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I have also seen many posts on this. I understand what the line is used for legally. The Kármán plane or a plane that is most efficient at the line regardless of the shape seems not to have a specific purpose but maybe I am wrong?

I left this link below as an example on how the line might be used:

Challenging the Kármán line from above

enter image description here

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closed as too broad by Jörg W Mittag, Uwe, Russell Borogove, Steve Linton, Muze Dec 26 '18 at 0:24

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • $\begingroup$ @uhoh fix it i think you may change it. $\endgroup$ – Muze Dec 26 '18 at 18:12
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    $\begingroup$ One particular user seems to be obsessing about it, and resisting urges to take a look into an orbital mechanics book. Thus, ignorance prevails and the circle repeats itself. $\endgroup$ – AtmosphericPrisonEscape Dec 27 '18 at 0:51
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For a number of reasons human reasons like giving someone astronaut wings, winning the X prize or for rules around national boundaries there is a need for a hard divider between 'space' and 'atmosphere' and the calculations for the Karman line provide a convenient mathematical way to do that. The fact that the Karman line of 100km used for most purposes is actually some distance from the ~85km point that Karman calculated is possibly telling in terms of how meaningful the value is other than it being a round number higher than aircraft can fly and lower than space craft can sensibly sustain orbit.

So the Karman line is a legal construct for space law and administration, and almost meaningless for actual engineering.

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  • $\begingroup$ What about for reentry? $\endgroup$ – Muze Dec 26 '18 at 18:14
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    $\begingroup$ @Muze - The only real impact to re-entry is that legally you would need airspace and traffic clearance for every nation your profile crosses below 100km, so a complication in doing space operations out of smaller nations. All other engineering for heat/drag etc needs to start well above that point and be tapered in. $\endgroup$ – GremlinWranger Dec 26 '18 at 22:55
  • $\begingroup$ but even that is a legal impact, not a physical one $\endgroup$ – JCRM Dec 27 '18 at 21:44
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It's a pretty abstract boundary, with gradual changes either side, of no practical use.

No "line" has never[sic] found a practical use in engineering, and it has been misinterpreted by lawyers as having some physical significance it does not in fact have.

"How High the Sky?" Thomas Gangale, p. 156

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