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If you're wondering why the air isn't full of supersonic passenger aircraft, one of the main reasons is that high bypass turbofan engines are much more efficien

If you're wondering why the air isn't full of supersonic passenger aircraft, one of the main reasons is that high bypass turbofan engines are much more efficient than turbojet engines like the Olympus engines in the concord. (And way quieter) Turns out a lower volume or air in a gas turbine engine moving at supersonic speed is much less efficient than a large volume of air moving at lower speed.

idlewords 12y ago
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.
- Tloewald 12y ago
  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.
  - idlewords 12y ago
    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.
    - Tloewald 12y ago
      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.
- RachelF 12y ago
  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.
rodgerd 12y ago
Really, it had a great deal more to do with politics: specifically, the Europeans had a supersonic airliner and the US didn't, so there was no way in hell the US government was going to do anything other than make it as difficult to use as possible.
- hollerith 12y ago
  Do you have any specific reasons to believe that the USG ever stopped or even simply tried to discourage the Concorde from flying to airports on the U.S. East Coast? As for flying over populated parts of the U.S. at supersonic speeds, not even the U.S. Air Force gets to do that. (Source: https://news.ycombinator.com/item?id=8209976 https://news.ycombinator.com/item?id=8209976)
  - WildUtah 12y ago
    I've been in a sonic boom under a navy plane in the continental USA, not adjacent to any military base. But it only happened once, so maybe it was a mistake.
- jpatokal 12y ago
  I take it you're referring to the "no sonic boom in my backyard" shenanigans that hampered flights to/from/over the US in the early days? These were sorted out for the key NY-Europe routes by 1977, just over a year from first commercial flight. https://en.wikipedia.org/wiki/Concorde#Operational_history https://en.wikipedia.org/wiki/Concorde#Operational_history
  - aidenn0 12y ago
    That only solved the transoceanic flights. No LAX to JFK flights for the Concorde.
- bootload 12y ago
  > the Europeans had a supersonic airliner and the US didn't Technically no, but the 747 is fast, almost too fast in these days of fuel economy. The 747 was designed by Boeing to do M.87 (Pan Am wanted M.9) , though it now sits around M.855 (913 km/h) [1] due mainly to the requirement of fuel efficiency over raw speed. Almost Mach 1. How did Boeing designers get this speed? Hint: The hump and area rule (min drag > M.8) [2] Now a DC8 was pushed to M1.012 under special conditions. [3] And as a point of interest, Air Force One (VC-25A) is rated at M0.92 (1015 km/h). [4] [1] http://www.boeing.com/boeing/commercial/747family/pf/pf_400er_prod.page http://www.boeing.com/boeing/commercial/747family/pf/pf_400e...? [2] http://www.aerospaceweb.org/question/performance/q0150a.shtml http://www.aerospaceweb.org/question/performance/q0150a.shtm... [3] https://en.wikipedia.org/wiki/Douglas_DC-8#Production_and_testing https://en.wikipedia.org/wiki/Douglas_DC-8#Production_and_te... [4] https://en.wikipedia.org/wiki/Boeing_VC-25#Specifications_.28VC-25A.29 https://en.wikipedia.org/wiki/Boeing_VC-25#Specifications_.2...
svdree 12y ago
Another reason might be that supersonic flying doesn't really gain you all that much. You still have to spend time getting to the airport, being there a couple of hours in advance, go through all the security measures etc, and jump through similar hoops upon arrival. Cutting your flight time in half sounds nice, but the effect on your actual travel time is a lot smaller.
- cstross 12y ago
  Please note that the Concorde requirement was set circa 1958-60, and the plane first flew in 1969; airlines were placing orders in the early 1970s. Back in those days, airline security was a lot more relaxed: despite numerous hijackings nobody had yet figured out that a couple of hundred tons of airliner and jet fuel made a potent self-guided cruise missile, so the usual ending of a hijacking was on the ground with a negotiated hand-over. It was still possible to get from drop-off at the airport to the Concorde departure lounge in under 30 minutes; and for the trip from Arrivals at JFK to Wall Street ... well, that's what helicopters are for. (If you can afford a Concorde ticket you can afford a helicopter limo service!) The immediate aftermath of 9/11 pushed the suggested minimum check-in time for trans-Atlantic flights up from 30-40 minutes to 3 hours, for everyone, VIP's included. So the folks who could afford a first class ticket -- the only kind of seat available on Concorde -- decamped for NetJets and other bizjet services, because the civil aviation security regulations didn't apply to private jets and you could still do the 30 minute from door-to-plane thing. (And then Airbus refused to renew Concorde's type certification at the 30 year mark, allegedly just as they were discussing development of a new second-generation SST with a large Japanese corporation ...)
  - Tloewald 12y ago
    Actually, airport security -- at least for international flights -- in the late 60s and early 70s was not more relaxed. Having traveled quite a bit as a child in those days, most international travel involved having all your bags searched by hand in front of you. The difference was that air traffic volumes were much lower, you spent less time in queues, and Concorde passengers were first class and got preferential treatment. The lightest airport security in my experience was in the mid-to-late 90s following deregulation. I flew from Sydney to Zurich in 1998 without breaking stride at either end (literally waved through every checkpoint).
SpacemanSpiff 12y ago
From page 2 of the discussion: "Anyway, back to some figues; at Mach 2, 50,000', the typical fuel burn per engine would be around 5 tonnes/hour, falling to around 4.2 tonnes/hour at 60,000'." So at 60,000 feet, total fuel flow would be: 4.2 tonnes/hour * 4 engines = 16.8 tonnes/hour 16.8 tonnes/hour * 326 gallons per tonne = 5477 gallons/hour 5477 gallons/hour * 1/1320mph = 4.15 gallons/mile, 0.24 miles/gallon 0.24 miles/gallon * 120 passengers = 28.8 passenger miles/gallon The 787 boasts around 120 passenger miles per gallon. So the Concorde uses 4x this much at highest efficiency. Not bad for 60's engineering at mach 2! I can see why it was not profitable to operate however, due to the increases in fuel cost since the aircraft was designed.
- imaginenore 12y ago
  So it's 4x less fuel-efficient. Fuel is a small percentage of the ticket price: http://www.darinlee.net/fuel/fuel_widget.html http://www.darinlee.net/fuel/fuel_widget.html But even if fuel represented 100% of the ticket price, these supersonic flights could catch all of first- and business-class market. Rich people and businessmen value their time.

