[jjk166]

Drag is higher, but jet engines are more efficient at higher speeds which compensates for the increased drag. Overall propulsive efficiency is at a minimum at mach 1, above that it starts increasing again. Beyond a certain point (supercruise) you get higher overall propulsive efficiency than at any subsonic point.

[TylerE]

Again incorrect! Look at the data. In what universe is having 12x the fuel burn (per unit of time) or 5x (by mile) “more efficient”.

A Supercuruising Concorde has higher fuel flow than a (much much larger) 747.

Your whole “engines more efficient at higher speed” argument is totally unsupported by facts. Supersonic flow is a major problem is jet engines. They have to use special very-high drag inlets to slow it down to subsonic velocity to actually combust.

The air being thinner doesn’t help, either. It’s basic chemistry... every unit of fuel you burn requires X units of oxygen. The density doesn’t really matter - except that denser air moving slower decelerates less and thus causes less drag.

[jjk166]

You're comparing apples and oranges. The concorde used turbojet engines with afterburners. That's 10 to 50x the fuel consumption of a comparable turbofan like the one a modern airliner uses. If you are using 5x the fuel with an engine that should be burning 10x the fuel, you must be using it in a way that is twice as efficient.

[TylerE]

No, that’s just dumb. BURNING MORE = LESS EFFICIENT

Vehicle A takes 10 gallons of fuel to move 4 passengers 500 miles.

Vehicle B takes 40 gallons of fuel to move 4 passengers 500 miles.

In what possible universe is vehicle B more fuel efficient than Vehicle A?

[jjk166]

Again, I'm not saying the concorde is a more efficient vehicle, I'm saying flying supersonically is more efficient than flying subsonically. The concorde was an extremely inneficient plane because it used inneficient engines. Were it to use engines with the same efficiency as modern turbofans, it would be more efficient overall.

I'm saying put vehicle A's engine in vehicle B to make some far more efficient vehicle C and it'll only take 5 gallons of fuel to move 4 passengers 500 miles.

[jandrese]

As wind resistance increases with the square of the speed and is the major resistive force when traveling by air it's really hard to become more efficient by going faster.

It's probably true that Concorde, with its supersonic optimized design would have been less efficient traveling at Mach 0.95, even when traveling at its optimum speed a subsonic optimized airliner is going to require significantly less fuel per passenger mile. I mean this is one reason Concorde failed: the oil crisis turned its high fuel consumption into a serious liability.

[jjk166]

Drag increases proportional to the square of velocity times the density. At higher speeds, you can fly at higher altitudes for a given lift to drag ratio. Higher altitudes -> lower air densities. Drag overall does increase, but at a slower rate than just looking at the velocity alone would suggest. The efficiency of jet engines increases with both altitude and airspeed (which is why subsonic airliners fly as close to the speed of sound as they can get btw). At and slightly above the speed of sound drag dominates, but engine efficiency eventually becomes more important again past Mack 1.6.

The important thing to remember here is that at the same time as the concorde was being developed, another technbology, the high bypass turbofan, was also developed. This looks like and is commonly referred to by the layperson as a jet engine, but the two are very different. A Turbofan is 10 to 50 times more efficient than an afterburning turbojet, regardless of speed. The concorde didn't have turbofans, the planes it was competing with did. You slap the concorde's engines onto any other airframe and fly at any other speed, it's still going to burn an order of magnitude more fuel than a turbofan aircraft.

Compare the concorde to turbojet powered aircraft it is competitive. The turbojet powered boeing 720 got 16 passenger miles per gallon and the 727 got 10. In that context the concorde's 17 at over twice the speed looks pretty good.

[TylerE]

And I say that " I'm saying flying supersonically is more efficient than flying subsonically." is a complete load of rubbish unsupported by science or data.

[jjk166]

If you study aerospace engineering you derive this in undergrad. Again, the whole reason supersonic transports were pursued in the 60s was to reduce fuel costs, and the attempts were abandoned when turbofans proved a better method of achieving that goal.

[valuearb]

Then why did the Valkyrie B-70 have a longer range flying supersonic than subsonic?