5 ms·

Regarding the “supersonic is now viable because LNG” argument, but for a different reason than usual. Even if supersonic flight becomes cheaper via new fuels o

Regarding the “supersonic is now viable because LNG” argument, but for a different reason than usual. Even if supersonic flight becomes cheaper via new fuels or propulsion, that doesn’t reset the baseline. The same advances (materials, engines, fuel handling, manufacturing) will also apply to subsonic aircraft, where the physics are already far more energy-efficient. So if supersonic gets “cheap,” traditional jets will get much cheaper. Airlines will always arbitrage toward the lowest energy-per-seat-km for most routes, and supersonic flight is structurally disadvantaged there (drag, noise, routing constraints). Historically, faster transport doesn’t replace slower transport wholesale; it creates a premium tier while pushing the mass market down to a lower cost/energy equilibrium. Concorde didn’t kill widebodies, widebodies got cheaper. My intuition: supersonic may of course exist as a niche (time-sensitive, premium), but its biggest impact would be indirect, accelerating efficiency gains that make conventional aviation even more dominant and cheaper.

foota 9mo ago
Hm... I don't know that I buy your argument, since just as you point out, traditional jets are already very optimized. One would assume there's less slack to pick up.
- api 9mo ago
  Fuel is a huge component of the cost of operating an airline, sometimes the largest component. LNG is a much cheaper fuel, so I can see it being adopted for mainstream aviation eventually. Existing jets could technically be converted, though the conservative nature of aviation would demand many years of testing before use on commercial flights. It's also a pathway to incremental decarbonizing of aviation. LNG releases less CO2 per unit energy than oil, and methane can be produced biologically or synthetically which offers a path to total (net) decarbonization.
  - masklinn 9mo ago
    > Even if fossil LNG is used, it releases less CO2 per unit energy. However released methane has a significantly worse greenhouse effect than CO2 (80x over 20 years, 28 over 100, 8 over 500 — this decreases because methane has an atmospheric lifetime of 12 years and decays to CO2). So leakage in the LNG chain is a massive problem.
    - mr_toad 9mo ago
      Jet engines don’t release the methane, they burn it, and they’re very efficient. And jets don’t leak fuel, that would be very hazardous.
      - masklinn 9mo ago
        LNG is not a nice liquid at room temp, so more use means more in transit and more complicated supply chains, meaning more leaks.
    - akoboldfrying 9mo ago
      Right, but are leakage rates high enough to make this a concern? Every methane molecule leaked is a methane molecule not burnt, so there's already a strong profit maximisation incentive to leak as little as possible (even before considering loftier goals like workplace safety or externally imposed regulation).
      - masklinn 9mo ago
        > Right, but are leakage rates high enough to make this a concern? According to a recent IEA report, 30% of the LMG supply chain’s greenhouse impact is methane leaks. > there's already a strong profit maximisation incentive to leak as little as possible That runs against the stronger profit maximisation incentive of doing as little maintenance and being as cheap as possible.
    - api 9mo ago
      A major difference is: there is an economic incentive to not leak methane since a leak is wasted fuel, while the economic incentive for CO2 is to make more of it.
      - masklinn 9mo ago
        > A major difference is: there is an economic incentive to not leak methane since a leak is wasted fuel That economic incentive only goes so far given the entire point of the discussion: LNG is cheap. Per the IEA's recent "Assessing Emissions from LNG Supply and Abatement Options": > Our analysis estimates total GHG emissions from the LNG supply chain are around 350 million tonnes of carbon dioxide equivalent (Mt CO2-eq) (this excludes emissions from combustion of the natural gas at the point of use). Around 70% of this is in the form of CO2 emissions which are either combusted or vented, and the remaining 30% is methane that escapes, unburnt, into the atmosphere. > ... > Globally, the average GHG emissions intensity of delivered LNG is just under 20 g CO2-eq/MJ, compared with an average of 12 g CO2/MJ for natural gas supply overall.
