Moving from gravity to zero g in a starship

Discussion in 'General Writing Discussion' started by alchemist, Jul 24, 2011.

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    alchemist

    alchemist Not on holidays

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    I am writing about a late 21st century starship, one with most of its complement in suspended animation, and I've come up against a design issue. Either this is a real problem for the engineers of the future, or I'm missing something simple.

    My design is straightforward enough. It's based on a long, slender tube (I call it the "spindle") with various sections around it. Some don't move, but four of them rotate (let's call these "wheels"), causing artificial gravity of various levels. My problem is, how does a person move from a wheel to the spindle, or vice versa?

    They step into an elevator at the rim of a wheel, at one g, and travel inward where they lose weight. The "spoke" (of the wheel) intersects the spindle at its roof (the spoke doesn't have to aim for the centre; it could aim for a little off-centre, like a bicycle wheel, but the problem remains), so the "roof" of the elevator opens and the occupant floats into the spindle. Seems easy?

    Unfortunately, for the spindle (and the ship) to have structural integrity, it must effectively be continuous from nose to tail. Where the wheel intersects it, the spindle can have a door. But the elevator roof, where it meets the spindle, is orbiting it. The occupant with the open elevator roof sees the spindle door slide past each time the wheel does a rotation. He could jump through, but risks being cut in half if he's too slow.

    The spindle can not have a "continuous" door around its circumference - what's holding the ship together then?

    Potential solutions---

    1. The whole ship rotates, so there's no difference between the spindle and wheels. I don't think this is feasible as I remember reading that the direction of spin of the wheels must be balanced i.e. one clockwise, the next counter, and so on. And whoever heard of a ship spinning through space like a corkscrew?

    2. Some sort of "smartdoor", that moves its position in the spindle wall, with the wheel's rotation, leaving constant access to the spindle. This would seem to require a fantastic level of tech, though.

    3. Generate my gravity from the engines. I've read about potential propulsion systems where this is a proposed benefit. It would mean gravity pulls everyone towards the tail end of the ship, and everywhere on board has gravity. There are two reasons I don't want to do this. One, it doesn't seem very feasible in the 21st century. Two, it takes away the scope for zero g fighting, which would be cool.

    So, any suggestions? I'd be amazed if this hasn't cropped up before. Yet it’s kept me awake at night and no amount of internet searching or diagram-making has helped, so I thought I'd ask the experts.
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    Ursa major

    Ursa major Bearly Believable Staff Member

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    I'd be more than interested in hearing of the disadvantages of having a ship corkscrewing** through space, as this is what my starships do.









    ** - Rifling might be a better word, particularly if the crew are pirates. ;):)
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    alchemist

    alchemist Not on holidays

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    I'm on my phone so can't find the thread, but it was something chrispenycate said, about the "wheels" needing to go in opposite directions. I forget the exact term, but it was akin to the ship being off-balance if they go in the same direction. Now, if I can rotate the whole ship, it's problem solved (hopefully).
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    Ursa major

    Ursa major Bearly Believable Staff Member

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    Each of my rotating starships have at least a pair of rings (matched in diameter) where gravity is provided (which differs from deck to deck, obviously). The main propulsion systems lie along and around (very close to) the ship's main axis of rotation.

    There may be problems with pods on the end of arms, which the counter-rotating scheme Chris mentioned may solve.
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    MstrTal

    MstrTal Valeyard

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    Can't the ship just rotate in sections? Forward section rotating clockwise, midsection counterclockwise and aft clockwise again? If the whole ship is segmented in its construction and designed so that each section can spin independently would there be a problem? Sort of like a kaleidoscope is a two pieced tube with one tube to look into and another to spin that holds the colored stained glass bits that tumble and change color and look pretty.
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    iansales

    iansales Active Member

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    You have the spokes connect to a hub which rotates. If the spindle must be continuous have it as a central column, so the core of the ship is composed of annular sections between hubs.

    There's nothing wrong with having the entire spacecraft rotate, incidentally. Plans for future NASA missions include variants where a pair of MPCVs rotate about each other to provide some degree of gravity.
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    alchemist

    alchemist Not on holidays

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    That's what I want it to look like, all right. What's tripping me up is, what sort of construction allows rotation AND the annular sections to stick together?

    Thanks Mstr and Ursa too. I'll look up kaleidoscopes later; perhaps it holds the key. As I said, there's a good chance I'm overlooking some very simple principle.
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    Vertigo

    Vertigo Mad Mountain Man

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    As said above if the main bulk of the ship is not rotating then you would need to have an even number of rotating sections, rotating in opposite direction.

    On the other had if the whole ship is rotating (corkscrewing as you say) then that should be fine except...

    I have considered having ships like this but the problem is with directional thrusters. There would have to be rings of these around the ship that would then have to fire sequentially in order to change your attitude and even then the change would be a little wobbly. So this would seem a little wasteful of resources and a little difficult to control.

    In the case of having rotating sections then, assuming the living area is reasonably far out from the spindle the rate of rotation would not be very fast and at the spindle you would have plenty of time to get through a door into the spindle. For safety an inner door (in your elevator) would only open whilst it is safe.

