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Old 25th July 2011, 03:51 PM   #16 (permalink)
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Re: Moving from gravity to zero g in a starship

The only problem with having the entire ship rotate that I can see is in turning corners, from the gyroscopic effect, and I'm sure that could be compensated for (no, not in "grab the steering wheel and pull us out of the line of that planetoid" space opera, but in "in eighteen months and two days our primary thrust axis needs to be 2.07° closer to Polaris").

I would hate to design an air-tight, frictionless bearing/seal which would last – oh, decades at least with the sort of mission lengths a craft like this could expect, and really difficult to bring to a halt for repairs. (Frictionless suggests magnetic bearings; I wonder if you could shape the field so it also held a ferrofluid layer in place solidly enough to resist not merely atmospheric pressure, but the extreme peaks of molecular displacement, and make a ferrofluid that resisted vacuum long enough?) Otherwise you could run a central shaft down the entire ship (structural and your main material conduits), thickened up where the outer skin is missing, maintaining rigidity. The bars leading from this central pillar to the rotating section might be a bit of an inconvenience to anyone trying to get along the ship at speed, but, if you build big enough, your carrousel only needs to turn at the same speed as a rotating restaurant; say, once every two hours. And that means your "holes that line up" biting your ankles off problem is a lot less than you've considered, too.

The main argument against propulsion gee forces is that you are expelling reaction mass continuously, rather than freewheel most of the trip (oh, and energy, too, but a decent energy source is essential to the project from the start) and, for more than about a hundredth of a g, an appreciable fraction of the vessel's total mass. Not easy to replace, then you spin the ship round mid voyage and go straight on to deccelerating just as hard…
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Old 25th July 2011, 04:25 PM   #17 (permalink)
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Re: Moving from gravity to zero g in a starship

Thanks, chris, for all the info. On this point...

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The bars leading from this central pillar to the rotating section might be a bit of an inconvenience to anyone trying to get along the ship at speed, but, if you build big enough, your carrousel only needs to turn at the same speed as a rotating restaurant; say, once every two hours. And that means your "holes that line up" biting your ankles off problem is a lot less than you've considered, too.
...that would mean a person is trapped in the rotating sections most of the time, if e.g. the door constitutes only 10% of the circumference, leaving a small (metaphorical and literal) window for entrance and exit. At the moment that probably doesn't suit my purposes, but it opens up new possibilities...

"The hatches won't line up for another two hours. And the bomb's going off in thirty minutes."

"Suit up, boys. We're going EVA."
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Old 25th July 2011, 08:30 PM   #18 (permalink)
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Re: Moving from gravity to zero g in a starship

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Originally Posted by chrispenycate View Post
The only problem with having the entire ship rotate that I can see is in turning corners, from the gyroscopic effect, and I'm sure that could be compensated for (no, not in "grab the steering wheel and pull us out of the line of that planetoid" space opera, but in "in eighteen months and two days our primary thrust axis needs to be 2.07° closer to Polaris").
There's a great deal of room between those two extremes, Chris, which makes it harder for those of us less well informed than you to know what these gyroscopic effects might be, how tolerable they are and how to match an acceptable environment with the need to manoeuvre.

Basically, how manoeuvrable can a rotating ship be, given near-g gravity on-board? Does it depend on the diameter of the habitat zone (and thus the angular velocity) or are things not so simple? Is simultaneous acceleration/deceleration a big factor?

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I would hate to design an air-tight, frictionless bearing/seal which would last – oh, decades at least with the sort of mission lengths a craft like this could expect, and really difficult to bring to a halt for repairs. (Frictionless suggests magnetic bearings; I wonder if you could shape the field so it also held a ferrofluid layer in place solidly enough to resist not merely atmospheric pressure, but the extreme peaks of molecular displacement, and make a ferrofluid that resisted vacuum long enough?) Otherwise you could run a central shaft down the entire ship (structural and your main material conduits), thickened up where the outer skin is missing, maintaining rigidity. The bars leading from this central pillar to the rotating section might be a bit of an inconvenience to anyone trying to get along the ship at speed, but, if you build big enough, your carrousel only needs to turn at the same speed as a rotating restaurant; say, once every two hours. And that means your "holes that line up" biting your ankles off problem is a lot less than you've considered, too.
My doubts about the effectiveness of such bearings led me away from having them and towards whole-ship rotation.
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Old 27th July 2011, 01:47 AM   #19 (permalink)
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Re: Moving from gravity to zero g in a starship

Me too Ursa, but I still have concerns about mounting and controlling directional thrusters when the whole hull is rotating. Each thruster is only momentarily facing in the right direction giving wobble needing many thrusters to avoid that.
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Old 27th July 2011, 09:13 AM   #20 (permalink)
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Re: Moving from gravity to zero g in a starship

On ships of the size required to avoid or minimise the adverse consequences of the Coriolis effect the (mine rotate at just over 2rpm maximum, meaning that they must be at least 400m in diameter), the use of multiple thrusters is not an issue. (Given the masses involved, the power required to change vector is more of a problem.)