- paganholiday 9mo ago
  Traditional jets have a long inventory and regulation cycle but for example retrofitting a A320 to LNG appears to save 20%: https://repository.tudelft.nl/record/uuid:63b89022-ac68-426d-8213-8470bd816e11 https://repository.tudelft.nl/record/uuid:63b89022-ac68-426d... Which still puts it behind the 787 let alone the generation that comes next.. But you aren't going to succeed at making any new inventory without every possible efficiency improvement to drive sales and retirement of older inventory.
MattGaiser 9mo ago
And even then, it is only so premium. As you could have a speedy economy seat on the Concorde or a lie flat bed on a widebody by the time Concorde left service. The speed benefit largely goes away if I can travel while sleeping.
- bobthepanda 9mo ago
  Halving travel times would be really good, the problem is that supersonic never had the range to make the difference meaningful. JFK-London in 3 hours vs 6 is pretty tolerable if you’re more comfortable for the 6 hours. SFO to Shanghai in 7 hours vs 14 would be a lot more compelling but Concorde could not do transpacific range.
  - thesumofall 9mo ago
    I actually prefer a 10+ hrs business class flight over a 6-7 hrs flight. At least you can get a full night of sleep
    - hdgvhicv 9mo ago
      Depends on the time zone change. From Europe to with Africa sure a 12 hour figggt is great. If I travel business London to singapore i get far worse jet lag than if I fly via the Middle East and break my journey for a few hours.
    - bobthepanda 9mo ago
      Past like hour 12 it becomes really draining. I used to fly JFK to HKG and that was an ordeal; I can’t imagine the newer super long haul like London to Perth.
  - HarHarVeryFunny 9mo ago
    That's the just flight time, but you've also got travel to/from the airport, parking, maybe shuttle bus/monorail, and checking/security/wait time at the airport as well. So, add an hour for door-door travel to airport, and 2-hr before flight check-in, and now the comparison isn't 3 vs 6hr but 9 vs 12hr, which doesn't sound so worthwhile, although no doubt there are customers for it. For longer flights it'd be much more attractive, but this is never going to be an affordable service for the masses.
HPsquared 9mo ago
Aeronautical engineering isn't that linear. A technology suited to one application may not be helpful in another. It's one of those "hardware is hard" fields.
masklinn 9mo ago
> Airlines will always arbitrage toward the lowest energy-per-seat-km for most routes That's a second order effect from fuel being the primary cost, and thus the primary lever to either make more profit or improve competitivity. If airlines could triple their profits by doubling their fuel burn they'd happily do that.
c_o_n_v_e_x 9mo ago
I agree with your market analysis. Private jets are often referred to as "time machines" given how much time HNW / exec travelers can save. There's a market segment that's willing to pay a high premium for reduced travel time.
- gerad 9mo ago
  For most of my trips, a huge % of the travel time is outside the actual flight time. Trip to the airport, security, boarding, waiting to take off, and reverse on the other side (with addition of potentially getting a rental car). This can be solved without supersonic solutions (e.g. flying private), but adoption is low for business travel – is it too expensive? Separately, I wonder if a lot of the demand is also obviated by in-air wifi.
  - vdqtp3 9mo ago
    > adoption is low for business travel – is it too expensive Yes. Most companies won't even spring for business/first class, which is 10-20% the cost of a charter. Unless your time is both limited and worth 4 digits per hour, it's not worth it.
  - s1gsegv 9mo ago
    You might also look at “semi-private” solutions like JSX, if they go where you want to go. Should significantly cut down on the time outside of the flight itself.
- expedition32 9mo ago
  Private jets don't go faster. They can just land closer to your destination and make you skip all the annoying airport stuff.
atoav 9mo ago
Want to reduce the time it takes to get somewhere? Reduce the security circus at airports. This will cut off way more of the travel time for the majority of flights, wothout the downsides of supersonic planes.
- 46493168 9mo ago
  TSA in the US is a jobs program
- ghaff 9mo ago
  You're going to have some security in any case and have since at least the 1970s. And it typically takes me <10 minutes even if I allow some extra time for the potential that it could be longer if it rarely is with pre-check.
