site updated 1/1/05

http://www.dyarstraights.com/msgundam/opentype.html

I can't speak to the accuracy of what is presented, I can only pass on
this quote:

"My purpose here is to detail the Real Science behind the science
fiction,..."

A lot of interesting O'Neill stuff.

RKH

I'm having trouble with the idea that a connected system of two gyroscopes
rotating in parallel but opposite directions has a gyrscopic inertia of zero.

Take a motorcycle, support it on a stand with powered rollers under each
wheel. Now, spin the rollers up until the cycle's wheels are turning at an
equivalent speed of 70 mph. The motorcycle will exhibit a large amount of
gyroscopic effect, i.e., it will be difficult to tip over.

Now stop the rollers, reverse the direction of the roller under the front
wheel so that the wheel spins backwards, and again spin everything up until
the wheels are turning at an equivalent 70 mph speed.

Are you saying that in this instance, the motorcycle will have no more
stability than if it was standing still with its wheels stopped?

And if that's not the case, how will spinning two Oneill cylinders side by
side but in opposite directions zero out gyroscopic inertia?

Steve

From: Steve Long [mailto:longsteven@...]

> Are you saying that in this instance, the motorcycle will
> have no more stability than if it was standing still with
> its wheels stopped?

True. The only difference between that situation and the situation of
the wheels being stopped is that when you turn or tip the motorcycle,
there's a certain amount of stress on the frame.

Regards,

Mike Combs

"Combs, Mike" wrote:
>
> From: Steve Long [mailto:longsteven@...]
>
> > Are you saying that in this instance, the motorcycle will
> > have no more stability than if it was standing still with
> > its wheels stopped?
>
> True. The only difference between that situation and the situation of
> the wheels being stopped is that when you turn or tip the motorcycle,
> there's a certain amount of stress on the frame.
>

I don't follow you. You are saying that a motor/bicycle with
counter-rotating wheels will fall over if unsupported, whereas one with the
wheels rotating in the same direction won't? And if you aren't saying
that, how are the "stresses on the frame" of a motorcycle different from
the stresses on the link between two Oneill habitats that are being forced
to keep their axes of rotation pointed directly at the sun as they orbit
around it? (Which I understand is why the idea of linking them together was
proposed in the first place.)

Steve

OK, I think I understand now.

It is not gyroscopic inertia that is canceled in a coupled,
counter-rotating system, but gyroscopic precession. Gyroscopes, when an
attempt is made to rotate them in a direction 90 degrees to their plane of
rotation, react by precessing their axis in the third direction, 90 degrees
to both their plane of spin and the force being applied to their axes. The
direction of this precession depends on the direction of rotation of the
gyroscope. For example, attempting to rotate the axis of a
clockwise-spinning gryoscope to the right may cause the end you're pushing
on to dip; rotating a counter-clockwise-spinning gyroscope in the same
direction will cause the same end to rise.

A coupled system of counter-rotating gyroscopes cancels this precession
*for the system as a whole* -- but each individual gyroscope is still
exerting its precessional force on the point at which it is attached to the
coupling mechanism.

For a pair of Level-3 Oneill habitats, that's going to be a lot of torque ...

Steve

From: Steve Long [mailto:longsteven@...]

> I don't follow you. You are saying that a motor/bicycle
> with counter-rotating wheels will fall over if unsupported,
> whereas one with the wheels rotating in the same direction
> won't?

Oh, I'm sure that the motorcycle with both wheels spinning in the same
direction will /eventually/ fall over. But the point is that gyroscopic
forces will come into play in how the motorcycle behaves. Not so with
two counter-rotating wheels, provided that we consider /the motorcycle
as a whole/ to be the system under discussion.

> And if you aren't saying that, how are the "stresses on
> the frame" of a motorcycle different from the stresses
> on the link between two Oneill habitats that are being
> forced to keep their axes of rotation pointed directly at
> the sun as they orbit around it?

Not at all different. Like with the tower under compression at one end,
and the cable under tension at the other, some portion of the
motorcycle's frame would see a compressive load, while another saw a
tensile force. The amount of force would vary with how rapidly we were
tipping or turning the cycle.

What's amazing to me WRT the O'Neill Cylinders is how modest those
forces are, despite the billion-ton mass of the habitat. But I guess
that's because the rate of turn is so modest (~1 deg/day at our distance
from the sun).

Regards,

Mike Combs

From: Steve Long [mailto:longsteven@...]

> For a pair of Level-3 Oneill habitats, that's going to be
> a lot of torque ...

Well, O'Neill compared the tower under compression to a radio mast here
on Earth, and said the cable under tension would only have the diameter
of a teacup.

