Why no tri or quad jet fighters?

[sferrin]

Acceleration, lower thermal signature and efficiency; at the cost of an afterburner, maybe altitude, weight..

None of those are "how to". Theoretically, if you made the compressor the rotor in an electric motor, that would give you an air supply. Then use electrical heating elements aft of the compressor to heat (expand) your gas stream. For missiles you could even do a Project Pluto but using an electric heating element in place of the nuclear reactor. (Which should be a clue as to why electric fighter engines will never happen.) The problem is even the best theoretical battery weighs WAYYY too much compared to an equivalent energy content of jet fuel. On top of that you keep that greater weight all the way to landing. No getting lighter as fuel burns off. There was an article in AvWeek a month or two ago about why you'll never see electric airliners. You might see some turbo electric applications for propulsion but then that's not what we're talking about. (And no Madrat, turbo electric won't work for propulsion in a fighter aircraft.)

[nutshell]

Acceleration, lower thermal signature and efficiency; at the cost of an afterburner, maybe altitude, weight..

None of those are "how to". Theoretically, if you made the compressor the rotor in an electric motor, that would give you an air supply. Then use electrical heating elements aft of the compressor to heat (expand) your gas stream. For missiles you could even do a Project Pluto but using an electric heating element in place of the nuclear reactor. (Which should be a clue as to why electric fighter engines will never happen.) The problem is even the best theoretical battery weighs WAYYY too much compared to an equivalent energy content of jet fuel. On top of that you keep that greater weight all the way to landing. No getting lighter as fuel burns off. There was an article in AvWeek a month or two ago about why you'll never see electric airliners. You might see some turbo electric applications for propulsion but then that's not what we're talking about. (And no Madrat, turbo electric won't work for propulsion in a fighter aircraft.)

Yup absolutely not doable.

We can barely make an electric car, let alone a fighter.
I can forsee an ERS in the near future tho.

[botsing]

Once wireless energy transfer gets efficient enough it will enable a whole new generation of drones with electric engines, airtime would be virtually limitless and with electric engines being relatively simple it will drop maintenance costs as well.

A carrier strike group could use a continuous high flying sensor platform for over the horizon visibility while it's refueling drones could carry more fuel for a longer period of time thereby increasing availability.

Just last year the first continuous maser at room temperature was developed, so there seems to be quite a revolution on the horizon for wireless energy transfer and it's many applications.

[collimatrix]

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.

[madrat]

The big advantage to electric motor technology is it's extreme lightweight for its high torque potential. I did earlier oversimplify it as thrust to weight. While a battery is a horrible weight penalty, it probably is somewhat necessary to stabilize the electric flow through the circuit, making regulation of the current simpler. The bigger picture is that you can rapidly convert electricity into kinetic energy. They may not have the endurance, but don't ever underestimate the benefit of enjoying an advantage to quickly utilize stored energy. You have a very short term anaerobic energy system in the body that makes it possible to lift things that would not be attainable through the glycolic or Kreb,'s cycle. Sferrin, you're mind is stuck in the past. Not only is electric motors perfectly possible for propulsion, they are simpler to engineer in projects such as the NASA project I mentioned earlier. I wouldn't dismiss the idea that other vehicles such as helicopters wouldn't begin to augment their lift and propulsion via electric motors where they could certainly increase control ability of the aircraft while decreasing main rotor drag and all the downside to a large rotor.

[sferrin]

Once wireless energy transfer gets efficient enough it will enable a whole new generation of drones with electric engines, airtime would be virtually limitless and with electric engines being relatively simple it will drop maintenance costs as well.

A carrier strike group could use a continuous high flying sensor platform for over the horizon visibility while it's refueling drones could carry more fuel for a longer period of time thereby increasing availability.

Just last year the first continuous maser at room temperature was developed, so there seems to be quite a revolution on the horizon for wireless energy transfer and it's many applications.

I'm skeptical that this will ever be a thing. Too many down sides.

