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I’ll be super excited to have low-to-null-boom supersonic transcontinental flights! But if this becomes a passenger plane, what kind of fuel efficiency would th

I’ll be super excited to have low-to-null-boom supersonic transcontinental flights! But if this becomes a passenger plane, what kind of fuel efficiency would that have? Are supersonic aircraft more or less efficient (oranges and apples I presume).

dexen 7y ago
>Are supersonic aircraft more or less efficient Surprisingly enough, supersonic planes have good fuel economy while supercruising - thanks to thin air at the high altitude, and speeds around Mach 2 ... 3 being good for turbojet engines. Expect worse taxiing, take-off and climb fuel economy, due to dense air and also suboptimal (high) angle of attack. This means supersonic planes are best suited for longer routes. Even the grandpa Concorde had good fuel economy in cruise, however it was overshadowed by atrocious fuel economy at low speeds, and the use of afterburners in transsonic region[1]. The designed but never built Concorde "B" would have notably better economy thanks to more advanced engines doing away with the afterburners. [1] https://www.quora.com/How-fuel-efficient-was-the-Concorde-compared-to-similar-sized-jets https://www.quora.com/How-fuel-efficient-was-the-Concorde-co...
- throwawayhhakdl 7y ago
  Is it accurate to say that high speed efficiency matters much more for any flight a supersonic jet would reasonably take
- baybal2 7y ago
  Yes, because of the second point, it makes sense to build new runways just to accommodate SSTs outside of normal landing procedures to let SST land while gliding down from their cruising altitude. Concordes were wasting double digit of their fuel reserve while flying in the holding pattern if runways were not available for some reason.
  - gsnedders 7y ago
    > Concordes were wasting double digit of their fuel reserve while flying in the holding pattern if runways were not available for some reason. Though note this was uncommon; they were typically cleared for a direct approach all the way from cruise to touchdown. Part of the reason why they rarely got put in the holding pattern was the fact that it would quickly turn into a low fuel emergency, and part was giving the SST preferential treatment.
- imglorp 7y ago
  The taxi phase, at least, can be mitigated with an electric tug motor built into nose gear. Engines can be off or idle until needed. https://en.wikipedia.org/wiki/WheelTug https://en.wikipedia.org/wiki/WheelTug
  - jobseeker990 7y ago
    Great point. Or motors in the rear wheels could probably get it up to 100MPH on the runway before the jets had to take over.
    - DuskStar 7y ago
      I'm not sure if those fuel savings would be worth getting rid of the integrated pre-takeoff engine thrust test, though.
      - blackflame 7y ago
        Yea what is generating the power for those motors?
        imglorp 7y ago
        APU, normally. A smaller turbine generator, just enough to run electrical and hydraulic things, usually, and maybe to start the main engines. I think they shut off APUs after the mains come up. They use almost nothing compared to, say, an Olympus engine on the Concorde at idle.
        blackflame 7y ago
