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Turbojets used to be more common in the 40's and 50's but once jet engine design matured they pretty much went extinct in favor of the more efficient turbofan. I know one advantage of the turbojets is the high exhaust velocity which allowed a lot of turbojet aircraft at the time to supercruise, turbofans typically need the assistance of an afterburner in able to reach similar speeds at the cost of a lot of fuel. It's very common to see faster fighter/interceptor type aircraft to use low bypass afterburning turbofans (for the purpose of this question, lets say about 1.5-2.0). This is for obvious reasons, the vast majority of an aircraft's lifetime is spent at subsonic speeds so the turbofan makes a lot more sense, and the afterburner is there for the rare occasion it's gotta go like a bat outta hell.

My question is, say you have two hypothetical aircraft, both identical with the exception of the engine, one with a turbojet (no afterburner) and one with an afterburning turbofan (like I said, bypass ratio is somewhere in the ballpark of 1.5-2.0), both aircraft weigh the same, have the same coefficient of drag, and have the exact same top speed, (something greater than Mach 1). So both jets have similar high speed performance, which jet should we expect to be more fuel efficient, the turbojet at max thrust, or the turbo fan with afterburner on, at max thrust?

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How do tubojets compare to afterburning turbofans in terms of efficiency?

The 5th slide from this presentation (original plot from Daniel P. Raymer, Aircraft Design: A Conceptual Approach), shows a qualitative comparison of the specific fuel consumption of several propulsion technologies:

 specific fuel consumption

As visible, until around Mach 1.6 the fuel consumption of a typical low bypass turbofan is better than a turbojet. Afterwards the pure turbojet becomes more efficient and the turbofan just gives up at around Mach 2, mainly due to aerodynamic limitations of the fan.

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I don't think there were "a lot" of turbojet aircraft that could supercruise. There were some (at least the F-106), but that doesn't constitute a lot. Please provide an authoritative list of more -- or a quote for this being more common.

The aircraft of that day had to be very clean (aerodynamically) because they did not have enough thrust otherwise. Consequently, most of the Century Series fighters carried their weapons internally. The B-58 mostly fits into this category, with a special conformal weapon, but using afterburning turbojets.

I suspect that the F-106's supercruise was under very specific conditions -- without weapons. Not in combat ready configuration. All of these aircraft still had afterburners.

The next generation of fighters (F-4, F-5 etc.) still used turbojets. They moved back to externally carried stores.

The other important case here is the Concorde -- obviously designed for efficiency (as much as a supersonic aircraft can be). It could supercruise on turbojets, but also had afterburners.

The F-14, 15, 16, 18, etc. all use afterburning turbofans. They are much more efficient than the preceeding turbojets. I also expect that they are superior in terms of engine specific thrust (thrust to engine weight ratio). Engine technology improved substantially throughout this time. They of course carry their stores externally.

5th gen aircraft go back to carrying their stores internally, but now for stealth reasons -- but they get an aerodynamic benefit. The F-22 and F-35 still have afterburners, but of course the F-22 can supercruise.

This is all to say -- there is a lot going on between these generations that mean you can't really make the comparison you're suggesting.

In order for the two engines to have equal thrust -- the non-afterburning one would have to be much larger than the one with afterburner.

An afterburner lets you add a tremendous amount of thrust -- with terrible TSFC (fuel consumption) for very little added engine weight -- it is a long hollow tube.

To get the same thrust without an afterburner, you would need to increase the diameter and length of an engine. It would have better TSFC, but would be larger, heavier, and more expensive.

We consider how much time you need to spend at maximum speed and consider that against the rest of the mission requirements. It does not seem that turbojets win out anymore.

Edit: Add a bunch of hopefully relevant performance data.

Here are some performance figures for the aircraft in question. Hopefully this is a fun addition to the conversation.

F-104. At this gross weight and load out, it does not appear to have any super cruise ability.

F-104

F-106. At this gross weight, it can barely supercruise at a narrow window of altitudes.

F-106

F-4. Unable to supercruise, but this shows the dramatic difference a change in load out can make -- the numbered bubbles are for different missions

F-4

F-15. Slight supercruise ability. This figure is nice because it shows Ps across the envelope -- both dry and wet. Notice the Ps 'island' at Mach 1.9 and 36,000 ft.

There is a thrust pinch transonically -- where drag rises much faster than thrust. This makes Ps very bad transonically, but can lead to an operating window that is better supersonically. This is why some aircraft would climb high subsonically and then dive through the transonic area to get to supersonic flight. This is actually the optimal time to climb procedure for some aircraft. Similarly, when aircraft do supercruise, they sometimes use AB to get through the thrust pinch and then oeprate on the other side.

F-15

XB-70 Some supercruise, but unsurprisingly it really was designed to go all out.

XB-70

If you look at the Wikipedia article on Supercruise, you won't see any of these aircraft listed. Instead, all the aircraft that appear in that list are super cruising at a much higher Mach number.

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  • $\begingroup$ Okay my ignorance is showing, I guess I assumed that super cruising turbojets were much more common then they actually were. I'm seeing the flaws in my question and you pretty much answered it. There's a ton of nuance to cross-generational comparisons like this and if I'm not mistaken, super cruising Isn't a whole lot easier for turbojets and the majority of supersonic turbojets relied on afterburners which kind of defeats the point of my original question. $\endgroup$
    – shrimp
    Commented May 1 at 22:02
  • $\begingroup$ "I suspect that the F-106's supercruise was under very specific conditions -- without weapons." If you meant internal vs external, F-106 weapons were all internal. 4x AIM-4 and 1x AIR-2A Genie. $\endgroup$
    – WPNSGuy
    Commented May 1 at 22:32
  • $\begingroup$ @WPNSGuy Agreed -- I was more thinking about flight at lower gross weight. I recall a coast to coast F-106 flight (possibly near retirement of the type) that was impressively fast. $\endgroup$ Commented May 1 at 22:59
  • $\begingroup$ And the Six still apparently holds the FAI single engine speed record (In a very unloaded condition, and a few engine mods). f-106deltadart.com/speedrecord.htm $\endgroup$
    – WPNSGuy
    Commented May 1 at 23:11

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