What is the trajectory of an object thrown within a rotating centrifuge?

[CakeOrDeath?]

Hi all. I posted a question here once before for a sci-fi writing project, and I'm back again to pick the brains of some people more left-brained than myself.

Here's the scenario: Imagine you're in a spacecraft with a rotating centrifuge, to create artificial gravity for the astronauts. It's probably easiest to use the familiar pop culture reference and think about the Discovery I from 2001: A Space Odyssey. The occupants are pushed to the outside, away from the axis of rotation.

Dave is tired of just sitting there eating his green and orange goop and gets the urge to throw his utensil toward the axis. How does the utensil act during its flight? Does it slow down near the center and then pick up speed, falling to the opposite side (the ceiling from Dave's perspective)?

Thanks in advance.

 

[Simon Bridge]

The "gravity" of the centrifuge is not a force field like it is on the Earth.
Probably the thing to think of is Sheridan's jump from the train in B5 ... he approaches the "ground" at a constant speed rather than accelerating like you'd expect on Earth. Air resistance not withstanding, he could, in principle, stop his speed with a jet pack and "hover" over the ground as the station rotated under him.

Back to the Discovery:
If Dave just dropped the utensil ... he holds it still and let's go ... think that the utensil just travels in a straight line in the direction of the spin until it hits the curved deck, while the deck rotates under it.

If Dave throws the utensil "upwards", it has the same tangential velocity as when he dropped it but also with a radial component: It basically flies off at an angle to Dave's "floor" but since Dave rotates under the Utensil while the utensil moves in a straight line, he sees the utensil follow a curved trajectory.

Dave would have to account for the curve if he wanted to actually "hit" the center.
You can have fun with drawing the position of Dave and the utensil at different times to see what it would look like.

 

[ImaLooser]

Hi all. I posted a question here once before for a sci-fi writing project, and I'm back again to pick the brains of some people more left-brained than myself.

Here's the scenario: Imagine you're in a spacecraft with a rotating centrifuge, to create artificial gravity for the astronauts. It's probably easiest to use the familiar pop culture reference and think about the Discovery I from 2001: A Space Odyssey. The occupants are pushed to the outside, away from the axis of rotation.

Dave is tired of just sitting there eating his green and orange goop and gets the urge to throw his utensil toward the axis. How does the utensil act during its flight? Does it slow down near the center and then pick up speed, falling to the opposite side (the ceiling from Dave's perspective)?

Thanks in advance.

Look in Wikipedia under Coriolis effect. There is a video there that explains it better than I ever could. Everything will seem follow a curved path.

 

[CakeOrDeath?]

Interesting stuff. Thanks guys!

 

[ImaLooser]

Interesting stuff. Thanks guys!

Another thing worth noting is that you'll have to turn the artificial gravity off whenever you want to maneuver the craft. Otherwise huge stresses will be generated.

 

[ImaLooser]

Interesting stuff. Thanks guys!

Another question is how to get it started. You'll have part of the craft rotating one way and the other part with opposite spin. And turning it on and off is expensive.

Of course, you can just ignore all these problems if you like. People have seen it in 2001, so it is traditional.

I read that such artificial gravity was considered for a space station, but they gave up because of these difficulties.

 

[Rear Naked]

Another question is how to get it started. You'll have part of the craft rotating one way and the other part with opposite spin. And turning it on and off is expensive.

Of course, you can just ignore all these problems if you like. People have seen it in 2001, so it is traditional.

I read that such artificial gravity was considered for a space station, but they gave up because of these difficulties.

Why would you need two rotating parts?

You could just put two opposite facing rockets positioned on opposite sides of the craft. Start em up, wait until the adequate rotational velocity is reached, then shut em off.

 

[Rear Naked]

Another thing worth noting is that you'll have to turn the artificial gravity off whenever you want to maneuver the craft. Otherwise huge stresses will be generated.

Is this true if the craft travels along the axis of the hoop?

 

[jbriggs444]

Why would you need two rotating parts?
You could just put two opposite facing rockets positioned on opposite sides of the craft. Start em up, wait until the adequate rotational velocity is reached, then shut em off.

If you plan starting and stopping rotation frequently (as you might want to do for maneuvering or docking) then using rockets to do it wastes reaction mass. Setting counter-rotating parts in motion only costs energy. That's what solar panels and nuclear reactors are for.

Applying the same principle in a slightly different application, you would typically want to use gyroscopes for stabilization in a communications satellite rather than depending on thrusters. Save your stationkeeping fuel for stationkeeping. [Or use it to boost you to geosynchronous orbit ala TDRS-1]

 

[BobG]

Is this true if the craft travels along the axis of the hoop?

I don't think so.

I think the rocket has to be lined up with the axis of the hoop. You perform a precession maneuver to align the axis so the rocket is pointing the proper direction.