[sferrin]

Sferrin, you're mind is stuck in the past. Not only is electric motors perfectly possible for propulsion, they are simpler to engineer in projects such as the NASA project I mentioned earlier. I wouldn't dismiss the idea that other vehicles such as helicopters wouldn't begin to augment their lift and propulsion via electric motors where they could certainly increase control ability of the aircraft while decreasing main rotor drag and all the downside to a large rotor.

Except we're not talking about helicopters, we're specifically taking about fighters (you know, the title of the thread). Now tell us, specifically, why and how electric motors would make better fighter engines than turbofans, AS YOU CLAIMED. Go!

[sferrin]

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.

This is more a function of design than some inherent physical law. The F-15 and F-14 were designed to carry a big radar and lots of weapons high, fast and a long distance. That took a lot of power which meant two engines. I doubt anybody would argue that the F-104, F-106 weren't fast and had excellent altitude capability. And the F-106 had good range as well. Swap out the J75 for an F119 in the F-106 and that would have been a BEAST. Swap out the radar for an APG-79 and the Falcons and Genie for AIM-120s- carried internally.

[michaelemouse]

collimatrix,

Thanks for the great response. How would the square-cube law apply to flight ceiling? If you scaled up a fighter in the way you described, would it be able to fly higher? I suspect it would but I'm wondering about the missing link in the causal chain from: larger volume to surface ratio to higher ceiling.

The SR-71 wasn't a fighter but gives us an example of what could be achieved in the 60s if you wanted an air-breathing aircraft to fly as fast and high as possible. Could a large fighter fly at 30km altitude today? At Mach 4+?

[sferrin]

collimatrix,

Thanks for the great response. How would the square-cube law apply to flight ceiling? If you scaled up a fighter in the way you described, would it be able to fly higher? I suspect it would but I'm wondering about the missing link in the causal chain from: larger volume to surface ratio to higher ceiling.

The SR-71 wasn't a fighter but gives us an example of what could be achieved in the 60s if you wanted an air-breathing aircraft to fly as fast and high as possible. Could a large fighter fly at 30km altitude today? At Mach 4+?

Always thought the original configuration of RASCAL made into a fighter would be awesome. 4 F100s modified with pre-compressor cooling to the degree the engines operated at 100,000 feet as if they were at sea-level. Mach 4. Zoom capability to 200,000 feet. Was roughly fighter sized (XF-108 / B-58-ish).

rascal03.jpg (15.21 KiB) Viewed 15107 times

http://stargazer2006.online.fr/space/pages/rascal.htm



"The Rascal design comprises a high-speed, high-altitude aircraft carrying a two-stage expendable rocket capable of boosting a 150kg (330lb) satellite into low-Earth orbit (LEO). The aircraft, which is designed by Scaled Composites, is powered by four turbojets incorporating mass injection pre-compressor cooling (MIPCC).

This injects liquid oxygen and water into the inlet to cool the compressor and increase massflow, allowing operation to higher speeds and altitudes.

The MPICC turbojets, based on Pratt & Whitney's F100 fighter engine, enable the aircraft to take off conventionally, accelerate to beyond Mach 3 then "zoom climb" to above 200,000ft (61,000m) - essentially outside the atmosphere - to deploy the expendable upper stage from its payload bay, before returning to a runway landing"

Okay, probably not fighter material.

[collimatrix]

This is more a function of design than some inherent physical law. The F-15 and F-14 were designed to carry a big radar and lots of weapons high, fast and a long distance. That took a lot of power which meant two engines. I doubt anybody would argue that the F-104, F-106 weren't fast and had excellent altitude capability. And the F-106 had good range as well. Swap out the J75 for an F119 in the F-106 and that would have been a BEAST. Swap out the radar for an APG-79 and the Falcons and Genie for AIM-120s- carried internally.

The designers weren't slaves to scaling laws, but you can still see them tangling with them in the designs. The F-15 has those proportionally enormous bat wings; they're much bigger relative to the size of the airframe than the wings on the F-16. They're like that, in part, because of square/cube scaling. Another reason is that the F-15 is conventionally stable and therefore requires larger control surfaces generally.

