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  #101  
Old June 23rd 18, 12:58 AM posted to sci.space.policy,sci.physics,rec.arts.sf.science
Alain Fournier[_3_]
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Default Towards routine, reusable space launch.

On Jun/22/2018 Ã* 5:45 PM, JF Mezei Wrote :
On 2018-06-22 07:18, Jeff Findley wrote:

Because it's flexible, duh. Imagine that you took 10 meters of fishing
line with a weight at the bottom then cut it in the middle. Would it
stay straight as it fell? Try it!



This does not consider/test the issue that the top of the cable is
travelling at faster speed and as it is being pulled down, will
accelerate further. If the top of the cable wants to travel faster than
the bottom part of cable, a tension will exist which would not exist in
your fishing line example, and that tension should keep the cable straight.


That tension you talk about is caused by the centrifugal pseudo-force.
In the fishing line example Jeff gave there is the gravitational force
that should pull the line down.

In both cases, when the fishing line or the elevator cable snaps the
elasticity pulls back with more force than the gravity or centrifugal
force. With the caveat that we don't know what material would be used
for the hypothetical space elevator, so we don't know how elastic it
would be. But I think it is a safe assumption that if such an elevator
was to ever be built, the material would be sufficiently elastic for
tens of thousands of kilo-meters of it to give a very strong pull back.


Alain Fournier
  #102  
Old June 23rd 18, 01:34 PM posted to sci.space.policy,sci.physics,rec.arts.sf.science
Jeff Findley[_6_]
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Default Towards routine, reusable space launch.

In article ,
says...

On 2018-06-22 07:18, Jeff Findley wrote:

Because it's flexible, duh. Imagine that you took 10 meters of fishing
line with a weight at the bottom then cut it in the middle. Would it
stay straight as it fell? Try it!



This does not consider/test the issue that the top of the cable is
travelling at faster speed and as it is being pulled down, will
accelerate further. If the top of the cable wants to travel faster than
the bottom part of cable, a tension will exist which would not exist in
your fishing line example, and that tension should keep the cable straight.


Depends a lot on exactly where it breaks. Also the drag from the
atmosphere of earth makes the dynamics a lot different than on a mostly
airless planet like Mars. Look again at the simulations and research
papers on this subject that are online.

Why did the Tacoma Narrows Bridge break up due to *wind*?


That cable would have to travel over 38,000km in vacuum before it got to
atmpsphere. So the top of cable (assuming breal at geostationary
altitude) wouldn't have wind gusts at resonnance frequency.


That's not what I meant. I meant that you can't know these things
intuitively. The example was a huge bridge made of concrete and steel
that was destroyed by wind. That's not intuitive!

You have to do the engineering analysis on these things. Again, look at
the research that's already been done.

Jeff
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These posts do not reflect the opinions of my family, friends,
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  #104  
Old June 23rd 18, 02:58 PM posted to sci.space.policy,sci.physics,rec.arts.sf.science
Alain Fournier[_3_]
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Default Towards routine, reusable space launch.

On Jun/23/2018 at 3:39 AM, JF Mezei wrote :
On 2018-06-22 19:58, Alain Fournier wrote:

In both cases, when the fishing line or the elevator cable snaps the
elasticity pulls back with more force than the gravity or centrifugal
force.


Centrifugal force is equal to gravity at genostationary orbit for this
setup. below that, gravity is bigger so it pulls cable down.

But what your argument does not consider is that the cable at
geostationary is travelling at roughly 9370 km/h. But throughout the
cable, all portions have the same radial speed (15° per hour, 360° per
24 hours).

As the topmost portion of the cable is pulled down, its speed increases
and it now has a radial speed greater than 15° per hour.


The lower end will pull cable down (gravity) and resist beiong pulled
forward (either because still anchored or being dragged on ground
(resistance).

The higher end will respond to being pulled down by increasing forward
velocity, thus tugging on cable to move horizontally. Those two forces
should keep cable fully extended and straight. It won't be snaking around.

Any elasticity in the cable means that when the initial break at
geostationaly happens, the elasticity will pull cable down more than
just gravity. But that extra force will also result in the top most
portion accelerating horizontally. So it isn't clear that as the
tension is released, the cable would "snake".


You are just missing the end of you last sentence. It isn't clear that
as the tension is released the cable would snake until you do
simulations to see what happens.


Alain Fournier
  #105  
Old June 24th 18, 05:28 AM posted to sci.space.policy
Scott M. Kozel[_2_]
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Default Towards routine, reusable space launch.

On Saturday, June 23, 2018 at 8:34:11 AM UTC-4, Jeff Findley wrote:

That's not what I meant. I meant that you can't know these things
intuitively. The example was a huge bridge made of concrete and steel
that was destroyed by wind. That's not intuitive!


Sure it is. A high enough wind will destroy almost anything.
  #106  
Old June 24th 18, 06:53 AM posted to sci.space.policy,sci.physics,rec.arts.sf.science
Sergio
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Posts: 37
Default Towards routine, reusable space launch.

