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Non-rotating Airlock
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MirariNefas
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PostPosted: Sun Jan 07, 2007 9:41 pm 
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Many space colony plans call for a non-rotating section of the structure. I can see how this would work if both sections are seperately pressurized and a specialized vehicle system is used for transfers, but there seem to be people who think this can be done in a single pressurized system. How?
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scienceguy
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PostPosted: Mon Jan 08, 2007 12:07 am 
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Maybe if you used rubber linings of the airlock with oil for lubrication. Otherwise you could use some smooth plastic that maintained a seal even when it is moving past itself. Just guesses.
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MirariNefas
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PostPosted: Mon Jan 08, 2007 12:22 am 
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Friction would necessarily wear it away though. I could see some sort of oil involved, but wouldn't that boil away into space? Especially with the high gas pressure behind it?
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PostPosted: Mon Jan 08, 2007 11:11 am 
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Assuming the centrifuge is used to simulate gravity, it would be disigned for crew comfort and located entirely within the non-rotating gyro-stabilized exterior envelope such that no airlock would be required and periodic lubrication could easily be performed. Entry and exit would be along the rotational axes. Observational instruments and communicating antennae would be mounted on the non-rotating exterior envelope; or, as Ezekiel would say: "a wheel within a wheel".
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MirariNefas
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PostPosted: Mon Jan 08, 2007 3:12 pm 
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Yes, I'd thought a non-rotating exterior envelope might be a good idea. I'm glad that somebody agrees with me.
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MirariNefas
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PostPosted: Sat Jan 13, 2007 1:14 am 
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How much energy would be involved in keeping such a wheel spinning against the friction of the exterior envelope, or, if suspended magnetically, how much against the friction of gas? How much rotational force would this impart on the envelope?
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PostPosted: Sat Jan 13, 2007 2:27 pm 
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Doesn't the energy required to keep spinning depend on the coefficient of friction? If the materials have wheels with which to roll past each other, the friction would be less. I don't offhand know of a formula for heat generated by friction, but I can see if I can look it up later.
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PostPosted: Sat Jan 13, 2007 5:33 pm 
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I looked it up, and the energy dissipated by friction is:

E = uNd

where

u is the coefficient of static friction
N is the normal force
d is the distance travelled

So if you have a radius of 100 m and the averaged normal force between the 2 rotating sections is 100 N and the coefficient of friction is 0.3, then the apparatus would dissipate 18 840 J per rotation. If the thing were to rotate once per minute for a six month journey to Mars, you would need 4.94 X 10^9 Joules of energy just to maintain the rotation. Presumably you could minimize this with lower friction.
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MirariNefas
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PostPosted: Sat Jan 13, 2007 7:03 pm 
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I was actually thinking of this for a space colony rather than a ship. I think you can see why I would be concerned in that case; a space colony is much larger, and supposed to be permanent.
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PostPosted: Sun Jan 14, 2007 9:08 am 
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There need not be that much difference between the centrifuge for the space colony and the vehicle. The dimension "d" need not be determined by the diameter of the centrifuge, but rather by the diameters of the entry and exit ports to/from the centrifuge (the bearings in the vehicle attach points for the centrifuge) and the angular momenta added and subtracted by the people entering and exiting the device. What goes on inside will generally follow the laws of conservation of momentum.

For instance, imagine the ports rimmed with a two meter geared collar and the rotating force being applied by a gear of appropriate diameter (20 or 30 centimeters at most) driven by a 1 kw electric motor. The dynamic coefficient of friction is normally less than the static coefficient, but I haven't seen any values for the coefficients of friction using the latest and best lubricants.

It may be worth considering building the rotor winding of the electric motor into the centrifuge and have the stator windings rigidly mounted to the external envelope of the vehicle thus eliminating gears and making automatic control of the centrifuge rotation more easily facilitated.

If it can be shown that a one kw motor will do the job, then multiply by the hours required for the mission to get the total kilowatt hours. Double the value, if you like, to handle contingencies.

By having an even number of centrifuges and having them counter rotate, the torque applied to the outer envelope could be managed more easily, but some vehicle oreintation propulsion will probably be required to keep the envelope non-rotational.
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PostPosted: Sun Jan 14, 2007 3:12 pm 
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By having an even number of centrifuges and having them counter rotate, the torque applied to the outer envelope could be managed more easily


This should be obvious, but I'd forgotten about it. Thanks.

I'm only half sure what a stator winding is, but I think I get the idea. good thinking.
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PostPosted: Mon Jan 15, 2007 11:33 am 
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I'm only half sure what a stator winding is, but I think I get the idea.
It's the one in the stationary part of the motor that interfaces external power to the rotating part of the motor.
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PostPosted: Mon Jan 15, 2007 6:19 pm 
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My only real experience here is taking apart radios. This would be the copper coil in an electric motor, right?
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Gourdhead
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PostPosted: Tue Jan 16, 2007 9:30 am 
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My only real experience here is taking apart radios. This would be the copper coil in an electric motor, right?
There are two sets of copper coils. One is stationary because it is rigidly attached to the housing. The other set is wound into the armature (for DC) or the rotor (for AC). It's the interactions of the magnetic fields of the two sets of coils that convert the magnetic power to the mechanical power. The electric current power to magnetic power takes place in the stator.
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MirariNefas
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PostPosted: Tue Jan 16, 2007 12:16 pm 
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This makes sense. Thank you.
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