The original "Mobile Suit Gundam" anime (1979-80, 42 half-hour
episodes) featured O'Neill "Island 3"-type space colonies, plus a
variety of space warships. On several occasions, the warships would
enter through an axial airlock, then fly around the interior.

<http://en.wikipedia.org/wiki/Mobile_Suit_Gundam>
<http://en.wikipedia.org/wiki/Island_3>

This is clearly a more extreme situation than residents cavorting
around the axial region with human-powered wings. Or is it?

A rotating cylinder, and anything in direct contact with the cylinder,
will experience centrifugal force. Obviously air must also couple to
the floor of a colony, otherwise residents would endure an unending
windblast.

But what if you're in the air *between* the axis and floor? What, if
anything, causes you to "fall" to the floor? If you had an initial
downward (outward) radial velocity, you'd soon pick up a tangential
velocity from the air; but do you accelerate radially?

My Visualization of the Rotating All :) is obviously incomplete.

***

These dynamics also apply to the climactic scene at the end of season
2 of "Babylon 5," when Captain Sheridan leaps from the axial tram just
before it explodes, and leisurely floats toward the floor (where he'll
have a sudden and very unwelcome finish), long enough for a dramatic
rescue by an un-canned Kosh.

They apply, in a more complicated fashion, to the rotating "towns" of
Karl Schroeder's "Virga" series (to date, _Sun of Suns_ and _Queen of
Candesce_), which are open-ended cans rotating within an oxygen
atmosphere contained within a planet-sized balloon. And in particular
to anybody who comes drifting in from the end at an angle that will
intersect the floor.

***

Note that "Gundam" does not have conventional SF-style antigravity. It
does have "Minovski particles," which explain everything else: they
block radio and radar (hence visual-range space combat), they
constitute lightweight radiation shielding (hence nuclear-powered
mecha and beam weapons), and they're funneled into lightsaber-like
weapons.

They might also serve as a "repulsorlift"-like cushion, because during
the series, the heroic _White Base_ travels around Earth's surface. I
think a "hypermagnet" was mentioned once or twice in dialogue, so
maybe it repels against Earth's geomagnetic field -- rather like the
flying carpet in the _Hyperion_ novels.

Nice trick, though. Earth's field is a puny 0.3-0.6 gauss, compared
to the 100 gauss of a toy iron magnet, 2,000 of a rare-earth magnet,
and 10,000-30,000 of an MRI chamber or maglev train.

You might like reading _Lifeline_ by Kevin J. Anderson and Doug
Beeson, 1990 Spectra. It's a hard SF look at the life and times of
three orbital colonies, including one small Island 3 type, including
descriptions of flying around in the center of the cylinder. I think
it's a good read personally.

On 2008-02-09, Phillip Thorne <pethorne@comcast.net> wrote:
> But what if you're in the air *between* the axis and floor? What,
> if anything, causes you to "fall" to the floor?

Initially nothing.


> If you had an initial downward (outward) radial velocity, you'd
> soon pick up a tangential velocity from the air; but do you
> accelerate radially?

There are two main ways to look at this: from the inertial reference
frame or the co-rotating one. Both give the same answer.

In the inertial frame, you start at rest. The air rushing past you
imparts some velocity to you, tangential to the colony's rotation
axis. If no subsequent force acted upon you, you would drift along
that line until you met the (rotating) outer shell. The picture is
actually more complicated because other forces will act upon you: the
air at each point along your path has a different velocity.

In the co-rotating frame, you start with a tangential velocity. As it
happens, the centrifugal force at your position is negated by the
coriolis force due to your velocity. However, as your speed is
reduced by drag the centrifugal force begins to dominate, so you
accelerate toward the stationary outer shell. The situation is
complicated by the changing direction and magnitude of drag as your
velocity changes, but you will end up hitting the "ground".


- Tim

:: If you had an initial downward (outward) radial velocity, you'd soon
:: pick up a tangential velocity from the air; but do you accelerate
:: radially?

If you aquire a velocity tangent to a circle, you will very soon have
a radial velocity wrt that circle. Compared to the spot on the circle
where your straight-line trajectory originated, you will be accelerating
radially, and decelerating tangentially, as,over time, your trajectory
becomes more and more radial and less andless tangential, compared
to the circle through your current position.

Note also that, as you get further from the center, the air will be
blowing past you with higher and higher velocities, so you'll continue
to accelerate tangentially, and that will be turned into radial velocity
in the above manner.

You can also look at it in rotating coordinates, but it's more complicated
and you'll get the same answers. As you accelerate tangentially, your
centrifugal "force" increases beyond the coriolis force that held you
steady, and you'll start heading for the wall.

Or look at it this way. If you are pushed tangentially, you won't go in
a circular path and stay the same distance from the center. Objects tend
to go in straight lines, and any straight line will intersect the wall.


Wayne Throop throopw@sheol.org http://sheol.org/throopw

I had to deal with the kind of physics we're discussing in a short story I
recently wrote:

Improbable Events
http://writings.mike-combs.com/improbable_events.htm

--


Regards,
Mike Combs
----------------------------------------------------------------------
By all that you hold dear on this good Earth
I bid you stand, Men of the West!
Aragorn