O'Neill habitat spin axis

I'm designing an O'Neill habitat for use in a game scenario, where I'm
trying to keep the science as plausible as possible. It's going to be
the classic "spinning tin can" design, surrounded by an array of solar
panels, mirrors and radiators (and directly in orbit around a star,
rather than a planet, though I could change that if there was reason
to do so). My question is about the spin axis.

I'd ideally like to point it at the sun. Then the nearside cap could
be coated in solar cells while the rest of the surface is painted
black to help radiate heat.

But in that case, it seems that a quarter orbit later it'll be side on
to the sun, another quarter orbit the opposite end will point at the
sun etc, since conservation of angular momentum will tend to keep the
spin axis pointing in the same direction relative to the rest of the
universe, not relative to the sun.

Is there any way to change that (I mean, reasonable ways, i.e. without
expending propellant or using huge gyroscopes etc)? For example,
Earth's axis precesses every 26,000 years IIRC; how does that square
with conservation of angular momentum? Is there a way a habitat's axis
could be made to "precess" through a full circle every year?

Or if not, is the best solution then to orient it vertically, let most
of the surface be mirror colored, and paint the end caps black?

(Another question: Has anyone investigated the question of what the
rate of leakage of volatiles from a habitat would be, and what would
be the limiting factors on how low you could make it?)

Thanks,

--
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How close is it going to be to the star? What is the orbital period?

If you make the habitat long enough, it'll just naturally want to keep
itself oriented with the long axis straight through the center of mass
of the primary. I believe NASA uses a long boom and counterweight to
keep satellites oriented towards the Earth.

That will still give you some interesting effects do to gyroscopic
inertia. That can be handled by hooking two counter-rotating together.
In fact, that has been proposed for O'Neill colonies. The pictures I
saw had the colonies set side-by-side, but it might be possible to put
one behind the other. You would have problems with the front colony's
mirrors shading the rear colony, though.

If simply making a really long colony won't do the job, using the same
long boom and weight trick that NASA uses ought to work. Or, instead of
a weight (or factory), you could put a colony pair on the other end of
the long boom.

If you do this, the boom will be under tension, and there will be some
tidal force at each end. It won't be much, but it might be worth
mentioning.

If I was designing an O'Neill colony for my WIP, I would do a google
search on "L5 society". I'm sure they have a web site, and I'm equally
sure that they have done lots of design work on various colonies. Also,
you might want to google on "O'Neill Colony".


Ray Drouillard



"Russell Wallace" wrote in message
...
I'm designing an O'Neill habitat for use in a game scenario, where I'm
trying to keep the science as plausible as possible. It's going to be
the classic "spinning tin can" design, surrounded by an array of solar
panels, mirrors and radiators (and directly in orbit around a star,
rather than a planet, though I could change that if there was reason
to do so). My question is about the spin axis.

I'd ideally like to point it at the sun. Then the nearside cap could
be coated in solar cells while the rest of the surface is painted
black to help radiate heat.

But in that case, it seems that a quarter orbit later it'll be side on
to the sun, another quarter orbit the opposite end will point at the
sun etc, since conservation of angular momentum will tend to keep the
spin axis pointing in the same direction relative to the rest of the
universe, not relative to the sun.

Is there any way to change that (I mean, reasonable ways, i.e. without
expending propellant or using huge gyroscopes etc)? For example,
Earth's axis precesses every 26,000 years IIRC; how does that square
with conservation of angular momentum? Is there a way a habitat's axis
could be made to "precess" through a full circle every year?

Or if not, is the best solution then to orient it vertically, let most
of the surface be mirror colored, and paint the end caps black?

(Another question: Has anyone investigated the question of what the
rate of leakage of volatiles from a habitat would be, and what would
be the limiting factors on how low you could make it?)

Thanks,

--
"Sore wa himitsu desu."
To reply by email, remove
the small snack from address.

On Mon, 23 Aug 2004 14:56:37 GMT
(Russell Wallace) wrote:

Or if not, is the best solution then to orient it vertically, let most
of the surface be mirror colored, and paint the end caps black?

How about including two free flying mirrors, one just beyond each end of the habitat. The mirrors use light pressure to orient themselves and control their position relative to the main habitat.

The light reflected into (through windows near tha axis or rotation) the cylinder provides natural sunlight. Some of it is intercepted by photovolatic cells.

As the relative position of the star changes the mirrors change geometry (and to some extent position) to continue redirecting sunlight into the habitat.
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Web Hosting | Internet Services

On Mon, 23 Aug 2004 14:56:37 +0000, Russell Wallace wrote:

Is there any way to change that (I mean, reasonable ways, i.e. without
expending propellant or using huge gyroscopes etc)? For example, Earth's
axis precesses every 26,000 years IIRC; how does that square with
conservation of angular momentum? Is there a way a habitat's axis could be
made to "precess" through a full circle every year?

Don't know if this would work, but...

