Al Globus et al. pointed out in their Kalpana One papers that a space habitat should be passively stable, i.e., its moment of inertia around the desired axis of rotation should be bigger (by a factor of 1.2, they say) than the moment of inertia around any other axis. Otherwise, the habitat will have a tendency to tumble.
This leads me to wonder: how important is this passive-stability constraint, anyway? Obviously it would be a Bad Thing if your habitat started to tumble, but perhaps not disasterous -- it would continue to spin around the desired axis, and just gain an additional spin around some other axis, which might tend to make the ground feel tilted (and things slightly heavier) until you got it under control. So, is there some way to quantify how likely this is, and how much thrust would be required to counteract it, for given moments of inertia?
This seems like a very important issue to settle, since if it really is a hard constraint, then most of our classic designs are useless and we need to explore our newly constrained design space more thoroughly. Conversely, if it's not a major issue, then we have a lot more freedom to optimize for other things. So I'd be surprised if this isn't something SSI has already studied.
Any pointers, however small, will be greatly appreciated.
Thanks,
- Joe
Joe Strout -- joe@...
Don't know about "tumbling". Do know about
"precession" and that comes about when a rotating
object is subject to a force at right angles to its
plane of rotation, ie, gravity. In free fall there is
no such force.
At any rate, there are several ways to ensure
stability in free fall of a rotating cylinder:
axis. Precessional forces should than counter balance.
2) Place your rotating cylinders inside a massive
shell, connect them to the shell via magnetic bearings
so that the external, non rotating shield can take up
any precessional load w/o itself being subject to
rotation.
I'm sure there are even better ideas out there, but
this is top of the head stuff,,,
GAry 7
> Don't know about "tumbling". Do know about
> "precession" and that comes about when a rotating
> object is subject to a force at right angles to its
> plane of rotation, ie, gravity.
It's possible that linking two cylinders together with sufficiently strong tension/compression structurs would solve this, but my grasp of dynamics isn't deep enough to be confident either way.
Best,
- Joe
Joe Strout -- joe@...
Ah, yes, you speak of the shifting of weights inside
the cylinder. That's a completely different horse.
Dynamic rebalancing has been considered using H2O
pumped around, perhaps using electrophoresis pumps to
quickly adjust the balance of this large body.
However, I expect that significantly altering the
balance of such a massive rotating object would be
almost as hard as affecting the earths wobble by
having all the humans on earth jump up and down at
once.
could the actions of a flea walking around inside it
cause it to wobble? Of course, moving large masses
around might require some planning, but isn't that
what we're good at?
Gary 7
> Ah, yes, you speak of the shifting of weights inside
> the cylinder. That's a completely different horse.
> We are discussing a multi mega ton gyroscope, eh? How
> could the actions of a flea walking around inside it
> cause it to wobble?
It would not. But if the multi mega ton gyroscope has I_x or I_y > I_z, then it's going to end up rotating around axis x or y rather than (the desired axis) z, absent some active attitude control. Exactly how big a problem this is, and how much attitude control will be required, is the question I'm hoping to address.
Best,
- Joe
Joe Strout -- joe@...
Wouldn't the stability "problem" drive the habitat configurations to
favor the toroidal habitats -- or very short cylinders?
Shep
> > Ah, yes, you speak of the shifting of weights inside
> > the cylinder. That's a completely different horse.
>
> Alas, no, that's not the issue I refer to either. This tendency
of rotating objects to rotate around the axis with the largest
moment of inertia applies perfectly well to rigid bodies. (Like the
rattleback, for example, though the perturbations there come from
contact with the surface it's resting on, so obviously the situation
in space is slightly different.)
>
> > We are discussing a multi mega ton gyroscope, eh? How
> > could the actions of a flea walking around inside it
> > cause it to wobble?
>
> It would not. But if the multi mega ton gyroscope has I_x or I_y
> I_z, then it's going to end up rotating around axis x or y rather
than (the desired axis) z, absent some active attitude control.
