michaelemouse wrote:Fighters with 2 engines are generally higher performing than single-engined fighters of the same generation. Has anyone tried making fighters with more than 2 jet engines in the last 50 years? Wouldn't more engines allow a large plane with more speed, range, payload, perhaps altitude? Engines do cost a high amount but looking at, say, the F-35, it doesn't seem like adding a second engine would have increased the cost by more than 10% while it would have definitely improved many performance metrics by more than that.
Relatedly, how much can fighters be scaled up? If we compare fighters like the F-15 with the F-22 or the F-16 with the F-35, fighters have been getting bigger and heavier. How much can they be scaled up and at what point would returns on size and weight diminish into the negative? Could 6th generation fighters have 40, 50 ton loaded weight and 50, 60 ton max. takeoff weight?
Back up a bit, what do you mean by "higher performance?"
If you mean that twin-engine fighters can generally fly faster, then, yes, I agree. If you mean that twin-engine fighters can generally fly further, then, yes, I agree. If you mean that twin-engine fighters generally have better instantaneous and sustained turn rates, then, no, I disagree.
The reason why the above is true has to do with scaling. Imagine that you can just freely scale an aircraft up or down in size. We'll ignore the fact that real engineering doesn't work this way at all, just imagine you could. The entire plane, wings, fuselage, engine are all enlarged or shrunk by some linear factor.
The top level speed of an aircraft is the speed at which the force of drag (which increases with airspeed) matches the force of thrust provided by the engine. Since the forces are equal and in opposite directions, they cancel out and the aircraft cannot accelerate anymore.
What happens if we take a fighter and scale it up? The force of drag is a product of the coefficient of drag (at a given airspeed) and the frontal area of the aircraft. The coefficient of drag is a function of the shape of the aircraft, and we're just scaling the plane up, so the drag coefficient stays exactly the same. So the zero lift drag at any airspeed will increase proportionally to the frontal area of the aircraft, which is a square function of how much the aircraft was scaled up.
But the
thrust isn't going to scale like that. Just scaling up the engine and maintaining power density (note: this is not a responsible way to design gas turbines outside of thought experiments) will mean that the thrust the fighter has will increase as a
cube function of the scaling factor. Cubes increase faster than squares. Therefore, all else being equal, a bigger fighter is a faster fighter.
But this is a pretty ridiculous level of simplification. You don't just scale planes up and down. For one thing, even if the plane is being scaled up and down, the
pilot stays the same size. And it follows that the ejection seat, and the cockpit, and the canopy, and the life support system and everything else that has to do with the pilot ought to stay the same size too. And since we're talking fighters, there are a bunch of other combat systems to consider. A jammer that can protect a seven ton fighter isn't really significantly lighter than the jammer required to protect a twelve ton fighter. A gun that's deemed adequate to kill enemy fighters and strafe ground targets for a seven ton fighter is likewise still adequate on a twelve ton fighter. The navigation, IFF, and a bunch of other avionics don't really need to be scaled up either. So if we make a bigger fighter, there's going to be a lot of weight left over, proportionally speaking, because not everything needed to be scaled up with the airframe.
That means that larger aircraft have more available weight for fuel storage. Therefore, all else being equal, a bigger fighter is a longer ranged fighter.
But what about maneuverability? This is where things stop favoring the heavier aircraft. The amount of Gs an aircraft can pull is a function of its maximum lift divided by its mass. Assuming uniform density as an aircraft is scaled up, the lift will increase as a square function of the scaling factor, since lift is wing area times lift coefficient times a bunch of other garbage. But, assuming uniform density, the mass of the aircraft will go up as a cubic factor. Therefore, all else being equal, a bigger fighter is a less maneuverable fighter.
The performance advantages you're noticing for twin engined fighters aren't a function of them having two engines, they're inherent advantages of larger aircraft. But there are scaling effects that favor big fighters, and there are also ones that hurt them. If a fighter were so large that it required three engines, it would suffer badly in the maneuverability department, and probably have other practical problems as well such as high landing speed.
Now, that said, there are some twin-engine fighters that aren't huge. The F-5 is quite small, for instance. The biggest advantage of having two engines on a small fighter is survivability. If one engine fails or gets shot, the other can keep operating and the fighter can limp back to base. At least, that is, if the engineers did their job right and the firewall that prevents disaster from spreading from one engine to another is up to the task. This argument was especially compelling in the bad old days when jet fighter engines weren't anywhere near as reliable as they are now. The loss rates heavily favored twins.
But that argument only favors
twins. A fighter with three engines isn't really any more survivable than a fighter with two engines. If a fighter loses an engine, a single-engine fighter becomes a glider, a twin ceases to be a fighter but can at least still fly back to somewhere safe and a three engine fighter... is in exactly the same situation as the twin. A three engine fighter will have more of its total thrust available if it loses an engine, but no fighter is going to try to fight on two thirds of its design power. It's all the additional cost of another engine, but for no real additional benefit.