From: Forrest Bishop (forrestb@ix.netcom.com)
Date: Sun Mar 09 1997 - 12:59:24 MST
>From: Michael Lorrey <retroman@tpk.net>
..
>Given the existence of Ti on the moon, I'd go for a TitanAl/Carbon
>matrix.
David Cary:
A metal/carbon composite. Interesting. We would have to worry about
unequal
thermal expansion and a few other things ...
I'm still amazed that a rope of "Buckyfiber" can hold over 100 times as
much weight as the same size rope made of Ti (500 times as much weight
as a
steel cable). (Bishop got this number off the web, so it must be true
:-).
[[Actually, I've heard the ‘two to three times diamond’s tensile’
from several sources. For a little more on Al/Ti/C
composites check out Tom McKendree's paper at the
Molecular Manufacturing Shortcut Group (MMSG) chapter of the
National Space Society. It is much easier to get the materials from an
asteroid. In
"The Interworld Rapid Transit System" is a proposal to shoot raw
materials
from various asteroids to the construction site. This paper will be in
*The Journal of the British Interplanetary Society*.]]
>The question is what the elasticity of the matrix it is cured
>into is.
Yes, the properties of small fibers don't help any; it's the bulk
material
properties that are important in this application.
Lorrey's comment has caused me to wonder how much these "Buckyfiber"
cables
will stretch when we start spinning the station. (Note that the answer
to
this question requires measuring the "modulus of elasticity", a
completely
different material property than and yet confusingly similar-sounding
to
Elastic Strength).
[[Young's Modulus (modulus of elasticity) for Buckyfiber is probably
similar to Carbon Fiber- on the order
of 100 Gpa (a total guess). One more sweet thing about Carbon Fiber is
the extremely
low coefficient of thermal expansion-- for some flavors it is even
slightly negative (translation:
the fiber doesn't expand in length when it is heated, it can even
contract). Composite material
engineers use a blend of fibers and matrix to create parts that don't
change their shape when
they are heated up. This property might also carry over to Buckyfiber.
Once the thing is spun up, slagged and pressurized, these expansion
effects retreat
to the second or third order.]]
Bishop makes the almost prescient guess that
>> >The
>> >fiber should be wound at a slight angle, maybe 10 degrees
Cary comments that
>> I think that 1980's technology "graphite-fiber" pressure cylinders
use the
>> optimum 26.6 degrees.
[[Yes, for cylinders under uniform hydrostatic load.]]
Or maybe lots of ropes or "sheets" connecting the axis to several
lateral
lines running the length of the cylinder. The slight extra curvature (I
have to climb to the top of the mountain to reach the rope anchor)
would
improve the safety factor.
[[A catenary-ish cross-section for the cylinder would be nice, although
harder to make. The presumption was the circumferential tensile
strength
is great enough to remove this requirement. ]]
If you merely inflate a really large sphere, so large that it just
starts
to touch the tops of the atmosphere on both sides, will the
differential
gas pressures automatically push sphere back toward axis if it ever
gets
misaligned ?
(The tiny amounts of gas leaking out the ends of the cylinder would
tend to
push the sphere along the axis *away* from the exact center of the
cylinder
..)
[[I looked at the idea of a thin membrane at the top of the atmosphere,
but didn't consider
an inflated version. A problem with transparent membranes is radiation
and
dust degradation. A nanotech membrane _might_ be able to self -repair
and still
have acceptable transparency.]]
>Nope, build it edge on to the sun, with no transparent sectors. the
ring
>shields itself from radiation, and the edge of the rim will have
fresnel
>lenses on both sides to focus light inward to the opposite edge.
Interesting. With a very "short" cylinder (almost looks like a toroid)
(length << radius), one could build a large (curved) mirror collecting
all
the light from full end-on area facing the sun (pi*r^2), and focus it
down
to the much smaller farmland area (2*pi*r*length), to get the full
Earth-standard amount of sunlight at distances much further away from
Sol
than Earth.
[[I think Lorrey means 'the axis of rotation collinear with the orbital
axis'.
Do note the period of the 1000 Km, one gravity design is .558 hour
(33.5
minutes).
>the rim can easily carry a large enough current
>to generate a good sized magnetic field. With the sun edge onto the
>ring, thermocouple electric effects will be a big contributor to this.
Hm. Interesting feature.
[[Hm. I like it!]]
>From: forrestb@ix.netcom.com (Forrest Bishop)
>Subject: Re: >H Open Air Space Habitats
..
>A loose strand on the outer surface of that rotating cylinder ...
would
>unwind, getting longer and more dangerous by the second.
>
>[[Another reason to build two cylinders side-by-side.
I don't understand this.
I originally thought you meant side-by-side like this:
spinward
^
N====S S====N
v
spinward
but that loose fiber would still be almost as dangerous if they were
side-by-side like this:
spinward
^
N====S
S====N
v
spinward
[[I meant the second version.
This reminds me of the story of
the Orbiting Astronomical Observatory (OAO), a space telescope launched
in
the late '60's. This (very expensive) package was spin-stabilized for
launch. When it
was in orbit, a small weight on a string wrapped around the body of the
satellite
was supposed to be released, like a yo-yo. As it unwound, it carried
off the angular momentum and
slowed down the spin rate. Well, a little problem came up when the
apogee motor
burned out. While its empty shell was still attached to the end of the
satellite, the
configuration was unstable about the long axis, and so the thing went
into a "flat spin".
So now ground control had a little dilemma: they knew that when they
released the apogee
motor that the satellite would flip back onto the right spin axis, but
there was a 50-50 chance
of it spinning the wrong way, meaning the yo-yo wouldn't work. Well,
Murphy stepped
up to the plate, and last I heard the observatory is still up there,
spinning away. If they
had installed a second yo-yo wound the other way this would have been
covered, but
all the spare yo-yos were left on the ground.]]
>Obviously if the ends are vented to space, the axis is in a vacuum --
>but I
>keep thinking that if we sealed up the ends (or the tube was very
>long), we
>would have (significant) atmosphere at the axis. Perhaps my intuition
>is
>leading me astray.
>
>[[What would keep it there?
>
Assuming the ends are sealed, what would pull atmosphere at the axis
away
from the axis ? I see that Earth's gravity pulls atmosphere at high
altitudes down towards the surface, (the force due to gravity varies by
less than a factor of 2 from the surface clear out to the typical Space
Shuttle orbit), but there is no such force near the cylinder's axis,
right?
[[No, not a force per se. But consider a particle in the space around
the axis (this
is vaguely similar to the Ringworld stability problem). Any non-zero
velocity
component will send it back to the walls or ends of the cylinder. This
will be
the case for gas-kinetic models. For continuum models, the force is
'centrifugal'
transferred via viscosity.
BTW, the Buckyfiber Open Air Habitat is an original proposal of mine,
and first
aired on the Extro and >H lists.
Forrest Bishop
]]
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