Exactly how big a problem this is, and how much attitude control
will be required, is the question I'm hoping to address.
> Wouldn't the stability "problem" drive the habitat configurations to
> favor the toroidal habitats -- or very short cylinders?
Best,
- Joe
Joe Strout -- joe@...
This issue has come up before, on our sister group.
It seems to me that as long as repairs are simple, cheap, and can be
done quickly, then it isn't that big a deal. Of course, it may not be
simple, cheap, and quick in the first few habitats. Thus the short
cylinders and tori would seem best for the early stages.
Then again, we might just toss out the whole rotational psuedogravity
thing, stuff the habitat full of utility fog, and tell it to hold us
"down."
> This issue has come up before, on our sister group.
>
> message in the thread on)
> It seems to me that as long as repairs are simple, cheap, and can be
> done quickly, then it isn't that big a deal. Of course, it may not be
> simple, cheap, and quick in the first few habitats. Thus the short
> cylinders and tori would seem best for the early stages.
Probably so. Still, I'm surprised noone has studied this in detail, for space habitats in particular. For example, if you have an Island One configuration, exactly how much compression and tension are the connecting members subjected to? When one breaks (or is sabotaged), what happens, and how quickly? If you wanted to try a single cylinder, how much would active stabilization cost you, and if your stabilizers failed, what exactly would happen?
These are questions that could be answered through simulation, I should think, and verified by comparison to real spacecraft. In some ways, the simulation would be much easier than a typical rigid-body mechanics problem, because there are no collisions involved. But I suppose the tricky part is calculating all the ways the system can dissipate kinetic energy -- it's this dissipation that causes a spinning object to flip over and spin about its largest moment of inertia. Energy can be dispersed in lots of subtle ways that are probably hard to quantify.
Off the top of my head, from what I've been reading, this tendency to a shift the axis of rotation could be quite severe. It'd also be rather disruptive to the colony, with "down" quickly becoming the direction that was formerly sideways. But I'd love to see a realistic simulation of this process.
I might be able to produce such a simulation, with a bit of help. Is there anyone here with more experience or training in mechanics that would like to collaborate on such a project?
Best,
- Joe
Joe Strout -- joe@...
Fog??? It's not a question of being " held down." That
does nothing for our muscle/bone health. If that was
the only problem, velcro treaded boots and walking
surfaces would be sufficient. We need the rotational
acceleration to provide enough stress to maintain our
1-G muscle/skeletal structures until we can maintain
that health with drugs, genetic manipulation, etc. I
expect we will wish to maintain our options to
return/visit high-G environments like earth/mars, etc,
for the foreseeable future and the ONLY way we have of
doing that at present is with the rotating structure.
Perhaps our future will provide artificial gravity,
but I expect we will have been building space colonies
a long time before that becomes technically feasible.
Then the obvious solution is to ensure that the
rotating body has length = to or < than the width. The
rotating cylinders can be stacked or strung along a
common non-rotational axis, but unconnected from each
other, allowing counter rotating cylinders of, say, 5
km diameter X 5 km length and strung along a 500 meter
diameter non-rotating tube. The structure can be made
quite long, perhaps as much as 100 km, though the
admission of light might become problematical, it
should be stable as no single rotating cylinder would
exceed the desired proportions.
Well, I have my own doubts that UFog could substitute for gravity over
the long term. But it is different than slapping Velcro on your
shoes. If I walk around my zero-G (yes, I know: micro-G) station in
my best Velcro hiking boots, then I'm stuck to the floor just fine.
But there is no force *pressing* me down. No stresses on the old
bones, only as much muscle as it takes to unstick the shoes with each
step, and to deal with inertia.
floor, *pushing* against my shoulders, *pushing* against me head,
*pushing* on my pot belly (I've started doing sit-ups, but it's still
there for now), and so on, then my muscles and bones get something
very much like they would get on Earth. I'm not even tall, like I
would normally be in freefall.
