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Old May 17th 2018, 02:52 PM   #1
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Spacecraft Verticle Re-entry From A Geostationary Orbit

Hi,

I have a question, and I hope it's in the right place. If not, hopefully the mods won't mind too much moving it....

I have a basic understanding of the concept of a space elevator, where a line is tethered to the Earth at one end and "floating" in space at the other (presumably at a terminus type spacecraft or counterweight. This would allow payloads to climb to or descend from Space.

So to my question. Why don't we re-enter space craft vertically from a geostationary orbit? Am I correct in thinking that the amount of ful that would be required to brake the craft so that it descended slowly enough to not overheat would be too impractical to carry? Or am I missing something more fundamental?

Thanks for reading, and apologies if this is a stupid question.

Best

DeltaIV
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Old May 17th 2018, 04:50 PM   #2
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The basic reason is that you would have to make sudden "right angle turn"! Your orbit is approximately perpendicular to a vertical line. It's hard enough to make a right angle turn around a corner in an auto. You can imagine how much hard for a craft at the speed necessary to stay in orbit.
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Old May 17th 2018, 09:30 PM   #3
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Originally Posted by HallsofIvy View Post
The basic reason is that you would have to make sudden "right angle turn"! Your orbit is approximately perpendicular to a vertical line. It's hard enough to make a right angle turn around a corner in an auto. You can imagine how much hard for a craft at the speed necessary to stay in orbit.

Why bother with a right turn?
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Old May 17th 2018, 09:42 PM   #4
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First consider an object inside the space elevator at geosynchronous orbit altitude , it will float freely ...to return to earth the elevator needs to pull itself down the cable , the object inside will contact the ceiling and the side of the elevator it will be pushed down and pushed sideways ... the side of the elevator will push on the object reducing it's speed ,as altitude is reduced .... the object will give up some speed pushing the elevator and cable forward , the earth will be pulled forward increasing it's rotational speed very slightly ....

A free object cannot just fall back to earth from orbit . First it needs a rocket thrust to push it in direction of the earth ... then it needs a thrust at right angles to slow it's horizontal speed .... lots of fuel ...no need ... better let the atmosphere dissipate all this unwanted kinetic energy in friction heat.
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Old May 18th 2018, 02:41 AM   #5
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Consider the tangential velocity of an object in Geostationary Orbit.
Now consider the tangential velocity of a second object directly under the first on the surface of the Earth.

The object in orbit will be travelling much faster than the object on the Earth.
So to come down (vertically) from orbit, an object would have to continually slow down.
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Old May 20th 2018, 04:06 PM   #6
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You're forgetting that the ship has orbital angular momentum which means its tangential speed must increase as it falls towards the Earth. Calculate it. I'm sure the answer is interesting.
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Old May 20th 2018, 07:47 PM   #7
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Originally Posted by Pmb View Post
You're forgetting that the ship has orbital angular momentum which means its tangential speed must increase as it falls towards the Earth. Calculate it. I'm sure the answer is interesting.
No it must Decrease we're not trying to keep this object in orbit ,but bring it to land on earth , so a force must push it back , reducing it's tangential speed.
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Old May 20th 2018, 08:37 PM   #8
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Originally Posted by oz93666 View Post
No it must Decrease we're not trying to keep this object in orbit ,but bring it to land on earth , so a force must push it back , reducing it's tangential speed.
I said its there, that's all. Its up to the OP to figure out how to over come it. But until not everyone has been forgetting it or ignoring it. Not good.
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Old May 21st 2018, 01:45 PM   #9
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Wink

I have come across indications in various bits and bobs I have read over the years that orbital mechanics can be counter intuitive.
But I must admit that I haven't attempted to try the maths for myself
(I'm far too lazy for that).
However, I'm quite happy to take Pete's word for it.
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