  - atoav 9mo ago
    Yes but compare it to trains or busses. If I want to take the train/bus I make sure I am at the station 10 minutes before the vehicle departs. In my experience the good time to arrive before a flight (with luggage to check in) is roughly 1 hour before (and this nearly wasn't enough in some cases). If we talk about a short flight that can add more than 50% to the flight duration on the ground for (1) putting the luggage somewhere, (2) going through the security funnel and (3) getting to the plane. Sure I get why things are shaped the way they are, but if I wanted to cut travel time I would first have a long deep look at that.
    - ponector 9mo ago
      If you are flying to/from popular destination like Europe in August, you can stuck for two hours just in the luggage drop queue. Extra hour for security check queue and one more for border/customs check queue. Another 20 minutes just to walk to the gate. Depends on the airport and your luck, of course.
      - ghaff 9mo ago
fooker 9mo ago
You are missing an important factor in the baseline here, the cost of time. Right now, a cheap 7 hour each way round trip between NYC and London is ~500$. Halve it to 3.5 hours each way with a supersonic plane, saving a total of 7 hours. Now, the real question is then, what's one hour of your time worth to you or whoever is paying for your flight? If improvements to subsonic aircrafts bring down the price to 200$ instead of $500, people would still be willing to pay 200$ + 7 * $HOURLY for a faster flight. Even with a low-ish estimate of $HOURLY = 50, it would make sense to take the supersonic fight if the price was $500, which it could conceivably be brought down to, and the market has already validated to be willing to pay.
- kibwen 9mo ago
  Business passengers aren't out here paying for their own tickets. Their employers are paying for those tickets, so the question is whether or not companies care about the time their salaried employees spend in the air, when those employees can be just as productive on the business-class wifi.
  - ghaff 9mo ago
    Assuming they're even micro-managing employee productivity to the degree that they really care about working on a plane. Personally, I never purchased plane wi-fi even when I could have expensed it.
  - fooker 9mo ago
    Most employers book the most convenient flight for their employees, and not really care much but saving a few hundred dollars.
- j1elo 9mo ago
  That's a reasonable argument for businesspeople, but it doesn't apply for the greater public. Because chances are that except in a minority of situations, they are on holidays and during that saved time they wouldn't be working at all anyways. People who could perfecty afford a $2,000 plane ticket still fly with $400 ones (as long as they are within reasonable standards), for example because they have a desired budget for a given trip, and the expensive option would blow it away, so they don't mind the extra time.
credit_guy 9mo ago
That is not quite true. The advantages of LNG are much more important for high supersonic jets (Mach 2.5 and higher) than for subsonic jets. There are disadvantages too, and they are quite significant for all jets, but altogether the tradeoff is worth it at high speed long endurance supersonic jets. Here's why. LNG offers 2 main benefits. The first is the higher energy density (53.6 MJ/kg vs 43 MJ/kg, so 25% more [1]). Airplanes are subject to the rocket equation, even if they are not rockets. The rocket equation says that the mass of the fueled vehicle is the mass of the vehicle at the end of the trip times the exponential of delta-v divided by the exhaust velocity. For airplanes, it is not exhaust velocity, but "effective exhaust velocity", because they borrow a lot of reaction mass from the atmosphere (the air used as oxidizer, and more importantly, the bypass air). The effective exhaust velocity is very high for subsonic airplanes, and much lower for high supersonic airplanes. The delta-v for subsonic airplanes is lower than the delta-v for supersonic airplanes because of the lower drag (although not as much lower as one would expect, because they need a higher attack angle). Overall, the benefit from the high energy density LNG is much more pronounced for high supersonic jets. The second benefit is the use of the cryogenic LNG to cool off the engine. For very high speed engines, this