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  #111  
Old June 25th 18, 12:15 PM posted to sci.space.policy
Jeff Findley[_6_]
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Default Towards routine, reusable space launch.

In article ,
says...

"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.


Agreed. There is movie footage of the bridge oscillating (quite wildly)
in the wind. It's pretty damn clear from this evidence that the wind
excited a mode of the bridge to the point of collapse. This collapse is
a "textbook case" of what happens when you don't understand or ignore
the modal dynamics of a large structure.

That's exactly what JF is doing. He's and waving away the modal
behavior of a cable like structure that is instantaneously released from
extreme tension.

I keep saying dynamic behavior like this is decidedly non-trivial
(again, I majored in dynamics and control), but JF won't listen and
instead keeps trying to hand wave away the complexity he doesn't
understand or is simply unwilling to accept.

Jeff
--
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These posts do not reflect the opinions of my family, friends,
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  #112  
Old June 25th 18, 07:31 PM posted to sci.space.policy
Fred J. McCall[_3_]
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Default Towards routine, reusable space launch.

JF Mezei wrote on Mon, 25 Jun 2018
12:48:25 -0400:

On 2018-06-25 07:15, Jeff Findley wrote:

That's exactly what JF is doing. He's and waving away the modal
behavior of a cable like structure that is instantaneously released from
extreme tension.


You are the one who pointed at a simulation showing the cable floating
freely and snaking in space. I argued this wouldn't be the case because
of orbital mechanics.


Yes, he pointed at a simulation that presumably takes some of those
dynamics he keeps trying to explain to you into account. You pointed
to handwavium.


--
"Some people get lost in thought because it's such unfamiliar
territory."
--G. Behn
  #113  
Old June 25th 18, 09:44 PM posted to sci.space.policy,sci.physics,rec.arts.sf.science
Thomas Koenig
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Posts: 47
Default Towards routine, reusable space launch.

JF Mezei schrieb:
On 2018-06-20 16:42, Sergio wrote:

Carbon nanotubes.


they are held in a matrix of epoxy glue. your 10,000 km of glue wont
support itself.


Carbon nanotubes do not replace the primary carbon fibre structure. They
are mixed with the epoxy to add strength to the matrix.


Or another material...

With the problem of disentangeling the tubes, of binding the
tubes to the matrix material (pulling out the fiber from the
matrix is a common failure mode), and of course the fibers may
have imperferfections which can _drastically_ (by quite a few
orders of magnitude) reduce their strenghts.

Is anybody aware of actual application of CNTs for materials,
an application which has made it into the marketplace and that
I could order today? I'm not, but I have not followed that
field for some time.
  #114  
Old June 25th 18, 10:17 PM posted to rec.arts.sf.science,sci.astro,sci.physics,sci.space.policy
Steve Willner
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Posts: 1,172
Default Towards routine, reusable space launch.

In article ,
Fred J. McCall writes:
Oversize fairings are easy.


No doubt the JWST engineers wish you had told them that.

[mirror size of reconnaissance satellites]


You can see the bloody things from Earth, after all.


See some of them, sure. Why do you think the census is complete?
What observations (equipment) are needed to measure their sizes?

Past a certain point a bigger mirror doesn't help
you for Earth observation. Atmosphere speckle becomes the driving
parameter and a bigger mirror doesn't help that.


Are you assuming LEO and visible wavelengths? I don't see why either
one necessarily represents all reconnaissance satellites. And even
with those assumptions, what about temporal resolution? Taking short
exposures most certainly helps mitigate seeing effects.
("Atmospheric speckle" is only one of those effects.)

No, they wouldn't. The next generation of recce satellites will use a
mirror right around 2.4 meters; the same size used since KH-11.


Source? All of them or only some? Other than better sensors and
onboard processing, how do newer telescopes differ from the older
generation? (As you no doubt know but some readers may not, two of
those were declared surplus and delivered to NASA.)

Distance has a lot to do with everything when
it comes to telescopes.


