In and out of orbit: Difference between revisions

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Above 6km or so becomes impractical for people to breathe enough oxygen.
Above 6km or so becomes impractical for people to breathe enough oxygen.


Above 10km or so it's ''inefficient'' to fly with jet engines, though the record of trying really hard is rather higher, order of 35km.  
Jet planes cruise around 12km because the decreased drag makes for better fuel economy, and not much higher for a mix of reasons (more than one of them related to pressure).
The record of trying really hard is rather higher, order of 35km.  
 
The same 35km-ish is also the highest recorded balloon flight{{verify}}
The same 35km-ish is also the highest recorded balloon flight{{verify}}


Above 100km or 200km the gas is thin enough that it would barely slow you down.
Above 100km or 200km the gas is thin enough that it would barely slow you down.
The likes of sputnik, MIR, ISS, and Hubble are somewhere in the 200 to 600km range.
The likes of sputnik, MIR, ISS, and Hubble are somewhere in the 200km to 600km range.


You can detect some tiny amount of gas ''at all'' up to order of 10000km{{verify}}, but you'd only care when you want your vacuum to be ''really'' empty.
You can detect some tiny amount of gas ''at all'' up to order of 10000km{{verify}}, but you'd only care when you want your vacuum to be ''really'' empty.
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On that scale, 100km away is comparatively close, and gravity is still roughly 95% of that at the surface.
On that scale, 100km away is comparatively close, and gravity is still roughly 95% of that at the surface.


Even at 2000km, far above the ISS, it's still roughly half.
Even at 2000km, far above the ISS (at ~400km), it's still roughly half.
 


While mathematically you could pretend it never quite goes to zero, yet in practice there is a distance at which the force becomes easy to counteract with small amounts of energy.
While mathematically you could pretend it never quite goes to zero, in practice there is a distance at which the force becomes easy to counteract with small amounts of energy.


This guides our understanding of '''microgravity''' (which has no exact definition - it's basically 'little, but not nothing').
This also guides our understanding of '''microgravity''' (which has no exact definition - it's basically 'little, but not nothing').




Assuming fuel is not an issue, it might be feasible to keep something up just by pointing a rocket down - yet given current technology, the point at which this becomes feasible with fuel you can actually take there and have is - ballpark - not closer than 200000km away[https://en.wikipedia.org/wiki/Micro-g_environment#Absence_of_gravity], which is ''well'' on your way to our moon.
Assuming fuel is not an issue, it might be feasible to keep something up just by pointing a rocket down - yet given current technology,  
So in practice fuel is ''still'' dumbly practical issue and is why we can't really do this for any amount of time.
but the point at which the force is low enough that you can keep this up for a long time is - ballpark - not closer than 200000km away[https://en.wikipedia.org/wiki/Micro-g_environment#Absence_of_gravity], which is ''well'' on your way to our moon. So in practice fuel is ''still'' dumbly practical issue and is why we can't really do this for any amount of time.




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: most [[https://en.wikipedia.org/wiki/Low_Earth_orbit low earth orbits] are between 200km and 2000km or so,  
: most [[https://en.wikipedia.org/wiki/Low_Earth_orbit low earth orbits] are between 200km and 2000km or so,  
: [https://en.wikipedia.org/wiki/Medium_Earth_orbit medium earth orbit] at the order of 20000km (~12 hour orbits),
: [https://en.wikipedia.org/wiki/Medium_Earth_orbit medium earth orbit] at the order of 20000km (~12 hour orbits),
: [https://en.wikipedia.org/wiki/Geostationary_orbit geostationary orbit] at approximately 35000km (~24 hour orbit, so when we make it rotate the same way as earth this seems fairly stationary to us),  
: [https://en.wikipedia.org/wiki/Geostationary_orbit geostationary orbit] at approximately 35000km (~24 hour orbit, so when we make it rotate the same way as earth this seems fairly stationary to us, hence the name),  
: [https://en.wikipedia.org/wiki/High_Earth_orbit high earth orbit] at the order of 100000km (and orbits lasting hundreds of hours).
: [https://en.wikipedia.org/wiki/High_Earth_orbit high earth orbit] at the order of 100000km (and orbits lasting hundreds of hours).


