4 ms·

> would burn up to 9x more fuel per passenger per kilometer That number seems a bit hyperbolic, even for Concorde, which was 1960s technology. Modern superson

> would burn up to 9x more fuel per passenger per kilometer That number seems a bit hyperbolic, even for Concorde, which was 1960s technology. Modern supersonics will be far lighter, use turbofans instead of turbojets, not require (or even have) afterburners etc. They also claim that they will be net-zero Carbon due to advances in sustainable aviation fuels, with processes like CO2-fuel direct air capture. The truth will probably be somewhere in-between.

heisenbit 3y ago
> They also claim that they will be net-zero Carbon due to advances in sustainable aviation fuels, with processes like CO2-fuel direct air capture. This fig leaf is transparent.
- Maxion 3y ago
  If I burn 10^21 gallons of sustainable, carbon neutral, aviation fuel every year, what would happen? Somehow I think the CO2 sensor on Mauna Loa would need to be dug out from a piece of percipitated carbon.
  - regularfry 3y ago
    Mostly what would happen is a lot of questions about where you are hiding that Dyson sphere you need to power the carbon capture.
- zmgsabst 3y ago
  How so? If the fuel is made from captured CO2, then there isn’t any net CO2 from burning it after.
  - ajuc 3y ago
    You could capture the CO2 and take a train. No matter how you get your clean fuel - burning more of it is worse for environment than burning less of it.
    - zmgsabst 3y ago
      Okay — but there’s no harm to the environment. You’re just not improving it. The grandparent claimed this was a “fig leaf”: instead, you made an ideological post about how we need to not do even environmentally neutral things — that doesn’t make carbon neutral air travel a “fig leaf”.
      - ajuc 3y ago
        You can say anything is carbon neutral if you use synthetic fuel. It's meaningless at best, and manipulative at worst. Producing synthetic fuel requires energy.
        zmgsabst 3y ago
        Okay — like solar? …or nuclear? …or hydroelectric? …maybe geothermal? And yes, that’s correct: Synthetic fuels mean that it’s carbon neutral — that’s a fact, not a fig leaf.
        ajuc 3y ago
        Like oil, natural gas and coal which is top 3 of our energy sources worldwide. Together in 2021 they amounted to 130 000 out of 160 000 TWh of energy produced. Wasting energy is bad. Even if we actually used synthetic fuel. Even if we actually moved to 100% renewable energy sources - we'd still have limited amount of energy. And before we do - it's just PR. Source: https://i.imgur.com/vZkz3sf.png https://i.imgur.com/vZkz3sf.png https://ourworldindata.org/energy-mix https://ourworldindata.org/energy-mix
        regularfry 3y ago
        What's missing from all this is how stunningly impractical the route from atmospheric CO2 to aviation fuel actually is. It's multiple GJ/tonne just to reverse atmospheric entropy and get the CO2 where you want it, and that's before you have to climb backwards up combustion's enthalpy cliff to turn it back into usable fuel. But the capture cost is a rounding error, even at 1GJ/tonne. People often have the wrong mental model of how energy dense aviation fuel actually is. A 747 burns 4 litres of fuel per second. Do the maths on aviation fuel energy content, and that comes out in the order of 120MW. Megawatts. With an M. 10^6. Those small modular reactors everyone's excited about? One of those. That's the thermodynamic range anything proposed has to operate in. Ok, so divide that by the number of passengers (say 400 to make the maths easy) and it's 300KW per passenger. Multiply by 9 for our fancy new supersonic plane: 2.7MW per passenger. Concorde had 128 seats; let's say ours has 100. 