What did you have in mind? I'd have said the key parameters are
angular resolution, temporal resolution, and sensitivity. Distance
affects requirements on those parameters, but I don't see that
distance _per se_ matters.

--
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  #115  
Old June 25th 18, 11:52 PM posted to rec.arts.sf.science,sci.astro,sci.physics,sci.space.policy
Fred J. McCall[_3_]
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Posts: 10,018
Default Towards routine, reusable space launch.

(Steve Willner) wrote on Mon, 25 Jun 2018
21:17:06 -0000 (UTC):

In article ,
Fred J. McCall writes:
Oversize fairings are easy.


No doubt the JWST engineers wish you had told them that.


Well, someone should have if mirror size alone was the reason for
making it folding.

[mirror size of reconnaissance satellites]


You can see the bloody things from Earth, after all.


See some of them, sure. Why do you think the census is complete?
What observations (equipment) are needed to measure their sizes?


What do you think they hide them behind? I think it's complete
because lots of people put lots of effort into it. And it takes a
telescope and some time to measure their size. This may be rocket
science, but it's EASY rocket science.

Past a certain point a bigger mirror doesn't help
you for Earth observation. Atmosphere speckle becomes the driving
parameter and a bigger mirror doesn't help that.


Are you assuming LEO and visible wavelengths? I don't see why either
one necessarily represents all reconnaissance satellites. And even
with those assumptions, what about temporal resolution? Taking short
exposures most certainly helps mitigate seeing effects.
("Atmospheric speckle" is only one of those effects.)


You can only 'mitigate' so far. Are you one of those people who
believes the movies that you can make out faces from orbit and that
you can 'improve' an image with processing beyond the information that
it contains?

No, they wouldn't. The next generation of recce satellites will use a
mirror right around 2.4 meters; the same size used since KH-11.


Source? All of them or only some? Other than better sensors and
onboard processing, how do newer telescopes differ from the older
generation? (As you no doubt know but some readers may not, two of
those were declared surplus and delivered to NASA.)


Sources aren't hard to find. Here's one.

https://spaceflightnow.com/2015/05/0...ching-in-2018/

Distance has a lot to do with everything when
it comes to telescopes.


What did you have in mind? I'd have said the key parameters are
angular resolution, temporal resolution, and sensitivity. Distance
affects requirements on those parameters, but I don't see that
distance _per se_ matters.


'Temporal resolution' is difficult, given that both the satellite and
the Earth's surface are moving relatively rapidly with regard to each
other. Increased sensitivity means increased image noise. Distance
matters. That's why as newer satellites launch the older ones are
moved to lower orbits.


--
"The reasonable man adapts himself to the world; the unreasonable
man persists in trying to adapt the world to himself. Therefore,
all progress depends on the unreasonable man."
--George Bernard Shaw
  #116  
Old June 25th 18, 11:53 PM posted to sci.space.policy
Fred J. McCall[_3_]
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Default Towards routine, reusable space launch.

JF Mezei wrote on Mon, 25 Jun 2018
18:29:22 -0400:

On 2018-06-25 14:31, Fred J. McCall wrote:

Yes, he pointed at a simulation that presumably takes some of those
dynamics he keeps trying to explain to you into account. You pointed
to handwavium.



"presumably". So you take some random animation off the web that doesn't
consider orbital mechanics as science, despite neither material nor
structure of the space elevator being known and thus elastic proporties
that can't be simulated.


You need to stop making **** up.


Yet, you choose to ignore orbital mechanics that apply to all matter is
orbit that is brought down and consider that to be "handwavium"


You wouldn't know an orbital mechanic if one tuned your car.


--
"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
  #117  
Old June 26th 18, 02:19 AM posted to sci.space.policy
Jeff Findley[_6_]
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Posts: 2,307
Default Towards routine, reusable space launch.

In article ,
says...

On 2018-06-25 07:15, Jeff Findley wrote:

That's exactly what JF is doing. He's and waving away the modal
behavior of a cable like structure that is instantaneously released from
extreme tension.



