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Ringworlds, Culture Orbitals and Halo Arrays are also open-faced. Centripedal force? How tall do atmosphere retaining walls on rotating space habitats need to

Ringworlds, Culture Orbitals and Halo Arrays are also open-faced. Centripedal force? How tall do atmosphere retaining walls on rotating space habitats need to be? https://worldbuilding.stackexchange.com/questions/119739/how-tall-do-atmosphere-retaining-walls-on-rotating-space-habitats-need-to-be https://worldbuilding.stackexchange.com/questions/119739/how...

ncmncm 5y ago
Short answer: really tall. 100 km is not enough.
marcosdumay 5y ago
I doubt it's possible. It doesn't matter what frictionless calculations say, the air will climb through any wall you create until it gets out of the station.
- hermitcrab 5y ago
  The atmosphere falls off exponentially (IIRC). No matter how high the wall is there will be some loss as energetic molecules reach the top of the wall and fall out of the side. But the loss would be very small for a wall that is 100km+ high as very few air molecules will be energetic enough to get that high. According to the stackoverflow entry above, a 10km high wall on a ringworld would leak about half the atmosphere every century. Obviously the wall would have to be airtight.
  - jandrese 5y ago
    Half of the atmosphere from a Ringworld is a stupendous amount of gas. 100 years is basically nothing on the timescales you need to think about when building a Ringworld. You might be able to slow down the loss of gas by putting lips on the tops of the walls and maybe even air jets blowing downward though. Or leave it mostly closed except for a few openings for spacecraft to enter or leave. Also, working out the practical considerations for a Ringworld is an exercise in futility anyway. You are already well beyond practical when you start building one.
  - marcosdumay 5y ago
    > The atmosphere falls off exponentially (IIRC). Yeah, with constant gravity and no friction. 100km isn't enough because no height is ever enough, because friction exists. Any wall will have a flow of air near it going away into space, as a result, the pressure will fall much slower than exponentially.
  - hermitcrab 5y ago
    >Half of the atmosphere from a Ringworld is a stupendous amount of gas Yes. So you would need to build it multiple of 10km. Assuming the pressure falls of 75% for each 10km (which seems about right from a quick glance at some tables), if it was 100km high the loss would be 0.25^10 of what it would be at 10km. So 100km wall would be ~million times less loss than a 10km wall.
  - hermitcrab 5y ago
    >Yeah, with constant gravity and no friction. The gravity would be constant if the rotation was constant. No sure what you mean by 'no friction'. Friction against the wall?
    - marcosdumay 5y ago
      > The gravity would be constant if the rotation was constant. Hum... No. The gravity reduces linearly with height. And yes, friction against the wall.
      - hermitcrab 5y ago
        I was referring to gravity being constant at the surface over time. I'm not convinced friction with the wall is going to result in any additional air loss. There is no 'upward' (towards the top of the wall) force component.
        ncmncm 5y ago
        You keep the walls in permanent shadow. Convection keeps the air near them moving downward.
        marcosdumay 5y ago
        > There is no 'upward' (towards the top of the wall) force component. Friction ensures that whatever air reaches a high altitude near a wall tends to stay at that altitude and spread again on the lower gravity. The difference in pressure between this higher place and a lower one bias the movement up, there is no upward force component on the dynamics of any few components you can choose, it's emergent.
        hermitcrab 5y ago
        I haven't heard of this effect. Do you have a reference?

ncmncm 5y ago

Short answer: really tall. 100 km is not enough.

marcosdumay 5y ago

I doubt it's possible. It doesn't matter what frictionless calculations say, the air will climb through any wall you create until it gets out of the station.

hermitcrab 5y ago
The atmosphere falls off exponentially (IIRC). No matter how high the wall is there will be some loss as energetic molecules reach the top of the wall and fall out of the side. But the loss would be very small for a wall that is 100km+ high as very few air molecules will be energetic enough to get that high. According to the stackoverflow entry above, a 10km high wall on a ringworld would leak about half the atmosphere every century. Obviously the wall would have to be airtight.
- jandrese 5y ago
  Half of the atmosphere from a Ringworld is a stupendous amount of gas. 100 years is basically nothing on the timescales you need to think about when building a Ringworld. You might be able to slow down the loss of gas by putting lips on the tops of the walls and maybe even air jets blowing downward though. Or leave it mostly closed except for a few openings for spacecraft to enter or leave. Also, working out the practical considerations for a Ringworld is an exercise in futility anyway. You are already well beyond practical when you start building one.
- marcosdumay 5y ago
  > The atmosphere falls off exponentially (IIRC). Yeah, with constant gravity and no friction. 100km isn't enough because no height is ever enough, because friction exists. Any wall will have a flow of air near it going away into space, as a result, the pressure will fall much slower than exponentially.
- hermitcrab 5y ago
  >Half of the atmosphere from a Ringworld is a stupendous amount of gas Yes. So you would need to build it multiple of 10km. Assuming the pressure falls of 75% for each 10km (which seems about right from a quick glance at some tables), if it was 100km high the loss would be 0.25^10 of what it would be at 10km. So 100km wall would be ~million times less loss than a 10km wall.
- hermitcrab 5y ago
  >Yeah, with constant gravity and no friction. The gravity would be constant if the rotation was constant. No sure what you mean by 'no friction'. Friction against the wall?
  - marcosdumay 5y ago
    > The gravity would be constant if the rotation was constant. Hum... No. The gravity reduces linearly with height. And yes, friction against the wall.
    - hermitcrab 5y ago
      I was referring to gravity being constant at the surface over time. I'm not convinced friction with the wall is going to result in any additional air loss. There is no 'upward' (towards the top of the wall) force component.
      - ncmncm 5y ago
        You keep the walls in permanent shadow. Convection keeps the air near them moving downward.
      - marcosdumay 5y ago
        > There is no 'upward' (towards the top of the wall) force component. Friction ensures that whatever air reaches a high altitude near a wall tends to stay at that altitude and spread again on the lower gravity. The difference in pressure between this higher place and a lower one bias the movement up, there is no upward force component on the dynamics of any few components you can choose, it's emergent.
        hermitcrab 5y ago
        I haven't heard of this effect. Do you have a reference?