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Author Topic: Would stopping completely in orbit mean a safer re-entry?  (Read 2661 times)

P Kirk McAfee

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P Kirk McAfee  asked the Naked Scientists:

Dear Naked Scientists,
 
I know that space vehicles zip around the globe fast enough for their centrifugal force to offset gravity.  To re-enter, they slow a bit and gravity starts over-powering centrifugal force and then they hit the atmosphere at some incredible speed.  But my understanding is that most of the speed upon hitting the atmosphere is from the residual speed of spinning around the globe and not from the effect of falling.  So I have some questions:
 
1: If they could (magically) come to a complete stop in orbit and then fall like a rock being dropped from space, would the re-entry speed, starting at zero, accelerate back up to the same incredible speed as they currently experience or would they fall much slower and never exceed much above the terminal velocity of a falling object in air?

2: What amount of additional energy would it take to slow the Shuttle to a stop to allow for such a gentle, direct fall?
 
Thanks.
 
Kirk McAfee
San Francisco

What do you think?


 

Offline syhprum

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Would stopping completely in orbit mean a safer re-entry?
« Reply #1 on: 08/10/2008 11:53:27 »
It is not magic required but lots of fuel, space vehicle operators landing on Earth have it easy having the dense atmosphere available to dissipate the orbital energy to get the things down.
« Last Edit: 08/10/2008 14:21:18 by syhprum »
 

Offline turnipsock

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Would stopping completely in orbit mean a safer re-entry?
« Reply #2 on: 08/10/2008 18:54:58 »
Well when joe jumped, he didn't burn up.

Joe Kittenger
 

Offline ukmicky

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Would stopping completely in orbit mean a safer re-entry?
« Reply #3 on: 08/10/2008 22:20:50 »
Good old joe didnt get very high and wasnt travelling anywhere near the speed required to stay in orbit before he jumped.

I believe (could be wrong) that he accelerated to nearly the speed of sound before he hit the dense layer of air in the atmosphere which then slowed him down. Not very fast in the terms of re-entry speeds and whilst friction from the air may have warmed him up a little its way off the speeds that cause space shuttles to burn.
« Last Edit: 09/10/2008 20:44:09 by ukmicky »
 

lyner

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Would stopping completely in orbit mean a safer re-entry?
« Reply #4 on: 08/10/2008 23:16:55 »
Although there are immense problems associated with stability, I believe that the best strategy for re entry should be to 'fly' in and reduce your energy very slowly in one long glide, possibly involving several orbits. This would reduce the rate of energy transfer to a much more reasonable value - red heat rather than white heat.
 

Offline Soul Surfer

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Would stopping completely in orbit mean a safer re-entry?
« Reply #5 on: 08/10/2008 23:24:29 »
It depends how high an orbit it is in.  You solve the problem by thinking of the potential and kinetic energies involved.  A body in orbit has kinetic energy from the velocity needed to maintain the orbit and the potential energy in falling to the surface of the earth. A body falling from a grat distance (several times the diameter of the earth) would reach escape velocity (about half as much again as low orbital velocity) if it was not impeded by the atmosphere and so there is very little difference in the energy dissapated between being in orbit or dropping like a stone. If it was a very low orbit say around i00 miles the fall would involve dissapating less potential energy but it would still get fast enough to require a heat shield.  IRBMs need to take re-entry heating into account even though their velocities are well sub orbital.
« Last Edit: 08/10/2008 23:27:11 by Soul Surfer »
 

lyner

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Would stopping completely in orbit mean a safer re-entry?
« Reply #6 on: 09/10/2008 10:00:15 »
Quote
but it would still get fast enough to require a heat shield.
The temperature of the surface would depend upon the rate of energy transfer. By flying in, and producing lift, you can reduce the rate of energy transfer by reducing the rate at which you lose GPE. There will be practical limits to just how slowly you can lose height, though.
 

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Would stopping completely in orbit mean a safer re-entry?
« Reply #6 on: 09/10/2008 10:00:15 »

 

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