A question about a rotating space structure

In summary: Unless you use a magic fuel source that doesn't deplete over time.In summary, a station that is not symmetrical will rotate around its centre of mass.
  • #1
LokiReise
7
0
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 ?

TORQUE AND MOMENT OF INERTIA !

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 separate 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 ?
 
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  • #2
LokiReise said:
The point is to make the structure rotate in place, not translate to a different location.
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.
 
  • #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
and
F2 = τ2 / r2

To determine the clockwise tangential forces that need to be applied to get it to rotate around this point ?
 
  • #4
LokiReise said:
Oh its not symmetrical because i say so for point of argument.
Oh, OK, I thought this was maybe for a story or something. Presumably, it's more of a math exercise then?

LokiReise said:
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 ?
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.
 
  • #5
DaveC426913 said:
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.

Oh you're right ! Cruel and unimaginable irony ! LOL

So yes a purely idealized scenario... damn that's 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
HAHAHAH
j/k I don't have a garage :P
HAHAHAH IT'S EARLY lol
 
  • #6
LokiReise said:
So yes a purely idealized scenario... damn that's 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 ?
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.
LokiReise said:
By the way your scathing sarcasm just gave me a eureka
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'. ;)
 
  • #7
DaveC426913 said:
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'. ;)

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..
 
  • #8
I'm still a little unclear about your setup.
LokiReise said:
If it ran in one swift very strong impulse to accelerate the structure,
Do you mean accelerate it rotationally, or translationally?

LokiReise said:
and then stopped...
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.

LokiReise said:
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 ?
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.

LokiReise said:
I mean its in space... but somehow when I think of this system, I find myself expecting the the energy will dissipate..
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:

1. What is a rotating space structure?

A rotating space structure is a man-made structure designed to rotate in outer space, typically in low Earth orbit. It can be used for various purposes such as research, communication, and transportation.

2. How does a rotating space structure work?

A rotating space structure works by utilizing the principles of centrifugal force. This force is created by the rotation of the structure, which allows objects and people inside to experience a simulated gravity similar to that on Earth.

3. What are the benefits of a rotating space structure?

There are several benefits to a rotating space structure. It can provide artificial gravity for long-term human habitation in space, reduce the negative effects of microgravity on the human body, and enable the construction of larger and more complex structures in space.

4. What challenges are involved in building and maintaining a rotating space structure?

Building and maintaining a rotating space structure can be challenging due to the harsh conditions in outer space, including extreme temperatures, radiation, and micrometeoroid impacts. It also requires advanced technology and resources, as well as regular maintenance and repairs.

5. Are there any existing rotating space structures?

Yes, there are several existing rotating space structures, including the International Space Station (ISS) and the Chinese Space Station (CSS). These structures have been used for research, observation, and living quarters for astronauts and cosmonauts.

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