>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
the instability of long cylinders?

Regards,

Mike Combs

From: Raven

> I should agree that this mirror design is problematic.

In addition to the problems you cite, these mirrors, subjected to
multiple G's of centrifugal force, cannot be allowed to sag if they are
to continue functioning as planar mirrors.

The mirrors on a Stanford Torus would only be subjected to around 1/2 G.
And there are other habitat designs with stationary mirrors.

> 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.

That's the way I look at it too.

Regards,

Mike Combs

>
> 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.

If I remember my physics correctly a cyclindrical gyroscope has a
magic length (I believe its 4.5 diameters but its been fifteen years
since I had to do that problem set) where the precision vector (the
thing that makes it want to go end-over end) is still has a component
pointing towards the axis (making the spin want to stay along the long
axis).

Now if a large force (say a substantial meteor) were to collide with
the cylinder, then the precision vector may be go past the circular
tangent and thus destablizing a such a cylinder momentarily but enough.

If the length is beyond the magic length the precision vector has a
component point away from the axis and will destablize.

>
> "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."
>
Actually the mirror design is completely impractical, but it is
possible to decouple the mirror completely and have a non-rotating
conical section instead. The mirror would still need to be strong
enough to allow the precision needed to track the sun, and the actual
mirror would have to be designed not to completely fry the interior
(for example having strips of mirror panels followed by non-reflecting
or empty strips) since such a cone would be focusing light in the
interior.

> 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.
>

Again that depends on how long you make the cyclinder. If it is
substanially less than the magic length (which I claim is about 4.5
multiples of the diameter) then no this would never happen.

> 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
> the instability of long cylinders?

Yes and no, the net angular moment is zero but the individual
cylinders have angular momentum and there is no reason why they can
fail individually.

Consider that a bridge has a net force on it of zero all the time (or
it would accelerate away) but that does mean the forces on the bridge
is zero. It means the sum of all the forces are zero. It can still
fail catastrophically (accelerate away from its position) if those
forces reach some critical point in a section of the bridge. This is
the same with the two cylinders. Together there is no net force on
them, but each has a large net force on them.

Bill

--- In spacesettlers, "Bill" wrote:

> If I remember my physics correctly a cyclindrical gyroscope has a
> magic length (I believe its 4.5 diameters but its been fifteen years

For the traditional Gerard K. O'Neill Island Three, the cylinder
diameter is given as four miles, and the length as twenty miles. This
make the length five or even six times the diameter, depending on
whether you think "twenty miles" means the length of windows and
valley floors (the part between endcaps), or the overall length,
endcaps and all. So we can't get out of it by claiming that the
cylinder is too short for such a thing to happen (we could get out of
it by making shorter cylinders).

Connecting the two cylinders together solves the problem, if you trust
the connections. How much you're willing to trust the connections
probably depends, in no small part, on what you think would happen if
they failed. I trust most people won't try to mug me, and so I go
outside and walk. And anyway, the most I'll lose is twenty dollars.
I might not be as trusting if I were carrying every cent I have to my
name, because if my trust is violated, I'm left penniless.

So I guess it comes down to the same thing I ask whenever this comes
up, and I will propose that scenario and ask that question in my next
post.

From: Bill

> If I remember my physics correctly a cyclindrical gyroscope has a
> magic length (I believe its 4.5 diameters but its been fifteen years
> since I had to do that problem set) where the precision vector (the
> thing that makes it want to go end-over end) is still has a component
> pointing towards the axis (making the spin want to stay along the long
> axis).

If true, it means that this particular single-cylinder habitat design is
probably OK:
http://ssi.org/assets/images/Ch06p086.gif
http://ssi.org/assets/images/Ch06p088.gif

> Actually the mirror design is completely impractical, but it is
> possible to decouple the mirror completely and have a non-rotating
> conical section instead. The mirror would still need to be strong
> enough to allow the precision needed to track the sun, and the actual
> mirror would have to be designed not to completely fry the interior
> (for example having strips of mirror panels followed by non-reflecting
> or empty strips) since such a cone would be focusing light in the
> interior.

I've had similar thoughts, but we would have to give up the appearance
of a normal sun in the sky, especially as you moved closer to the
sunward end of the cylinder. Another thing I've though about was a
continuous mirror with a kind of bugle flare on it. Now we'd get a
continuous, single sun, but it's shape would depart markedly from a disc
shape. But that might be acceptable to many.

> Yes and no, the net angular moment is zero but the individual
> cylinders have angular momentum and there is no reason why they can
> fail individually.

Yeah, but I was thinking that the linking structures (one tower under
compression at one end, and one cable under tension at the other) would
restrain the cylinders, preventing the spin axis from departing from the
structural axis. In other words they would keep the wobble from getting
started.

Regards,

Mike Combs

From: Xenophile

> depending on
> whether you think "twenty miles" means the length of windows and
> valley floors (the part between endcaps), or the overall length,
> endcaps and all.

