Artificial Gravity in Science Fiction

[ZapperZ]

We get this type of question now and then, even in the main physics forums. One such question in the Sci Fi forum is from this one:

https://www.physicsforums.com/threa...possible-rotating-wheel-space-station.950770/

So I don't know of people have seen this video from a few years back, but it should be nice to have a record of it. This is especially true if people are not aware of other problems associated with having a spinning space station to generate such artificial gravity.



He talks a bit too fast, but you can always pause, and replay, what he says at your own pace.

Zz.

 

[DrClaude]

He talks a bit too fast, but you can always pause, and replay, what he says at your own pace.

You can also slow down the playback speed.

 

[Ryan_m_b]

For the budding hard-SF author or just anyone interested in rotating space stations/vessels this site is a good resource:

https://www.artificial-gravity.com/sw/SpinCalc/

 

[Kevin the Crackpot]

I came up with a possible idea about artificial gravity.

There have been pictures of leaves, small frogs (none the worse for wear) actually levitating on top of a super-strong magnet.

This is possible because of the diamagnetism of water (remember that a water molecule is more positive on one side, and more negative on the other).

This causes a water molecule to be repelled by a strong magnetic field, so a tiny frog can be levitated if a superconducting magnet is strong enough and in the correct shape.

In working out the math of how such a thing is possible, one must use Earnshaw's Theorem.

I had the idea that if the magnetic field was strong enough, it could work as a kind of artificial gravity in a spaceship by keeping you pressed against the floor . . . but any forgotten paperclips in your pocket could become lethal projectiles under these conditions.

I've also wondered if this diamagnetism of water could--somehow--allow a person to walk across a neutron star (there is tremendous gravity, but counter-balanced by a tremendous magnetic field) if one could avoid being fried by the hot surface.

I haven't worked out the math of such a situation, so I don't know if this would work.

 

[Tiran]

I had the idea that if the magnetic field was strong enough, it could work as a kind of artificial gravity in a spaceship by keeping you pressed against the floor . . . but any forgotten paperclips in your pocket could become lethal projectiles under these conditions.

I think you would find that the amount of attraction you are talking about to make a person feel "weight" would like cause cellular functions to stop and neurons from firing.

The frog isn't being levitated by a full G of repellant force - the surface tension of the water is doing a lot of the work.

 

[Kevin the Crackpot]

I think you would find that the amount of attraction you are talking about to make a person feel "weight" would like cause cellular functions to stop and neurons from firing.

The frog isn't being levitated by a full G of repellant force - the surface tension of the water is doing a lot of the work.

I imagine that you're correct about an enourmously strong magnetic field interfering with the functions of the body, but I was confused about your point with the surface tension.

The frogs I was discussing are levitated in thin air by the magnetic field . . . not supported on the surface of water like a water glider insect.

 

[Tiran]

I imagine that you're correct about an enourmously strong magnetic field interfering with the functions of the body, but I was confused about your point with the surface tension.

The frogs I was discussing are levitated in thin air by the magnetic field . . . not supported on the surface of water like a water glider insect.

Sorry, I misunderstood.

I imagine the frog's lack of mass per horizontal surface area is also a major factor - you could levitate a human's mass the same as the frog if you distributed a person as a 1/4" blanket of flesh horizontally over a 12x12 foot area. But people are tall vertical stacks of matter, and your magnet gets less effective with distance so it would have to be orders of magnitude stronger.

 

[Kevin the Crackpot]

Sorry, I misunderstood.

I imagine the frog's lack of mass per horizontal surface area is also a major factor - you could levitate a human's mass the same as the frog if you distributed a person as a 1/4" blanket of flesh horizontally over a 12x12 foot area. But people are tall vertical stacks of matter, and your magnet gets less effective with distance so it would have to be orders of magnitude stronger.

I imagine that you're correct.

Also, it occurs to me that if there was a magnetic field strong enough to levitate a person, then his or her body might get ripped apart.