However, are these itti-bitti robots doing anything about my blood?
If weightlessness has anything to do with observed changes to the
immune system, then I cave to wonder exactly what UFog could do about
that. My guess is "not much." Still, it's something to think about.
Imagine the shapes and forms habitats could assume in such a case.
> Then the obvious solution is to ensure that the
> rotating body has length = to or < than the width.
But this is a severe design constraint, so before applying such a solution, we should make sure we really understand how much of a problem it is.
> The rotating cylinders can be stacked or strung along a
> common non-rotational axis, but unconnected from each
> other, allowing counter rotating cylinders of, say, 5
> km diameter X 5 km length and strung along a 500 meter
> diameter non-rotating tube.
How are they both strung along this tube, and also unconnected from each other? If they're all connected to the tube, then I'd say they're all connected. However, that's OK since in this case each individual habitat is stable (well, not if they're 5 km long, but perhaps if 2.5 km long for a 5 km radius).
I do think connecting multiple habitats together, like cities combining to form a state, is a good idea. I also think that by the time we're talking about habitats with linear dimensions in the kilometers, that the stability constraint isn't that big a deal -- living in a valley 2.5 km wide is no big hardship.
But I'm more concerned about early habitats, with a radius of 250 m (about the smallest you can make it for a comfortable 1G), and a length on the order of 250 or 300 m. Now you're living in a much narrower valley. If you launch Google Earth and turn on the scale bar, and then look at small towns, to see what you can fit into a 250 m strip, it's not very much.
In fact, there's perhaps an interesting challenge for the group: can you find any town or village of at least 5000 people, which is constrained by geography into a narrow strip on the order of 250 m wide?
Best,
- Joe
Joe Strout -- joe@...
Frankly, this is the first I've heard of U Fog. How do
the micro bots exert force in free fall? Are they
interconnected with electric/magnetic field effects?
Or what?
> Frankly, this is the first I've heard of U Fog. How do the micro
> bots exert force in free fall? Are they interconnected with
> electric/magnetic field effects?
> Or what?
>
> GAry 7
hands. They would exert force in free fall by pushing against me in
one direction and walls/roof in the other.
http://en.wikipedia.org/wiki/Utility_fog
http://www.kurzweilai.net/articles/art0220.html?printable=1
> In fact, there's perhaps an interesting challenge for the group: can
> you find any town or village of at least 5000 people, which is
> constrained by geography into a narrow strip on the order of 250 m
> wide?
criterion. Paro has a population of 4500, but I can't tell how wide it
is, because Google Earth doesn't have good resolution in Bhutan, and I
can't find any online street maps.
Also, last night I flipped through a book that reminded me of the cliff
dwellings at Mesa Verde, Colorado. It seems that the largest of those
(Cliff Palace or Long House) are smaller in area than you want, but
they have high aspect ratios and had peak populations near 1000:
http://www.nps.gov/meve/cliff_dwellings/cliff_dwellings_home.htm
-Phil
>
>> In fact, there's perhaps an interesting challenge for the group: can
>> you find any town or village of at least 5000 people, which is
>> constrained by geography into a narrow strip on the order of 250 m
>> wide?
>
> There are probably some towns in valleys in Bhutan that fit that
> criterion. Paro has a population of 4500, but I can't tell how wide it
> is, because Google Earth doesn't have good resolution in Bhutan, and I
> can't find any online street maps.
>
> Also, last night I flipped through a book that reminded me of the cliff
> dwellings at Mesa Verde, Colorado. It seems that the largest of those
> (Cliff Palace or Long House) are smaller in area than you want, but
> they have high aspect ratios and had peak populations near 1000:
> http://www.nps.gov/meve/cliff_dwellings/cliff_dwellings_home.htm
> http://www.nps.gov/meve/cliff_dwellings/cliff_dwellings_home.htm>
>
> -Phil
>
seemed like they were cramed onto a narrow strip of land. It ran
Back in the day when all water was drawn from the river or a well people
simply did not build any higher than they had too - too expensive to
haul the water to where it had to be.