You are the one who pointed at a simulation showing the cable floating
freely and snaking in space. I argued this wouldn't be the case because
of orbital mechanics.

You replied "because of the wind".

There is no wind in space, and this was about the cable breaking at
geostationary altitude with over 38,000km of cable falling down and how
it would fall on earth (east, west, or all in one pile).


I suppose Google is broken again, SMH. Oh well, it's working for me:

Dynamics of Space Elevator after Tether Rupture
Vladimir S. Aslanov1
and Alexander S. Ledkov2
Samara State Aerospace University, Samara, 443086, Russian
Arun K. Misra 3
McGill University, Montreal, H3A 2K6, Canada
and
Anna D. Guerman4
University of Beira Interior, Covilhã, Portugal
http://aslanov.ssau.ru/papers/Aslanov_JGCD_2013b.pdf

You can skip past the math right to:

Fig. 13 Formation of a loop
Fig. 14 Profile of the tether at t=3500s

From above:

Analysis of space elevator dynamics after failure presented
here shows that the problem is rather complex and worth
studying in detail. We focus on the case when the tether is
destroyed by space debris in the vicinity of the
geostationary orbit, and consider motion of the upper and
lower parts of the system after rupture. The mathematical
model developed for this purpose represents the flexible
heavy tether of circular cross-section as a set of massive
points connected by massless viscoelastic bars. The model
takes into account the interaction of the tether with the
Earth atmosphere during the fall. The approximate expression
(18) for boundary value of distance to the center of mass of
the top part was obtained. If the distance to the center of
mass of the upper part surpasses the boundary value, this
part will escape the Earth on a trajectory close to
hyperbolic. For lower part numerical simulations show
that the aerodynamic force changes significantly the tether
behavior. After the tether enters the atmosphere, most of
it slows down and falls smoothly; however, one can notice
also quite unexpected motions, such as formation of large
loops that can get out the atmosphere. Analysis reveals
that some segments of the tether reach the ground with
rather large velocities. One can conclude that the rupture
of the space elevator ribbon can jeopardize both spacecraft
and objects on the Earths surface located close to the
equatorial plane.

So, atmospheric effects taken into account and the thing exhibits "quite
unexpected motions, such as formation of large loops that can get out
the atmosphere".

Jeff
--
All opinions posted by me on Usenet News are mine, and mine alone.
These posts do not reflect the opinions of my family, friends,
employer, or any organization that I am a member of.
  #118  
Old June 26th 18, 06:09 AM posted to sci.space.policy
Fred J. McCall[_3_]
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Default Towards routine, reusable space launch.

JF Mezei wrote on Tue, 26 Jun 2018
00:55:02 -0400:

On 2018-06-25 21:19, Jeff Findley wrote:

lower parts of the system after rupture. The mathematical
model developed for this purpose represents the flexible
heavy tether of circular cross-section as a set of massive
points connected by massless viscoelastic bars.


So virtual thing made of unobtainium and which has elements without any
mass.


So a model that behaves the way something real would behave. Do you
not have a single clue about how simulation and modeling work?


hyperbolic. For lower part numerical simulations show
that the aerodynamic force changes significantly the tether
behavior. After the tether enters the atmosphere, most of
it slows down and falls smoothly;


The simulation showed the upper sections of the tether where it broke
off near geostationary altitude. It showed it snaking, floating in space.

Meaningful atmosphere is roughly 20m in altitude, or if you want to
include stratosphere, 50km in altitude.


You say all that like it means something.


So, in the cable falling because it broke at geostationary orbit
scenario, you have some lateral forces in the first 50km and eventually,
so air resistance to the structure falling sideways or diagonally with
perhaps a terminal velocity.

But it will fall, and it will pull down on all the rest of the 39,000 of
cable. But that cable will first and foremost be affected by orbital
mechanics since it is way above atmosphere. And pulling down an object
that has forwrad motion causes it to accelerate that forward motion.