I've relied () on this Wiki article: http://en.wikipedia.org/wiki/Artific...avity#Rotation. It includes the following paragraph:

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The Coriolis effect gives an apparent force that acts on objects that move. This force tends to curve the motion in the opposite sense to the habitat's spin. Effects produced by the Coriolis effect act on the inner ear and can cause dizziness, nausea and disorientation. Experiments have shown that slower rates of rotation reduce the Coriolis forces and its effects. It is generally believed that at 2 rpm or less no adverse effects from the Coriolis forces will occur, at higher rates some people can become accustomed to it and some do not, but at rates above 7 rpm few if any can become accustomed. It is not yet known if very long exposures to high levels of Coriolis forces can increase the likelihood of becoming accustomed. The nausea-inducing effects of Coriolis forces can also be mitigated by restraining movement of the head.
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Old 27th July 2011, 01:20 PM   #21 (permalink)
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Re: Moving from gravity to zero g in a starship

That's interesting Ursa - I'm considering something much more massive though

My thoughts are on a nomad civilisation taking potentially centuries between stars. They would have a fleet of "ships" that would be hollowed out asteroid habitats with very large (very low acceleration) motors, with slow spins on their axis. Direction changes would be very slow taking maybe years to complete and I even consider that it might be easier to put engines at either end rather than try and flip the whole asteroid to switch to deccelration. Actually they would try to avoid anything other than very minor direction changes of direction using (big) thrusters and would rather use stars or planets to get their trajectory (and initial velocity) set up for the next interstallar crossing. I kind of imagine picking up fuel at star "stop overs" and literally coating the entire asteroid in a deep layer of ice from comets/asteroids in that solar system. Bear in mind these habitats would be their home (their orignal star system is no more) and are intended to last virtually indefinitely.

Actually thinking about it maybe on that sort of scale it would be easier to have very slight adjustments to the main motor affect direction changes rather than use directional thrusters. Possibly through magnetic/force fields of some sort rather than actual physical movement of the motor.
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Old 27th July 2011, 02:13 PM   #22 (permalink)
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Re: Moving from gravity to zero g in a starship

This is a very interesting thread. My immediate thought was a bearing arrangement, but thats already been mentioned and its not insubstantial problems discussed. I had another idea, but its slipped. I'm trying to remember it while I type but it is not working.

Now here is a very strange suggestion - lets call it a "rotor articulated personnel shaft":

How about if you could have a segmented elevator shaft that bends and flexes, closing for its switch between two or more doors on opposite sides of the centre? so they get in, move towards the hub, then wait as it swings to the next door in its circuit? so it stays connected to one door long enough for a person to exit or enter, (as the tube bends with the rings turning - it's fixed to the ring but moves around the hub, but it STOPS at the exits for a time) and as the ring hits a set point rotates to catch up with the spokes? a third of the circuit if it was three spokes a quarter if it was four etc? Obviously hub diameter and ring speed would effect the viability a hugs amount.

Still can't remember the suggestion I started out with... if it comes back to me I'll repost. great thread tho!
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Old 27th July 2011, 02:57 PM   #23 (permalink)
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Re: Moving from gravity to zero g in a starship

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This is a very interesting thread. ..... great thread tho!
And my biggest worry before posting this was that it would reveal my thickness. I'm glad to see that it appears people had to think before suggesting (excellent) ideas, or had had to deal with similar issues themselves.

Thanks all.
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Old 29th July 2011, 02:34 AM   #24 (permalink)
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Re: Moving from gravity to zero g in a starship

Well, the simplest design is to just rotate the whole ship. The second simplest is to have a single rotating section. That's right, just one. I'll get back to this but there are some problems that have to be overcome:
  • The seals between the two sections must be air tight and have very low friction at the same time.
  • Since the main engines are on the spindle, the bearings must provide rotation and transfer the thrust of the engines to the wheel or it will be left behind when the engines are used.
The two counter-spinning sections design has a big problem. When you turn the ship, a gyroscopic force at right angles to the apply force is produced. With two counter-spinning sections, they'll produce equal and opposite forces but these forces must be transfer to the spindle where they meet and cancel each other out. In other words, where the spindle meets a wheel (or its hub), it must be strong enough to transfer this force from the wheel to the spindle. And the part of the spindle between the wheels must be strong enough to transfer these forces along its length so they can meet and cancel each other out.

Two changes to your design should be made.