270MW for the whole thing. Now, a transatlantic flight might be 10 hours. Assume a spherical cow in a vacuum and 270MW x 10h = 2.7GWh. Say two flights per day doubles it, but they're only 5 hours each supersonic, so halve back down and that cancels out. Round to 3GWh to make the maths easy. In joules: 11TJ. With a T. 10^12. Now, we need to fuel this thing. Which means we need to convert enough energy into enough fuel. If we completely ignore the energy cost of actually capturing the carbon, 11TJ is the absolute lower bound. It will take more than that, but in the interests of generosity let's put that to one side. 86400 seconds per day gives us 127MW. So the good news is that the total energy requirements for this one aircraft to run on fuel made of already captured carbon are within the bounds of what we know how to build. You can, if you are a government, choose to buy these things. The bad news is that you need one per aircraft, for the eventual ability to fling 100 meatsacks across the Atlantic a bit faster. I would suspect, looking at the costs (in terms of energy, money, space) that this is not a choice a rational actor would make.
  - Maxion 3y ago
    Capturing Co2 requires energy, laws of thermodynamics say there is no way to get free energy. Any Co2 capture system will undoubtedly be responsible for some small amount of Co2 emissions.
carabiner 3y ago
The 9x factor seems in the ballpark when you consider a Mach 2 jetliner will be going 2.5-3x faster than a subsonic one, and drag increases with the square of the airspeed.
- implements 3y ago
  At constant altitude. I believe Concorde was actually more fuel efficient per passenger mile than a 747 when flying Mach 2 up near 60,000ft where the air was very thin, but as it took half the flight’s fuel simply to get that high and that fast the overall efficiency was much lower.
  - labcomputer 3y ago
    I am extremely skeptical of that claim. It might be true for the original 747-100 (with GE’s first turbofan engine) in unfavorable conditions, but it certainly isn’t for any 747 made in the last 40 years with even a semi-optimal flight plan. Wikipedia gives the TSFC of Concorde’s Rolls-Royce 593 engines as 1.195 lb/lbf-hr and 0.6 lb/lbf-hr for a 747-400’s Rolls-Royce RB211. So that’s 45% less fuel burn per pound of thrust in favor of the 747. Furthermore, drag increases as the cube of airspeed (which corresponds to fuel per mile scaling as speed squared). That means at constant air density, Concorde would consume (2.37)^2 = 5.6 times more fuel (not accounting for the difference in engines) A 747 cruising very low at 30k feet would have ~4 the air density to contend with, but even that doesn’t make up for the speed difference. 747-400’s routinely cruise at over 40k feet… Unfortunately, you can’t just wave your hands and shout “supersonic turbofan”, because high bypass ratios (what accounts for subsonic efficiency gains since the 1960’s) are incompatible with supersonic flight. Edit, one more thing: The General Electric GEnx found on modern 747-8’s are 15% more efficient than the old 747-400’s engines! What is true is that a Concorde flying at subsonic speeds is more efficient per passenger mile than a 747. But that’s mostly due to the very high fineness ratio and drag-reduction-above-all-else design philosophy.
hef19898 3y ago
The Concorde is still pretty top notch, tech wise. As far as supersonic and supercruise jet engines go, they are all military. Civilian engine development went all in improving noise levels and fuel economy. And without a viable market, no engine OEM will invest the billions needed to develop a civil supersonic engine. So it is either a huge market, or various governments foot the RnD bills. Both of which are far, far away. All technological promises regarding civilian supersonics only exist in fancy renderings and pitch decks, there is no substance behind. Even less than there is with the various eVTOLs, at least those actually fly. Well, some of them that is...

