# Zero G thought experiment

1. Jul 7, 2009

### texasblitzem

Hello everyone in PF land, I'm a long time reader, first time poster. My friend and I were conducting a thought experiment involving a spacecraft with a rotating section to produce artificial gravity. The rotating section would be accessed by a tube section that does not rotate. (I've attached an illustration showing our craft.) In our illustration, astronauts B and C would experience 1G due to the rotation of the craft, and astronaut A would experience zeroG because the access tube is not rotating. Is our assumption correct so far? Now, we are assuming that as astronaut A travels through
the access tube to the rotating section, he will continue to float in a zeroG "zone" in the center of the rotating structure. If this not the case, what would happen to astronaut A as he entered the rotating section? Would he be forced to the inner wall where astronauts B and C are standing?
Could he float in between B and C?

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2. Jul 7, 2009

### leroyjenkens

I think he would float in the center because nothing would be pushing him to the outer walls of the spinning section.

3. Jul 7, 2009

### AIR&SPACE

Correct! In fact, he wouldn't even have to be in the "center zone."

The spinning section does not produce gravity. Rather it accelerates objects interacting with it constantly towards the axis of rotation. Gravity also causes accelerations and we as humans just attribute accelerations that we feel to gravity.

It's much like two magnets. Since they can be affected by magnetic fields, they apply forces on one another which gives them an acceleration.

Anyone just floating through at any point while not touching anything on the surface would be no more affected by it than if it were a huge stationary magnet. However, if C3PO were to try to float through, he WOULD be affected by the magnet, and would feel as though gravity were pulling him.

4. Jul 7, 2009

### Staff: Mentor

Note that in practice, this zone would be unstable, as the air in the spacecraft would move with the rotation and a person inside the "zero G zone" would begin to rotate as well, due to the same aerodynamic friction. So any deviation from perfectly centered on the rotation axis (prefection = impossible) will cause the person to begin to fall toward the sides of the craft, sowly, as it is accelerated by the air.

5. Jul 8, 2009

### texasblitzem

Thanks guys, that info is beginning to make sense.

Ok, to be clear, you will only feels the effects of this pseudo-gravity if you are interacting with the rotating section itself. In other words you could be floating inches away from the inner wall that is rotating by you?

Now what would happen if astronaut B or C jumped up from their position. Would they float across to the other side or fall back to the inner wall? I think they would float across, since their is no real gravity to pull them back to the inner wall.

I know none of this stuff is pertinent to anything, but I enjoy imagining myself in this spacecraft playing in this environment.

6. Jul 8, 2009

### BAnders1

The acceleration felt by B and C is caused by the fact that they are rotating with respect to an observer in an inertial frame of reference (the access tube, for example). They are rotating because the friction between their feet and the walls of the ship is "pulling" them in a circle.

If the astronauts were to jump, something interesting would happen. Since they are in a noninertial frame, they would experience fictitious forces such as the Coriolis force. If they tried to jump to the opposite side, the ship could rotate halfway and they could land at the same spot they jumped from.

To an observer in the access tube, they would see the astronaut jump from one wall to the opposite side. To an observer moving with the walls, they would see the jumper moving in a small circular trajectory, then landing right where they began.

7. Jul 9, 2009

### texasblitzem

That would be a sight to see! I would give anything to spend an hour in an environment like this. Shoot, I would give anything just to watch other people play in this "playground".
NASA should build one and put a 24hr webcam in so we can watch astronauts have fun in it.

8. Jul 12, 2009

### flatmaster

Here's another thought. The small atmosphere inside the capsule should act in a similar matter to earth's atmosphere with real gravity. the closer you get to the center, the higher you go in artificial elevation. The air becomes thinner.

And if you were able to run fast enough around the outside in the opposite direction of rotation, you would stop gravity!

9. Jul 15, 2009

### Rasalhague

How does this work? How does friction mimic the effect of gravity on air, and are there any notable differences; is it that the air near the ground is being dragged along by friction with the ground, and how does that cause the air to be thinner higher up?

Pictures of larger rotating space habitats often show clouds.

Do these form in exactly the same way as in the earth's atmosphere; their shape looks familiar? Would rain fall from these clouds, deviating according to the Coriolis effect (i.e. appearing to curl in the opposite direction to the spin because the ground will have a greater tangential speed than the cloud)? Would steam rising from those lakes in the picture tend to curl in the same direction as the spin, for the same reason?