Regards,

Mike Combs

"Combs, Mike" wrote:
>
> Well, O'Neill compared the tower under compression to a radio mast here
> on Earth, and said the cable under tension would only have the diameter
> of a teacup.
>

If you align the pair one above the other with respect to the orbital plane
and spun them appropriately, you'd only need a cable at the ends that wants
to precess apart, and some kind of a rigid brace at the ends that want to
precess together. No twisting at all. But you'd have to work to keep the
whole coupled system from rotating around its solar-pointing axis. If the
two habitats drifted far from their above-and-below orientation, you'd get
a twisting precession vector that would require bracing in three dimensions
to counteract.

I kept envisioning habitats that were side-by-side instead of
above-and-below relative to their orbital plane, which would make it much
harder to deal with the precession. Took me a while, but I finally figured
it out!

Steve

Yeah, I'm pretty sure above and below the orbital plane was O'Neill's
assumption.

Regards,

Mike Combs

I've read here, from time to time, some concern about the cylinder's
wanting to tumble. But I assume that it wouldn't happen instantly.

OK, let's put it this way: I live in one of these Islands. I am
subscribed, through my cell phone, to a service that will let me know
the moment anything happens to the compression tower or the tension
cable. I have the phone out, as I intend to make a call (order
pizza), and am looking at the screen, about to dial, when all of a
sudden THIS shows up on the screen:

BOTH THE COMPRESSION TOWER AND THE TENSION CABLE HAVE BEEN COMPLETELY
SEVERED

Is my most natural reaction (assuming that I know all about this sort
of thing, which the real me does not) "Oh my God! We're all gonna be
dead in less than a minute!!" or "Gee, I hope bureaucratic red tape
nonsense doesn't keep them from fixing that in a week or so. Things
could get messy." Maybe something in between?

Xenophile (who will get to Mikki D's later, really he will)

From: Xenophile [mailto:xenophile2002@...]

> Is my most natural reaction (assuming that I know all
> about this sort of thing, which the real me does not)
> "Oh my God! We're all gonna be dead in less than a minute!!"
> or "Gee, I hope bureaucratic red tape nonsense doesn't keep
> them from fixing that in a week or so. Things could get messy."
> Maybe something in between?

Much closer to the latter than the former, I would think. Considering
that the pair is only turning at ~ 1 deg per day, I don't think the
cylinders would start tilting toward each other any faster than that.

Regards,

Mike Combs

On Fri, 04 Feb 2005 13:45:26 -0500, Steve Long wrote:
>
> OK, I think I understand now.
>
> It is not gyroscopic inertia that is canceled in a coupled,
> counter-rotating system, but gyroscopic precession.

Yup.

> Gyroscopes, when an
> attempt is made to rotate them in a direction 90 degrees to their plane of
> rotation, react by precessing their axis in the third direction, 90 degrees
> to both their plane of spin and the force being applied to their axes. The
> direction of this precession depends on the direction of rotation of the
> gyroscope.

Yup.

> For example, attempting to rotate the axis of a
> clockwise-spinning gryoscope to the right may cause the end you're pushing
> on to dip; rotating a counter-clockwise-spinning gyroscope in the same
> direction will cause the same end to rise.
>
> A coupled system of counter-rotating gyroscopes cancels this precession
> *for the system as a whole* -- but each individual gyroscope is still
> exerting its precessional force on the point at which it is attached to the
> coupling mechanism.

Yes, so the motorbike example will only work if you lock up the
steering mechanism.

> For a pair of Level-3 Oneill habitats, that's going to be a lot of torque ...

Not really. The precession of the whole habitat-pair is typically very
slow, mostly to track the sun, so you're only talking a turn per year.

Incidentally, the technique for pointing the habitats is really cute-
just varying the distance between the two habitats at one end and the
spin speed of one of the habitats and you can point wherever you want.

> Steve

--
-Ian Woollard

Professor Frink: "We studied traffic patterns and found that drivers
move the fastest
through yellow lights. So now, we just have red and yellow lights."

On Fri, 04 Feb 2005 20:30:48 -0000, Xenophile wrote:
> I've read here, from time to time, some concern about the cylinder's
> wanting to tumble. But I assume that it wouldn't happen instantly.

No, it just gets worse and worse over time, until it eventually ends
up spinning around the center. I think you would want to get rid of
the wobble as early as possible with pumped water or counterweights.

> BOTH THE COMPRESSION TOWER AND THE TENSION CABLE HAVE BEEN
> COMPLETELY SEVERED

Although the two are stabilised to a small degree by their connection
at the axis, it is still very important that the habitats be
individually stable and not wobble much. That linkage is only a teeny
tiny linkage to help point the habitats at the sun, it is *not* to
stop wobble in either habitat. If your habitat wobbles enough that
snapping the linkage causes a problem- you need to evacuate!

In any case, pretty much, if either habitat was seriously unbalanced,
stability of the joint system is not guaranteed at all, even with
unrealistically strong linkages.

Nevertherless loss of the linkage probably has no short term
consequences (say, over the course of a few days).

> Xenophile (who will get to Mikki D's later, really he will)

--
-Ian Woollard

Professor Frink: "We studied traffic patterns and found that drivers
move the fastest
through yellow lights. So now, we just have red and yellow lights."