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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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  • $\begingroup$ @uhoh fix it i think you may change it. $\endgroup$
    – Muze
    Commented Dec 26, 2018 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
    Commented Dec 27, 2018 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
    Commented Dec 26, 2018 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
    Commented Dec 26, 2018 at 22:55
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    $\begingroup$ but even that is a legal impact, not a physical one $\endgroup$
    – user20636
    Commented Dec 27, 2018 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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