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There have been several questions recently about the details of the definition of the Karman Line and the history of those calculations. This is not about those issues.

The question is: In practical, mission planning terms, is the Karman Line at 100km ever actually used?

I am not asking about a replacement definition for the line. Nor about alternative ideas of what the line could be. I am asking about whether having any line is useful at all.

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  • $\begingroup$ It is exactly as useful as the length of "a westward climb along the eastern and northern slopes of Mount Penteli to the pass of Dionysos, and then a straight southward downhill path to Athens", which is another challenging trip which "includes a very steep initial climb" and results in a patch. $\endgroup$
    – uhoh
    Oct 28 '18 at 6:45
  • $\begingroup$ There is a range around the Karman line. Within this range, no airplane could fly using the aerodynamical lift force and no satellite may stay in orbit longer without using thrust to fight drag. This range is just crossed by satellites inserted to orbit or reentering the atmosphere. But there is no exact definition of this range. $\endgroup$
    – Uwe
    Oct 28 '18 at 9:36
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The Karman line is used for two things:

  1. to legally separate the airspace above a country (over which the country has jurisdiction) from space (over which the country has no jurisdiction).
  2. to determine who has been "in space" and gets to be called an astronaut (but the USA uses a different definition, 80 km, to make sure the X-15 pilots were included).
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  • $\begingroup$ Not all X-15 pilots could be included by using the 80 km definition. The pilot of flight 64 Neil A. Armstrong had to wait some years for his astronaut badge. $\endgroup$
    – Uwe
    Oct 28 '18 at 8:56
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    $\begingroup$ Armstrong may not have cared a whole lot about his X-15 astronaut badge :) $\endgroup$ Oct 28 '18 at 13:26
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    $\begingroup$ The legal distinction is the most important thing about the Karman line. Launching things into another country's airspace is a very touchy subject, while satellites will necessarily fly over many countries. National airspace being legally cut off at a specific altitude is absolute necessity to make spaceflight politically possible. $\endgroup$
    – SF.
    Oct 28 '18 at 15:12
  • $\begingroup$ There may be countries who use the 100-km-mark as the upper national airspace boundary, but I dunno of any. There is no internationally binding boundary between national space and outer space. The American FAA is using the 50-mi-mark. According to Wikipedia, the Space Shuttle flew above another country (probably Mexico) less than 50 mi high without any uproar. On the other hand, some equatorial countries declared their national airspace up to geostationary orbit (which is higher than 20,000 mi). $\endgroup$ Jun 9 '20 at 17:59
  • $\begingroup$ @Uwe and OrganicMarble Neil Armstrong never crossed the 50-mi-mark in the X-15. His highest X-15 flight went to 207,500 ft. $\endgroup$ Jun 9 '20 at 18:00
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There is no practical use, because in practice, fixed-wing aircraft cannot fly horizontally in the mesosphere (50-85 km / 31-52 mi) anymore. The Kármán line assumes it, stating that airflight turns pointless at 100 km because stall speed reaches orbital velocity.

While this is true in theory (for altitudes 52-62 mi (84-100 km) depending on aircraft, latitude, and solar activity), a spaceplane cannot keep flying horizontally at lower altitudes anymore, because when trying to do so at too high altitudes it would need to go at speeds that would heat it up too much, due to aerodynamic friction and supersonic flight, making leveled flight in the mesosphere impossible without permanent heat protection from all sides. An air- or spaceplane trying to fly horizontally has to go faster with altitude as air gets thinner. At a certain altitude it would reach the point where it would either have to fly so fast that friction would produce enough heat that would damage it when unprotected, or when flying slower it would stall. This point is reached around the stratopause (31 mi / 50 km altitude) so a plane flying above the stratopause will necessarily have what it takes to be a spacecraft.

History proves this as there never has been a plane that could reach the mesosphere only but not go higher. Planes that could reach the mesosphere (such as the X-15 and the SpaceShipOne, Two and Three) all went higher into the thermosphere or are planned to do so. And the record for the highest leveled flight is currently set by the unmanned AeroVironment Helios plane which flew to 18.3 mi / 29.5 km, well within the stratosphere.

Therefore, in practice the Kármán line is useless as there won't be planes that will attempt to fly horizontally at say 70 km (43.5 mi) without going higher, unless one would build such plane that can't go higher and make them have incredible heat protection if one would attempt that for some reason. But at 70 km altitude, "flying" would mostly be gliding and propulsion barely needed anymore to reach a faraway goal.

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