Spin-cast a mirror in space?

Found an interesting article after a Google search:

The shape of a liquid surface in a uniformly rotating cylinder in the
presence of surface tension.
http://maeresearch.ucsd.edu/~vlubard...s/Acta2013.pdf

This calculates the shape of the meniscus under both gravity and zero
gravity.

A topic I'm interested in is whether the method of making large parabolic
mirrors on Earth by rotating the glass in molten form to form a parabolic
meniscus then allowing it to solidify can also work in space.

This will have an advantage over transporting the already formed mirrors
into space because for large mirrors you have to concerned about the size of
the rocket fairing. But in fact in zero g you would have an advantage in
that you wouldn't have to worry about the mass and cost of the support
structures and of the mirror deforming under it's own weight.

You could emulate the Earth's gravity during the formation stage in space
by using either centrifugal force due to rotation around a second axis or by
using linear acceleration. Rotation around a second axis though could create
instabilities. On the other hand doing a linear acceleration for the weeks
of cooling time would require a prohibitive amount of propellant.

That is why I wondered if it is possible to do in zero gravity just using a
rotation around a single axis as on Earth. In the article ther were able to
only solve numerically the equations for the zero gravity case. So my
questions is, is it possible to set the starting parameters such that the
meniscus shape is a good approximation to parabolic? Note it would also be
sufficient to get a good approximation to a spherical surface since then you
can use a combination of spherical mirrors to cancel out the distortions due
to a non-parabolic surface:

Spherical Aberration.
https://starizona.com/acb/basics/equ...spherical.aspx

It may be possible to get it to work no matter the shape of the curved
meniscus by using a mirror of similar shape to cancel out the aberrations
due to the non-parabolic shape. For instance, the Hubble uses a combination
of hyperbolic mirrors to cancel out aberrations.



Bob Clark


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"Robert Clark" wrote in message ...

Yes, that is how large mirrors are made on Earth. But the massive size of
the support equipment needed because of the Earth's gravity make them
impractical on Earth beyond a certain size for a single mirror, about 8
meters for a single mirror. Larger telescopes instead are made segmented.

The advantage of doing it in space though is you have zero gravity so you
would not need the massive support structures.

But you are correct about the instabilities. This is illustrated in this
video:

Torque free motion of a prolate axi-symmetric rigid body.
https://www.youtube.com/watch?v=s9wiRjUKctU

What's happening is when you have two rotations around axes both through a
common center, the result is a rotation around an axis at a diagonal line
between the two. It then has the appearance of gyrating wildly, while what
is really happening is it is rotating around an axis that is not on an axis
of symmetry of the body.

BTW, this is not exactly the same effect but it is cool:

Watch: WTF is going on with this object spinning in zero gravity?
Go home, physics. You are drunk.
BEC CREW 21 AUG 2015
http://www.sciencealert.com/watch-wt...n-zero-gravity

For our scenario, it probably could work to apply some restoring force to
maintain both rotations separately. But likely this force would be so large
that you might as well have applied a linear acceleration.

The reason why I was considering the centrifugal force case instead of using
linear acceleration is that this would require a rocket thruster operating
for a long period, perhaps weeks. But if this was by chemical propulsion the
propellant required would be prohibitive. On the other hand, if you used
electric propulsion such as ion thrusters, this would be an extremely small
thrust and acceleration.


Bob Clark

----------------------------------------------------------------------------------------------------------------------------------
Finally, nanotechnology can now fulfill its potential to revolutionize
21st-century technology, from the space elevator, to private, orbital
launchers, to 'flying cars'.
This crowdfunding campaign is to prove it:

Nanotech: from air to space.
https://www.indiegogo.com/projects/n...ce/x/13319568/
----------------------------------------------------------------------------------------------------------------------------------
"dlzc" wrote in message
...

Dear Robert Clark:

On Sunday, May 15, 2016 at 7:54:35 AM UTC-7, Robert Clark wrote:
I wonder what the fluid would look like in a two-axis
rotation system.
That is how very large parabolic mirrors are produced on Earth, using
gravity in place of he second spin axis.
That is, you would have the fluid rotating around a
vertical axis through the center.
The problem is, it is unstable, and unstable in the cooling time of most
liquids. Additionally, with a short "major spin arm", you'd not end up with
a section of a parabola, I wouldn't think.
But also have the entire spacecraft subjected to
a centrifugal rotation via a connection to a
tether with a counterweight at the other end,
as with proposals to simulate gravity with long
space missions.

To simulate this on Earth...
No point in trying, because on Earth, we already make mirrors and mirror
panels this way.

David A. Smith
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