is huge. So huge that the famous (but never materialized) SABRE engine was supposed to use liquid hydrogen, which is stored at much lower temperatures. The disadvantage of LNG is, surprisingly, not the need for cryogenic storage. It is the lower volumetric energy density. It is 22% lower than that of jet fuel. The rocket equation does not care about volumes, only about mass, but larger volumes means bigger airplanes, so more drag. So, for subsonic airplanes the advantages of LNG are not all that important, while the bulkier tanks are a pretty big downside. For high supersonic jets, the advantages of LNG are so high that they simply open up possibilities that are not there with jet fuel. The fact that the LNG is cheaper is a nice thing to have, but it's really not that important, since the economics of high supersonic jets are more impacted by the construction cost and very high maintenance cost than by the fuel cost. [1] https://en.wikipedia.org/wiki/Energy_density#Chemical_reactions_(oxidation) https://en.wikipedia.org/wiki/Energy_density#Chemical_reacti...
- vrighter 9mo ago
  Are you sure the numbers are correct? Wikipedia lists LNG as 53.6 MJ/kg and 22.2 MJ/L and jet fuel as 43MJ/kg and 35MJ/L. 53.6 / 43 = 1.247 22.2 / 35 = 0.634 Energy density by mass is higher than jet fuel. But it is higher by less than jet fuel's energy density by volume is higher than LNG. And LNG requires heavier tanks. So all in all, if it's not absolutely needed because of the cooling advantages, LNG can store less energy in the fuel tanks.
tw04 9mo ago
> Historically, faster transport doesn’t replace slower transport wholesale; it creates a premium tier while pushing the mass market down to a lower cost/energy equilibrium. If that were true, we’d all be taking trains and boats everywhere. We aren’t.
- Manuel_D 9mo ago
  The difference between a supersonic jet and and a subsonic jet is about 2x. Between NY and London that's a 3.5 hour flight vs a 7 hour flight (plus the overhead of security, traveling to and from the airport, boarding, taxiing, etc. which brings down the proportional cost). By comparison, a boat would take 7-8 days. The disparity in time saved between supersonic and subsonic flight is pretty trivial in comparison to the time saved between a boat and a subsonic plane.
  - tw04 9mo ago
    Presumably there are no flights from Paris to Munich then? There are. A flight is about an hour and a half, a train is about 5 hours. Far more people fly between the two every day than take a train.
    - Manuel_D 9mo ago
      A 5x disparity is still a lot more than a 2x disparity. In fact, considerably less than 2x given the overhead time of traveling to the airport, security, boarding, etc. How many people would pay 8x the ticket price for a 45 minute flight from Paris to Munich (The Concorde was ~8x the ticket price of economy subsonic flight tickets)?
    - nottorp 9mo ago
      Isn't the flight cheaper as well?
K0balt 9mo ago
Surprisingly, at least in theory, and probably in practice with better technology, supersonic travel can be as efficient or even more efficient than subsonic flight. Supersonic travel opens up higher altitudes, higher altitudes means less air resistance. The ultra high altitudes of LEO satellites showcase the steelman example, traveling effortlessly through the vanishingly thin atmosphere at hypersonic speeds with extreme efficiency even though the fuel expenditure to get them there was high. For more reasonable hypersonic travel, at 100k feet, the “wind” force at 3375mph is only as much as you would feel at 400 mph at sea level… so you can exert the force needed to fly at 400mph, but for that same energy you are going 3375mph. Of course there is a lot of tech needed to take advantage of these efficiencies, but it’s not a matter of faster = less efficient. As for economies, a jet that can fly LA to NY in 70 minutes, with an hour of turn at each end, could make 10 trips a day, potentially cutting the number of aircraft needed to cover a given route or route rotation by a factor of 4. Obviously this is not currently practical on so many levels, but there is nothing fundamentally stopping us from achieving that level of service, given enough knowledge and technical capability. If we ever want to achieve that level of understanding and competence, we will have to work on it when it seems impractical. Remember, it was in a single persons lifetime between flying precariously in glorified kites and supersonic flight.

