In the Kalpana One paper, we briefly sketched out a concept for a
dynamic stabilization system. Weights would be suspended around the
outside of the station on cables, and reeled in and out in order to
rebalance the station as masses were added, removed, or moved around
inside. The movement of these masses would be under computer control
and based on sensors (presumably accelerometers) around the station.
wrong -- the same effect would be achieved much more simply by having a
river/lake (or if you prefer, a series of reservoirs connected by pipes)
around the equator of the station.
Suppose things shift and the center of mass (COM) of the station moves a
little bit in one direction. The habitat must spin around its COM, of
course, which means that we're now spinning off-center. The part of the
river furthest from the new center now feels more centripetal force than
the part closest to the new center. As a result, water would flow
towards that furthest point. But water is heavy -- so this would shift
the COM in that direction, i.e. back towards the center.
It seems to me that this would automatically correct any imbalance, at
least within some range of imbalances. If so, it's a nice approach
because it's completely passive; no software glitch, hack, or power
failure can screw it up. And many habitat designs feature a central
lake or river anyway, because they add considerable aesthetic value.
Am I missing anything? Anybody with experience in dynamics want to
chime in with an opinion on this?
Thanks,
- Joe
As I run this through my brain, I feel that a central river (or sea, if it is salty and has waves instead of being fresh and flowing) would exacerbate the problem, not reduce it. But I can't exactly say why, so I'm quite likely wrong. If I'm right, I sure wish somebody would explain to me why.
For those of us too lazy or ill equipped to do the the math, visualize a bucket of water situated on a spinning platter. As the platter spins the water will tend to flow up the rim of the bucket as it experiences greater acceleration. Thus I agree with Joe's assertion that the water in a river (or connected reservoirs) will tend to flow towards a point experiencing the greater acceleration, which is the point furthest from the COM. Thus the river would have a stabilizing effect.
In the more general case of a cylinder, how will the weight/spring system fare against the reservoir approach in regulating balance in situations where mass distribution varies not around the axis of rotation, but also longitudinally?
Date: Fri, 25 Jul 2014 17:42:39 -0700
As I run this through my brain, I feel that a central river (or sea, if it is salty and has waves instead of being fresh and flowing) would exacerbate the problem, not reduce it. But I can't exactly say why, so I'm quite likely wrong. If I'm right, I sure wish somebody would explain to me why.
In space,,,NOTHING is automatic...pumping water from a point off center to stabilize rotation is one way taking care of business or,,,you can just make it so large and massive that small masses have no discernible effect on stability...
Gary 7
Ah yes! OK, I see it now.
I had been thinking that the accumulation of extra mass (the water) would pull the system even further out of whack, but the key is that as the CoM moves in ONE direction the water flows in the OTHER direction. The extra mass to one side pulls the CoM in it's own direction, in the process pulling the CoM through the geometric center. In a severe case, it might pull the CoM through the geometric center and a little beyond, but if that happens then water will flow the other way and pull it right back again. And since the water doesn't teleport, it flows at a finite speed, the whole effect is to dampen out the offset.
> I had been thinking that the accumulation of extra mass (the water)
> would pull the system even further out of whack, but the key is that as
> the CoM moves in ONE direction the water
> flows in the OTHER direction. The extra mass to one side pulls the CoM
> in it's own direction, in the process pulling the CoM through the
> geometric center. In a severe case, it might pull the CoM through the
> geometric center and a little beyond, but if that happens then water
> will flow the other way and pull it right back again. And since the
> water doesn't teleport, it flows at a finite speed, the whole effect is
> to dampen out the offset.
Of course all this sloshing around consumes energy, which would come out
of the spin, and have to be made up somehow. This is another reason why
a good design should probably have two counter-rotating sections, so you
can spin them up with simple electric motors. But even if it's a single
section, and you have to use rockets, you could use some high-efficiency
thrusters (e.g. ion engines), and it's probably not a big deal.
> It takes some brain-wrapping, but I see it now. This is really quite
> brilliant and I'm amazed at how simple it is. This is almost certainly
> how it will be done, and all it'll take to keep people safe is an
> advisory to stay away from the river bank (or beach) for a while.
In reality I would expect these currents to be rather subtle. Only if
you suddenly dropped a very large mass on one side of the habitat would
there be any significant waves (and in that case, somebody should get
fired!).
Best,
- Joe
This might work and a passive scheme is preferable to an active one, but any and all stabilization schemes require extensive and detailed analysis. In particular, the vibrational resonance must be well understood. The advantage of the weights and cables is that you program them so that problems can be fixed fairly easily.
On Jul 24, 2014, at 3:22 PM, Joe Strout joe@... [ssi_list] ssi_list@... wrote: In the Kalpana One paper, we briefly sketched out a concept for adynamic stabilization system. Weights would be suspended around theoutside of the station on cables, and reeled in and out in order torebalance the station as masses were added, removed, or moved aroundinside. The movement of these masses would be under computer controland based on sensors (presumably accelerometers) around the station.But today, it occurred to me that -- unless I'm looking at this allwrong -- the same effect would be achieved much more simply by having ariver/lake (or if you prefer, a series of reservoirs connected by pipes)around the equator of the station.Suppose things shift and the center of mass (COM) of the station moves alittle bit in one direction. The habitat must spin around its COM, ofcourse, which means that we're now spinning off-center. The part of theriver furthest from the new center now feels more centripetal force thanthe part closest to the new center. As a result, water would flowtowards that furthest point. But water is heavy -- so this would shiftthe COM in that direction, i.e. back towards the center.It seems to me that this would automatically correct any imbalance, atleast within some range of imbalances. If so, it's a nice approachbecause it's completely passive; no software glitch, hack, or powerfailure can screw it up. And many habitat designs feature a centrallake or river anyway, because they add considerable aesthetic value.Am I missing anything? Anybody with experience in dynamics want tochime in with an opinion on this?Thanks,- Joe
On Jul 26, 2014, at 8:54 AM, ronfox@... [ssi_list] ssi_list@... wrote: For those of us too lazy or ill equipped to do the the math, visualize a bucket of water situated on a spinning platter.
This is a poor model. Actually, not even close. In our case all of the water is constrained to the walls of the cylinder (locally two dimensional) and moves around to different parts of the cylinders walls as the cylinder wobbles.
As the platter spins the water will tend to flow up the rim of the bucket as it experiences greater acceleration. Thus I agree with Joe's assertion that the water in a river (or connected reservoirs) will tend to flow towards a point experiencing the greater acceleration, which is the point furthest from the COM. Thus the river would have a stabilizing effect.
> On Jul 26, 2014, at 8:54 AM, ronfox@...
> ronfox@... [ssi_list] ssi_list@...
>
>> For those of us too lazy or ill equipped to do the the math, visualize
>> a bucket of water situated on a spinning platter.
>
> This is a poor model. Actually, not even close. In our case all of the
> water is constrained to the walls of the cylinder (locally two
> dimensional) and moves around to different parts of the cylinders walls
> as the cylinder wobbles.
completely filling the race, so there is room for them to redistribute
the weight by moving around.
Could make an interesting science fair project.
- Joe
Maybe a protest, with a counter-protest, so everybody hops on their bicycles and rides to the same spot as fast as they can to make sure The Truth is well-represented. But it should be easy to design the river or sea so that the worst that happens is that some surfer gets a slightly larger or smaller wave than expected, going in a slightly different direction than expected.
UPDATE
I am an inept surfer, but I have now done it. Once.