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A question about a rotating space structure

  1. Nov 12, 2014 #1
    So I was thinking to myself, if i had a somewhat disproportionate tripodal space structure, how would i make it rotate around a chosen point where the arms of the structure (hallways and passes most likely) converged ?


    Then I thought about something else, obviously if i applied rotational force to one arm, the other two would most likely form the pivot point and the actual rotation would be occurring around their center of mass.

    But what if it was only a bipodal structure, with one sides mass being precisely twice that of the side being rotated ?

    My question is, if I rotate by applying a impulse which changes direction to approximate the force of torque, vs apply two seperate but equal tangential forces on each side, does it behave the same or does translation occur for the whole structure ?

    The point is to make the structure rotate in place, not translate to a different location.

    Anyone ?
  2. jcsd
  3. Nov 12, 2014 #2


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    To be clear, regardless of the shape of the station, it will rotate about its centre of mass. The only way to get translative motion is to apply a force, either externally, or via thrust.

    I am not entirely sure of the shape of your proposed station. Can you sketch it?

    And, why is it not symmetrical? Your engineers are in for a world of hurt trying to keep it stable and in one piece.
  4. Nov 12, 2014 #3
    Oh its not symmetrical because i say so for point of argument.

    Second, given it does not have a stationary pivot point, the reason i asked is because i imagined applying thrust from two different points of the extensions.

    So always around the center of mass ?

    no matter where the force is applied ?

    Could I not say....

    And say, I want the angular acceleration of
    Where I want the rotation point to be at 20 meters on an arm which is 120 m long with masses M1 and M2

    τ1 = F1 r1
    τ2 = F2 r2
    τ = Iα

    Where r1 = 100 m
    and r2 = 20 m

    and say τ = mr2α

    and then say I = M1r1 + M2r2
    and say I wanted α= 20° π/180

    and say τ1 = M1r12α
    and say τ2= M2r22α

    and then say to make the object rotate around the exact point of interest....

    F1 = τ1 / r1
    F2 = τ2 / r2

    To determine the clockwise tangential forces that need to be applied to get it to rotate around this point ?
  5. Nov 13, 2014 #4


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    Oh, OK, I thought this was maybe for a story or something. Presumably, it's more of a math exercise then?

    No, I was assuming a station that did not have a magical fuel source that could run continuously without depletion. (Any practical fuel source will deplete over time, resulting in a change in both mass and centre of mass).

    So, you're looking at a purely idealized scenario then.
  6. Nov 13, 2014 #5
    Oh you're right ! Cruel and unimaginable irony ! LOL

    So yes a purely idealized scenario............ damn thats annoying.. the whole center of mass changing part due to fuel loss........ grrr.......... wouldn't that only offset it slightly ? I mean given the relative size of all things involved ?

    By the way your scathing sarcasm just gave me a eureka :p A BIG Eureka LOL I never thought of that.

    It actually was for a game btw. So idealized works.

    Well that and my intercontinental ballistic missile I have in me garage :P
    j/k I don't have a garage :P
  7. Nov 13, 2014 #6


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    Well, it would lose mass in proportion to how much mass it expelled to provide thrust (depending on your engine type). If it's running continually, then that 'slight' mass would eventually become 'most' mass.

    And the stresses on the station would also be factored to the amount of eccentricity. After a while your station's occupants would feel a notice rock and roll and it would eventually shake itself to pieces. Unless of course, you had carefully distributed fuel storage tanks, and bled them carefully.

    But for an asymmetrical structure, the engineering to build and then keep all this stable would skyrocket.

    But upon re-reading, I can see how it might have read as if sarcastic.

    Apologies. It was not meant to be sarcastic. In an idealized scenario, there is nothing wrong with magical elements. Many idealized physics problems involve frictionless surfaces, zero air resistance and spherical chickens of uniform density.

    This is why I was trying to determine the nature of your question as to whether it was practical or ideal.

    Maybe I could have used the word 'idealized' instead of 'magical'. ;)
  8. Nov 13, 2014 #7
    No worries about the sarcasm :P Intentional or otherwise, I saw it as an excuse to banter a little heheheh besides it did help me recognize a considerable issue in a practical application of these principles. You seem like a very smart guy.

    If it ran in one swift very strong impulse to accelerate the structure, and then stopped.... would this create a sort of ideal rotational inertia (it would just stay in motion until something ran into it, or until retro thrusters fired) or would the effect wear off, and the momentum die down ? I mean its in space.... but somehow when I think of this system, I find myself expecting the the energy will dissipate..
  9. Nov 14, 2014 #8


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    I'm still a little unclear about your setup.
    Do you mean accelerate it rotationally, or translationally?

    If you stop the thrust, it will immediately begin rotating about its centre of mass. The only way to have it rotate about another point is to keep the thruster firing in perpetuity.

    This is where it's getting murky.

    I think what you're trying to suggest is that, if you fire some thrusters to start the station rotating about some arbitrary point, you can turn off the thrusters once it's spinning and hope that the station will continue, at least for a while, to rotate about that point.

    It will not. The moment you turn off the thruster, rotation will occur about the centre of mass.

    The effect will cease immediately.

    The thrust is not imparting a rotation on the station, it is acting to accelerate a (part of a) body already rotating. It is destabilizing it. This is why it might be quite a strain on the station, as well as very difficult to maintain.

    Think of a simpler system - just a plain ol' rocket engine firing in a straight line. As long as the engine is firing, the rocket is accelerating (just like the point on your station where the thruster is attached). The moment you turn off the rocket engine, does to rocket continue to accelerate, only dissipating its acceleration over time? No. Engine off = acceleration zero.

    Same with the station: engine off = thruster attachment point acceleration drops to zero = rotation about CoM.
    Last edited: Nov 14, 2014
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