Instability of long cylinders

A long cylinder...

The idea is appealing if one assumes a simple spherical orbit. The
math is simple and the long cylinder is elegant looking, the stuff of
which scifi is made of.

However, to keep that cylinder stable will require a number of factors
to be address. We can break these down into two categories. First,
architecting the long cylinder against any force, and second,
identifying internal and external forces.

Assume a box...any first year mechanical engineer knows that five (5)
sides of a box need to be cross braced otherwise the box cannot handle
forces from all directions. This concept extended to a cylinder
introduces asymmetries that require reducing the long cylinder to a
series of boxes. Yes, we could use trusses or wires or even clear
cylinders of fabric for cross bracing, but we will still need to
address the issues of basic box bracing.

We can use external bracing as well. In order to achieve an internal
clear cylinder this is probably best aesthically. We can apply bridge
techniques using cables. We can create a series of cylinders like a
six pointed jack and brace them, and we can even do something like the
Baseball configuration of Deep Space Nine. You still end up with a
boxy like construction.

I'll skip internal forces since external are going to be far more
interesting.

Any long cylinder in space is going to have a number of forces acting
on it no matter how small. However, due to the megaconstruction every
force is amplified.

The simple orbit is going to be affected by moon and sun gravitational
forces. In addition, the earth gravitational forces will vary slightly
in a circular orbit except most orbits are defined by an ellipse. So
all the gravitional sources are varying in time.

Next is the solar wind and the earth magnetic field. These are the
most significant forces in Space Weather and would also need to be
considered not only as a mechnical force against the whole craft, but
as a force that is charging the craft.

That brings up an interesting point. The Tether problem. For a long
cylinder traveling through a magnetic field (earth, sun, solar flare,
CME, etc) there will be induced by the m-field a significant electric
difference between the ends, and possibly a number of nodes due to
standing waves of ions.

The solar wind by itself will create a differential force.

Any space debris will create localized forces.

Any comet, asteroid, CME or other mass hitting, even glancing, the
cylinder will introduce an instability.

Any disruption to the center of mass will also have an effect.

Correcting these instabilities is best done passively since any active
structures, corrections or outright defenses are going to cost energy.

So, to your question. Once you shut off all the ONeill stabilizers,
what happens...

A cylinder 4 miles wide (diameter) is possible using truss structures
made of carbon fiber. A long cylinder, more than 8 miles in length,
might create more problems than it resolves.

Two counter rotating cylinders would provide some stability due to
gyroscopic forces. However, given the list of forces above, other
dynamic stability solutions would still be needed.

Finally, there is going to be assumed risk. We do not have the
technology of Ringworld Engineers, Dyson's Spheres or Deathstars just
yet. We barely have the technology to kludge together ISS, let alone
put a space station like 2001:A Space Odyssey into orbit. Should the
design last 100 or 1000 years? Should it be designed against all known
possibilities by years ("100 year flood", earth magnetic field
flipping, or EMF total loss) or by magnitude of event (CME, space
trash).

While technology advances allow us to see the future possibilities of
these space megaprojects from an engineering standpoint, the largest
instability remains: cost. And that assumes political will and no
interference from any species.

The cost just to ship the materials into earth orbit, either from the
ground or elsewhere, is huge. Currently, the $/lb (USD) cost for NASA
is 20k-30k and for the commercial folks even to LEO is $5k to $10k

Dollars per pound is the ticket price of entering space. And staying
there as well.

David

>
> From time to time on these maillists the subject has come up that
> spheres and tori might be better shapes for habitats than long
cylinders
> because of the latter's tendency to eventually tumble end-for-end.
> While on the newsgroup rec.arts.sf.science somebody was talking
about
> the ultimate size for space habitats given current materials, and I
was
> mentioning the dimensions of Island 3. This prompted James A.
Donald to
> say:
> "Island 3 is not practical. Being longer than it is
> wide, will be unstable. Will flip over, killing
> everyone inside."
>
> To which I responded:
>
> "Eh, you're referring to a real-life aspect of physics, but
needlessly
> hyperbolizing it. It's more like if civilization fell in an O'Neill
> cylinder, and the stabilization systems failed, and if you went away
and
>
> came back after a few decades, it would be tumbling end-for-end
rather
> than
> on its long axis.
>
> "Frankly, I consider the mirror design of Island 3 to come closer to
> making
> it impractical than the simple fact that it's longer than it is
wide.
>
> "But the point relevant to Knobby's point was that cylinders 4 miles
in
> diameter are possible with present materials."
>
> He came back to say that "It would tumbling be end for end in about
ten
> minutes or half an hour" and further went on to say that an above-
ground
> swimming pool breaking and dumping its water would be sufficient to
> destabilize the cylinder.
>
> So what say you guys? A problem if the automatic stabilization
system
> remains off-line for weeks or months, or a catastrophic flip within
the
> hour, killing millions?
>
> But ultimately, it occurs that my very first response should have
been
> that an O'Neill Cylinder(s) habitat would actually have 0 net
angular
> momentum because it's actually two counter-rotating cylinders. In
The
> High Frontier, O'Neill only mentioned this as a solution to the
problem
> of turning the axis of the habitat so as to track the sun, but could
it
> be that this design solution was also proposed for the second reason
of