idlewords 12y ago

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.

Tloewald 12y ago
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.
- idlewords 12y ago
  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.
  - Tloewald 12y ago
    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.
RachelF 12y ago
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.

rodgerd 12y ago

Really, it had a great deal more to do with politics: specifically, the Europeans had a supersonic airliner and the US didn't, so there was no way in hell the US government was going to do anything other than make it as difficult to use as possible.

hollerith 12y ago
Do you have any specific reasons to believe that the USG ever stopped or even simply tried to discourage the Concorde from flying to airports on the U.S. East Coast? As for flying over populated parts of the U.S. at supersonic speeds, not even the U.S. Air Force gets to do that. (Source: https://news.ycombinator.com/item?id=8209976 https://news.ycombinator.com/item?id=8209976)
- WildUtah 12y ago
  I've been in a sonic boom under a navy plane in the continental USA, not adjacent to any military base. But it only happened once, so maybe it was a mistake.
jpatokal 12y ago
I take it you're referring to the "no sonic boom in my backyard" shenanigans that hampered flights to/from/over the US in the early days? These were sorted out for the key NY-Europe routes by 1977, just over a year from first commercial flight. https://en.wikipedia.org/wiki/Concorde#Operational_history https://en.wikipedia.org/wiki/Concorde#Operational_history
- aidenn0 12y ago
  That only solved the transoceanic flights. No LAX to JFK flights for the Concorde.
bootload 12y ago
> the Europeans had a supersonic airliner and the US didn't Technically no, but the 747 is fast, almost too fast in these days of fuel economy. The 747 was designed by Boeing to do M.87 (Pan Am wanted M.9) , though it now sits around M.855 (913 km/h) [1] due mainly to the requirement of fuel efficiency over raw speed. Almost Mach 1. How did Boeing designers get this speed? Hint: The hump and area rule (min drag > M.8) [2] Now a DC8 was pushed to M1.012 under special conditions. [3] And as a point of interest, Air Force One (VC-25A) is rated at M0.92 (1015 km/h). [4] [1] http://www.boeing.com/boeing/commercial/747family/pf/pf_400er_prod.page http://www.boeing.com/boeing/commercial/747family/pf/pf_400e...? [2] http://www.aerospaceweb.org/question/performance/q0150a.shtml http://www.aerospaceweb.org/question/performance/q0150a.shtm... [3] https://en.wikipedia.org/wiki/Douglas_DC-8#Production_and_testing https://en.wikipedia.org/wiki/Douglas_DC-8#Production_and_te... [4] https://en.wikipedia.org/wiki/Boeing_VC-25#Specifications_.28VC-25A.29 https://en.wikipedia.org/wiki/Boeing_VC-25#Specifications_.2...

