Number of engines on an aircraft

[Kryptid]

Here's something I've been wondering about for some time. Why has it historically been the case for medium-to-heavy fighters to have two engines instead of a single, larger engine? It seems like it should be possible to develop a single engine capable of generating sufficient thrust to power the aircraft (take a look at the monster engines that were proposed for the US SST program). Would they take up too much space in the fuselage? Is the survivability of two engines considered necessary over one?

The theory that I'm favoring at the moment focuses on cost and adaptability/availability. It seems to me like it would be more affordable to develop a smaller engine because (1) having two in each airplane means the order for engines is larger and development costs might be spread out more that way (2) having smaller engines means that other aircraft can potentially use them in addition to the aircraft they were originally developed for. If the F100 had been developed as a large engine to power the F-15, it couldn't have been used in the F-16, for example. Plus, using the same engine in multiple aircraft is cheaper than developing a new one for each plane.

Any thoughts?

 

[Stargazer]

I have another theory for you, much simpler.

Since bombers are subject to flak and the cannons of interceptors, having several engines makes it possible for an aircraft to return to home base even with one engine down...

 

[hagaricus]

Yeah, I sortof see this the other way around: That multiple engines giving redundancy & engine-out safety is the general & desirable rule, and only where another consideration, say weight, overwhelmingly dictates a single powerplant, is this used.

 

[BAROBA]

Also it make a more stable flightpath possible.
A fighter needs to be able to turn quickly,a bomber needs to stay level.

Just my 2 cents

 

[Avimimus]

In WWI two-seaters had their own class of larger engines (which often seem to have had worse power/weight ratios).

I'd suspect that in WWII, and the early jet era, the technology didn't exist to produce engines that were larger and sufficiently reliable. It is simply easier to use small components (with greater strength/force ratios for internal forces) and have some redundancy.

P.S.
Correct me if I'm wrong about internal forces effecting engine reliability with scaling.

 

[sferrin]

I have another theory for you, much simpler.

Since bombers are subject to flak and the cannons of interceptors, having several engines makes it possible for an aircraft to return to home base even with one engine down...

Or an even simpler. It's cheaper and easier to use the same engine for both, just use more of them on the bomber. That's how you get 8 J57s on the B-52 and one or two on fighters. Same with the XB-70/XF-108, etc.

 

[Bill Walker]

I think the driving factor has most often been engine availability. The preliminary designers comes up with a weight, based on meeting specifications for range, armament, etc, and then figures the horsepower (or thrust) needed to take this weight to the specification performance. the next step is to see what engine can produce this thrust. If you can't find one, you try two. If two won't do you might even look at three or four.

I remember seeing some papers long ago that showed that even in the 1930s designers were aware of the statistics about engine failure rates, and realized that having two engines did not produce a great increase in overall mission reliability. Your chances of an engine failure (for purely mechanical reasons) go up slightly with two engines, and the remaining engine is not always going to insure safe completion of the flight. (Hence the old joke: after the first engine fails, the purpose of the second engine is to deliver you to the crash site.) I suspect that many designers went to two engines somewhat reluctantly, and only because they couldn't find an off-the-shelf proven single engine of the required power.

The whole idea of multiple engines improving combat survivability seems to have only become important to designers (and specification writers) later in the Second World War.

 

[LowObservable]

If you look at engines for the same role at the same technology level, there is something of a square-cube law involved: the weight increases more than in direct proportion with the thrust. The practical reasons for this are numerous, but one factor is that blade aero speeds tend to have a max level. That means that larger-diameter engines run at lower RPM, so with more power + lower RPM you have a lot more torque = heavier shafts and bearings.

Note for instance that the EJ200 and F414 have higher T/W than the GE F110-GE-132, which is close to the same technology level.

Also, twins have some advantages from the configuration viewpoint. They are shorter (nice if you have a delta or canard) and since duct curvature limits are based on diameter, it's easier to incorporate serpentine ducts for reduced RCS (see Typhoon). There's also something to be said for having a central spine protected by the engines for your system runs.