Instability of long cylinders

spacesettlers@... ssi_list@...

> 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."

I have had the same concerns. Shifting of mass inside cannot be avoided:
people, goods, water, winds. Such a cylinder rotating alone will be
inherently unstable. The more because an end cap that begins to swing will
see mass pooling on the outside of the wobble, in a principle somewhat
similar to that which causes rivers in flat areas to form horseshoe bends.
Of course a system of movable counterweights could do for Island Three
what stabilizers do for cruise ships in heavy seas (or the flight computer
does for a modern fly-by-wire fighter plane with its inverted-V wings), but
if the ship's stabilizers fail for a short time, the consequence will be a
number of passengers becoming sick and green-faced until the stabilizers
come online again. If Island Three's stabilizers fail there is the risk
that a wobble comes out of control, causing the habitat to tumble and
disintegrate, killing all inside.
I should certainly feel safer in a space habitat with a passive
anti-tumble system than with an active one. As a parallel, a nuclear core
has control rods connected to electromagnets such that if the electric power
goes, the control rods automatically fall into the core, halting the chain
reaction. This is far safer than control rods that have to be actively
inserted by electric motors even if something snafus.

> "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.

I should agree that this mirror design is problematic. If the mirrors
move, then only at the 45 degree angle will the entire valley opposite each
mirror be directly illuminated. In addition, the tips of the mirrors would
feel centrifugal effects up to five or so g's. Many cables would be needed
to support each mirror, like the cables of a suspension bridge. Each cable
would need to be reeled in and out every day. One cable failing to reel in
would not be a problem, but one failing to reel out in the morning would
either have to be jettisoned, or halt the whole opening of the mirror. Nor
would *I* like to be a repair worker fixing something at the tip of one of
those mirrors, especially if the problem included a mirror being stuck in
the full open position. A robotic repair worker might be flung off at such
a speed that its impact on the neighbouring cylinder would be no niggling
thing.
But there is no real reason to quarrel over this aspect of Island 3's
design now. By the time the relevant engineering desicions need be made, we
will have had decades of experience of building smaller habitats, as well as
having seen vast advances in robotic technology compared to today.

One possible idea is to keep each mirror pivoting around an axis going
lengthwise along the mirror. This axis is kept fixed, always at 45 degrees
wrt the axis of the habitat. In the habitat's morning the mirror rotates so
that the sun appears to come over the edge of one of the opposite valleys.
Then the rotation is slowed so that the image of the sun spends the rest of
the day crossing the fairly narrow width of the window opposite the valley.
In the evening the rotation is sped up again for a swifter "sunset".
This would of course not be close to mimicking the traverse of the sun
from nearly east to nearly west on a day on Earth, but it would be much more
easily done engineering-wise, and would mimick *some* movement of the sun.

> "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
> the instability of long cylinders?

But for leukemia we could have asked him. My own opinion (not based on
detailed engineering calculations, a task to which I'm unequal) is that
twin, counter-rotating habitats would be an adequate, passive anti-tumble
system. Perhaps something with magnetic bearings designed for ten times the
expected loads, and multiple girders and magnetic bearings so that if one
broke for whatever reason the others would be sufficient. Thus, I would not
move into an Island 3 type cylinder that rotated alone, with a 45 degree
mirror to track the sun like the Standford Torus proposal. I would move
into the one of a twin cylinder.
Electric eddy currents in the bearings might slow down the rotation of
each cylinder, so that a small force would be needed to sustain the
rotation. Of course the motors supplying this force might fail, but this at
least would *not* be a potentially catastrophic failure, since years would
probably elapse before a noticeable portion of the angular momentum had bled
off. A plot for a story, perhaps? :-)

Jon Lennart Beck.