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So, 100km is not ''that'' far away.
So, 100km is not ''that'' far away.


Sure, it's well above commercial airplanes which typically cruise around (10km), but that's just because they're specifically designed to fly in air - airfoils or jet engines won't work at 100km, so getting high with a plane must come largely from momentum, and is a little easier if you've strapped on a rocket too.
Sure, it's well above commercial airplanes which typically cruise around (~10km), but that's just because they're specifically designed to fly in air - airfoils or jet engines won't work at 100km, so getting high with a plane must come largely from momentum.  It helps if you've strapped on a rocket.
 
So yes, it takes a bunch of energy, and also a bunch of coordination.


Yet in theory, if you can somehow keep pointing a moderate rocket down, it's within the realm of manageable,
something you can do with enough determination, and somewhat-specialized airplanes have done this since roughly the sixties.


Sure it takes a bunch of energy, and a bunch of coordination, and some determination,
but pointing a moderate rocket down, is within the realm of manageable, and somewhat-specialized airplanes have done this since roughly the sixties.


...which have then quickly come back down.
...which have then quickly come back down.
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So if you're at 100km, and want to stay at 100km, and point a rocket down, you have as bad of an issue as you did at launch.  
So if you're at 100km, and want to stay at 100km, and point a rocket down, you're spending almost the same amount of fuel as you did at launch.  
You have to keep it on permanently on, so it will run out of fuel quickly, probably within minutes (you can't just take more, because fuel has weight, so takes more fuel to get and keep up there. This is an nonlinear relationship, but it's not one that is going to make you happy).
And to balance the force you have to keep it permanently on.


{{comment|(Even a lot of ''fictional'' spaceships aren't made for this. Some ignore it, some handwave it away.  Other acknowledge it by saying their spaceship really isn't made to work in atmosphere, or work around it by having some way to sustain high thrust ''and'' to power that, both for long enough to be practical.)}}
So it will run out of fuel quickly, probably within minutes. No, you can't just take more, because fuel has weight, so takes more fuel to get and keep up there.
Mathematically this is an nonlinear relationship, practically it's one that is ''not'' going to make you happy.


{{comment|(Even a lot of ''fictional'' spaceships aren't made for this.  Some ignore it, some handwave it away.  Other acknowledge it by saying their spaceship really isn't made to work in atmosphere, or work around it by having some way to sustain high thrust ''and'' to power for long enough to be practical.)}}




It turns out that if you want to stay up there for a longer time, probably the single most energy-efficient way to stay up there for a long time is to fall sideways so fast that it balances with the falling downwards.


We like to call that 'orbit', because that sounds less terrifying. Also it's fewer letters.
It turns out that if you want to stay up there for a longer time, probably the single most energy-efficient way is to fall sideways so fast that it balances with the falling downwards.
 
We like to call that 'orbit', because that sounds less terrifying. Also it's fewer letters.


This is also roughly why escape velocity is called escape ''velocity'', not escape distance.
This is also roughly why escape velocity is called escape ''velocity'', not escape distance.
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Further out you can go slower, but not by very much actually.  
Further out you can go slower, but not by very much actually.  
Geosynchronous orbit means ~24hr orbit time, which is 35000km away for earth (several earth diameters away from earth)), and still implies about 3km/s. Hhigh earth orbit is still 2km/s, and even the moon, 380000km away and a dozen years if you had to walk it, goes 1km/s in its orbit around us.
Geosynchronous orbit means ~24hr orbit time, which is 35000km away for earth (several earth diameters away from earth)), and still implies about 3km/s.  
 
High earth orbit is still 2km/s, and even the moon, 380000km away (a dozen years if you had to walk it), still goes 1km/s in its orbit around us.
 
So it turns out that when you have a rocket getting you to orbit, most of its fuel doesn't get you high, it gets you moving sideways fast enough.
 




https://what-if.xkcd.com/58/
So it turns out that when you have a rocket getting you to orbit, most of its fuel doesn't get you high, it gets you moving sideways fast enough. [https://what-if.xkcd.com/58/]




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[[Category:Nerding around]]
[[Category:Nerding around]]
[[Category:Physics]]
[[Category:Space]]

Latest revision as of 15:20, 14 July 2023


How high up is space?

How hard is it to get to space?

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