Would objects dropped by someone standing on the floor accelerate pretty much as on earth (aside from the Coriolis effect), if the habitat was spun fast enough for it to feel like earth gravity to someone standing on the floor? The Wikipedia article on the O'Neill cylinder space habitat design says, "The central axis of the habitat would be a zero gravity region," so is the idea of someone drifting along just above the floor and feeling no acceleration only possible in a vacuum? If someone drifted away from the axis in an atmosphere (thin or otherwise), would they inevitably accelerate towards the ground (and how would the air cause that), or would they be liable to orbit the axis?

http://en.wikipedia.org/wiki/O'Neill_cylinder

10. Jul 15, 2009

### texasblitzem

What would happen in this O'neill cylinder, if a helicopter took off from the inner surface and tried to hover a few feet off the ground? Would the helicopter hover over the same spot as the cylinder rotated? Or from the reference frame of the pilot, would it appear that he was moving with respect to the surface? Then what would happen if he kept increasing his altitude, could he reach the zero G region? What does a helicopter do in a zero G environment?---Have any astronauts played with a toy helicopter while in orbit?

11. Jul 15, 2009

### Rasalhague

(1) The air close to the ground on the Earth seems to rotate with the planet, so I'm guessing it'd be the same in the rotating habitat, and the helicopter would hover over one spot on the ground as it would on Earth. But I'm not sure; hopefully someone more knowledgeable than me can confirm or deny that. I'm curious about weather in general in such an environment, and how friction - or whatever it is - causes these effects.

(2) If I've got this right, moving parallel to the axis, there wouldn't be any Coriolis effect on the helicopter. Moving in the same direction as the habitat was spinning, the Coriolis effect would tend to force the helicopter downwards (so the pilot would have to compensate for that. Moving against the direction of spin, the Coriolis effect would tend to force the helicopter up.

(I think this is because when you move in a straight line perpendicular to the axis and the radius, you're approaching the ground some way round the cylinder from you; and when you go with the spin, the spot of ground you're approaching is one closer to you, and the spot you were initially aiming for is moving away from you further round the rim, so you'll tend to fall short, giving you the impression of being forced down. And when you set off counterspinwise, travelling above the ground, perpendicular to the axis and the radius, then the spot of ground you were initially aiming for will be moving towards you, and so you travel further, than your instincts expect, giving you the impression of lift.)

(3) The helicopter would rise, as it would in the earth's atmosphere, except that the Coriolis effect would cause it to deviate from the vertical, curling off in the direction that the cylinder was rotating; so the pilot would have to compensate for the Coriolis effect if they wanted to go straight up. And if the pilot wanted to go straight down (away from the axis) towards the ground, they'd have to compensate for the Coriolis effect pushing the helicopter in the opposite direction to the way the cylinder was rotating.

(I think this is because when you're rotating in time with the habitat, as you are when you're on the ground, you have a greater tangential (linear) speed than objects above rotating in time with the habitat at a smaller distance from the axis. As you rise towards the axis, with that same tangential velocity, you're no longer rotating in time with the habitat, but seem to be thrown forwards in the direction the spin. And when you move from near the axis down towards the ground, you start off with a lower tangential speed than the spot on the ground that you're headed for, so you seem to be being forced off course antispinwards, and need to compensate by increasing your tangential speed in the direction of spin.)

(4) As long as there was air, I think the blades would still generate "lift", in whatever direction the pilot wanted. Again, I'm not an expert, so don't take this as definitive!

12. Jul 15, 2009

### Rasalhague

http://mvl.mit.edu/AG/experience.html [Broken]

Last edited by a moderator: May 4, 2017
13. Jul 15, 2009

### texasblitzem

Thanks for those interesting explanations Rasalhague! You've given me a lot of info to mull over and process. I'm sure I'll think up some more questions about other scenarios that could arise in this strange environment.
Is the gravitational situation similar in the O'neill cylinder and the Stanford torus?

14. Jul 15, 2009

### Rasalhague

I've just come across a book, Artificial Gravity, by Gilles Clément and Angelia P. Bukley, on Google Books.

Am I right in thinking that the Coriolis force in fig. 4 on p. 42 is pointing in the wrong direction? Unless I'm mistaken,

$$-2m\mathbf{\Omega}\times\mathbf{v}$$

should point into the page (since omega, the angular velocity of the centrifuge, points up and v, the linear velocity vector of the person, points left), which would agree with their point c (pp. 42-43), that says the Coriolis force causes deviation opposed to the direction of rotation for someone moving out from axis to rim; and with the correct illustrations of ladder-climbing on p. 44.

I think there's also a typo in eq. 3 on p. 40 for the (pseudo)gravity gradient between the feet and head of a person standing on the rim.

$$\frac{\omega^{2}r(r-h)}{\omega^{2}r} = \frac{r-h}{r}$$

r is the radius of the rim, h the person's height, so r - h is the radial distance from the axis of their head. They need to lose the first r on the left, don't they?