svdree 12y ago

Another reason might be that supersonic flying doesn't really gain you all that much. You still have to spend time getting to the airport, being there a couple of hours in advance, go through all the security measures etc, and jump through similar hoops upon arrival. Cutting your flight time in half sounds nice, but the effect on your actual travel time is a lot smaller.

cstross 12y ago
Please note that the Concorde requirement was set circa 1958-60, and the plane first flew in 1969; airlines were placing orders in the early 1970s. Back in those days, airline security was a lot more relaxed: despite numerous hijackings nobody had yet figured out that a couple of hundred tons of airliner and jet fuel made a potent self-guided cruise missile, so the usual ending of a hijacking was on the ground with a negotiated hand-over. It was still possible to get from drop-off at the airport to the Concorde departure lounge in under 30 minutes; and for the trip from Arrivals at JFK to Wall Street ... well, that's what helicopters are for. (If you can afford a Concorde ticket you can afford a helicopter limo service!) The immediate aftermath of 9/11 pushed the suggested minimum check-in time for trans-Atlantic flights up from 30-40 minutes to 3 hours, for everyone, VIP's included. So the folks who could afford a first class ticket -- the only kind of seat available on Concorde -- decamped for NetJets and other bizjet services, because the civil aviation security regulations didn't apply to private jets and you could still do the 30 minute from door-to-plane thing. (And then Airbus refused to renew Concorde's type certification at the 30 year mark, allegedly just as they were discussing development of a new second-generation SST with a large Japanese corporation ...)
- Tloewald 12y ago
  Actually, airport security -- at least for international flights -- in the late 60s and early 70s was not more relaxed. Having traveled quite a bit as a child in those days, most international travel involved having all your bags searched by hand in front of you. The difference was that air traffic volumes were much lower, you spent less time in queues, and Concorde passengers were first class and got preferential treatment. The lightest airport security in my experience was in the mid-to-late 90s following deregulation. I flew from Sydney to Zurich in 1998 without breaking stride at either end (literally waved through every checkpoint).

SpacemanSpiff 12y ago

From page 2 of the discussion: "Anyway, back to some figues; at Mach 2, 50,000', the typical fuel burn per engine would be around 5 tonnes/hour, falling to around 4.2 tonnes/hour at 60,000'." So at 60,000 feet, total fuel flow would be: 4.2 tonnes/hour * 4 engines = 16.8 tonnes/hour 16.8 tonnes/hour * 326 gallons per tonne = 5477 gallons/hour 5477 gallons/hour * 1/1320mph = 4.15 gallons/mile, 0.24 miles/gallon 0.24 miles/gallon * 120 passengers = 28.8 passenger miles/gallon The 787 boasts around 120 passenger miles per gallon. So the Concorde uses 4x this much at highest efficiency. Not bad for 60's engineering at mach 2! I can see why it was not profitable to operate however, due to the increases in fuel cost since the aircraft was designed.

imaginenore 12y ago
So it's 4x less fuel-efficient. Fuel is a small percentage of the ticket price: http://www.darinlee.net/fuel/fuel_widget.html http://www.darinlee.net/fuel/fuel_widget.html But even if fuel represented 100% of the ticket price, these supersonic flights could catch all of first- and business-class market. Rich people and businessmen value their time.