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Comment by Ajedi32 | Hacker News Reader

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.

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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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