So why the lack of three-engined fighters?

[Avimimus]

Yes, cost and maintenance of course. It is better to have two larger engines... so only on a few occasions were three engined proposals made during the Cold War (particularly for long range interceptors or during the very early jet era when there were technological lags).

But - what about WWII? There was the wartime rush, plenty of pressure for quick fixes. There was also a need for interceptors.

Imagine a twin boom design with an engine at the front of each boom and one pusher mounted on the rear fuselage.

You get:
- Excellent pilot visibility
- The benefits of a twin-boom design
- Shared frontal cross-section between all engines and internal volume
- A third engine to allow one or two engined cruise (without having thrust as far out on the wing)
- Center of gravity is still reasonably far forward (relative to a pure pusher)
- Plenty of excess power for climb/attack

So why can't I think of a single design?

 

[Steve Pace]

Wasn't the proposed USAF version of the USN Vigilante a three-engine design? -SP

 

[sferrin]

Wasn't the proposed USAF version of the USN Vigilante a three-engine design? -SP

Yep.

 

[Jemiba]

You could have used three low powered engines, but the effort for manufacturing them probably would be
higher, than for a single high power engine. May have been a way for countries like Italy, which had problems with
developing higher powered engines, of course.
And a fighter with three high performnace engines probably would have needed a heavy structure for the wings
AND for the fuselage. So it may have been fast, but heavy and probably just as agile as a heavy bomber.
And especially during wartimes, engines often were a scarce commodity, so a three-engined fighter may have provoked
some questions by those responsible for supply.

 

[Tailspin Turtle]

For one thing, for WW II fighters, think of the workload involved in air-to-air combat imposed by three throttles, mixture controls, RPM controls, cowl flaps, superchargers, oil cooler management, etc. and the increased likelihood of an engine failure, which would introduce a change in handling qualities and reduction in performance. (At some point with an increasing number of reciprocating engines, you add a flight engineer to manage them.) There is also a weight penalty of the additional set or sets of accessories and systems. Note that there weren't all that many twin-engine fighters, either; my guess is that the P-38 wouldn't have had two engines if there had been one available that provided almost as much power. Probably one reason that the twin-engine Grumman F5F lost to the Vought F4U was that the big R-2800 was successfully qualified. The attempt to take advantage of the power of two of them in the F7F wasn't very successful although some of that had to do with the introduction of the jet-powered fighters. Also see http://www.chuckhawks.com/twin_engine_fighter_planes.htm

 

[Sundog]

For WW2 aircraft, the other problem is integrating the armament. Besides everything already mentioned above.

As for jet fighters, don't forget the drag is directly proportional to frontal area. As such, integrating three engines is going to be problematic. But once again, there's the costs involved. Notice most passenger planes are going to two engines as well. You could design a three engined fighter, but it just hasn't been necessary, with the exception of Battlestar Galactica and the North American design already mentioned.

 

[Apophenia]

Are we only talking about completed airframes or the unbuilt-project end of things as well? At least one of the Piaggio P.125 iterations of the Santangelo Combattimento concept fits Avimimus' described layout -- twin-booms with engines at the front of each and a third engine in the tail of the central fuselage pod.

 

[lastdingo]

Propeller-age aircraft developers avoided having the propeller circles overlapping, except with few tandem installations (large distance between pull and push prop).
This means the wing-mounted engined of a 3-engined fighters would need to be farther out than usually (compare how close the propeller circles are to each other in the XF5F Skyrocket).

This reduces the agility around the longitudinal axis; the aircraft would roll as sluggish in both directions as a Camel against its torque.


It's rather more intriguing why there were no three-engined high speed long range photo reconnaissance aircraft.

 

[Paradaxos]

I've been thinking about a potential large-ish three engined, two seat carrier based fighter-bomber (more like a F-111 in flight regime perhaps). Each engine would, on its own be capable to making the aircraft fly below cruise speed while all three engines on at once would provide a high T:W ratio enabling the aircraft to launch from a ski-jump type carrier deck carrying a rather large payload.