Different substances have different degrees of diamagnetism, so adjacent substances with different degrees of diamagnetism might be pulled apart from each other.

I don't know.

I still thought it would be cool to walk across a neutron star one day.

Oh well.

 

[member 656954]

I came up with a possible idea about artificial gravity.

There have been pictures of leaves, small frogs (none the worse for wear) actually levitating on top of a super-strong magnet.

This is possible because of the diamagnetism of water (remember that a water molecule is more positive on one side, and more negative on the other).

This causes a water molecule to be repelled by a strong magnetic field, so a tiny frog can be levitated if a superconducting magnet is strong enough and in the correct shape.

...

This concept was used in a sci-fi novel some time ago...and I'm thinking John C. Wright's "Count to a Trillion" but my memory might be playing tricks...as a way for the characters to fly all over the place. The novel made it seem like fun, but I'd hate to think what would have happened in a power out

 

[member 656954]

OK, I've read this thread and a few others, and what I can't see is the 'docking procedure' for such a space station.

I'm working on the second "Guardian" novel and am considering a tubular space station, 12km in length and 6km in diameter, which for a simulated 1G on the interior surface means I only need 0.386 RPM so that's a reasonable figure.

But, how do you enter the thing? That rate of spin imparts almost 900 km/h tangential speed, which is not a lot in the scheme of things, but to maintain in a circle as you approach? It seems unfeasible.

To get around this, most sci-fi stories have zero gee airlocks at the ends. But, from my understanding, the centripetal force (aka 'artificial gravity') only acts upon objects in contact with the interior surface. It's not like a planet, where gravity builds up as you get closer.

So, you enter via the air lock at the end, in zero gee. And you're in zero gee until you make contact with the rotating interior wall (floor?). Which suddenly imparts 1G load upon you...or almost 900 km/h tangential speed?

I can fluff this for the sake of the story, it's sci-fi after all, but how exactly do you safely enter one of these things?

 

[Vanadium 50]

The hub has a linear velocity that is much smaller than the rim. Similarly, the apparent gravity is very small at the hub compared to the rim.

 

[Ryan_m_b]

So, you enter via the air lock at the end, in zero gee. And you're in zero gee until you make contact with the rotating interior wall (floor?). Which suddenly imparts 1G load upon you...or almost 900 km/h tangential speed?

If you take an elevator from the hub, drive down some switchbacks or even walk down a long stair case your angular velocity will steadily increase. It will feel like the gravity is slowly building up until you meet the rim.

 

[DaveC426913]

But, how do you enter the thing? That rate of spin imparts almost 900 km/h tangential speed, which is not a lot in the scheme of things, but to maintain in a circle as you approach? It seems unfeasible.So, you enter via the air lock at the end, in zero gee. And you're in zero gee until you make contact with the rotating interior wall (floor?). Which suddenly imparts 1G load upon you...or almost 900 km/h tangential speed?

You have a runway!

1. Bay is rotating. Shuttle is not.
2. Shuttle enters Bay, along axis, nose-in.
3. Shuttle turns 90 degrees (yaw). Extends landing gear (wheels!)
4. Bay is now slipping under nose of shuttle at full speed.
5. Shuttle thrusts to descend (away from axis). (see diagram)
6. Shuttle wheels touch bay runway.
7. Shuttle starts to brake.
8. Shuttle gradually spins up to space station rotation.

It will feel (and look) to passengers almost exactly like a normal aircraft landing (except starting off in zero-g).

Step 5:

Actually, it would be more efficient if the landing "gear" were part of the station, not part of the shuttle.
All you need is a clamp that spins down to non-rotating speed. It grabs the shuttle and then spins back up.

 

[Nik_2213]

"I can fluff this for the sake of the story, it's sci-fi after all, but how exactly do you safely enter one of these things?"

Watch 2001 ??