Brian Dunbar
System Administrator
Liftport - The Space Elevator Company
brian.dunbar@...
aim: bdunbar1967
this email is: [ ] bloggable [x] ask first [ ] private
Remember.
But move forward, too. Light a candle, yes. But also drive a rivet.
~Lileks
The town of Blairmore, Alberta, Canada has a
population of about 6500, and is about 500 meters wide
by 3 kilometers long. It is nestled in the Crowsnest
Pass in the Rocky Mountains. I'd say that is on the
order of 250 meters wide (ie within one order of
magnitude). If there were tall buildings in that town
the population could be higher or more densely packed,
but from what I saw nothing is over two stories.
> >
> > On Jul 14, 2006, at 2:37 PM, joe@...
> >
> >> In fact, there's perhaps an interesting challenge
> for the group: can
> >> you find any town or village of at least 5000
> people, which is
> >> constrained by geography into a narrow strip on
> the order of 250 m
> >> wide?
> >
> > There are probably some towns in valleys in Bhutan
> that fit that
> > criterion. Paro has a population of 4500, but I
> can't tell how wide it
> > is, because Google Earth doesn't have good
> resolution in Bhutan, and I
> > can't find any online street maps.
> >
> > Also, last night I flipped through a book that
> reminded me of the cliff
> > dwellings at Mesa Verde, Colorado. It seems that
> the largest of those
> > (Cliff Palace or Long House) are smaller in area
> than you want, but
> > they have high aspect ratios and had peak
> populations near 1000:
> >
http://www.nps.gov/meve/cliff_dwellings/cliff_dwellings_home.htm
> >
http://www.nps.gov/meve/cliff_dwellings/cliff_dwellings_home.htm>
The question of "endcaps" assumes the living cylinder
is a complete, closed system. I have proposed encasing
the rotating portion within a passive, non rotating,
outer slag cylinder, that could itself be capped and
act as a pressurized enclosure for the rotating "life
enclosure". The end caps could then be an inverted
geodesic structure of Borosylicate glass, very tough
and radiation resistant triangular "plugs" of glass,
3m wide by 3m long by three meters thick, inserted
into a geodesic framework made of aluminum.
Penetration of any single glass plug would result in
an insignificant air loss until it could be replaced.
Larger penetrants, those pieces of space debris large
enough to cause damage to sections of the geodesic
larger than 1 plug, could be detectable by radar and
vaporized or at least broken up with small lasers.
Ablation of the outer layers of the rocks should be
sufficient to deflect the debris and avoid impact with
our glass endcaps.
bearings within the passive shell, transferring load
to that shell and reducing the inner cylinder load
considerably. The passive shell then provides
radiation protection, acts as an air containment
vessel and load containment structure, plus any micro
g environmental requirements could be constrained
within that shell, even to the growth of air recycling
plants, which as far as I know can be grown in the
absence of gravity. Additional plant/animal life could
be grown within the rotating cylinder as required. As
long as the cylinders come as a counter rotating pair,
there would be no energy transfer to the shell and the
life cylinders would be rotationally stable at
Length =< 2Radii.
Gary 7
I think you'll find that with the below design, air drag will create a significant problem. The peer-reviewed studies sometimes looked at separate, stationary slag shields, and I've elsewhere seen it proposed that a stationary outer structure take up some portion of the load bearing, but the assumption is always that there's a vacuum between the rotating inner structure and the stationary outer structure.
is a complete, closed system. I have proposed encasing
the rotating portion within a passive, non rotating,
outer slag cylinder, that could itself be capped and
act as a pressurized enclosure for the rotating "life
enclosure". The end caps could then be an inverted
geodesic structure of Borosylicate glass, very tough
and radiation resistant triangular "plugs" of glass,
3m wide by 3m long by three meters thick, inserted
into a geodesic framework made of aluminum.