So in space, that should be the primary factor to affect cable
behaviour. And since the cable is going down, any motion imparted in the
first 50km of cable will be 1-dampened by the anchor point (or drag on
ground) 2- have very little chance of "snaking up" the whole length of
the 39,000km of the cable.

Once the cable has mostly fallen to the ground and all you got left is a
few hundred km of cable left, then yeah, atmopshere will play a large
role because it affects a large part of the cable.


Of course you're absolutely correct, Mayfly, and everyone else is
wrong and dynamics from things like elasticity are just irrelevant.
Keep flapping.


--
"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
  #119  
Old June 26th 18, 11:58 AM posted to sci.space.policy
Fred J. McCall[_3_]
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Default Towards routine, reusable space launch.

JF Mezei wrote on Tue, 26 Jun 2018
03:49:31 -0400:

On 2018-06-26 01:09, Fred J. McCall wrote:

Of course you're absolutely correct, Mayfly, and everyone else is
wrong and dynamics from things like elasticity are just irrelevant.
Keep flapping.


Am I incorrect in stating that as the cable is pulled down, the higher
end portions will accelerate horizontally?


Depends on what you mean by 'accelerate'. You've been incredibly
sloppy in differentiating angular acceleration vs linear and
tangential acceleration.


If I am so far off, why are you not able to explain why orbital
mechanics don't play a role in this?


Nobody said that, that's why. They just don't play the only role,
which is what your argument assumes.


--
"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
  #120  
Old June 26th 18, 12:29 PM posted to sci.space.policy
Jeff Findley[_6_]
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Posts: 2,307
Default Towards routine, reusable space launch.

In article ,
says...

On 2018-06-25 21:19, Jeff Findley wrote:

lower parts of the system after rupture. The mathematical
model developed for this purpose represents the flexible
heavy tether of circular cross-section as a set of massive
points connected by massless viscoelastic bars.


So virtual thing made of unobtainium and which has elements without any
mass.


I see you didn't comprehend what you're reading. Try again. Note in
the above quote that the "heavy tether of circular cross-section" is
modeled "as a set of massive points". These points, that have mass, are
"connected by massless viscoelastic bars"

The mass is modeled as points. This is a common simplification in an
analysis such as this.

hyperbolic. For lower part numerical simulations show
that the aerodynamic force changes significantly the tether
behavior. After the tether enters the atmosphere, most of
it slows down and falls smoothly;



The simulation showed the upper sections of the tether where it broke
off near geostationary altitude. It showed it snaking, floating in space.

Meaningful atmosphere is roughly 20m in altitude, or if you want to
include stratosphere, 50km in altitude.

So, in the cable falling because it broke at geostationary orbit
scenario, you have some lateral forces in the first 50km and eventually,
so air resistance to the structure falling sideways or diagonally with
perhaps a terminal velocity.

But it will fall, and it will pull down on all the rest of the 39,000 of
cable. But that cable will first and foremost be affected by orbital
mechanics since it is way above atmosphere. And pulling down an object
that has forwrad motion causes it to accelerate that forward motion.

So in space, that should be the primary factor to affect cable
behaviour. And since the cable is going down, any motion imparted in the
first 50km of cable will be 1-dampened by the anchor point (or drag on
ground) 2- have very little chance of "snaking up" the whole length of
the 39,000km of the cable.

Once the cable has mostly fallen to the ground and all you got left is a
few hundred km of cable left, then yeah, atmopshere will play a large
role because it affects a large part of the cable.


I see that it's pointless trying to give you an actual acedemic paper
describing an actual analysis performed by actual qualified researchers.
You clearly don't understand what was presented and go right back to the
handwavium. Ugh.

Hopefully other readers will get more out of the link I posted.

Jeff
--
All opinions posted by me on Usenet News are mine, and mine alone.
These posts do not reflect the opinions of my family, friends,
employer, or any organization that I am a member of.
 




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