heisenbit 3y ago

> They also claim that they will be net-zero Carbon due to advances in sustainable aviation fuels, with processes like CO2-fuel direct air capture. This fig leaf is transparent.

Maxion 3y ago
If I burn 10^21 gallons of sustainable, carbon neutral, aviation fuel every year, what would happen? Somehow I think the CO2 sensor on Mauna Loa would need to be dug out from a piece of percipitated carbon.
- regularfry 3y ago
  Mostly what would happen is a lot of questions about where you are hiding that Dyson sphere you need to power the carbon capture.
zmgsabst 3y ago
How so? If the fuel is made from captured CO2, then there isn’t any net CO2 from burning it after.
- ajuc 3y ago
  You could capture the CO2 and take a train. No matter how you get your clean fuel - burning more of it is worse for environment than burning less of it.
  - zmgsabst 3y ago
    Okay — but there’s no harm to the environment. You’re just not improving it. The grandparent claimed this was a “fig leaf”: instead, you made an ideological post about how we need to not do even environmentally neutral things — that doesn’t make carbon neutral air travel a “fig leaf”.
    - ajuc 3y ago
      You can say anything is carbon neutral if you use synthetic fuel. It's meaningless at best, and manipulative at worst. Producing synthetic fuel requires energy.
      - zmgsabst 3y ago
        Okay — like solar? …or nuclear? …or hydroelectric? …maybe geothermal? And yes, that’s correct: Synthetic fuels mean that it’s carbon neutral — that’s a fact, not a fig leaf.
        ajuc 3y ago
        Like oil, natural gas and coal which is top 3 of our energy sources worldwide. Together in 2021 they amounted to 130 000 out of 160 000 TWh of energy produced. Wasting energy is bad. Even if we actually used synthetic fuel. Even if we actually moved to 100% renewable energy sources - we'd still have limited amount of energy. And before we do - it's just PR. Source: https://i.imgur.com/vZkz3sf.png https://i.imgur.com/vZkz3sf.png https://ourworldindata.org/energy-mix https://ourworldindata.org/energy-mix
        regularfry 3y ago
        What's missing from all this is how stunningly impractical the route from atmospheric CO2 to aviation fuel actually is. It's multiple GJ/tonne just to reverse atmospheric entropy and get the CO2 where you want it, and that's before you have to climb backwards up combustion's enthalpy cliff to turn it back into usable fuel. But the capture cost is a rounding error, even at 1GJ/tonne. People often have the wrong mental model of how energy dense aviation fuel actually is. A 747 burns 4 litres of fuel per second. Do the maths on aviation fuel energy content, and that comes out in the order of 120MW. Megawatts. With an M. 10^6. Those small modular reactors everyone's excited about? One of those. That's the thermodynamic range anything proposed has to operate in. Ok, so divide that by the number of passengers (say 400 to make the maths easy) and it's 300KW per passenger. Multiply by 9 for our fancy new supersonic plane: 2.7MW per passenger. Concorde had 128 seats; let's say ours has 100. 270MW for the whole thing. Now, a transatlantic flight might be 10 hours. Assume a spherical cow in a vacuum and 270MW x 10h = 2.7GWh. Say two flights per day doubles it, but they're only 5 hours each supersonic, so halve back down and that cancels out. Round to 3GWh to make the maths easy. In joules: 11TJ. With a T. 10^12. Now, we need to fuel this thing. Which means we need to convert enough energy into enough fuel. If we completely ignore the energy cost of actually capturing the carbon, 11TJ is the absolute lower bound. It will take more than that, but in the interests of generosity let's put that to one side. 86400 seconds per day gives us 127MW. So the good news is that the total energy requirements for this one aircraft to run on fuel made of already captured carbon are within the bounds of what we know how to build. You can, if you are a government, choose to buy these things. The bad news is that you need one per aircraft, for the eventual ability to fling 100 meatsacks across the Atlantic a bit faster. I would suspect, looking at the costs (in terms of energy, money, space) that this is not a choice a rational actor would make.
- Maxion 3y ago
  Capturing Co2 requires energy, laws of thermodynamics say there is no way to get free energy. Any Co2 capture system will undoubtedly be responsible for some small amount of Co2 emissions.

carabiner 3y ago

The 9x factor seems in the ballpark when you consider a Mach 2 jetliner will be going 2.5-3x faster than a subsonic one, and drag increases with the square of the airspeed.

implements 3y ago
At constant altitude. I believe Concorde was actually more fuel efficient per passenger mile than a 747 when flying Mach 2 up near 60,000ft where the air was very thin, but as it took half the flight’s fuel simply to get that high and that fast the overall efficiency was much lower.
- labcomputer 3y ago
  I am extremely skeptical of that claim. It might be true for the original 747-100 (with GE’s first turbofan engine) in unfavorable conditions, but it certainly isn’t for any 747 made in the last 40 years with even a semi-optimal flight plan. Wikipedia gives the TSFC of Concorde’s Rolls-Royce 593 engines as 1.195 lb/lbf-hr and 0.6 lb/lbf-hr for a 747-400’s Rolls-Royce RB211. So that’s 45% less fuel burn per pound of thrust in favor of the 747. Furthermore, drag increases as the cube of airspeed (which corresponds to fuel per mile scaling as speed squared). That means at constant air density, Concorde would consume (2.37)^2 = 5.6 times more fuel (not accounting for the difference in engines) A 747 cruising very low at 30k feet would have ~4 the air density to contend with, but even that doesn’t make up for the speed difference. 747-400’s routinely cruise at over 40k feet… Unfortunately, you can’t just wave your hands and shout “supersonic turbofan”, because high bypass ratios (what accounts for subsonic efficiency gains since the 1960’s) are incompatible with supersonic flight. Edit, one more thing: The General Electric GEnx found on modern 747-8’s are 15% more efficient than the old 747-400’s engines! What is true is that a Concorde flying at subsonic speeds is more efficient per passenger mile than a 747. But that’s mostly due to the very high fineness ratio and drag-reduction-above-all-else design philosophy.

hef19898 3y ago

The Concorde is still pretty top notch, tech wise. As far as supersonic and supercruise jet engines go, they are all military. Civilian engine development went all in improving noise levels and fuel economy. And without a viable market, no engine OEM will invest the billions needed to develop a civil supersonic engine. So it is either a huge market, or various governments foot the RnD bills. Both of which are far, far away. All technological promises regarding civilian supersonics only exist in fancy renderings and pitch decks, there is no substance behind. Even less than there is with the various eVTOLs, at least those actually fly. Well, some of them that is...