Penetration of any single glass plug would result in
an insignificant air loss until it could be replaced.
Larger penetrants, those pieces of space debris large
enough to cause damage to sections of the geodesic
larger than 1 plug, could be detectable by radar and
vaporized or at least broken up with small lasers.
Ablation of the outer layers of the rocks should be
sufficient to deflect the debris and avoid impact with
our glass endcaps.
The rotating inner cylinder could ride on magnetic
bearings within the passive shell, transferring load
to that shell and reducing the inner cylinder load
considerably. The passive shell then provides
radiation protection, acts as an air containment
vessel and load containment structure, plus any micro
g environmental requirements could be constrained
within that shell, even to the growth of air recycling
plants, which as far as I know can be grown in the
absence of gravity. Additional plant/animal life could
be grown within the rotating cylinder as required. As
long as the cylinders come as a counter rotating pair,
there would be no energy transfer to the shell and the
life cylinders would be rotationally stable at
Length =
How much energy would be expended by a rotating
cylinder, with R= 2500 M, L= 5000 M? As I recall, the
rotational Velocity would be around 60 km/hour.
cylinders,refreshing the rotational V would be a
simple matter requiring both cylinders to be
synchronous. The air drag would be necessary to mix
the air in the passive containment vessal and prevent
accumulation of CO2 and other inert gases. Drag then
becomes just another energy investment, like fans in
free fall, to maintain dynamic atmospheric mixing,
necessary if we're to grow plants on the inside wall
of the passive cylinder. I just like the idea of using
all available space for SOMETHING, from as simple as
an air containment vessal to as complex as a micro G
shirt sleeve manufacturing environment to air
purification via plants growing in that free fall
area.
I really need more input than the statement "a
significant" problem. HOW significant IS significant?
How much energy cost are we talking about? The point
is, there doesn't seem to be any reason other than the
drag mentioned for not using a pressurized passive
shield vessal. It could solve a great many other
problems, from the use of non-rotating end caps, to
structural support of the active cylinder, to air
storage, to air refreshment with green growy things,
to micro g manufacturing,,,how do all those uses
balance out against the energy investment to overcome
drag?
Gary 7
I can't really give you an answer other than "significant enough that the original researchers in this field chose not to go in this direction". I don't consider myself smarter than an entire room full of space scientists and engineers working under NASA, so I just pretty much go along with whatever conclusions came out of that (and got peer-reviewed).
Mike Combs
significant" problem. HOW significant IS significant?
The idea of 'so we have air drag; we compensate for it' is new to me.
This could be because it's a lousy idea. Or maybe it *was* a lousy
idea in the 1970's, but isn't anymore. I'd sure like to see some
thought put towards it. If it's a lousy idea, then we show it to be
and move on to something else.
enough advantages to make up for it. My suspition is that there are
not enough such advantages, but then I've been wrong before. Don't
tell anybody.
Which reminds me of a paraphrase of Dr. Richard
Feynman that goes something like," Don't trust
nobody!"
suspect they were accounting for energy costs to
maintain spin and may have been concerned about
turbulence. Turbulence near the rotating surface COULD
be significant, but energy transfer to the air, thence
to the passive shell should minimize turbulence,
especially in a large structure in which the air gap
from internal cylinder to passive shell is 30 Meters
or so, ie, about 1 % of the radius of the rotating
cylinder. Pleanty of room to dampen the turbulence.
Energy transfer to the shell is conteracted by the
contra rotating cylinder. But real numbers, based on
total affected air mass and velocity of the cylinder,
should tell the tale and affected air mass depends
upon the atmospheric pressure/density as well as the
efficiency of energy transfer which should have some
dependency upon humidity. So, are we talking about a
pressure of 2.7 PSI of pure O2 or 14.7 PSI of an N2+O2
mix? (I missed references to the desired air mix.)
GAry 7