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Comment by hnuser123456 | Hacker News Reader

Huh. The longest flights are around 10,000 miles. They usually cost over $1000. Fuel apparently accounts for about 25% of ticket price on long haul, so $250 in fuel normally. To do that in 4 hours is to travel 2,500 mph. Naively, traveling twice the speed requires 8x the power, so going over 4x the usual 550mph should mean over 64x more fuel burn, or $16,000 in fuel alone. Maybe a bit less since drag doesn't grow quite as quickly above transonic, call it $10,000. But if a ticket's only $100, I guess they've figured out how to get gas for 0.25% of typical prices.

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The air density decreases exponentially with the altitude, while the drag only increases quadratically with speed. It is entirely possible that there is an altitude, maybe 70km, where it is much more economical to fly (at supersonic speeds) than the current subsonic planes. Most likely the CEO of Boom ran the numbers, and the $100 ticket price is doable, at least if you exclude things like profit, capital depreciation, insurance, etc.

> Most likely the CEO of Boom ran the numbers, and the $100 ticket price is doable

Most likely it's aspirational, something to market to investors and potential employees.

> something to market to investors and potential employees

Neither the investors nor the potential employees strike me as gullible. By the way, the $100 ticket price target was not for the first aircraft, see [1]:

  > The four hour, $100 dream is Boom’s long-term aim, two or three generations of aircraft down the line.

[1] https://www.cnn.com/travel/article/boom-supersonic-four-hour...

Yes, and self driving cars are right around the corner!

I don't get the analogy. The Boom CEO explicitly stated that $100 tickets are not around the corner. Two or three generations down the line means four decades at a minimum, if we think one generation takes 20 years. Lots of things can happen in 4 decades, like: significant advances in ramjet engines, rotation detonation engines become mainstream, people get comfortable with windowless aircraft (so there's no need for drooping nose Concorde-style), airports could start being equipped with arresting wires, like aircraft carriers today, airplanes without the vertical tail fin become common place, stronger and lighter composites become available, and, who knows, maybe even some jets will start running on hydrogen rather than jet fuel (hydrogen having about 3 times more energy density per unit of mass). I have to admit the even with all these things, $100 per ticket to any place in the world still seems like a stretch, but I'm willing to give the guy the benefit of the doubt.

>Naively, traveling twice the speed requires 8x the power, so going over 4x the usual 550mph should mean over 64x more fuel burn

You've forgotten to cancel the denominator. If you use the drag relation of speed to power, you're multiplying by time, but the time is reduced by the speed. It would be more straightforward to use the F ~ v^2 relation between speed and force. So going 4x as fast for the same distance would require 16x the fuel, while going 4x as fast for the same time would require 64x the fuel. But the latter would obviously never happen in practice as you'd circumnavigate the Earth.

Oops, good catch, thank you. I got too sloppy with the napkin math.

Once you get out of the atmosphere, drag (and fuel consumption) is ~0. So theoretically possible, but I'm not sure if that's what he was talking about. Certainly Overture won't be capable of that.

Fair, I hadn't considered the intercontinental ballistic passenger missile approach.

Isn't that specifically one of the types of travel predicted to be made possible by reusable rockets capable of landing on the ground? From Florida to Japan in 45 minutes type of thing

Yes point to point travel was a market for Starship. I think they’ve mostly backed off that though, as Starlink offers an easier market opportunity and just as much revenue potential.

The supersonic plane would have advantages over the rocket approach though. Rockers require long, inconvenient transfers to offshore launch facilities. (But would have the selling point of a microgravity transit.)

Reaction Engines in the UK spent over 35 years working mostly on that concept (though when they eventually went bust trying to scale up last year I think they were focused on reusable space launch business model which is ironically more realistic)

No, they were working on the latter (skylon) most of the time, though the new management that came in after their £60M investment quickly dropped SSTO in favour of more immediate RoI applications. The passenger plane was LAPCAT which was a paper study commissioned by the EU. They did some interesting real work too, such as designing and testing a hypersonic engine combustion chamber that could reduce NOx emissions, which would be a big problem in any ‘conventional’ (eg scramjet) hypersonic engine.

ICPM! That's a new acronym you just coined!

> Fair, I hadn't considered the intercontinental ballistic passenger missile approach.

The terminal deceleration on an ICBM trajectory would be lethal. Ballistic passenger transport at global distances has to be almost orbital so the entry is sufficiently shallow.

Once you get out of the atmosphere, lift is ~0 too.

If you're going fast enough, you don't need lift.

But judging by "in four hours" I'm guessing he's imagining something somewhere in between those two extremes. High enough to substantially reduce drag, low enough that you don't need to approach orbital velocity to maintain altitude.

"Fast enough" is very nearly orbital speed, though. Suborbital range is very short on the lower end, and increases rapidly and nonlinearly later. E.g. if you can boost to 2km/s (~ Mach 7), this gives you, I kid you not, around 200km of ballistic range. It's either atmospheric flight or orbital flight, and there's nothing really useful in between.

GP is not talking about a ballistic trajectory though.

One possibility is a trajectory that's a series of skips.

There actually is still significant lift. We define the edge of the atmosphere to be where the lift to drag ratio of a plane would be less than 1 below orbital velocity (ie if you were going fast enough to lift your weight with conventional wings you'd be in orbit), so you can't fly conventionally in space but lift might still be generating a force which is significant compared to your craft's weight.

Well the assumption was that there is no drag because the air density is so low. You can’t just say there’s no drag but still assume that you get lift. Your lift/drag ratio won’t go up infinitely just because you’re flying higher.

GP's assumption was travelling through space to avoid drag which doesn't necessarily imply generating lift in space.

My comment was not a support of that argument, but a clarification that simply being in space does not automatically mean no aerodynamic forces. I'm also not saying L/D increases, actually the opposite happens at higher speeds and altitudes.

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