engine into the XP-54 Swoose Goose.

 

[Jemiba]

I know, it's just a trick, as are piston engined types with additional jet or rocket engines, or twin
engined fighters with the third engine driving the supercharger, but ... the Leduc 011, proposed in 1939.
Maybe comparable to a conventional twin engined type, with an additional engine, driving a pusher prop
for achieving low fuel consumption/longer range.
(from "Rene Leduc-Pionier De La Propulsion A Reaction", Jean Lacroze, Philippe Ricco)

 

[Nik]

A couple of thoughts...
Unless have enough through-hub guns and/or interrupter gear, three front engines would only suit a 'turret fighter'.
( 'Too few' front-firing guns are only good for dissuading fighters rather than culling bombers... )
Upside, perhaps thus fast enough to climb, chase, catch and kill bombers. Down-side, better ways to do such: Thee engines give you three single-engined fighters, or one and a twin...

Need all three front engines be same ? Logically, the wing-engines should match, but nose engine may be smaller / larger, perhaps with prop-feathering. Perhaps turning opposite to the wing engines. Perhaps shut down nose engine for loiter / cruise...

Regarding an in-fuselage engine, I'm reminded sub-hunting and mag-mine sweeping fighter-bombers had a rather big generator in the bomb-bay to drive Leigh-light or sweep coil.

IMHO, a third 'mid-engine' driving nose and/or tail prop would introduce ghastly 'long drive-shaft' issues that thwarted umpteen designs...

A twin-boom configuration with two 'pullers' and a central pusher might work, sorta-kinda, leaving nose free for armament...

 

[sienar]

Power to weight ratio of late war fighters assuming 200lb pilot, full ammo and fuel equal to ~1hr (max hp x 0.55). EDIT: twins are BOLD

3.36​	Yak-3U​
3.44​	Hornet​
3.66​	Ki-83​
3.89​	Spitful​
3.94​	Ki-96​
3.97​	La-9​
4.05​	XP-72​
4.07​	Yak-3​
4.07​	A7M2​
4.13​	BF.109K-4​
4.25​	Ta-154​
4.28​	F8F-2​
4.35​	CA-15​
4.39​	Ki-100​
4.41​	I.Ae. 30​
4.56​	Tempest​
4.57​	F7F-1​
4.58​	J7W1​
4.64​	J5N​
4.67​	Yak-9P​
4.72​	MC.205​
4.77​	P-38L​
4.85​	Ki-94​
4.89​	Do.335​
4.89​	Re.2005​
4.9​	G.55​
5.11​	Ta-152​
5.42​	MB.5​
5.54​	P-51H​

Some late war twins are actually very competitive on these terms. Conceivable a 3 engine aircraft could be competitive in power/weight terms too.

 

[Arjen]

The whole point of three engines would be to compensate for your engine makers not being able to produce engines with enough power for a twin engine installation to suffice. Three tractor propellers pose a challenge to forward firing guns. One or more pusher props offer a way out for the guns - tricycle landing gear would be advisable - but if every engine has its own prop, with three engines spread spanwise you're left with quite a bit of rotational inertia. This impacts on agility. The dedicated supercharger engine sans prop adds power at high altitude, but the French, Germans and Soviets only produced such aircraft in single-digit numbers. Here I have to speculate on possible causes:
- too heavy
- overly complicated, unreliable
- fuel hungry
- better, cheaper alternatives: engines with integrated superchargers such as installed in Pe-8 production aircraft

Whatever the reasons, the idea wasn't pursued further by any of the three nations.

My instinct is that a twin engine fighter with a third, separate engine for supercharging will be overly complex, too heavy, thus fuel hungry and a dog to fly. Better to concentrate on lightweight construction (see A6M) and/or aerodynamic efficiency to compensate for low power engines. Or go the Italian way and scrounge higher power engines from an ally.

 

[Arjen]

From OKB Tupolev by Yefim Gordon and Vladimir Rigmant, Midland 2005 on the ANT-42, later redesignated TB-7, then Pe-8:

As the the ANT-42 was to be a high-altitude machine, special attention had to be given to increasing the altitude capability of its engines. The lack of developed and reliable turbocompressors for aircraft engines in the USSR now led to the installation of a fifth engine in the upper centre fuselage, an M-100 [licence-built Hispano-Suiza 12Y] driving a special ATsN supercharger to supply the four Mikulin AM-34FRN wing-mounted engines.
[...]
As series production of the ATsN could not be organised, Factory No. 124 was only able to fit these units to the first four machines. It was decided to build the TB-7 without this vital component, although without it the machine lost its major advantage of high speed at high altitude.

Production was halted and restarted in 1940, a small run was delivered with Charomskiy turbocharged diesels. In parallel, other TB-7s were fitted with AM-35A engines.

The diesel engines proved unreliable operationally, and from 1941 all new TB-7s were fitted (and existing examples re-engined) with AM-35As. In 1942 the factory began fitting 1,330/1,700-hp Shvetsov M-82 radials.

93 Pe-8s were produced.

 

[Nicknick]

It is easy to find reasons and evidence against multi engine fighters, but to give this discussion any sense, we should try to find good arguments for three engine fighters.

For single engine fighters, a separate charging engine would make little sense, because it eliminates the charging system of only one engine. The numbers of twin engine fighters of WW2 is extremely small and even much smaller if we take out the turbocharged P-38, which, for obvious reasons, is not a candidate for a separate charging engine. Because of that, there are only few possible applications.

The Mosquito could have been a suitable candidate, here the superfluous bomb bay for fighter applications could have been used to install a third charging engine plus charging system, thus increasing the power output of the existing engines without increasing the mechanical and thermal loads. The high altitude capabilities could have been increased significantly. Despite that, there wasn’t really a need to do so, it already could intercept any propeller driven German bomber but even with a third engine, it wouldn’t have been able to intercept jet planes.

 

[Arjen]

High-altitude Mosquito PR.32 and PR.34 had 1,690hp RR Merlin 113/114 engines, RR's mechanical superchargers were up to the job the Soviet ATsN was designed for. To RR, the ATsN was a solution looking for a problem.
The USSR developed the mechanically supercharged Mikulin AM-35 used by the high-altitude Pe-8 and MiG-3.

 

[Nicknick]

Taking the power load for the supercharging away from the main engines, would still have improved the performance significantly. The variable drive which is enabled by an extra charging engine makes it also more efficient.

If there would have been a demand for an ultra high flying and even faster Mosquito, this would have been a suitable approach with relative litte development effort.

 

[Arjen]

Or build the DH.101 with 2 x Sabre, alternatively the DH.102 with 2 x Griffon. And be overtaken by jets.

 

[Nicknick]

The Sabre wasn't equiped with a charging system for high altitudes, a third engine for charging would have made a nice combination with it....

As I mentioned before:

"(...) but even with a third engine, it wouldn’t have been able to intercept jet planes"

thank you again for repeating it...

 

[Arjen]

You're welcome