        An apu doesn't generate nearly enough power to taxi the aircraft. Since a large APU is essentially another turbine, you're back to using a turbine to propel the aircraft. If this really were a place to save money then the simplest solution would just have the tug pull it all the way to the runway. That way the aircraft doesn't need to take on additional weight for motors/apus which will sap fuel economy during flight.
- mshook 7y ago
  To give some perspective, the hourly fuel consumption for Concorde is almost identical at Mach 2 than at max subsonic speed... And Concorde powerplant had at some point the highest overall thermal efficiency at max speed: https://en.wikipedia.org/wiki/Rolls-Royce/Snecma_Olympus_593 https://en.wikipedia.org/wiki/Rolls-Royce/Snecma_Olympus_593 The overall thermal efficiency of the engine in supersonic cruising flight (supercruise) was about 43%, which at the time was the highest figure recorded for any normal thermodynamic machine.
- everdev 7y ago
  > Expect worse taxiing, take-off and climb fuel economy, due to dense air and also suboptimal (high) angle of attack. This reminds me, in the book "Skunk Works", SR-71 Blackbird pilots mentioned their schedule of mid-air refueling 20-30min after takeoff. After that they could make it 3-5hrs at high altitude before refueling again.
  - dexen 7y ago
    The SR-71 did take off with much less than full fuel load; IIRC to keep the mass low enough for safe T/O abort or maybe for a safe go-around. As for books, I wholeheartedly recommend the excellent https://www.barnesandnoble.com/w/sled-driver-brian-shul/1123287835 https://www.barnesandnoble.com/w/sled-driver-brian-shul/1123... written by one of the pilots.
    - blackflame 7y ago
      Yea the SR-71 has very poor lift at slow speeds because of its lack of wing hence why it needs the chines on the forward section of the fuselage.
  - skykooler 7y ago
    This is because the Blackbird took off with only partially filled tanks, to lower the weight and therefore the takeoff speed. It was not designed to take off or land with full tanks.
    - gpderetta 7y ago
      Also the tanks leaked untill thermal expansion at supersonic speed sealed them.
      - utopian3 7y ago
        Wow, shouldn’t that be a safety concern? Fuel leaking all over the airbase prior to takeoff?
Robotbeat 7y ago
Fundamentally, about half as efficient. You can compensate by cramming in seats closer or trying exotic designs. And there's some efficiency buy-back by operating at higher altitudes where the engine can expand to lower pressures (and thus increase the pressure ratio), but overall, roughly half as efficient is a decent guesstimate. Then again, conventional aircraft are about double the efficiency they used to be, so supersonic aircraft today could conceivably be approximately the same efficiency as transonic aircraft of yesteryear. I think exotic designs could enable decent range low supersonic electric flight (i.e. Mach 1.5 or something). Going to need good batteries, though, and extremely high aspect ratio wings.
- blackflame 7y ago
  A lot of the fuel efficiency of modern aircraft comes from having giant turbofans instead of turbojets. By attaching a giant fan the front of your compressor and redirecting most of the air around the engine you can get great efficiency at subsonic speeds. However, the nature of the fan has fundamental limitations when approaching the speed of sound and turbo/ram jets are basically just pouring gas in a tube of precompressed air.

