Question on the space elevator

I'm a bit confused about the orbital mechanics relevant to an elevator.

Suppose we had an elevator. Several tens of thousands of miles
of cable/ribbon run from the surface of the earth through the point of
weightlessness (ie geosynchronous orbit). There is sufficient mass
on the other side of that point to balance the weight of the ribbon
on the earthward side. To put the same point differently: the center
of mass is orbiting geosynchronously.

Now we decide to use the elevator. We attach a 20 ton climber and
turn her on. Immediately 20 tons are added to the downward force on
the cable. Let us suppose for a minute that nothing is done to
compensate for this new mass.

What happens to the cable and why?
http://www.pobox.com/~hapgood

In article ,
Fred Hapgood wrote:

Suppose we had an elevator. Several tens of thousands of miles
of cable/ribbon run from the surface of the earth through the point of
weightlessness (ie geosynchronous orbit). There is sufficient mass
on the other side of that point to balance the weight of the ribbon
on the earthward side. To put the same point differently: the center
of mass is orbiting geosynchronously.

Now we decide to use the elevator. We attach a 20 ton climber and
turn her on. Immediately 20 tons are added to the downward force on
the cable. Let us suppose for a minute that nothing is done to
compensate for this new mass.

What happens to the cable and why?

Nothing -- or something immeasurably close to nothing. We're talking
about 20 extra tons on a structure that is (off the top of my head)
hundreds of thousands of tons. You'll get bigger effects from other
forces, such as tides.

I do imagine the elevator would need some station-keeping to adjust for
accumulated small sources of drift over time. Fortunately it has a
cheap way to obtain propellant.

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In article ,
Fred Hapgood wrote:
...cable/ribbon run from the surface of the earth through the point of
weightlessness (ie geosynchronous orbit). There is sufficient mass
on the other side of that point to balance the weight of the ribbon
on the earthward side...

Not just sufficient mass to balance it, but some extra. You want the
surface anchor to be in tension.

To put the same point differently: the center
of mass is orbiting geosynchronously.

No, the center of mass should be beyond geostationary orbit. (Actually,
there are complications because "center of mass" and "center of gravity"
are not the same thing in systems this large. But disregard that for
simplicity...)

Now we decide to use the elevator. We attach a 20 ton climber and
turn her on. Immediately 20 tons are added to the downward force on
the cable. Let us suppose for a minute that nothing is done to
compensate for this new mass.
What happens to the cable and why?

If the cable is already in tension at the anchor, nothing much -- that's
*why* the cable is in tension. You want the center of mass of the system
to remain outside geostationary orbit *after* you add all the climbers
and their payloads.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

On Tue, 23 Mar 2004 07:08:26 -0600, Joe Strout wrote:

In article ,

Nothing -- or something immeasurably close to nothing. We're talking
about 20 extra tons on a structure that is (off the top of my head)
hundreds of thousands of tons. You'll get bigger effects from other
forces, such as tides.

My curiosity is not practical but theoretical. I don't quite understand
how the elevator deals with the interaction of orbital mechanics and
centripetal force. My way of presenting my ignorance is to picture a
functioning elevator and then imagine tugging on the cable, such that the
center of mass is pulled lower. What happens and why?

I used the figure of 20 tons because the dynamics governing a shift
of a few millimeters would inform me as much as a larger effect. But
if it helps to picture the situation you should feel free to select any
number you like.

Here's another pictu suppose you have two functioning elevators
and for some bizarre reason you want to bring them down, wrapping
one around the earth clockwise and the other counter-clockwise. You
have a dial that can move the center of gravity of either elevator
to any altitude. How do you use that dial to get the effect you want,
and why does it work??

Fred
http://www.pobox.com/~hapgood

I think calling it a "cable" highly misleading?

That may be how it acts, once a miricle happens and
it is standing there attached to the ground.

But how does it get there in the first place?

Is there some other way to grow a beanstalk than
from the ground up?



