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Comment by ucha | Hacker News Reader

To everyone saying this will not work because it's more expensive... jets were more expensive than turboprops. And even though the former use more fuel, we prefer them.

The costs of flying an airplane isn't proportional to its fuel usage. The faster an aircraft is, the more flights it can perform per day.

The carbon emissions impact of flying a gas-guzzling supersonic aircraft aren't evident either. Of course, more gas is used per trip but fewer planes need to be manufactured. Since there is no supersonic business jet, it could also make sense for some people who used to fly private for the speed and convenience to reconsider as they may get faster to their destination by flying supersonic.

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> The costs of flying an airplane isn't proportional to its fuel usage. The faster an aircraft is, the more flights it can perform per day.

It is indeed not proportional, but not in the way you are thinking. Drag (and ~fuel consumption) scales with velocity squared, so a plane flying twice at fast (and neglecting any time at the airport, which would make the argument even worse) would use four times as much fuel. I.e., even if twice the amount of passengers would be served, it would be done for four times the fuel consumption and four times the carbon emissions (or twice the fuel consumption per trip).

That doesn't have much to do with the point I'm making. I'm saying if you double fuel usage, you don't double the cost of using the plane.

As cost of fuel is only a percentage of the price of the ticket, it's pretty obvious that there is a threshold as a percentage of total ticket cost under which spending 4x more in fuel to fly say 1.5x more passengers (because the airplane isn't flying 24/7) makes business sense.

That is obviously one of the reasons why they are starting with business class tickets because, fuel consists of a smaller percentage of ticket cost.

Yeah, there is an epsilon on top of fuel. But maybe you missed the obvious fact that all aircraft are getting slower rather than faster over the last decades, so that threshold is in the opposite direction of what you're proposing.

And there is zero logic behind your second obviousm, as your premise is already wrong. The reason they start with business class is because you can charge more per seat. Seems pretty obvious.

Fuel cost, depending on routes, number of business seats, seat occupancy is at about 25% of total costs [0]. 75% is what you call epsilon? Aircraft speed as barely budged since we transitioned from turboprop to jets. That's something you could call epsilon.

You're right that they start with business seats because they cost more. Business class seats cost 3-4x time economy while occupying less than 2x the space so the cost of fuel as a percentage of the ticket price is lower. It might not be obvious to you but I'm happy to explain it :)

[0] https://www.mckinsey.com/industries/travel-logistics-and-inf...

> 2x the space so the cost of fuel as a percentage of the ticket price is lower. It might not be obvious to you but I'm happy to explain it :)

See, your claim is that fuel cost is the sole reason they do this. I'd argue it's obvious they'd do that even if all fuel was free.

> Aircraft speed as barely budged since we transitioned from turboprop to jets.

Ah, it's again one of those nonlinearities you seem to have trouble with. See, the cost increase is, again, not proportional to speed. On top of the quadratic scaling, you have a very nonlinear and steep (not-proportional!) increase in drag coefficient. So, when you look up that what I say is true, but you want to weasel your way out by saying 'it's not by much', you're missing that the impact on drag (and fuel) is substantial.

And fuel cost is typically more than double the capital cost in an airline's budget at subsonic speeds

And the proposed aircraft are less than half the size of the aircraft they'd most likely replace, so actually sell fewer tickets on double the flight numbers

> a plane flying twice at fast would use four times as much fuel

Not that this only applies if they fly at the same altitude. If you fly higher you can avoid that. That of course causes other problems but it is a relevant factor.

The fuel burned by an aircraft is almost certainly the dominant portion of its lifecycle carbon emissions. I expect the carbon emissions due to its manufacture are negligible by comparison. Have you seen a source either way?

You're right.

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