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I Space station artificial gravity - how to spin up to speed?

  1. Jan 12, 2017 #1

    Al_

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    If a space station has artificial gravity created by spinning, how can it best be spun up to speed? Little attitude rockets could do it, but they would use up fuel, and limit your ability to change the spin rate in future. What if you had an external wheel that you spin up very fast in the opposite direction? If you did this, would it cancel out the gyroscope effect because the total angular momentum of the whole would be zero, and make it easier to orient the station with respect to the Sun, spacecraft, etc.?
     
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  3. Jan 12, 2017 #2

    Drakkith

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    Sure. This is known as a reaction wheel (though it isn't "external" since it would be inside the station). You might be able to spin one part of the station in one direction, and another part of the station in the opposite direction, with both parts exerting torque on each other to spin up to speed.
     
  4. Jan 12, 2017 #3

    Al_

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    Thanks. I am thinking that the wheel could be mounted outside the station on an axle.
    Maybe it could contain some kind of payload or machinery, to avoid having to carry extra mass.
    But it would be difficult to connect to stuff spinning round that fast.
    Maybe magnetic bearings could be used to eliminate vibration?
     
  5. Jan 12, 2017 #4

    Drakkith

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    You can mount it inside or outside. It doesn't really matter.

    Sure. You can make the reaction wheel a large module or room that is spun up in the opposite direction as the rest of the station. This is similar to what I said above with having separate parts of the station spinning in different directions.

    I don't know what kind of bearings they would use in space. Magnetic bearings would certainly be a possibility.
     
  6. Jan 16, 2017 #5

    sophiecentaur

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    I'm assuming that the reaction wheel diameter would have to be smaller than the ship of the suspension would get in the way of 'navigation' around the ship.
    also, it would be a much more stable arrangement if the CMs of the annular station and the reaction wheel were both in the same plane and on the common axis. But would it be a problem to have the wheel at the centre of the station? If it were to be an embarrassment (vibration and safety issues), it could always be moved away using its one on-board motors.
    I was thinking about angular momentum conservation and the big ratio between the main station diameter and that of the reaction wheel. The mass of the reaction wheel could be considered as 'lost' to the system in the same way that rocket propellant mass needs to be lost. The rotation rate would need to faster than the ship's rotation by a factor of the ratio of the Moments of Inertia of the two. Double whammy here because the inertia wheel has small diameter and it's a lot lower mass. What sort of angular velocity could be considered? It wouldn't be too hard to do some rough sums and compare the results with just having tangential thrusters. Of course, if you kept the reaction wheel on board, you could always slow the station down again when you wanted to, for free, by locking the two together again. (Regenerative braking even!!)
     
  7. Jan 16, 2017 #6

    mfb

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    A counter-rotating wheel would need a lot of mass. If it is dead mass, make it as large as possible to reduce its mass. That might limit spacecraft docking to one side. If it is a symmetric space station, with the two parts rotating in opposite directions, all those problems do not arise. Moving between the space stations would need some rotating air-tight connector, but you want such a connector for spacecraft docking anyway.
     
  8. Jan 16, 2017 #7

    sophiecentaur

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    What would be the advantage of this method, even if it could be made to work? The mass would need to be transported into orbit and even that would involve an Energy Premium.
    The tangential force needed could be very small if the spin up time could be made a matter of days or weeks. That would open up the possibility of Ion or Photon drive, with a PV (Solar) energy source.
     
  9. Jan 16, 2017 #8

    anorlunda

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    For energy purposes, I like @Drakkith 's suggestion best. Spin parts of the station in different directions. No additional mass for a wheel.

    I guess the disadvantage of that would be safety with objects or people crossing from one half to the other half. It would also be difficult to share plumbing and wiring between the parts.
     
  10. Jan 16, 2017 #9

    sophiecentaur

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    That would be a better idea than having a reaction wheel. The energy to bring a large mass all that way would surely be much more than the rotational energy the mechanism would provide.
    But quite high speeds would be involved. For 1g at the periphery and 100m radius, the speed would be over 30m/s (relative 60m/s). That could involve an exciting transition, going from one arm to the other - not something to do every day, even for a hard space crew member.
    But the spin up energy would not be (particularly) high so where's the return for such inconvenience?
     
