Celestia Forums

Got a question about spaceship design, I'm hoping someone here might know the answer:

One of my friends had an idea for a spaceship that used rotating cylinders to create gravity. However, the idea was that the ship itself is a flat slab, with two rotating cylinders embedded within it. Both cylinders are oriented with the long axis parallel to the direction of motion of the ship, symmetric around the centreline of the slab hull. And finally the cylinders rotate in opposite directions to eachother. Like this:



My question is whether this setup would actually work. The idea is that if the ship starts accelerating then the cylinders stop spinning and gravity acts according to the acceleration direction of the ship.

I'm suspicious about this, wouldn't it be better to have a single spinning hull (a la O'Neill station) or a rotating ring setup?

Malenfant wrote:My question is whether this setup would actually work. The idea is that if the ship starts accelerating then the cylinders stop spinning and gravity acts according to the acceleration direction of the ship.

Well, here's what WOULD work: The rotating of the inner cylinders would create the impression, to whoever was standing on the inside, that the were being pressed into the "floor" just like gravity (with two caveats: one, the opposite is actually occuring in that it is the floor that would be pressing into occupants and two, things might be wobbly for them depending on the circumfrence of the rotating cylinder - bigger being better).

Secondly, sure, once the ship started accelerating, whoever was on the inside of the cylinder would, again, be pushed into the floor in a gravity-like way.

The problem?

Each acceleration (the rotation and the forward movement) would be "pushing" the occupant into two completely DIFFERENT "floors". Even worse, the combination of the two - depending on the rate of the accerlation and speed of rotation - would create an unending combination of "floors".

So pratically, at one moment the floor could be "outward" of the cylinder. At another, the floor would be the rear end of the ship. Or any combination in between.

Please note, however, I am not an expert in these matters. However, hopefully, I did the "heavy lifting" of answering your question (and it is accurate) - so that someone truly qualified could just weigh in and say, "Yes."

Well, I was already aware that you couldn't have the cylinders spinning while the ship was accelerating for precisely the reasons you state.

My question was really more along the lines of whether such a setup would (a) be practical and (b) be actually stable physically.

Malenfant wrote:Well, I was already aware that you couldn't have the cylinders spinning while the ship was accelerating for precisely the reasons you state.

My question was really more along the lines of whether such a setup would (a) be practical and (b) be actually stable physically.

lol... I actually was kinda figuring you already knew that. But as for A...well, "practical" is an interesting word - since we're not even able to get humans into orbit AT ALL at the moment. As for b... as long as the cylinders were rotating in opposite directions, why not?

If it helps, I think this setup may be referred to as a 'hamster cage' setup. I'm still trying to research this though.

By 'practical' I guess I meant "does this offer any advantages (or disadvantages) over other artificial gravity generation methods like spun hulls or spin capsules or rotating rings?". But yeah, I'm starting to think that maybe this method isn't so improbable from a physics standpoint.

Counter-rotating rings with equal mass can (in principle) be spun up and down simultaneously without inducing rotation into the surrounding framework. A single spinning wheel either requires the entire ship to spin rigidly (rotation induced by thrusters) or for the framework to rotate in the opposite direction (rotation induced by forces between the two).

Any attempts to do course corrections while the rings are rotating will cause the system to precess, complicating things substantially. It'd be much simpler to stop all rotations before making corrections. Thrust parallel to the axis of rotation shouldn't cause precession, but getting such things perfectly right is unlikely.

And what happens when one of the bearings freezes up?

selden wrote:Any attempts to do course corrections while the rings are rotating will cause the system to precess, complicating things substantially. It'd be much simpler to stop all rotations before making corrections. Thrust parallel to the axis of rotation shouldn't cause precession, but getting such things perfectly right is unlikely.

I'm currently developing a solution to this problem involving my O'Neill cylinders. I'll leave it as an exercise for the reader as to exactly how I will do this. I will post the result when complete so watch this space: http://celestiaproject.net/forum/viewtopic.php ... highlight=

selden wrote:A single spinning wheel either requires the entire ship to spin rigidly (rotation induced by thrusters) or for the framework to rotate in the opposite direction (rotation induced by forces between the two).

Ouch, didn't think of that! Though I'm not entirely sure what you're referring to as the framework here. You mean there'd have to be an inner core of the ship along the rotation axis of the ring that would have to be spinning?

And obviously, the rotation has to stop if the ship is undergoing thrust/acceleration.

Malenfant wrote:

selden wrote:A single spinning wheel either requires the entire ship to spin rigidly (rotation induced by thrusters) or for the framework to rotate in the opposite direction (rotation induced by forces between the two).