foota 9mo ago

Hm... I don't know that I buy your argument, since just as you point out, traditional jets are already very optimized. One would assume there's less slack to pick up.

api 9mo ago
Fuel is a huge component of the cost of operating an airline, sometimes the largest component. LNG is a much cheaper fuel, so I can see it being adopted for mainstream aviation eventually. Existing jets could technically be converted, though the conservative nature of aviation would demand many years of testing before use on commercial flights. It's also a pathway to incremental decarbonizing of aviation. LNG releases less CO2 per unit energy than oil, and methane can be produced biologically or synthetically which offers a path to total (net) decarbonization.
- masklinn 9mo ago
  > Even if fossil LNG is used, it releases less CO2 per unit energy. However released methane has a significantly worse greenhouse effect than CO2 (80x over 20 years, 28 over 100, 8 over 500 — this decreases because methane has an atmospheric lifetime of 12 years and decays to CO2). So leakage in the LNG chain is a massive problem.
  - mr_toad 9mo ago
    Jet engines don’t release the methane, they burn it, and they’re very efficient. And jets don’t leak fuel, that would be very hazardous.
    - masklinn 9mo ago
      LNG is not a nice liquid at room temp, so more use means more in transit and more complicated supply chains, meaning more leaks.
  - akoboldfrying 9mo ago
    Right, but are leakage rates high enough to make this a concern? Every methane molecule leaked is a methane molecule not burnt, so there's already a strong profit maximisation incentive to leak as little as possible (even before considering loftier goals like workplace safety or externally imposed regulation).
    - masklinn 9mo ago
      > Right, but are leakage rates high enough to make this a concern? According to a recent IEA report, 30% of the LMG supply chain’s greenhouse impact is methane leaks. > there's already a strong profit maximisation incentive to leak as little as possible That runs against the stronger profit maximisation incentive of doing as little maintenance and being as cheap as possible.
  - api 9mo ago
    A major difference is: there is an economic incentive to not leak methane since a leak is wasted fuel, while the economic incentive for CO2 is to make more of it.
    - masklinn 9mo ago
      > A major difference is: there is an economic incentive to not leak methane since a leak is wasted fuel That economic incentive only goes so far given the entire point of the discussion: LNG is cheap. Per the IEA's recent "Assessing Emissions from LNG Supply and Abatement Options": > Our analysis estimates total GHG emissions from the LNG supply chain are around 350 million tonnes of carbon dioxide equivalent (Mt CO2-eq) (this excludes emissions from combustion of the natural gas at the point of use). Around 70% of this is in the form of CO2 emissions which are either combusted or vented, and the remaining 30% is methane that escapes, unburnt, into the atmosphere. > ... > Globally, the average GHG emissions intensity of delivered LNG is just under 20 g CO2-eq/MJ, compared with an average of 12 g CO2/MJ for natural gas supply overall.
paganholiday 9mo ago
Traditional jets have a long inventory and regulation cycle but for example retrofitting a A320 to LNG appears to save 20%: https://repository.tudelft.nl/record/uuid:63b89022-ac68-426d-8213-8470bd816e11 https://repository.tudelft.nl/record/uuid:63b89022-ac68-426d... Which still puts it behind the 787 let alone the generation that comes next.. But you aren't going to succeed at making any new inventory without every possible efficiency improvement to drive sales and retirement of older inventory.

MattGaiser 9mo ago

And even then, it is only so premium. As you could have a speedy economy seat on the Concorde or a lie flat bed on a widebody by the time Concorde left service. The speed benefit largely goes away if I can travel while sleeping.

bobthepanda 9mo ago
Halving travel times would be really good, the problem is that supersonic never had the range to make the difference meaningful. JFK-London in 3 hours vs 6 is pretty tolerable if you’re more comfortable for the 6 hours. SFO to Shanghai in 7 hours vs 14 would be a lot more compelling but Concorde could not do transpacific range.
- thesumofall 9mo ago
  I actually prefer a 10+ hrs business class flight over a 6-7 hrs flight. At least you can get a full night of sleep
  - hdgvhicv 9mo ago
    Depends on the time zone change. From Europe to with Africa sure a 12 hour figggt is great. If I travel business London to singapore i get far worse jet lag than if I fly via the Middle East and break my journey for a few hours.
  - bobthepanda 9mo ago
    Past like hour 12 it becomes really draining. I used to fly JFK to HKG and that was an ordeal; I can’t imagine the newer super long haul like London to Perth.
- HarHarVeryFunny 9mo ago
  That's the just flight time, but you've also got travel to/from the airport, parking, maybe shuttle bus/monorail, and checking/security/wait time at the airport as well. So, add an hour for door-door travel to airport, and 2-hr before flight check-in, and now the comparison isn't 3 vs 6hr but 9 vs 12hr, which doesn't sound so worthwhile, although no doubt there are customers for it. For longer flights it'd be much more attractive, but this is never going to be an affordable service for the masses.

HPsquared 9mo ago

Aeronautical engineering isn't that linear. A technology suited to one application may not be helpful in another. It's one of those "hardware is hard" fields.

masklinn 9mo ago

> Airlines will always arbitrage toward the lowest energy-per-seat-km for most routes That's a second order effect from fuel being the primary cost, and thus the primary lever to either make more profit or improve competitivity. If airlines could triple their profits by doubling their fuel burn they'd happily do that.

c_o_n_v_e_x 9mo ago

I agree with your market analysis. Private jets are often referred to as "time machines" given how much time HNW / exec travelers can save. There's a market segment that's willing to pay a high premium for reduced travel time.