I'm willing to bet the 20 miles refers to the latter. Reason being that
I remember one early reference saying 16 miles (which I'm betting refers
to your former), then later on most every reference was saying 20 miles.

I'm in favor of including the endcaps in the length because, unlike the
minds behind the alternative Lewis One model, I'd consider the endcaps
valuable and useful territory.

Regards,

Mike Combs

--- In spacesettlers@yahoogroups.com, "Xenophile"
>
> --- In spacesettlers, "Bill" wrote:
>
> > If I remember my physics correctly a cyclindrical gyroscope has a
> > magic length (I believe its 4.5 diameters but its been fifteen years
>
> For the traditional Gerard K. O'Neill Island Three, the cylinder
> diameter is given as four miles, and the length as twenty miles. This
> make the length five or even six times the diameter, depending on
> whether you think "twenty miles" means the length of windows and
> valley floors (the part between endcaps), or the overall length,
> endcaps and all. So we can't get out of it by claiming that the
> cylinder is too short for such a thing to happen (we could get out of
> it by making shorter cylinders).
>

The traditional one may still be a valid length. In my problem set we
were discussing a solid cylinder not a cylindrical shell.

More importantly the design that ONeill came up with was part of a
after class physics seminar for his first year students. I can make
myself believe that part of designing this, ONeill, or at least one of
the physics students, solved the same problem I did for a shell
instead of a solid cylinder.

Short answer since there is a minimum length a cylinder must be before
it destabilzes, anyone building such a habitat would be wise to stay
underneath it as well as putting active dampeners, and a few other
fail safe mechanisms.

Bill

--- In spacesettlers@yahoogroups.com, "Combs, Mike" wrote:
>
> From: Bill
>
> > If I remember my physics correctly a cyclindrical gyroscope has a
> > magic length (I believe its 4.5 diameters but its been fifteen years
> > since I had to do that problem set) where the precision vector (the
> > thing that makes it want to go end-over end) is still has a component
> > pointing towards the axis (making the spin want to stay along the long
> > axis).
>
> If true, it means that this particular single-cylinder habitat design is
> probably OK:
> http://ssi.org/assets/images/Ch06p086.gif
> http://ssi.org/assets/images/Ch06p088.gif

Please remember that the people that are designing this stuff are
scientists and engineers with, I hope, a fairly indepth knowledge of
Newtonian Physics. Thus when asking about simple problems like overall
dynamic stability, one should at least believe that these people
solved the dynamic models especially when the numbers seem reasonable.

>
> > Actually the mirror design is completely impractical, but it is
> > possible to decouple the mirror completely and have a non-rotating
> > conical section instead. The mirror would still need to be strong
> > enough to allow the precision needed to track the sun, and the actual
> > mirror would have to be designed not to completely fry the interior
> > (for example having strips of mirror panels followed by non-reflecting
> > or empty strips) since such a cone would be focusing light in the
> > interior.
>
> I've had similar thoughts, but we would have to give up the appearance
> of a normal sun in the sky, especially as you moved closer to the
> sunward end of the cylinder. Another thing I've though about was a
> continuous mirror with a kind of bugle flare on it. Now we'd get a
> continuous, single sun, but it's shape would depart markedly from a disc
> shape. But that might be acceptable to many.

It should look like an ellipse with the long side of the ellpse
parallel to the axis of the cylinder.

>
> > Yes and no, the net angular moment is zero but the individual
> > cylinders have angular momentum and there is no reason why they can
> > fail individually.
>
> Yeah, but I was thinking that the linking structures (one tower under
> compression at one end, and one cable under tension at the other) would
> restrain the cylinders, preventing the spin axis from departing from the
> structural axis. In other words they would keep the wobble from getting
> started.

Yes you are right, that the tension and compression towers would
dampen the forces that would start the wobble, but that doesn't really
solve the problem. You are transfering the moment of the cylinders
forming a stress cycle in the towers. The wobble may actually cause
the towers to fail catastrophically if the stress the wobble produces
is higher than the yield strength of the metal in the towers. The
question then must be asked is the strength of the tension and
compression towers greater than the force the wobble will produce.
Also a wobble may induce fatigue cracking in the towers causing them
to fail.

All these kinds of failure modes would need to be analyzed and
appropriate safeties would need to be added during the design phase of
the project. On the bright side, this kind of structure need only be
designed once.

I doubt anyone would seriously promote an island 3 design prior to a
century after the construction of the first habitats.

Bill

From: Bill

> Please remember that the people that are designing this stuff are
> scientists and engineers with, I hope, a fairly indepth knowledge of
> Newtonian Physics. Thus when asking about simple problems like overall
> dynamic stability, one should at least believe that these people
> solved the dynamic models especially when the numbers seem reasonable.

Yeah, that seemed to be an assumption the guy I was arguing with was
unwilling to make.

Regards,

Mike Combs