The idea would be that the aircraft would launch from its flight deck and then select which engine to shut down based on its mission requirements, it would be large enough to fit quite a large quantity of internal fuel which could provide long loitering periods on a single engine while having provision for healthy quantities of stores. The idea being that this would be an aircraft that the Royal Navy could adopt as a carrier based, long range ground attack aircraft for the Queen Elizabeth class, making up for some of the deficiencies of the F-35B while still allowing the latter to live up to its strengths (it would certainly be a better close-in fighter and would have the stealth advantage if the mission required it)

The proposal here might result in a very heavy, not particularly maneuverable aeroplane but bringing a lot of capability in the ground attack and standoff air defence roles (a bit like the US Navy's misileer concept using AIM-120s) while retaining pretty decent range at cruise speed on one or two engines depending on payload.

I would if any of you think this is a plausible idea?

 

[Kryptid]

I've been thinking about a potential large-ish three engined, two seat carrier based fighter-bomber (more like a F-111 in flight regime perhaps). Each engine would, on its own be capable to making the aircraft fly below cruise speed while all three engines on at once would provide a high T:W ratio enabling the aircraft to launch from a ski-jump type carrier deck carrying a rather large payload.

The idea would be that the aircraft would launch from its flight deck and then select which engine to shut down based on its mission requirements, it would be large enough to fit quite a large quantity of internal fuel which could provide long loitering periods on a single engine while having provision for healthy quantities of stores. The idea being that this would be an aircraft that the Royal Navy could adopt as a carrier based, long range ground attack aircraft for the Queen Elizabeth class, making up for some of the deficiencies of the F-35B while still allowing the latter to live up to its strengths (it would certainly be a better close-in fighter and would have the stealth advantage if the mission required it)

The proposal here might result in a very heavy, not particularly maneuverable aeroplane but bringing a lot of capability in the ground attack and standoff air defence roles (a bit like the US Navy's misileer concept using AIM-120s) while retaining pretty decent range at cruise speed on one or two engines depending on payload.

I would if any of you think this is a plausible idea?

The two cold engines would generate a lot of ram drag since they aren't processing any air. If you could close their inlets off in a streamlined fashion in flight, that would help alleviate the drag problem. Another drag problem would arise from the nozzles, however. Without producing exhaust, there would be a low pressure region directly behind the nozzles. A fully-extensible plug nozzle might help relieve the issue. Then you'd have to find some way to make all of this low-observable. With all of the extra moving parts, that could be a challenge.

Otherwise, the idea of shutting down unused engines in flight for increased efficiency does have merit. I recall Dan Raymer mentioning one such example in his design book. He posited a VTOL transport aircraft which used 4 turbofans which had enough power to lift it vertically but had too much excess power in cruise flight. For that reason, each engine would have to run at an unrealistically low power setting which would harm specific fuel consumption. He stated that shutting 3 engines down in cruise flight would be a preferred solution (assuming drag issues could be solved).

 

[F-14D]

Actually, I don't think the three engine concept would be that viable, especially in a tactical jet. You'd have engine(s) that would only be used part of the time but you'd always have to lug around the weight, space and complexity of the extra engine(s) and the associated plumbing, controls, structure and you'd have to have the space for the extra engine. That space could be more efficiently used for payload, fuel or just eliminated altogether. The A-6 had really great range and it did it on two relatively thirsty turbojets. This is why lift + lift/cruise never gained traction for V/STOVL. The lift engines were more efficient than oversizing the main engine to do it all, but the penalties they brought with them more than offset their advantages. Given engine technology today, if you're designing from scratch, you might as well go for the twin, or even a single. Otherwise, design you a/c around an existing engine, of which there are many excellent ones.

The cases where 3 engined tactical jets were proposed, the XB-51 and NR-349, that was more a function of no suitable sized engine being available for a twin layout that could meet the needs of the design. You'd be better off just sticking with a twin. Actually, by the time we got to the F-4, one of the biggest reasons for going with a twin over a single was again that given the performance and payload required, there weren't many engine of the necessary performance available for a single engined aircraft. Still, if you looked at the performance of the XF4H and XF8U-3, they were pretty close. The single engined XF8U-3 actually exceeded the performance of the F4H in some areas (though not as much as legend has it) and actually had longer range with the baseline payload, but it wasn't as versatile and didn't have as much growth factor.

Regarding the F-35B, keep in mind the Marines note that it's got a range greater than that of the Super Hornet with comparable loads. Not sure what penalties it pays relative to a tri-engined fighter, given the really high fuel burn of the latter and the penalties I mentioned above. Consider the fuel burn used in a waveoff by a three engine fighter with all burning at the max. Plus, the F-35B doesn't need to reserve as much fuel for overhead loiter, waveoffs and bolters as would the tri-fighter.

