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Comment by _moof | Hacker News Reader

The Kármán line isn't arbitrary - or at least it wasn't when Kármán first conceived of it. It's supposed to be the altitude at which centrifugal force becomes more significant than aerodynamic lift. That's a definite altitude with significant implications.

Whether this definition is meaningful for a suborbital flight is debatable.

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Yes, but, like all the other factual bases for a definition, it is a concept that does not yield an exact number.

Er, not really. There is a specific, identifiable altitude where this happens.

Centrifugal force is a function of angular velocity, and atmospheric density varies with the weather and seasons.

From the paper imglorp linked to: "in reality the von Karman criterion defines a line whose altitude varies with position and time (because of variations in atmospheric density due to solar activity) and with the lift coefficient of the spacecraft."

https://www.sciencedirect.com/science/article/pii/S009457651...

I'm aware. I'm not saying the altitude is the exact same altitude at all times under all conditions. I'm saying there is a specific altitude where centrifugal force becomes the dominant force if you want to stay aloft. And that was Kármán's original conception. Yes, that altitude will vary with the atmospheric tides (and with C_L, as another commenter points out). But it is still a definite altitude and not at all arbitrary.

arbitrary: "based on random choice or personal whim, rather than any reason or system"

It is not based on random choice or personal whim. It is the result of a systematic analysis of the relationship between the forces required to sustain flight.

>I'm not saying the altitude is the exact same altitude at all times under all conditions.

You were disputing a post in which this was literally the only point I made: https://news.ycombinator.com/item?id=27801257

Nothing says "arbitrary" more than a quantity that was deliberately rounded up 9km just for the heck of it. Things can be guided by math/logic and still have arbitrary elements. The Karmen line definition is a logical definition with certain arbitrary constant selections, and therefore it's not specific. This doesn't mean it's random, only that there are band of valid definitions.

It's correct to state a boundary defined by centrifugal force dominance is somewhere between 80ish and 100ish km, and the specific number is somewhat arbitrary. It's not correct to say its for 20km (for example), which would be a truly random selection.

The specific identifiable altitude still requires an arbitrary selection of lift coefficient and vehicle mass in order to calculate. The calculation is vehicle dependent, not general. The values used in the original definition of the general case were set to be "typical", which yielded 91km. Because of the arbitrariness of this definition, it was defined to be 100km as that was "more memorable"

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