Big pipes on the exterior containing fluid with one-way valves every so
often.

Sunlight on the sunward-facing side heats up the fluid, which expands the
only way it can, through the valves, providing net force in other
direction, habitat rotates exposing new fluid, fluid now in shade cools,
sucks fluid from other side.

Net result, sunlight into rotation.

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= Mitchell --- viruses, so please don't add me to your address book.
================================================== =====================

Russell Wallace wrote:

I'd ideally like to point it at the sun... a quarter orbit later it'll be
side on to the sun... etc

Yep. That's why it's called *conservation* of angular momentum
grin. The Earth precesses because it has a torque on it, a
gravitational torque from the Sun & Moon (these dominate) and other
planets (OK, it takes two things: the Earth has to have a "handle" of
some sort as well, such as the equatorial bulge due to it's rotation).
To get a faster precession, reduce the angular momentum or increase
the torques. But to push it to a year... well, I'd have to do crunch
the numbers. I'm certain you couldn't do it gravitationally, but you
might be able to do it via light pressure or mass exchange
(import/export) from the habitat.
By far the simpiler way it to use two counter-rotating habitats.
Net angular momentum is therefore zero (or as low as you like it, by
making one habitat very slightly smaller/lighter than the other),
allowing it to be much easier to turn. Think the flywheel solution,
but with half the habitat as the flywheel (& people living in it).

Or if not, is the best solution then to orient it vertically, let most
of the surface be mirror colored, and paint the end caps black?

Are you trying to collect usable power, or handle the thermal
control issue?

(Another question: Has anyone investigated the question of what the
rate of leakage of volatiles from a habitat would be, and what would
be the limiting factors on how low you could make it?)

Good question. I don't know. Leakage rates are going to be large in
terms of mass, but small in terms of fraction of volatiles on-board,
and if you had the transport cabability to set up such a system,
importing the metric tons of gases needed shouldn't be too much of an
issue.

--
Brian Davis

(Russell Wallace) wrote in message ...
I'm designing an O'Neill habitat for use in a game scenario,
where I'm trying to keep the science as plausible as possible.

Instead of saying "to keep the science as plausible as possible", it
might be better to say "to keep the science as low tech as possible
according to the scientific information that are released to the
public today".

It's going to be the classic "spinning tin can" design,
surrounded by an array of solar panels, mirrors and radiators

Well... I think that it would be much better and easier to use an
independent powerplant instead of solar panels and artificial
lightings instead of mirrors. But that's okay on using solar panels
and mirrors if you want it low tech.

It should be noted that there're some structural weakness in the
originial O'Neill designs, the movie "G-Saviour" feature some
refinements on the designs.

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

(and directly in orbit around a star, rather than a planet,
though I could change that if there was reason to do so).

Well... Putting it in orbit around a planet or near a planet might
make it easy for the people in the space colony to shuttle back and
forth from and to the planet.

Anyway. Since you're want to keep things as low tech as possible, it
might be better if these space colonies are built at the exact site
where the construction materials were gathered. Kinda like making a
brick house with the bricks were made from the soil next to the
construction site, or even constructing a house from a hilly land.
This method is easier rather than moving asteroids into prefered orbit
and then work on them, or even shuttling the materials from somekind
moon or asteroid.

For example, in Sun-Earth's L4 point and Sun-Earth's L5 point there
are plenty of asteroids trapped there, and these asteroids pretty much
stay there. These points with these asteroids should be good
construction sites, plus since that they are on the same orbit around
the star as the planet is, it would made shuttle flights between these
Lagrange points and the planet quite easy.

http://www.astro.uwo.ca/~wiegert/etrojans/etrojans.html

But then again, it might be scary if the Trojan asteroids are actually
more like the Trojan horse.

My question is about the spin axis.

I'd ideally like to point it at the sun. Then the nearside cap could
be coated in solar cells while the rest of the surface is painted
black to help radiate heat.

But in that case, it seems that a quarter orbit later it'll be side on
to the sun, another quarter orbit the opposite end will point at the
sun etc, since conservation of angular momentum will tend to keep the
spin axis pointing in the same direction relative to the rest of the
universe, not relative to the sun.

The solution is easy to make it not pointing in the same direction all
the time, put another cylinder that rotate in the opposite direction,
that way the rotation of the two cylinders will cancel each other out.

http://lifesci3.arc.nasa.gov/SpaceSe...AC75-1085.jpeg

Though there's still the matter of it keeping it aiming at the primary
star / sun, it will be a quite a hard thing to do.

For example, the Earth's Moon for example has the uncanny ability to
roughly keep the same face pointed at Earth, this and some other
things (like the moon distance was quite right enough to cause a solar
eclipse that almost totally covered the sun) lead some people to
believe that it might be a ancient space colony.

Is there any way to change that (I mean, reasonable ways, i.e.
without expending propellant or using huge gyroscopes etc)?

If you're talking about low tech ways, you will have more luck on a
spherical space colony than a cylindrical one.