 

[pathology_doc]

Of course the other thing to consider is that back in the early days of the high-powered, rapid-reaction jet interceptor, the additional engine (and sometimes two) was, as often as not, a rocket. As history actually worked out, of course, it would have been a waste - nothing ever happened for which the rocket-boosted interceptors would have shone, and nothing was ultimately lost by not having them (with the exception of the Me163, of course, which was actually built and flown and saw combat) - but the designers couldn't have known that.


The rocket avoids the whole ram-drag problem and, depending on the mission profile, also the nozzle base drag issue (on the outward-bound intercept phase, that rocket is almost certainly going to be lit up for the climb to height or straight-line speed). What happens on the way back home when the rocket fuel is all gone and you're carrying around the dead weight of the engine and associated systems isn't as important (assuming there's a home to go back to, which there arguably might not have been if such fighters had ever been built and called upon to do what they were designed for).

 

[riggerrob]

Dear Kryptid,
Lockheed P-3 Orion have 4 turboprop engines, the same Allison T56 engines as the C-130 transport. They use all 4,000 hp. for take-off and high-speed transit to the search area, then often shut down one engine to increase low-and-slow loiter time. Most often they shut down the number 1 engine (left, outboard) because it has no generator and the lack of exhaust plume improves visibility of left rear observer. Depending upon the mission, weight, weather, etc. they might also shut down the number 4 engine (right, outboard) to conserve more fuel.

 

[BullpupRafale]

I think F-5E can be used as example since "economic" cruising thrust of each engine was 8kN for total of 16kN hence 50% of thrust that considerably impacts specific fuel consumption ratio of lbf generated per lbs of fuel injected.

For that reason a third engine would make sense if you want greater endurance and range when cruising truly economically.

Another is that you won't need afterburner, hence F-5E could have supercruise capability.

 

[iverson]

Three engines do not make a lot of sense unless you just can't get the required power from one or two.

A three-engined airplane that cannot maintain flight on a single engine less reliable/survivable than a twin but three times as complex. A three-engined airplane that can fly on one engine is three ties as reliable/survivable but over-powered, over-comlicated, and overly expensive.

As I recall, North American based contender for the Improved Manned Interceptor competition on the Vigilante because it was the only modern North American airframe available at short notice. But in its stock form, the Vigilante was unable to compete with the YF-12 or even improve upon the F-106. So North American did what it could and fitted the third J-79 to get the power that it could not get from two, accepting the drawbacks of the layout in order to have some to offer the Air Force.

 

[BlackBat242]

 

[Archibald]

Never knew that production of RA-5C ended so late: November 1970.

 

[Scott Kenny]

Dear Kryptid,
Lockheed P-3 Orion have 4 turboprop engines, the same Allison T56 engines as the C-130 transport. They use all 4,000 hp. for take-off and high-speed transit to the search area, then often shut down one engine to increase low-and-slow loiter time. Most often they shut down the number 1 engine (left, outboard) because it has no generator and the lack of exhaust plume improves visibility of right left rear observer. Depending upon the mission, weight, weather, etc. they might also shut down the number 4 engine (right, outboard) to conserve more fuel.

I've seen a P-3 with 3 engines shut down before. Way out in the middle of the Pacific. And they used to shut down all 4 at air shows till the safety nannies complained about it.

Of course the other thing to consider is that back in the early days of the high-powered, rapid-reaction jet interceptor, the additional engine (and sometimes two) was, as often as not, a rocket. As history actually worked out, of course, it would have been a waste - nothing ever happened for which the rocket-boosted interceptors would have shone, and nothing was ultimately lost by not having them (with the exception of the Me163, of course, which was actually built and flown and saw combat) - but the designers couldn't have known that.


The rocket avoids the whole ram-drag problem and, depending on the mission profile, also the nozzle base drag issue (on the outward-bound intercept phase, that rocket is almost certainly going to be lit up for the climb to height or straight-line speed). What happens on the way back home when the rocket fuel is all gone and you're carrying around the dead weight of the engine and associated systems isn't as important (assuming there's a home to go back to, which there arguably might not have been if such fighters had ever been built and called upon to do what they were designed for).

Agreed, the place the 3-engined fighters make the most sense is as an interceptor. Rocket on for takeoff and climb, plus as much speed as you can get horizontally. Though I wonder just how long most of the rocket assisted interceptors would burn, everything I've read has suggested 120sec or so.