Richard

My curiosity is not practical but theoretical. I don't quite understand
how the elevator deals with the interaction of orbital mechanics and
centripetal force. My way of presenting my ignorance is to picture a
functioning elevator and then imagine tugging on the cable, such that the
center of mass is pulled lower. What happens and why?


well , this way , I imagine

pull on the ribbon from ground , the center of mass lowers , gets on a
slightly faster orbit,

because of the growing tension on the cable this movement is topped and
the center of mass is slowed back to it's position (?)

--

for the problem of momentum conservation , when moving masses away from
ground to space, I imagine the operation ot the elevator would result
in slowing down (not much !!) the rotation of earth



I also imagine that the principal issue regarding the construction of
the elevator is the material the ribbons ar made of ..

and if such a material can ever be made , I think it would change the
face of Earth much before the 60.000 km long elevator runs up

imagine what impact such a material would have on architecture,
bulding, bridges design .. etc ...

--
Julie
"please save Yuri"
http://membres.lycos.fr/andromedanews

In article ,
Richard Lamb wrote:
But how does it get there in the first place?
Is there some other way to grow a beanstalk than
from the ground up?

You want the center of mass to always be at or above geostationary
altitude, so the cable -- yes, "cable" -- is always in tension. Long thin
structures are far stronger in tension than in compression. So no, you
don't build it from the ground up; it's lowered from above (possibly one
strand at a time rather than all at once), not raised from below.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

Henry Spencer wrote:

In article ,
Richard Lamb wrote:
But how does it get there in the first place?
Is there some other way to grow a beanstalk than
from the ground up?

You want the center of mass to always be at or above geostationary
altitude, so the cable -- yes, "cable" -- is always in tension. Long thin
structures are far stronger in tension than in compression. So no, you
don't build it from the ground up; it's lowered from above (possibly one
strand at a time rather than all at once), not raised from below.
--


You are going to drop if from the sky?



Ok, let me rephrase the questions...

What are the reasons NOT to built it from the bottom up?

In article ,
Richard Lamb wrote:
don't build it from the ground up; it's lowered from above (possibly one
strand at a time rather than all at once), not raised from below.

You are going to drop if from the sky?

"Lower" it from the sky, please -- it will be supported at all times, from
above.

Ok, let me rephrase the questions...
What are the reasons NOT to built it from the bottom up?

The fact that it's orders of magnitude harder to do that way. Any
structure that tall and that thin, supported from its base, will have a
very strong tendency to buckle -- squirm sideways out from under the load.
Building a tower hundreds or thousands of kilometers high is not utterly
impossible, but it involves elaborate dynamic control and perhaps even
dynamic support, not just strong materials. None of this is an issue for
a structure supported from the top, which just hangs there, held straight
by its own weight.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

Richard Lamb writes:

Henry Spencer wrote:
In article ,
Richard Lamb wrote:

But how does it get there in the first place?
Is there some other way to grow a beanstalk than
from the ground up?

You want the center of mass to always be at or above geostationary
altitude, so the cable -- yes, "cable" -- is always in tension.
Long thin structures are far stronger in tension than in compression.
So no, you don't build it from the ground up; it's lowered from above
(possibly one strand at a time rather than all at once), not raised from
below.


You are going to drop if from the sky?

Yes, exactly.


Ok, let me rephrase the questions...

What are the reasons NOT to built it from the bottom up?

1.) While there _might_ be materials strong enough to take the tension
of a "beanstalk" hanging from the sky, =NOTHING= is strong enough
to take the compressional load of a "tower" built from the ground up ---
even theoretically.

2.) Structures built in compression are _UNSTABLE TO BUCKLING_ for loads
that are a relatively small fraction of their maximum compression strength,
as may be easily demonstrated by attempting to extend a metal tape-measure
vertically. By contrast, structures built in tension are unconditionally
stable, as long as one does not exceed their tensile yield strength.


-- Gordon D. Pusch

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