On 6/23/2018 2:39 AM, JF Mezei wrote:
On 2018-06-22 19:58, Alain Fournier wrote:

In both cases, when the fishing line or the elevator cable snaps the
elasticity pulls back with more force than the gravity or centrifugal
force.


Centrifugal force is equal to gravity at genostationary orbit for this
setup. below that, gravity is bigger so it pulls cable down.


nope.
The orbiting object just needs to orbit faster to stay at the same
height above the earth, and it does not go down.


But what your argument does not consider is that the cable at
geostationary is travelling at roughly 9370 km/h. But throughout the
cable, all portions have the same radial speed (15° per hour, 360° per
24 hours).


the cable will bend under these conditions if very long. as the top
part is going too fast to be geostationary, and the bottom is going too
slow to be geostationary for the cable to remain normal to the Earths
surface. Short cable is ok, but not a 10,000 km one.



As the topmost portion of the cable is pulled down, its speed increases
and it now has a radial speed greater than 15° per hour.


The lower end will pull cable down (gravity) and resist beiong pulled
forward (either because still anchored or being dragged on ground
(resistance).

The higher end will respond to being pulled down by increasing forward
velocity, thus tugging on cable to move horizontally. Those two forces
should keep cable fully extended and straight. It won't be snaking around.

Any elasticity in the cable means that when the initial break at
geostationaly happens, the elasticity will pull cable down more than
just gravity. But that extra force will also result in the top most
portion accelerating horizontally. So it isn't clear that as the
tension is released, the cable would "snake".




  #108  
Old June 24th 18, 09:19 PM posted to sci.space.policy
Fred J. McCall[_3_]
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Default Towards routine, reusable space launch.

"Scott M. Kozel" wrote on Sat, 23 Jun 2018
21:28:31 -0700 (PDT):

On Saturday, June 23, 2018 at 8:34:11 AM UTC-4, Jeff Findley wrote:

That's not what I meant. I meant that you can't know these things
intuitively. The example was a huge bridge made of concrete and steel
that was destroyed by wind. That's not intuitive!


Sure it is. A high enough wind will destroy almost anything.


Except that isn't what destroyed it. Winds when the Tacoma Narrows
Bridge collapsed were 40 MPH and less.


--
"Ignorance is preferable to error, and he is less remote from the
truth who believes nothing than he who believes what is wrong."
-- Thomas Jefferson
  #109  
Old June 25th 18, 05:31 AM posted to sci.space.policy
Scott M. Kozel[_2_]
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Posts: 160
Default Towards routine, reusable space launch.

On Sunday, June 24, 2018 at 4:19:49 PM UTC-4, Fred J. McCall wrote:
"Scott M. Kozel" wrote on Sat, 23 Jun 2018
21:28:31 -0700 (PDT):

On Saturday, June 23, 2018 at 8:34:11 AM UTC-4, Jeff Findley wrote:

That's not what I meant. I meant that you can't know these things
intuitively. The example was a huge bridge made of concrete and steel
that was destroyed by wind. That's not intuitive!


Sure it is. A high enough wind will destroy almost anything.


Except that isn't what destroyed it. Winds when the Tacoma Narrows
Bridge collapsed were 40 MPH and less.


"For over six decades, engineers have studied the collapse of the 1940 Tacoma
Narrows Bridge. The experts disagree, at least on some aspects of the
explanation. A definitive description that meets unanimous agreement has not
been reached. The exact cause of the bridge's failure remains a mystery."

https://www.wsdot.wa.gov/TNBhistory/...e/machine3.htm
  #110  
Old June 25th 18, 09:50 AM posted to sci.space.policy
Fred J. McCall[_3_]
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Posts: 10,018
Default Towards routine, reusable space launch.

"Scott M. Kozel" wrote on Sun, 24 Jun 2018
21:31:15 -0700 (PDT):

On Sunday, June 24, 2018 at 4:19:49 PM UTC-4, Fred J. McCall wrote:
"Scott M. Kozel" wrote on Sat, 23 Jun 2018
21:28:31 -0700 (PDT):

On Saturday, June 23, 2018 at 8:34:11 AM UTC-4, Jeff Findley wrote:

That's not what I meant. I meant that you can't know these things
intuitively. The example was a huge bridge made of concrete and steel
that was destroyed by wind. That's not intuitive!


Sure it is. A high enough wind will destroy almost anything.


Except that isn't what destroyed it. Winds when the Tacoma Narrows
Bridge collapsed were 40 MPH and less.


"For over six decades, engineers have studied the collapse of the 1940 Tacoma
Narrows Bridge. The experts disagree, at least on some aspects of the
explanation. A definitive description that meets unanimous agreement has not
been reached. The exact cause of the bridge's failure remains a mystery."

https://www.wsdot.wa.gov/TNBhistory/.../machine3.htmr


Disagree on SOME aspects. That doesn't change the fact that it was
NOT high winds that brought it down.


--
"Some people get lost in thought because it's such unfamiliar
territory."
--G. Behn
 




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