When you launch a satellite to high altitudes (geosynchronous, for example), the satellite is launched into a low altitude parking orbit, then an upper stage rocket is fired to put it into a transfer orbit (with apogee at the desired altitude of the satellite), and then a final rocket (often called an apogee kick motor) is fired to circularize the orbit.

All of the equipment on the satellite is usually folded up during this process and the satellite is spin stabilized. The spin axis is lined up with the rocket(s). You aim the entire satellite by precessing the spin axis before each rocket firing.

No reason that would change just because you're talking science fiction instead of science fact.

You definitely don't want to be firing rockets to start and stop the rotation of your hoop. Not unless you have a gas station that can service spacecraft somewhere along your route.

The smart plan is to use rockets to spin the hoop up initially, and then solar generated electricity to spin a wheel, with the hoop automatically spinning the opposite direction (or stopping) as conservation of angular momentum. You want to use rockets to initially spin the hoop up, because the gyroscopic stability can come in handy for aiming your thruster and maintaining that aim all through the thruster firing. After that, somebody is always carrying that angular momentum - either the hoop or the wheel.

The last is just personal opinion, since there's several different schemes for handling this type of problem.

 

[CakeOrDeath?]

Another thing worth noting is that you'll have to turn the artificial gravity off whenever you want to maneuver the craft. Otherwise huge stresses will be generated.

This is actually a great bit of info for my project. Thank you very much!

 

[CakeOrDeath?]

Another question for you guys. Is it feasible to rotate an entire fuselage toward the same end, or is there a plausibility concern there?

Bearing in mind that it is science-fiction.

 

[Simon Bridge]

Another question is how to get it started. You'll have part of the craft rotating one way and the other part with opposite spin. And turning it on and off is expensive.

Of course, you can just ignore all these problems if you like. People have seen it in 2001, so it is traditional.

Clark gave Discovery a flywheel for spinning up and spinning down the living quarters. Presumably the initial spin-up was wrt to the flywheel ... stations can be spun up via rockets about the rim plausibly removing the need for a counter-rotating section (though they'd be handy).

There are a lot of problems with rotation for gravity and any engineer will have to do a cost-benefit analysis at some point. We have a decent history of space exploration now and techniques for coping without gravity so what is the problem that is supposedly being solved here? In TV and movie sci-fi it is hard to simulate free fall but you can simulate 1g artificial gravity by using the real thing for free.

Anyway - as far as the original question is concerned I believe a low-math approach has been supplied.

 

[Simon Bridge]

Another question for you guys. Is it feasible to rotate an entire fuselage toward the same end, or is there a plausibility concern there?

Bearing in mind that it is science-fiction.

That is technically feasible - you'd have a cylindrical-symmetry ship with the main thrusters on the axis. Cherryh had something like that for the battlewagons in Downbelow Station (they had an exostructure but were mostly rotating cylinder). Technically, Niven's Ringworld was a huge one and for that matter so is the Earth-Sun system (though rotation don't supply gravity there). You still have the angular momentum conservation issues with spin-up and manovering. Think of any of the SF rotating space stations with engines at the hub.

 

[Chronos]

Acceleration is a derivative of velocity. As such, it is irrelevant in relativity. See the 'clock paradox' for further discussion.

 

[Simon Bridge]

Acceleration SR and GR?
http://math.ucr.edu/home/baez/physics/Relativity/SR/acceleration.html

 

[CakeOrDeath?]

That is technically feasible - you'd have a cylindrical-symmetry ship with the main thrusters on the axis. Cherryh had something like that for the battlewagons in Downbelow Station (they had an exostructure but were mostly rotating cylinder). Technically, Niven's Ringworld was a huge one and for that matter so is the Earth-Sun system (though rotation don't supply gravity there). You still have the angular momentum conservation issues with spin-up and manovering. Think of any of the SF rotating space stations with engines at the hub.

A cylindrical fuselage is what I was thinking. I'll look into the Niven and Cherryh, thanks.

 

[ImaLooser]

A cylindrical fuselage is what I was thinking. I'll look into the Niven and Cherryh, thanks.

I just spent an hour writing a thoughtful reply. Because of my "inactivity" physicsforums logged me out and threw away the reply. Maybe some other day. Anyway, its all about precession.

 

[Simon Bridge]

http://en.wikipedia.org/wiki/Precession
The rotation is a factor when you want to maneuver.
How they deal with it will be a nice bit of detail for a SF story.

 

[DrGreg]

I just spent an hour writing a thoughtful reply. Because of my "inactivity" physicsforums logged me out and threw away the reply.

Next time you log in, ensure the "Remember Me?" check box is checked. (Or if you object to that for some reason, prepare your answer offline and paste it in when finished.)