gerad 9mo ago
For most of my trips, a huge % of the travel time is outside the actual flight time. Trip to the airport, security, boarding, waiting to take off, and reverse on the other side (with addition of potentially getting a rental car). This can be solved without supersonic solutions (e.g. flying private), but adoption is low for business travel – is it too expensive? Separately, I wonder if a lot of the demand is also obviated by in-air wifi.
- vdqtp3 9mo ago
  > adoption is low for business travel – is it too expensive Yes. Most companies won't even spring for business/first class, which is 10-20% the cost of a charter. Unless your time is both limited and worth 4 digits per hour, it's not worth it.
- s1gsegv 9mo ago
  You might also look at “semi-private” solutions like JSX, if they go where you want to go. Should significantly cut down on the time outside of the flight itself.
expedition32 9mo ago
Private jets don't go faster. They can just land closer to your destination and make you skip all the annoying airport stuff.

atoav 9mo ago

Want to reduce the time it takes to get somewhere? Reduce the security circus at airports. This will cut off way more of the travel time for the majority of flights, wothout the downsides of supersonic planes.

46493168 9mo ago
TSA in the US is a jobs program
ghaff 9mo ago
You're going to have some security in any case and have since at least the 1970s. And it typically takes me <10 minutes even if I allow some extra time for the potential that it could be longer if it rarely is with pre-check.
- atoav 9mo ago
  Yes but compare it to trains or busses. If I want to take the train/bus I make sure I am at the station 10 minutes before the vehicle departs. In my experience the good time to arrive before a flight (with luggage to check in) is roughly 1 hour before (and this nearly wasn't enough in some cases). If we talk about a short flight that can add more than 50% to the flight duration on the ground for (1) putting the luggage somewhere, (2) going through the security funnel and (3) getting to the plane. Sure I get why things are shaped the way they are, but if I wanted to cut travel time I would first have a long deep look at that.
  - ponector 9mo ago
    If you are flying to/from popular destination like Europe in August, you can stuck for two hours just in the luggage drop queue. Extra hour for security check queue and one more for border/customs check queue. Another 20 minutes just to walk to the gate. Depends on the airport and your luck, of course.
    - ghaff 9mo ago

fooker 9mo ago

You are missing an important factor in the baseline here, the cost of time. Right now, a cheap 7 hour each way round trip between NYC and London is ~500$. Halve it to 3.5 hours each way with a supersonic plane, saving a total of 7 hours. Now, the real question is then, what's one hour of your time worth to you or whoever is paying for your flight? If improvements to subsonic aircrafts bring down the price to 200$ instead of $500, people would still be willing to pay 200$ + 7 * $HOURLY for a faster flight. Even with a low-ish estimate of $HOURLY = 50, it would make sense to take the supersonic fight if the price was $500, which it could conceivably be brought down to, and the market has already validated to be willing to pay.

kibwen 9mo ago
Business passengers aren't out here paying for their own tickets. Their employers are paying for those tickets, so the question is whether or not companies care about the time their salaried employees spend in the air, when those employees can be just as productive on the business-class wifi.
- ghaff 9mo ago
  Assuming they're even micro-managing employee productivity to the degree that they really care about working on a plane. Personally, I never purchased plane wi-fi even when I could have expensed it.
- fooker 9mo ago
  Most employers book the most convenient flight for their employees, and not really care much but saving a few hundred dollars.
j1elo 9mo ago
That's a reasonable argument for businesspeople, but it doesn't apply for the greater public. Because chances are that except in a minority of situations, they are on holidays and during that saved time they wouldn't be working at all anyways. People who could perfecty afford a $2,000 plane ticket still fly with $400 ones (as long as they are within reasonable standards), for example because they have a desired budget for a given trip, and the expensive option would blow it away, so they don't mind the extra time.