Apart from the various Wikipedia articles (artificial gravity, centrifugal force, coriolis effect, rotating reference frame, fictional force, etc.), Theodore W. Hall has written a lot of interesting articles on the subject of building rotating space habitats. There's a discussion in one of the ladder in 2001, and how - in real life - it would seem to be curving off to the side at a ridiculously unclimable angle!

http://www.twhall.com/

Another great page with lots of pictures relating to the design of rotating spacecraft:

http://www.projectrho.com/rocket/rocket3u.html

I remember seeing ages ago a video clip of (Gemini era?) astronauts bouncing around off the walls of a drum-like centrifuge, but I forget the details. I haven't managed to find the link, but I'll post it if I do. All I can find about artificial gravity experiments of that era that were actually done in space was one involving a tether which didn't generate a humanly perceptible effect, so perhaps the clip I saw was an experiment on earth - but maybe I'm getting it mixed up with something else... There are some good videos on YouTube / Google Video about the Coriolis effect. It's very counterintuitive; I had to watch those animations again and again until it started to make sense.

On the question of weather, there's a whole bunch of old educational films here

http://web.mit.edu/hml/ncfmf.html

about fluid dynamics. The one entitled Rotating flows has some funky effects, including ink suspended in clear fluid, and under some conditions, the ink stretches into these long strands called Taylor columns, aligned parallel to the axis. There's another one where the fluid moves in seperate rings, rotating alternately clockwise and anticlockwise. I wonder how much of this would apply to a breathable atmosphere. Presumably one big difference would be that the atmosphere would be compressible. Would clouds form in long strings like the ink in the demonstration? Would rain fall (more or less) as on earth?

And finally, can anyone recommend any good science fictional treatments of these themes?

Last edited by a moderator: Apr 24, 2017
15. Jul 15, 2009

### Rasalhague

I suppose the biggest differences are that the Stanford torus is smaller, and when you moved from place to place inside it, apart from short journeys parallel to the axis of rotation, you'd be mostly going east or west (with or against the spin). The article on the O'Neill cylinder mentions mild inner ear effects and deflection of objects drops from human height by a few centimeters. With a smaller radius, the Stanford torus would have to rotate faster to create the same strength of artificial gravity, and so these effects would be more obvious, although still well within human comfort levels for 1g, judging by what I've read. Very small habitats, just a few metres across, spun up to generate 1g are liable to be nauseating - although I've read suggestions for spinning up gradually to allow astronauts to become acclimatised. Theodore Hall discusses the relative merits of different shaped spinning crew modules for small spaceships. At smaller scales, apparently, it can make a big difference whether you have to move about mainly parallel to the axis (no Coriolis effect), or in other directions (potentially significant Coriolis effect).

BY the way, I just noticed, after watching that fluid dynamics video, that the clouds in the artist's impression of the O'Neill cylinder

http://en.wikipedia.org/wiki/Island_Three

are aligned in the directin of spin, rather than parallel to the axis. I wonder if there's a reason for this. The clouds in the Rendezvous with Rama picture seem more random. I wonder if the far side would really look so clear or whether there'd be more Rayleigh scattering of sunlight.

16. Jul 16, 2009

### texasblitzem

Thanks Rasalhague, you've given me alot to read. I look forward to exploring these links.

17. Jul 16, 2009

### DaveC426913

While the helicopter is sitting on the ground, it is held in place due to its rotation along with the cylinder.
As it lifted off, it would first have to cancel that movement. It would have to accelerate in the opposite direction of the spin of the cylinder. It would then have also work against the motion of the air it is flying in.

In effect, except for subtle coriolis effects, the helicopter would have to fly exactly as if it were on a planet: lift off, accelerate in one direction to a certain speed, including air resistance.

Yes.
It would simply move in whatever direction "up" is for the pilot. It basically becomes a propellor-driven craft with the pilot facing 90 degrees to the direction of travel.

Although it would have a slight advantage over a plane, since a helicopter has a built-in counter-counter-rotating prop to cancel the counter-rotation. A plane in zero g would suffer from barrel rolls due to the prop rotation.

18. Jul 16, 2009

### texasblitzem

Ok, say the helicopter takes off in this O'neill cylinder, straight up from his position on the ground. His altitude increases and he eventually reaches the zero G region along the axis of rotation. What happens if he keeps going? He's headed towards the other side of the cylinder, but upside down!?(in reference to the side hes approaching) At some point would he have to re-orient the copter to be in a correct attitude wrt to the other side? Im laughing just imagining this scene in my head, looking up and seeing an upside-down helicopter coming at me.

19. Jul 16, 2009

### DaveC426913

It would be no stranger than looking up and seeing tiny people, houses, rivers and trees clinging upsidedown to the arched roof over your head.

Come to think of it, I am not sure if a helicopter upsidedown in a low-G environment could right itself. They can in do some serious rolls and pitches in a high-G environment, true, but I'll bet that depends on a strong tendency for its mass to want to be downwards.

20. Jul 16, 2009

### texasblitzem

I totally hadn't thought of that. My dreams are going to be a lot more interesting from now on.

Now I'm imagining an elevator that spans the diameter of the cylinder, and what fun one might have inside as it traverses through the center and to the other side.:surprised