IIRC, AC Clarke was so weary of people asking a similar question, that he wrote the *definitive* axial docking sequence...

Watch the auto-land system re-aligning the shuttle from its 'inertial' frame to the station's 'rotating' frame.
Job Done.

Plan_B is to have, between stationary axial hangar and rotating station, a transfer area, like a lift or 'people mover'. People accept express lifts (elevators), rotating foyers, escalators etc. Just takes a little 'getting used to'...

 

[DaveC426913]

Watch the auto-land system re-aligning the shuttle from its 'inertial' frame to the station's 'rotating' frame.
Job Done.

Problem with that system is that it is not scaleable.
Since transfer from inertial frame to rotating frame occurs at the axis, you can only have a single ship in the docking procedure at a time. No ship can even begin docking until the previous one is debarked and moved to a side bay.
Land them on circumferential runways and you've got an airport that can scale as large as you want just by adding parallel runways.

 

[Ryan_m_b]

Problem with that system is that it is not scaleable.

If you have a very large spinning habitat you can also build a counter-rotating space port at the end. No need to mess around with runways or trying to dock on the spinning outer/inner surface. Just have a disk-shaped space port jutting from the end like a mushroom cap on a coke can. Space craft dock, people and items travel through the port in zero-g. They then take an elevator or a curved track down to the inner surface. See this quick mock up for more detail:

 

[DaveC426913]

Yep. That works too.
And has the decided advantages of:

1. leaving all the scary approach, maneuvering and touch-down procedures on the outside of the station.
2. All differing-velocity-interface hijinks are in a single, restricted well-controlled and low-energy location.
3. Any small problems, and the shuttle can just back away in any convenient direction.
4. Any big problems, and the habitat is well-protected from damage behind the landing cap.

(Though I can't make heads or tails of your diagram. Here's my take)

 

[Ryan_m_b]

Yep. That works too.

(Though I can't make heads or tails of your diagram)

Here's a version with annotations and illustrations

The diagram is a cross section of a rotating space habitat. The center black circle is a counter rotating space port on the axis. Spacecraft would dock to it, passengers and cargo travel through in zero G. To get to the inner surface of the rotating space port they could take an elevator down to it which would gradually increase in apparent gravity as the axial rotation of the elevator carriage increases. The track represents a similar option except rather than going straight down carriages can ride on a curve.

Make sense?

View attachment 238221

Counter-rotating rather than non-rotating which cancels out the rotation but otherwise yes. I doubt spacecraft would land like that on it as they don't need to land like planes but any method of them slowly approaching and latching on would work.

 

[Nik_2213]

IMHO, given orbital micro-debris etc, your un-spun dock should be a big, big cylindrical hangar with an axial door. Without an 'up' or 'down', you may use all the internal 'drum' wall area...

For convenience, it may be implemented as a polygonal drum, with a plane section for each 'pad'...

Also, because it is not rotating, it is both scaleable and potentially modular...

 

[DaveC426913]

IMHO, given orbital micro-debris etc, your un-spun dock should be a big, big cylindrical hangar with an axial door. Without an 'up' or 'down', you may use all the internal 'drum' wall area...

As per posts 13 and 15.
But I'm leaning strongly toward Ryan's idea.

Why have the shuttle doing all their maneuvering inside a cramped space? That's a recipe for bumper cars with dents that won't buff out.
Why not let them have all of space to maneuver? (and blow up, it if it comes to that)
It's not like they need protection from cross winds and rain. Off in space is the safest place to do tricky maneuvers.

 

[member 656954]

Thanks for all the pointers and suggestions. I'd totally forgotten about the sequence in "2001", that'll help cement the concept for me. Plus, reading your posts, I only just realized that if there is a ladder or elevator attached to the 'floor', then if you start using it your frame of reference will include the centripetal force, however weak at the center of axis, which will increase as you 'descend'. That was my main mental stumbling block with this concept, so having it sorted is much appreciated