One, the main engine reduced in size and at least two engines are placed on each wheel. When you want the ship to accelerate, all engines fire at once but they only provide enough force to accelerate the part they are attached to. They are carefully control so that the acceleration of each part is the exact same during the burn and the forces transferred between them is minimized.

Two, each part gets its own manoeuvring engines and when the ship is turned, each is carefully controlled so all the parts turn in the same direction, at the same time. This, again, reduces the forces between them.

Or, you can simply have the whole ship spin.
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Old 7th August 2011, 10:45 AM   #25 (permalink)
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Re: Moving from gravity to zero g in a starship

Quote:
Actually thinking about it maybe on that sort of scale it would be easier to have very slight adjustments to the main motor affect direction changes rather than use directional thrusters. Possibly through magnetic/force fields of some sort rather than actual physical movement of the motor.


You can turn the ship, as long as you’re in no hurry, using the same gyroscopic effect that is tending to hold it stable on course, making it difficult to steer.
Somewhere near the centre of gravity of the vessel (non-critical, but mechanically simpler) we mount a frame supporting two massive flywheels, their axes at right angles to each other and both to the principal axis of rotation of the ship. The bearings have to be good; several tons of force will need to be transferred through them, without effecting their constant rotation. Then we spin up the entire structure in the opposite direction to the spin, so the framework is stationary relative to the inertial frame of reference.
Now, if we spin up one of the wheels, the nose will start to swivel; not in the obvious direction, as the entire ship is rotating, which shifts the direction of the force, but it’s calculable. As long as the flywheel keeps spinning, the principal axis of the ship (along which the thrust of the main motors acts) will keep on slowly moving, obeying Newton; when the energy is pulled back out, and the wheel stops, the ship will continue on its new vector, with minimal loss of energy, and no use of reaction mass as would be the case if you used thrusters to steer.
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Old 7th August 2011, 01:19 PM   #26 (permalink)
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Re: Moving from gravity to zero g in a starship

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You can turn the ship, as long as you’re in no hurry, using the same gyroscopic effect that is tending to hold it stable on course, making it difficult to steer.
Sorry, but it's easy to steer. The same principle is currently used in helicopters. Just apply the tilting forces 90° in front of the way you want it to tilt.
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Old 7th August 2011, 07:10 PM   #27 (permalink)
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Re: Moving from gravity to zero g in a starship

My understanding was that helipcopters steered by altering the angle of its blades; I don't see how that can be applied to a spaceship in vacuum.

Chris, is that the same sort of thing as proposed for turning O'Neill cylinders (http://en.wikipedia.org/wiki/O%27neill_cylinders) to keep them end on to the sun as they orbit? What sort of mass would those flywheels have to be relative to the mass of the craft for a reasonable (slow) turn?
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Old 7th August 2011, 07:36 PM   #28 (permalink)
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Re: Moving from gravity to zero g in a starship

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My understanding was that helipcopters steered by altering the angle of its blades; I don't see how that can be applied to a spaceship in vacuum.
Because the force to alter the angle of the blades must be applied 90° in front of where you want them altered. To tilt a rotating wheel on a spaceship, you must applied the tilting forces 90° in front of the direction you want it tilted. The solutions to each problem are very similar.
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Old 7th August 2011, 09:03 PM   #29 (permalink)
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Re: Moving from gravity to zero g in a starship

No, it looks as if they're using the gyrostabilisation of the rotating cylinders themselves for minor corrections, not full two dimensional realignment.

If you can put the majority of the mass in the outer ring, spin it up to twenty or thirty rpm (which should be easy enough) and make the diameter a reasonable fraction of the diameter of the ship (not the life support ring), say half to two thirds, you should be able to get by with about one percent of the total ship mass. A very big machine, but relative to bracing the entire length for off axis forces I suspect worthwhile. I'm more worried about the forces acting through the bearing as the movement starts and stops. (You don't want to know how many approximations I made in those calculations.)

Obviously, having a constriction like this right in the middle of the ship, around which all passagewazs, service channels, information ducts, atmosphere tubes, everything that has to get from one end to the other of the ship have to be diverted is not convenient, but I'd really like to avoid applying those forces at the end of a moment arm.
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Old 7th August 2011, 09:30 PM   #30 (permalink)
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Re: Moving from gravity to zero g in a starship

So you want a starship to have gravity, but at some moment for a fight scene you want it taken away?

Since it is science fiction why not there just be some special electromagnetic engine that gives the ship gravity. Then for the fight scene it gets turned off for some reason and then there is no gravity.

Don't worry about how much tech something takes because it is starship after all. Although, I highly doubt we'll have something that will be able to get to any star in a relatively small amount of time in the 21st century. Remember, the closest star is 4 light years away. Even going at the speed of light (which isn't possible because the ship would be turned into energy) it would still take four years. That leaves only the possibility of worm holes that bend space.
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