credit_guy 9mo ago

That is not quite true. The advantages of LNG are much more important for high supersonic jets (Mach 2.5 and higher) than for subsonic jets. There are disadvantages too, and they are quite significant for all jets, but altogether the tradeoff is worth it at high speed long endurance supersonic jets. Here's why. LNG offers 2 main benefits. The first is the higher energy density (53.6 MJ/kg vs 43 MJ/kg, so 25% more [1]). Airplanes are subject to the rocket equation, even if they are not rockets. The rocket equation says that the mass of the fueled vehicle is the mass of the vehicle at the end of the trip times the exponential of delta-v divided by the exhaust velocity. For airplanes, it is not exhaust velocity, but "effective exhaust velocity", because they borrow a lot of reaction mass from the atmosphere (the air used as oxidizer, and more importantly, the bypass air). The effective exhaust velocity is very high for subsonic airplanes, and much lower for high supersonic airplanes. The delta-v for subsonic airplanes is lower than the delta-v for supersonic airplanes because of the lower drag (although not as much lower as one would expect, because they need a higher attack angle). Overall, the benefit from the high energy density LNG is much more pronounced for high supersonic jets. The second benefit is the use of the cryogenic LNG to cool off the engine. For very high speed engines, this is huge. So huge that the famous (but never materialized) SABRE engine was supposed to use liquid hydrogen, which is stored at much lower temperatures. The disadvantage of LNG is, surprisingly, not the need for cryogenic storage. It is the lower volumetric energy density. It is 22% lower than that of jet fuel. The rocket equation does not care about volumes, only about mass, but larger volumes means bigger airplanes, so more drag. So, for subsonic airplanes the advantages of LNG are not all that important, while the bulkier tanks are a pretty big downside. For high supersonic jets, the advantages of LNG are so high that they simply open up possibilities that are not there with jet fuel. The fact that the LNG is cheaper is a nice thing to have, but it's really not that important, since the economics of high supersonic jets are more impacted by the construction cost and very high maintenance cost than by the fuel cost. [1] https://en.wikipedia.org/wiki/Energy_density#Chemical_reactions_(oxidation) https://en.wikipedia.org/wiki/Energy_density#Chemical_reacti...

vrighter 9mo ago
Are you sure the numbers are correct? Wikipedia lists LNG as 53.6 MJ/kg and 22.2 MJ/L and jet fuel as 43MJ/kg and 35MJ/L. 53.6 / 43 = 1.247 22.2 / 35 = 0.634 Energy density by mass is higher than jet fuel. But it is higher by less than jet fuel's energy density by volume is higher than LNG. And LNG requires heavier tanks. So all in all, if it's not absolutely needed because of the cooling advantages, LNG can store less energy in the fuel tanks.

tw04 9mo ago

> Historically, faster transport doesn’t replace slower transport wholesale; it creates a premium tier while pushing the mass market down to a lower cost/energy equilibrium. If that were true, we’d all be taking trains and boats everywhere. We aren’t.

Manuel_D 9mo ago
The difference between a supersonic jet and and a subsonic jet is about 2x. Between NY and London that's a 3.5 hour flight vs a 7 hour flight (plus the overhead of security, traveling to and from the airport, boarding, taxiing, etc. which brings down the proportional cost). By comparison, a boat would take 7-8 days. The disparity in time saved between supersonic and subsonic flight is pretty trivial in comparison to the time saved between a boat and a subsonic plane.
- tw04 9mo ago
  Presumably there are no flights from Paris to Munich then? There are. A flight is about an hour and a half, a train is about 5 hours. Far more people fly between the two every day than take a train.
  - Manuel_D 9mo ago
    A 5x disparity is still a lot more than a 2x disparity. In fact, considerably less than 2x given the overhead time of traveling to the airport, security, boarding, etc. How many people would pay 8x the ticket price for a 45 minute flight from Paris to Munich (The Concorde was ~8x the ticket price of economy subsonic flight tickets)?
  - nottorp 9mo ago
    Isn't the flight cheaper as well?

K0balt 9mo ago

Surprisingly, at least in theory, and probably in practice with better technology, supersonic travel can be as efficient or even more efficient than subsonic flight. Supersonic travel opens up higher altitudes, higher altitudes means less air resistance. The ultra high altitudes of LEO satellites showcase the steelman example, traveling effortlessly through the vanishingly thin atmosphere at hypersonic speeds with extreme efficiency even though the fuel expenditure to get them there was high. For more reasonable hypersonic travel, at 100k feet, the “wind” force at 3375mph is only as much as you would feel at 400 mph at sea level… so you can exert the force needed to fly at 400mph, but for that same energy you are going 3375mph. Of course there is a lot of tech needed to take advantage of these efficiencies, but it’s not a matter of faster = less efficient. As for economies, a jet that can fly LA to NY in 70 minutes, with an hour of turn at each end, could make 10 trips a day, potentially cutting the number of aircraft needed to cover a given route or route rotation by a factor of 4. Obviously this is not currently practical on so many levels, but there is nothing fundamentally stopping us from achieving that level of service, given enough knowledge and technical capability. If we ever want to achieve that level of understanding and competence, we will have to work on it when it seems impractical. Remember, it was in a single persons lifetime between flying precariously in glorified kites and supersonic flight.