  11. Jan 16, 2017 #10

    anorlunda

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    I thing when you consider all factors in addition to energy plus operational flexibility, the "old fashioned" thruster rockets are best.
     
  12. Jan 17, 2017 #11

    mfb

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    Which mass? If it is a second part of the space station, there is no additional mass.

    Spinning it up with thrust would be my suggestion as well, but that's not what OP was asking about.
    Huh? You would cross at the center, where the two parts are connected. Relative velocity: Negligible.

    Spinning a space station up to 30 m/s (velocity at the rim) needs ~1% of the space station mass as fuel for chemical rockets - affordable. Ion thrusters using 0.1% of the station mass would need 450 kJ/kg of station mass. At a power consumption of 0.1W/kg (ISS-like), it would need 2 months at 100% efficiency, and several months at a more realistic efficiency.
     
  13. Jan 17, 2017 #12

    sophiecentaur

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    The mass of a reaction wheel.
    We have discussed what was suggested and, as we seem to have rejected it, we are discussing alternatives. Better than just saying 'no can do', I should have thought
    Yes that makes sense. I was sort of assuming that the hub would contain the reactor and would best be avoided. But the docking of supply craft would have to be at the centre in any case. It would, of course, mean a spot of zero gravity during the transfer but I guess you could get used to anything like that pretty soon.
    Are you ignoring the fact that the rotational energy has to be supplied from some source, however you do the job? We'd need to discuss the relative efficiency of the methods and I was assuming that time would not be a high priority. Once the main masses in the station were in place, the spin up could start. Components would be 'lowered' from the central dock. Alternatively, the docking could be on the periphery - a sort of tangential approach, which could be done easily with computerised navigation. The heavy bits could be 'bringing their own' momentum with them to the periphery. (I'd need to think that through as there would be the question of balancing the supplied loads etc). Quite a sexy idea though, don't you think?
     
  14. Jan 17, 2017 #13

    mfb

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    I am not ignoring it. I tried to find some realistic estimate for the power such a station would have, and used the ISS power density. It does not matter where the energy comes from - solar power, nuclear power, and even every hypothetical future power source will all lead to the same results. If you make the power source appropriate for the station size, you can spin it up with 0.1% of the station mass within a few months. Take more propellant and you can do it faster.

    Docking at the periphery would be like docking a rocket (coming from below) to a ceiling on Earth. Possible, but unnecessarily complicated and risky if you can dock at a zero-g section.

    Humans can easily handle short periods of zero-g. They will have that in the approaching spacecraft as well. And machines are not a problem either. If they have to run in zero-g, they need special designs, otherwise just take care of all liquids and don't switch them on before they have their proper acceleration again.
     
  15. Jan 17, 2017 #14

    sophiecentaur

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    That's when you have to eject a mass. Two rotating halves avoid the need to consume mass but it may not be very relevant as it's only a one-off cost.

    You are spoiling my fun now. But would it be that risky? It could be a fail safe situation - perhaps safer, actually than aiming for the middle of the wheel. Complexity is a relative thing when you have the use of machine intelligence. The ship and wheel would always need a near-zero relative velocity for any docking and it would even be possible to use a tether (just thought of that) to grab the craft as it coasts slowly past. I have done the equivalent many times, picking up a mooring with wind and tide to deal with. Once you are hooked on, you can do things at your leisure. When you think about landing on an aircraft carrier in bad weather, the space docking is a relative piece of cake, I would reckon.
     
  16. Jan 17, 2017 #15

    mfb

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    Reaching a precise velocity is easier if you don't have your rocket running at full thrust. Tethers would still need some connection procedure at low relative velocities.

    Docking with high thrust also makes every previous mass saving effort irrelevant. A single second of 1g thrust needs ~0.3% of the spacecraft mass. And you cannot dock in a single second. The most fuel-efficient procedure would start with a high-speed (30m/s) approach towards the space station - which is dangerous already, get your course wrong and you crash into the station. Afterwards, fire your main thrusters and use the RCS system to fly in a curve together with the station until you are docked. Your spacecraft suddenly needs a strong engine, something spacecrafts rarely need. It also means docking is limited to a single place - opposite of the main engine. The space station would need additional structural strength at the connection points, would need some movable masses to keep the center of gravity balanced, and the connection points would be under strong tension. The spacecraft would have to be designed to handle tension (instead of just compression, as normally).
    No. Just no.
     