Ouch, didn't think of that! Though I'm not entirely sure what you're referring to as the framework here. You mean there'd have to be an inner core of the ship along the rotation axis of the ring that would have to be spinning?

And obviously, the rotation has to stop if the ship is undergoing thrust/acceleration.

Selden's just talking about a fundamental rule of physics.... "For every action there's an equal and opposite reaction."
Think about how you're going to spinup your cylinder. Whatever it's attached to must as a result rotate in the opposite direction. There's no way around this fundamental fact.

Chuft-Captain wrote:Selden's just talking about a fundamental rule of physics.... "For every action there's an equal and opposite reaction."
Think about how you're going to spinup your cylinder. Whatever it's attached to must as a result rotate in the opposite direction. There's no way around this fundamental fact.

Well, there's an obvious way around it that I can see: The rotating parts could easily be made to revolve independently of the main body of the ship. I'm not quite seeing how that alone would somehow force the rest of the ship to rotate the other way.

I think counterbalancing is required for gyroscopic purposes - which I think means that if you're not counterbalanced then the ship will tend to veer in the direction of rotation all the time. That's not related to Newton's Third Law as far as I know.

Malenfant wrote:...if you're not counterbalanced then the ship will tend to veer in the direction of rotation all the time

No, that's not right. The nature of a gyrosope is that it will always try to point in the same direction (which is why they are used for navigation and attitude control in spacecraft, satellites, etc)
If you try to alter the pointing of a gyroscope, it will cause the precession which Selden referred to (at a 90 degree angle to the force)
Try this at home by taking the front wheel off your bicycle and spinning it on the end of your finger.

Hey guys,
Author C. Clark used a cylindrical ship in his Rama series. I believe the cylinders were about a mile wide by 50 miles long, but I could be wrong about those dimensions. More then big enough to prevent the wiggles and more than roomy enough to inhabit. Now figuring on how to light the whole interior might be an issue. Own and moving the whole thing would be another one.

Don. Edwards

John Dollan designed a freight ship with a small rotating centrifuge; here is my model of it

Don. Edwards wrote:Author C. Clark used a cylindrical ship in his Rama series. I believe the cylinders were about a mile wide by 50 miles long, but I could be wrong about those dimensions. More then big enough to prevent the wiggles and more than roomy enough to inhabit. Now figuring on how to light the whole interior might be an issue. Own and moving the whole thing would be another one.

RAMA statistics:
Spaceguard catalogued object 31/439 according to the year and order of it's discovery.
Length: 50km
Diameter: 20km
Period: 4 minutes
Mass: At least 10 trillion tons

FYI: I am not the alien who designed it.

eburacum45 wrote:John Dollan designed a freight ship with a small rotating centrifuge; here is my model of it

So what is rotating there? The arms with the two boxes at the front presumably are rotating around the central axis of the ship, but from what Selden said it sounds like the rest of the ship has to rotate the other way to counterbalance that and avoid gyro effects?

The ship in the Red Planet movie was based on this concept:



IIRC, it seems to me that the exploration ship in the follow-up movie to 2001 a space odyssee was also based on some kind of "rotating arms" principle.

The spaceship "Alexei Leonov" in 2010 : Odyssey II was based on the same concept of John Dollan's freight ship. It may have a system for counterbalancing the rotation on the arms...

Don't forget that a lot of movie "physics" is bogus. Actions are portrayed in a certain way because they look good, not because they're realistic. Often good SF productions include a science advisor, but a good image will win out over realism every time. e.g. sounds in space and banked, swooping turns by fighters in Star Wars.

selden wrote:Don't forget that a lot of movie "physics" is bogus. Actions are portrayed in a certain way because they look good, not because they're realistic. Often good SF productions include a science advisor, but a good image will win out over realism every time. e.g. sounds in space and banked, swooping turns by fighters in Star Wars.

Sure,
it just seems that the concept of counter-rotating arms or wheels has always been popular in Sci-Fi.
After all, it is also in a way used in the real world to control rotor torque by some helicopters (no need for an additional tail lateral propeller) and attack nuclear submarines for obvious reasons given the power of the propulsion system (thousands tons displacement accelerating like a Ferrari, well almost!).
About sound, Star Wars would be utterly boring without. And aerodynamic flight is probably more instinctive and elegant (and good for the pilot to absorb Gs).

selden wrote:Don't forget that a lot of movie "physics" is bogus. Actions are portrayed in a certain way because they look good, not because they're realistic. Often good SF productions include a science advisor, but a good image will win out over realism every time. e.g. sounds in space and banked, swooping turns by fighters in Star Wars.

The new Battlestar Galactica still has sound in space but they've finally done away with the swooping turns. However, the turns they do take with directional rockets would most likely rip the pilots apart. Ahh, scifi.