    Another approach would be to have the spokes not connecting directly to the spindle but to a hollow cylinder. The cylinder would be connected to the spindle at either end which is where the "bearings" would be. Now you would exit from your elevator into the hollow cylinder with the spindle passing through its centre and could then easily move to a door into the spindle itself. Crikey hope that's understandable, a picture would be easier...
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    MstrTal

    MstrTal Valeyard

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    I was just thinking of something along the lines of Vertigo. Connecting the Rings to the central "Spindle" cylinder could be one or two hollow passage cylinders, gantry tubes that are an integral component of both ring and spindle. If I where to draw a parallel with something take a look at a Pepsi or other soda can and see how the edge is beveled up.

    Imagine that upward bevel is a collar on the outside of the spindle on either side with a hollow center. Now attached to that collar is a tube that connects to another matching collar on the ring. Inside these tubes and collars you can place whatever you like elevators, blast doors, slideways, crew compartments, or anything you can imagine. For extra support you can always attach more pylons between your rings and spindle. Some could even be hollow to further necessitate the transfer of electrical conduits, life support systems and sanitation as well as system redundancy or sysnor arrays.
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    Vertigo

    Vertigo Mad Mountain Man

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    OK here's a rough simple picture I hope. Remember that close to the spindle you would effectively be coming out of your elecator in zero g so all you've got to do now is float over to the door in your spindle. Funnily enough MstrTal I was thinking of coke tins and the like as well! In fact it is quite liekly that you would have al sorts of stuff going on inside the cylinder. Any operations best performed in zero g could be done here as it would be so close to it as would make little difference.

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    Glitch

    Glitch #452

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    The continuous door idea isn't all that bad. The section only has to be a few metres long, then you can have the internal structure.

    If I recall correctly, the 2000 film Mission to Mars had a ship with rotating and non-rotating sections. http://www.imdb.com/title/tt0183523

    If you are only looking for zero gravity for your fight scene, you could have a problem with the engines, perhaps sabotage.
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    Nik

    Nik Speaker to Cats

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    There's another alternative...

    The hollow spokes are fastened to a toroid which wraps the central hull, and has heavy duty bearings. Fore and aft, there is a similar toroid which wraps the central hull and can be rotated to match facing hatches with the spokes' toroid. Beyond each of them, there's a similar toroid that does not rotate.

    Coming from spoke, you clamber through bulkhead hatches. Press the call button if a transfer toroid is not waiting. Match speeds, align and clamp seals, open hatches, go through, seal up, slow toroid to halt, align and clamp to stationary toroid, open hatches and go through into spine of ship...

    You have two 'transfer' toroids lest one jam or for extra speed...

    You don't *really* need those stationary toroids, but making a 'pringle-shaped' seal is orders of magnitude harder than matching plane faces.

    FWIW, if the spokes & hab are right at the bow, you could have a flexible tube connector, as seen in a SciAm 'Amateur Scientist' column, long, long ago. However, I'd rather not trust my life to such a writhing monster...

    Plan B is to spin the whole ship apart from de-spun comms antennae on a rotating collar. This is simplest way, and does not require multiple failure points...

    FWIW, though not relevant to your in-system design, my Convention's City-class starships had two spun drum habs and a stationary 'service & hangar' module arrayed around load-bearing spine. You went out the hollow axis into the drum's outrigger...
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    alchemist

    alchemist Not on holidays

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    The hollow cylinder appears to be my winner! Simple and clever, I wish I could have thought of it myself. Thanks Vertigo and MstrTal (thanks for the diagram too, Vertigo, although I understood it well enough from your description). I had planned zero g modules along the length of the spindle, so it fits neatly with what I have.

    I had thought of using just a slow rotation speed (with Glitch's elongated doors), but in real life it just seems so dangerous. Someday, someone will move too slowly and get shorn in two. Put in a safety of stopping rotation if anyone's in the doorway and it's perhaps too easy for someone to block the doorway and cause a lot of inconvenience.

    Thanks to Nik also. Clever, but a bit too elaborate for my purposes, although the flexible tube connector sounds like the ultimate playground slide.
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    Vertigo

    Vertigo Mad Mountain Man

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    I was thinking the same about someone getting the chop! And I doubt you could stop the spin in time; it would likely take hours or even days to start and stop without doing structural damage.
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    MstrTal

    MstrTal Valeyard

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    Not to mention the practicality of it. Just spit-balling here but if some sort of emergency fail safe where to kick in that effects the ships systems to that degree all sorts of other sub-systems would then by default be initiated. I mean one could safely assume that anything that effects the ships mission, propulsion, gravity, structural integrity, core systems or cryo-complement is going to set off all sorts of bells and whistles. Stopping the rotation of one of its core components and thereby effecting some or all of the ships gravity could also set off pre-programed security protocols. Might even wake up the the cryo-sleeping command staff cause an unforeseen system shut down or any number of things. One never knows. :D
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    chrispenycate

    chrispenycate resident pedantissimo Staff Member

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    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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    alchemist

    alchemist Not on holidays

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

    ...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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    Ursa major

    Ursa major Bearly Believable Staff Member

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    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?

    My doubts about the effectiveness of such bearings led me away from having them and towards whole-ship rotation.
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    Vertigo

    Vertigo Mad Mountain Man

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    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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    Ursa major

    Ursa major Bearly Believable Staff Member

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    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 :)eek:) on this Wiki article: http://en.wikipedia.org/wiki/Artificial_gravity#Rotation. It includes the following paragraph:

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