  17. Jan 17, 2017 #16

    sophiecentaur

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    I think you are misunderstanding my idea here. The visiting craft would not necessarily be contributing to the station's existing angular momentum; no acceleration of the wheel would be involved. By docking at the periphery, it would avoid the need for the station to supply the added mass with any angular momentum. Even with a zero velocity docking, the system wins out compared with docking at the centre and then being taken out to the periphery, which would require angular momentum to be supplied (to keep the rotation rate constant)
    I appreciate that some structural strength would be needed in order to deal with the balance problem. But the balance problem would apply to the delivered payload, wherever it landed. Two docking platforms, one on each side of the station could take care of imbalance. It wouldn't be too much of a stretch to arrange for the arriving craft to have two halves which could dock, one at a time awn the two opposite sides of the station - just waiting for a half revolution so they both supply the same momentum contribution. Balance would be a major consideration throughout the operation of the station, particularly during construction, so I can't see that a relative light ship arriving would be a major problem. Would a metre or so, of wobble, be a serious issue if a ship with 1% if the total mass were to land on the edge? I don't think so. I was suggesting that every consitiuent part of the station could be delivered to the station in this manner. The station could expand for the initial framwork construction phase.
     
  18. Jan 17, 2017 #17

    mfb

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    I never suggested that. I just outlined the easiest possible procedure to dock at the rotating periphery. The spacecraft has to co-rotate, and as long as it is not docked that involves firing thrusters.

    If you leave with the same mass as you join the spacecraft, docking at the center doesn't change the rotation speed. Once the station is built and spun up, there is nothing that would constantly increase the mass of the station.

    Delivered payload is lighter than a spacecraft.

    If you absolutely have to add significant mass with each arriving spacecraft, let them also add some fuel to the station that is used to keep it spinning. That will need less fuel than docking to the moving outside.
    That sounds really complicated. And I still don't see any advantage.
     
  19. Jan 17, 2017 #18

    sophiecentaur

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    Yep, I can tell. :smile:
    Let me justify the tangential docking.
    It is energy neutral. The relative speed (relative to the periphery) can be near zero as the ship is approaching the CM of the station. No 'boost' needed, to catch up the rim. Navigation can bring you to the ship at any velocity you want. When the ship leaves, it is as if it had never stopped off; it leaves at the tangential velocity and just carries on forward. The approach timing would need to be good but I really can't see that being a problem in the future (or even now). All that video game palaver of docking would not be needed. As with all other space manoeuvres, it's actually more straightforward than where wind a waves are present. The tangential speed is actually negligible compared with the orbital speed so you have pretty well exactly the same navigational issues in order to dock anywhere on the station. You are probably right that the energy involved is low enough for the fuel issue to be insignificant, though.
    If all stores, construction materials and visiting crew are to be brought to the hub, first, everything would need to be carried on an almost superfluous full capacity lift arrangement. Goods and staff arriving at the periphery would be carried round on rails (needed with both systems).
    The ring structure of the station would already be strong as it has to cope with things weighing what they do on Earth. But a resilient capture / coupling would be perfect feasible for coping with minor speed adjustments on contact. I am assuming that the station would be far bigger than ISS (that's obvious if we're talking artificial gravity) so things would be very different. It is worth while exploring all possibilities for a revolutionary (no pun intended) system.
     
  20. Jan 17, 2017 #19

    anorlunda

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    Thinking back to the OP, I think what he had in mind was spinning up using a replaceable power source (like solar) as opposed to using irreplaceable reaction mass in a rocket. In other words, not all power sources are equal in features other than energy.

    Rockets do use mass that needs an outside source to replace, but they are very versatile. Consider the case when a rotating station developed a wobble. That would be hard to correct with a torque at the central hub, whereas rockets could be aimed in any direction to correct almost anything.
     
  21. Jan 17, 2017 #20

    sophiecentaur

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    Just like in the tyre shop!
    Absolutely
     
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