Anyway. The current I.S.S. maintain its attitudes with gyroscopes, so
I guess that maybe putting huge gyroscopes should at least will fit in
nicely with the level of technology you're talking about. The whole
thing after all is already a huge gyroscope.

Or if not, is the best solution then to orient it vertically, let most
of the surface be mirror colored, and paint the end caps black?

Maybe the type of space colony below is much better suited:

http://spaceinfo.jaxa.jp/note/kouso/..._colony_e.html

Thanks,

Brian Davis wrote:

Yep. That's why it's called *conservation* of angular momentum
grin. The Earth precesses because it has a torque on it, a
gravitational torque from the Sun & Moon (these dominate) and other
planets (OK, it takes two things: the Earth has to have a "handle" of
some sort as well, such as the equatorial bulge due to it's rotation).
To get a faster precession, reduce the angular momentum or increase
the torques. But to push it to a year... well, I'd have to do crunch
the numbers. I'm certain you couldn't do it gravitationally, but you
might be able to do it via light pressure or mass exchange
(import/export) from the habitat.

The precession angular velocity is equal to the ratio of the torque to
the angular momentum. To increase the precession rate from 2 pi/(26 000
yr) to 2 pi/(1 yr) is of course an increase in the angular velocity of
26 000, so you'd have to increase the torque/angular momentum ratio by
26 000 to get the precession rate up that fast.

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\__/ War is a continuation of policy by other means.
-- Karl von Clausewitz

Russell Wallace wrote:
I'm designing an O'Neill habitat for use in a game scenario, where I'm
trying to keep the science as plausible as possible. It's going to be
the classic "spinning tin can" design, surrounded by an array of solar
panels, mirrors and radiators (and directly in orbit around a star,
rather than a planet, though I could change that if there was reason
to do so). My question is about the spin axis.

I'd ideally like to point it at the sun. Then the nearside cap could
be coated in solar cells while the rest of the surface is painted
black to help radiate heat.

O'Neil's Island Three design, as described in High Frontier, has this basic
orientation and configuration. He uses a solar boiler rather than
photovoltaic cells for power; radiant cooling is I think rather neglected in
the short version in the book.

As several people noted, the trick to keeping the station oriented sunward
is to couple a pair a pair of cylinders together and counter-rotate them.
That solves your conservation of momentum issues. Precision pointing can be
done by adjusting the relative rotation rate slightly.

--
Tom Schoene Replace "invalid" with "net" to e-mail
"Our country, right or wrong. When right, to be kept right, when
wrong to be put right." - Senator Carl Schurz, 1872

Brian Davis wrote:

Russell Wallace wrote:

I'd ideally like to point it at the sun... a quarter orbit later it'll be
side on to the sun... etc

Yep. That's why it's called *conservation* of angular momentum
grin. The Earth precesses because it has a torque on it, a
gravitational torque from the Sun & Moon (these dominate) and other
planets (OK, it takes two things: the Earth has to have a "handle" of
some sort as well, such as the equatorial bulge due to it's rotation).

[snip]

By far the simpiler way it to use two counter-rotating habitats.
Net angular momentum is therefore zero (or as low as you like it, by
making one habitat very slightly smaller/lighter than the other),
allowing it to be much easier to turn. Think the flywheel solution,
but with half the habitat as the flywheel (& people living in it).

The usual design has the the two habitats slightly non-parallel,
so there is a net angular momentum. The connection between them
at one end is under tension and at the other under compression.

--
Bill Woods

"If you examine my 16-year record in the Senate, you'll see
that I'm just as effective when I'm not there as I was when
I was there," said Mr. Kerry. "... I think it's disingenuous
for Gov. Romney to suggest that my absence from the Senate
harms America in any way."
http://www.scrappleface.com/MT/archi...37.html#001737

Bill Woods wrote in message ...
Brian Davis wrote:

Russell Wallace wrote:

I'd ideally like to point it at the sun... a quarter orbit later it'll be
side on to the sun... etc

Yep. That's why it's called *conservation* of angular momentum
grin. The Earth precesses because it has a torque on it, a
gravitational torque from the Sun & Moon (these dominate) and other
planets (OK, it takes two things: the Earth has to have a "handle" of
some sort as well, such as the equatorial bulge due to it's rotation).

[snip]

By far the simpiler way it to use two counter-rotating habitats.
Net angular momentum is therefore zero (or as low as you like it, by
making one habitat very slightly smaller/lighter than the other),
allowing it to be much easier to turn. Think the flywheel solution,
but with half the habitat as the flywheel (& people living in it).

The usual design has the the two habitats slightly non-parallel,
so there is a net angular momentum. The connection between them
at one end is under tension and at the other under compression.

I thought that the two cylinders were parallel, there being no need
for a net angular momentum. I belive your last statement is correct,
though, since such a force arrangement would cause both cylinders to
precess so as to keep one end pointed at the Sun.