Comment by twawaaay | Hacker News Reader

Comment by twawaaay | Hacker News Reader

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> At 100 km most of the atmosphere (99.9999%) is below you. 100 km also marks the Karman Line (shown in the main graphic), generally considered the “official” start of outer space. Above this altitude aerodynamic flight is no longer possible. Wings are useless. Only rocket thrusters and gravity can affect your motion.

I would like to nitpick here a bit.

100km is just "administrative" end of atmosphere. The atmosphere does not really have an end. If there are any hard, measurable boundaries it would be where solar winds meet our atmosphere (kinda like discussion about when did Voyagers leave Solar system). But this boundary does not have defined shape, moves a lot due to solar wind or Earth magnetic field situation.

And we do actually use our atmosphere above 100km to "affect motion". We place satellites in LEO well above 100km to assure they will eventually fall to Earth.

And anybody who played KSP knows how useful it is to skim the atmosphere for aerocapture...

Nothing you said contradicts anything the article said. What are you nitpicking, exactly?

> The atmosphere does not really have an end.

Article clearly doesn't state it has an end. They said 99.9999% is below you. They then say the Karman Line is considered the "official" (IN QUOTES) start of outer space, implying that this is a somewhat arbitrary definition.

> And we do actually use our atmosphere above 100km to "affect motion". We place satellites in LEO well above 100km to assure they will eventually fall to Earth.

The article clearly stated "affecting motion" in relation to aerodynamic flight ("aerodynamic flight is no longer possible"). Wings are useless to generate lift at those altitudes. I wouldn't classify falling to Earth as "aerodynamic flight".

> Wings are useless to generate lift at those altitudes.

They aren't, though. They still work, just much less well. There isn't one altitude where wings suddenly stop working entirely - there is however an altitude where generating enough lift to sustain flight would require going so fast that you're already in orbit (regardless of lift), according to wikipedia it's somewhere around 80km.

The Karman line has nothing to do with lift, it's just an arbitrary, round number.

> The Karman line has nothing to do with lift, it's just an arbitrary, round number.

I think you got this upside-down. The Kármán line is named after an engineer called Theodore von Kármán who studied the problem of how high aeronautical flight is possible theoretically. That has everything to do with lift, and speed an orbiting body needs to stay in orbit. That is the Kármán line.

Then yes of course organisations need clear and bright lines for things like "when do we give someone an astronaut badge?" For that different organisations come up with nice round threshold numbers. And those numbers have nothing to do with lift. But those numbers are not the Kármán line. They are just convenient approximations of it for administrative purposes.

It's like if a country's tax authority declares that for tax purposes you shall calculate the area of a circle using the first 5 digits of PI. That doesn't mean that suddenly the PI become not a transcendental number. It is just an approximation for practical reasons.

>there is however an altitude where generating enough lift to sustain flight would require going so fast that you're already in orbit

or you could increase the size of the wings?

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