[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.

[jandrese]

Air pressure difference between FL350 (traditional airliners) and FL600 (Concorde) is about 1/2. But the speed difference was about 2.5x, so overall you would expect Concorde to need to expend about 3.125 times as much energy per flight hour. Even with the flight time divided by 2.5 you're burning more fuel for the trip.

Interesting to consider a Concorde like design today using a slightly scaled up version of the P&W F135 supercruise capable turbofans.

But you'd still be competing with highly efficient turbofans like the GEnx series in a cost dominated aviation market. Also you still have issues like not being able to fly over land in the US that seriously limit the potential market.

[jjk166]

> Air pressure difference between FL350 (traditional airliners) and FL600 (Concorde) is about 1/2. But the speed difference was about 2.5x, so overall you would expect Concorde to need to expend about 3.125 times as much energy per flight hour.

This is false. First, drag is proportional to air density, not pressure, with density at FL600 being 30% of FL350. Drag is increased by 1.728 times[1], which means thrust required is increased by 1.728 times, but energy required is not linearly proportional to thrust required. As a jet engine moves at higher speeds and through less dense air, it's efficiency increases and therefore the power required to produce a unit of thrust decreases. So even though you need 1.728 times the thrust, you need less that 1.728 times the power. Even if it were the same amount of power, at 2.44 times the speed you are only expending that power for 41% of the time, and thus the energy consumption would be 73%.

There are plenty of very valid concerns with regards to the concorde and supersonic transport in general. For starters, you can't simply slap a modern turbofan on there and call it a day, you'd have to sacrifice decades of lessons learned to make high bypass turbofans of the necessary size, so they're still going to be inferior to more refined engines on subsonic aircraft. But the belief that the laws of physics force SSTs to be ridiculous gas guzzlers and no amount of technological refinement can overcome it is misguided at best.

[1] This is actually an oversimplification. Drag does not scale perfectly with qV^2 through the transonic and low supersonic regimes. Wave drag dramatically increases drag close to Mach 1 (which as an aside is the source of the term sound barrier). Wave drag becomes less significant past Mach 1.4 though and by the time you get to supercruise qV^2 is once again a good approximation.

[TylerE]

So your argument continues to be that the aircraft that is provable, and AS YOU ADMIT, is less efficient, is somehow "more efficient" because "reasons".

[jjk166]

Again, that's not my argument. I will continue to assume good faith one last time.

Drag is proportional to density times velocity squared

flying through air with 30% the density at 2.5 times the speed thus produces (.3)x(2.5^2) = 1.875 times the drag.

Using 1.875 times the power for 1/2.5 = 0.4 times as long requires (0.4)x(1.875) = 0.75 times as much energy.

This is before you consider that jet engines moving at higher speeds and higher altitudes are more fuel efficient.

The 737 uses turbofans which are much more efficient than the concorde's turbojets. Trying to compare their fuel consumption is meaningless unless you account for this. If you compare the concorde to a subsonic turbojet of its era, like the 727, the concorde burns significantly less fuel per passenger mile.

[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?