dexen 7y ago

>Are supersonic aircraft more or less efficient Surprisingly enough, supersonic planes have good fuel economy while supercruising - thanks to thin air at the high altitude, and speeds around Mach 2 ... 3 being good for turbojet engines. Expect worse taxiing, take-off and climb fuel economy, due to dense air and also suboptimal (high) angle of attack. This means supersonic planes are best suited for longer routes. Even the grandpa Concorde had good fuel economy in cruise, however it was overshadowed by atrocious fuel economy at low speeds, and the use of afterburners in transsonic region[1]. The designed but never built Concorde "B" would have notably better economy thanks to more advanced engines doing away with the afterburners. [1] https://www.quora.com/How-fuel-efficient-was-the-Concorde-compared-to-similar-sized-jets https://www.quora.com/How-fuel-efficient-was-the-Concorde-co...

throwawayhhakdl 7y ago
Is it accurate to say that high speed efficiency matters much more for any flight a supersonic jet would reasonably take
baybal2 7y ago
Yes, because of the second point, it makes sense to build new runways just to accommodate SSTs outside of normal landing procedures to let SST land while gliding down from their cruising altitude. Concordes were wasting double digit of their fuel reserve while flying in the holding pattern if runways were not available for some reason.
- gsnedders 7y ago
  > Concordes were wasting double digit of their fuel reserve while flying in the holding pattern if runways were not available for some reason. Though note this was uncommon; they were typically cleared for a direct approach all the way from cruise to touchdown. Part of the reason why they rarely got put in the holding pattern was the fact that it would quickly turn into a low fuel emergency, and part was giving the SST preferential treatment.
imglorp 7y ago
The taxi phase, at least, can be mitigated with an electric tug motor built into nose gear. Engines can be off or idle until needed. https://en.wikipedia.org/wiki/WheelTug https://en.wikipedia.org/wiki/WheelTug
- jobseeker990 7y ago
  Great point. Or motors in the rear wheels could probably get it up to 100MPH on the runway before the jets had to take over.
  - DuskStar 7y ago
    I'm not sure if those fuel savings would be worth getting rid of the integrated pre-takeoff engine thrust test, though.
    - blackflame 7y ago
      Yea what is generating the power for those motors?
      - imglorp 7y ago
        APU, normally. A smaller turbine generator, just enough to run electrical and hydraulic things, usually, and maybe to start the main engines. I think they shut off APUs after the mains come up. They use almost nothing compared to, say, an Olympus engine on the Concorde at idle.
        blackflame 7y ago
        An apu doesn't generate nearly enough power to taxi the aircraft. Since a large APU is essentially another turbine, you're back to using a turbine to propel the aircraft. If this really were a place to save money then the simplest solution would just have the tug pull it all the way to the runway. That way the aircraft doesn't need to take on additional weight for motors/apus which will sap fuel economy during flight.
mshook 7y ago
To give some perspective, the hourly fuel consumption for Concorde is almost identical at Mach 2 than at max subsonic speed... And Concorde powerplant had at some point the highest overall thermal efficiency at max speed: https://en.wikipedia.org/wiki/Rolls-Royce/Snecma_Olympus_593 https://en.wikipedia.org/wiki/Rolls-Royce/Snecma_Olympus_593 The overall thermal efficiency of the engine in supersonic cruising flight (supercruise) was about 43%, which at the time was the highest figure recorded for any normal thermodynamic machine.
everdev 7y ago
> Expect worse taxiing, take-off and climb fuel economy, due to dense air and also suboptimal (high) angle of attack. This reminds me, in the book "Skunk Works", SR-71 Blackbird pilots mentioned their schedule of mid-air refueling 20-30min after takeoff. After that they could make it 3-5hrs at high altitude before refueling again.
- dexen 7y ago
  The SR-71 did take off with much less than full fuel load; IIRC to keep the mass low enough for safe T/O abort or maybe for a safe go-around. As for books, I wholeheartedly recommend the excellent https://www.barnesandnoble.com/w/sled-driver-brian-shul/1123287835 https://www.barnesandnoble.com/w/sled-driver-brian-shul/1123... written by one of the pilots.
  - blackflame 7y ago
    Yea the SR-71 has very poor lift at slow speeds because of its lack of wing hence why it needs the chines on the forward section of the fuselage.
- skykooler 7y ago
  This is because the Blackbird took off with only partially filled tanks, to lower the weight and therefore the takeoff speed. It was not designed to take off or land with full tanks.
  - gpderetta 7y ago
    Also the tanks leaked untill thermal expansion at supersonic speed sealed them.
    - utopian3 7y ago
      Wow, shouldn’t that be a safety concern? Fuel leaking all over the airbase prior to takeoff?

Robotbeat 7y ago

Fundamentally, about half as efficient. You can compensate by cramming in seats closer or trying exotic designs. And there's some efficiency buy-back by operating at higher altitudes where the engine can expand to lower pressures (and thus increase the pressure ratio), but overall, roughly half as efficient is a decent guesstimate. Then again, conventional aircraft are about double the efficiency they used to be, so supersonic aircraft today could conceivably be approximately the same efficiency as transonic aircraft of yesteryear. I think exotic designs could enable decent range low supersonic electric flight (i.e. Mach 1.5 or something). Going to need good batteries, though, and extremely high aspect ratio wings.

blackflame 7y ago
A lot of the fuel efficiency of modern aircraft comes from having giant turbofans instead of turbojets. By attaching a giant fan the front of your compressor and redirecting most of the air around the engine you can get great efficiency at subsonic speeds. However, the nature of the fan has fundamental limitations when approaching the speed of sound and turbo/ram jets are basically just pouring gas in a tube of precompressed air.