The F-14 is swing-wing, which means that with the wings forward it has a better lift slope than its wing loading would suggest. However, it also has the big pancake to help generate lift at high speeds. Again, heavy fighters need help getting lift from somewhere.

The F-104 is fast, kind of (a lot of those earlier jets' top speeds are theoretical as they were actually fuel-limited), but you can see that it has incredibly tiny wings to minimize drag, while the Phantom had more or less the same top speed and was more sanely proportioned. Also, the F-104 had a very austere avionics compliment compared to the Phantom, and quite a lot less range. And these aircraft used variant of the same engine, so comparing them is going to be about as apples-to-apples as real world comparisons are going to get.

Delta-winged fighters like the F-106 usually have above-average range for a reason that isn't square/cube scaling, but is related to it. Delta wings have very long chord length, so for a given chord length to chord thickness ratio (which informs the transonic characteristics of the wing), a delta wing will have absolutely much more chord thickness. This means that delta wings have a better volume to surface area ratio than swept wings or straight wings, which means that they make better fuel tanks than swept wings or straight wings and lead to an aircraft with higher fuel fraction and therefore better range.

collimatrix,

Thanks for the great response. How would the square-cube law apply to flight ceiling? If you scaled up a fighter in the way you described, would it be able to fly higher? I suspect it would but I'm wondering about the missing link in the causal chain from: larger volume to surface ratio to higher ceiling.

The SR-71 wasn't a fighter but gives us an example of what could be achieved in the 60s if you wanted an air-breathing aircraft to fly as fast and high as possible. Could a large fighter fly at 30km altitude today? At Mach 4+?

I think these days flight ceiling is less a function of lift generation, and more a function of keeping the engines from choking and keeping the pilot alive. So it isn't informed very much by the size of the aircraft.

If you want a plane that's able to fly to high altitude and actually fight up there as well, I think that plane looks a hell of a lot like a Raptor. Huge wings to provide decent lift even in high, thin air, engines with a very low bypass ratio usually work better at altitude, and thrust vectoring to give a little bit of additional control authority.

[basher54321]

Beside the point but clearly some profiles where the F-4 was the one lacking in range.

Carrying one bomb (guess what kind) with 4 tanks an F-104 will go about half again as far as an F-4 on a Low-Low-Low sortie. And it will do it faster, too.

http://www.916-starfighter.de/Ruminatio ... 0F-104.htm



Although true for a lot of aircraft I remember reading about the Norweigans developing or practising a Mach 2 F-104G intercept profile for intercepting very high altitude targets - will need to find the ref.

[sferrin]

Beside the point but clearly some profiles where the F-4 was the one lacking in range.

Carrying one bomb (guess what kind) with 4 tanks an F-104 will go about half again as far as an F-4 on a Low-Low-Low sortie. And it will do it faster, too.

http://www.916-starfighter.de/Ruminatio ... 0F-104.htm



Although true for a lot of aircraft I remember reading about the Norweigans developing or practising a Mach 2 F-104G intercept profile for intercepting very high altitude targets - will need to find the ref.

Here's an F-104 launching a Genie from Mach 1.7 and 56,000 feet:



Then there are stories of cruise in min afterburner at Mach 2 and 73,000 feet, doing gun attacks on U-2s at altitude, etc.

[basher54321]

Great video that


The Norway intercepts are here http://www.starfighter.no/hi-alt.html - will stick in the F-104 thread as there is one.

[madrat]

The F-101 was probably the most comparable to F-106. The F-101 did out distance the F-106 nearly as much as F-106 clearly had more top end speed. The frontal area of the two is similar. The J57 didn't have near the high speed performance and probably explains a large amount of the speed difference. Both were about the same length. The F-101 did carry a bigger internal fuel load so no doubt that contributes to the range difference. So two similarly powered jet fighters with drastically different results by design.