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Comment by crazygringo | Hacker News Reader

Oh interesting.

Looking it up, the Concorde flew at 60,000 ft, compared to normal planes at 30,000 ft.

And atmospheric pressure at 60k ft is less than a quarter of what it is at 30k.

Is it possible to fly twice the speed of a regular aircraft but without using that much more fuel, by flying higher? Or does the plane have to burn even more fuel to get up to that altitude and maintain lift in a thinner atmosphere?

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Yes. It's one of the reasons liquid hydrogen is potentially a better fuel than kerosene. You need a larger volume of fuel, but you carry less mass which makes cruise more efficient, and hydrogen burns so lean that you can fly around twice as high. You would cruise around 70k ft, instead of the usual 35k.

You do need more energy to go that high and fast, but it take a bit of analysis to figure out if it's problematic. Most likely it would be driven by the mission profile as to whether or not the trade-off is worth making.

One of the bits of trivia about the Concorde: It nominally flew at 60k feet, but a "cosmic radiation" sensor was added, and if went above some limit, they would descend to 47k feet or lower.

Not sure the quotes are necessary. That's exactly what it was.

"In the days when the supersonic transport was in active service, and cruising at between 60 and 68,000 feet, the estimated radiation received by the crew was 50-130 mSv/yr. thus, obviously, as newer generations of aircraft cruise ever higher, by the time we reach altitudes above 60,000 feet, it is entirely possible that, especially with crews flying trans-Atlantic or transpolar routes, the acceptable maximum safe dose of radiation per year will be exceeded. In addition, these numbers do not take into account the possibility of pregnancy in female crewmembers. "

See Van Allen Belts.

I haven’t ever thought of cosmic radiation in that way before. It would be interesting to know more data on the rate of bit flips when flying at altitude.

If I edit my photos on the flight home, am I more likely to corrupt my files with bit flips?

Yes, even at normal flight altitudes rays energetic enough to flip bits are hundreds of times more common than at sea level. It would be a fairly poor idea to put your new datacenter in La Paz (or Utah for that matter ... attention: NSA).

I just imagined this as Nethack, where a pilot readout suddenly says, "Oh wow! Everything looks so cosmic!" and then everyone/everything in the cabin begins hallucinating.

Not twice as much I think, but it's definitly more efficent to fly higher if it's in the efficent envelope of these engines , just alone cause winds up there are way way faster which can be very favorable, because of the Coriolis Effect

Take it just a little further: why not ballistic flight? Sure, they call it the "vomit comet" for a reason but with the right marketing and some investment in gravol, I feel like the zero g portion of the flight should be a selling point.

The Pentagon is investigating using SpaceX's Starship for point to point travel...

There's also no reason to keep everyone in the same capsule once you're in space: you could have multiple independent reentry vehicles. Fire off a rocket somewhere West of Chicago, send a dozen people to Stockholm, a dozen to London, a bit of cargo to Prague.

As long as nothing ever seems to be mysteriously off course and heading for Moscow it sounds like a great idea. Is it easy to distinguish a ballistic missile full of passengers from the more bad kind?

Fit them with transponders and broadcast schedules well in advance of flights. The destination can veto a flight at any time before launch, and have inspectors at the point of launch. In this way, Moscow (etc) could keep arrivals down to a limit that their antiballistic defenses can handle, and ensure that only people-carriers are being put into their airspace. An attacker would be limited to a single kinetic suicide attack before their target goes on high alert.

But I was more curious about the fuel efficiency...

Which is offset by the need to climb through 30,000 ft on the way to 60,000 ft.

It turns out that higher cruising altitude isn’t nearly as useful as atmospheric pressure might suggest. Aircraft end up optimized for their cruising altitude, but there’s a lot of tradeoffs when targeting a higher altitude.

Doesn't that also imply that it would be possible to fly a modified plane at the regular speed of a regular aircraft at that much higher altitude, and use way less fuel? Airlines' main cost driver is fuel, so it seems like they'd take advantage of that as much as possible.

I think that would require a different plane design since they'd have less lift at that altitude.

It's not that easy.

If you go higher, the speed of sound decreases so you have to go slower to avoid going above the critical Mach number for your aircraft. Normal airliners have to stay quite a bit below Mach 1 to avoid any part of the airflow going supersonic, since that would create big issues like shock waves making the aircraft uncontrollable.

But due to the air being really thin, you also have to go faster. Otherwise your wings will not generate enough lift to keep flying.

At some point you cannot go faster and you cannot go slower, that effect is called Coffin Corner and limits how high a subsonic airliner could fly: https://en.wikipedia.org/wiki/Coffin_corner_(aerodynamics)

There’s a balance. And the Concorde had to have weird stuff to get that high but yes - you can get better performance higher to a point.

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