# Re: >H Open Air Space Habitats

Forrest Bishop (forrestb@ix.netcom.com)
Sun, 2 Mar 1997 22:04:33 -0600 (CST)

>From David Cary,

"I saw a small storm gathering on the floor overhead."
..
>Open Air Space Habitats
..
>The only example this author
>knows of is Larry Niven's "Ringworld",

Then you haven't seen _Legacy_ by Greg Bear.
from chapter 1:
"The entrance to the first chamber yawned wide, and we flew into
brilliant
tubelight.
50 Km in diameter and 30 Km deep, ...

>The smaller the diameter of the rotating ring or cylinder, the less
demands
>are put on its main structural material.

..I want equations :-). ...

"floor-level air pressure" p in a rotating
cylinder
stress_h = r*(pressure/t + density*a)
stress_a = r* pressure/2t

[[Those is them!]]
..
> This fabulous new material is the long sought Carbon-Carbon chain
>molecule.

So what's so fabulous about it ? Fill in the blanks in my chart:

..Elastic Strength (stress_max)(in tension)
150 Gpa ("Buckyfiber")(Is this right ?)

[[Buckyfiber appears to have two to threee times the tensile strength
of
diamond (50 Gpa). I got this particular value off the web (so it must
be
true, right?).]]

Density
???? Mg/m^3 C

[[I used a "standin" value of 3500 Kg/m^3. The safety factor is a
reasonably slowly
varying function of this, for densities between 2000-5000 kg/m^3.]]

>The gathered ends of Buckyfibers are led off of the spools (which are
also
>spinning) and
>brought to rendezvous with the outer surface of the spinning hoop.

Perhaps it would be easier to lay down stuff on the inner surface,
allowing
the radial acceleration (the imaginary "centrifugal force") to hold the
loose fibers in place for a while, until we glue them down.

If the end of the strand comes off the spool too soon, or the strand
breaks...
A loose strand on the outer surface of that rotating cylinder ... would
unwind, getting longer and more dangerous by the second.

Makes for good drama :-/.

[[Another reason to build two cylinders side-by-side. Also, these
fibers are envisioned
as being wound on as a multiple set, and at an angle. If one breaks, it
should (?) get
entangled by the others next to it and brought down.]]
..
> Leaving this cylinder spinning slowly, we bring in fleets of these
>Buckytube spools. The
>fiber should be wound at a slight angle, maybe 10 degrees

I think that 1980's technology "graphite-fiber" pressure cylinders use
the
optimum 26.6 degrees. If I knew the ("floor-level") air pressure, wall
thickness, and desired floor pseudo-gravity I think I could calculate
the
optimum angle (between 0 and 26.6 degrees).

[[Sure- first you have to calculate the atmosphere load on the ends,
and
some average hoop load. The 10 degrees is just a punt.]]
..
> The interior volume of this world can be left open to space,
meaning
>each point on the
>interior living surface has about 150 Km of atmosphere above it, and
then
>700 Km of
>nothing. Looking upward, at an angle, one can still see the stars.

So, I could place my own personal communication satellites about 200 Km
"up", and they would just stay there ?
Interesting that there is *nothing* at the axis, it truly is an
imaginary line.
I like this. It allows you to build a spacedock in the exact center of
the
vessel, well-protected, from which you can either (a) push off and exit
the
station through either end, or (b) ride the elevators down to the
surface
for some heavy time.

[[Just so.]]

> That was a lot of work, but what is a World worth?

You might mention the 3 ways to get light to the plants;
(a) direct sunlight through the hole in the ends (doesn't work for
extremely long cylinders)
(b) direct sunlight through transparent sectors in the cylinder (the
transparent sectors obviously let light in through the holes in a "net"
of
filaments, with no slag attached to those sectors ... there are many
variants; one of them is that the sunlight filters through the layer of
atmosphere twice, once on the transparent side and once on the farmland
side).
(c) big light generators on the inside. This gives the best radiation
shielding to the plants. The energy presumably comes from solar cells
while
near a star, nuclear energy while traveling between stars.

[[I might, but space and time limitations in the article may not permit
this.
There is a drawing that goes with this piece that shows these
things.]]

>From: Carl Feynman <carlf@atg.com>
>Subject: Re: >H Open Air Space Habitats

>>and another three
>>meters of slag should be sprayed on the outside, for radiation
protection.
>
>Why spin the slag? Put it in a nonrotating shell well away from the
>cylinder, and you save big on structural mass.

Also, the maximum stresses (due to rotation) are on the outside. If
this
"slag" is less strong than the carbon fiber, it will flake off when the
cylinder is brought up to speed. Very dangerous to ships in the area.

[[The slag (this term is a holdover from when we made space habitats
out
of metal) is subject to one gravity directed radially out. A little
fiber
reinforcement should suffice to hold it in place. Consider that it
deforms
plastically to relieve hoop stress.]]

I've heard an interesting study in which it was claimed that, in some
cases, *no* shielding was better than any reasonable amount of
shielding.
The argument was that, with no shielding, a high-energy cosmic ray
would
hit and utterly destroy one DNA molecule in an organism. However, with
"small" amounts of shielding, the high-energy cosmic ray would hit the
shield and generate lots and lots of low-energy particles, which would
spread out and damage many, many cells of the organism. The authors
argued
that the amount of shielding which would be thick enough to catch all
these
low-energy particles was totally unreasonable.

[[Yes, a little shielding is worse than none for the above reason. The
author may
have been referring to spacecraft rather than non-propelled structures.
Many long
duration mission proposals use a small "storm shelter" for solar storms
consisting
of a meter or two of material. I *think* 6 meters of stuff is adequate
against the
lower energy flux.]]

.. 150 Gpa (!) as the Ultimate Tensile Stre=
ngth
>of Buckyfiber. I don=EDt know its speed of sound.]]

I think the speed of sound is irrelevant.

[[For pertubation analyses. I was not familiar with casting the
performance
in terms of tangential velocity, and thought Carl was referring to
perturbations.]]

A 1.0 Mm radius, 1 gee spacecraft has a v = sqrt(a*r) = 3.13 Km/s (well
under v_max), so your numbers look physically possible (although the
safety
factor looks kind of small, less than 2).

>[[I gave a safety factor of three to the shell for the slag and the
>atmosphere- so it will have
>to be thicker if you want serious dirt. I was assuming a few inches
>average.]]

[[Oops. I had the slag turned off when I did the safety factor. Guess
you
don't want me designing your house eh?]]

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