Each engine type has a range where it's superior. Turbojets are much more efficient at high speeds, while high-bypass turbofans are better at low speeds. The Concorde burned remarkably little fuel at Mach 2, but used up ludicrous amounts of fuel at idle, taxi and takeoff. Read the thread, it's all in there.
If you read further into the thread, a pilot says that the high fuel consumption during takeoff was a consequence of the aircraft's weight being higher, and higher air density at lower altitudes -- the engines were not more efficient at higher speeds as an earlier post suggested.
Note that the idle fuel consumption (1t/hour; not sure whether that figure was per engine or for the entire plane) was not higher than cruising (4-6t/hour) -- it was just high in absolute terms.
Thank you; I didn't mean to imply the plane used more fuel at idle than at supersonic cruise.
Fuel consumption during takeoff and climb was something like 25 tons an hour. Part of this is because the Concorde's wing has high drag at high angles of attack, most of it was due to the engines needing to be in full afterburner. The plane needed full afterburner to take off even without a full load of fuel on board.
At low speeds, most of the turbofans' power goes to creating a really high-speed stream of exhaust. That's what I mean about the engines being fundamentally more efficient (at converting fuel into forward motion) at higher speeds, where the exhaust speed more closely matches the speed of the plane.
Yes, someone (M2dude, I think) says that 65% of the thrust, at speed, was provided by the intakes. That said, at lower speed, all that work to bring air up to speed is accelerating the plane as well.
Even so, since drag tends to increase as the square of velocity, and in proportion with density -- you're spending a lot more effort to maintain velocity at higher speeds. The afterburners are mentioned as increasing fuel consumption by 80% and power output by 17%. The high fuel consumption comes down to the plane accelerating, climbing, being in denser and warmer air, and heavier -- all at once. While the engines might be marginally more efficient in some sense at high speed, this would be cancelled out by the vastly higher drag at low altitude/speed.
An enjoyable thread, so much good discussion, no trolls or stupid comments, mostly professionals and interested amateurs discussing a topic they love, and know lots about.
