4 ms·

...meaning a diameter of almost half a kilometer, or over a quarter mile-- more than four times the longest part of the current ISS, which took over 12 years an

...meaning a diameter of almost half a kilometer, or over a quarter mile-- more than four times the longest part of the current ISS, which took over 12 years and 150 billion dollars (and is essentially flat when compared to the cylinder shape needed for AG). I can see why this might be a bit of a challenge, the time and costs involved would be enormous.

iwwr 11y ago
See my reply here: https://news.ycombinator.com/item?id=10649760 https://news.ycombinator.com/item?id=10649760
pavel_lishin 11y ago
Why would you need a full cylinder? Living quarters on one side, (rocket stage) counterweight on the other, tether in between. You'd probably want all that stuff reeled in together for acceleration and deceleration, but you can unspool the whole thing while you're coasting in between.
- Xylakant 11y ago
  so how do we stop the rotation while reeling in for acceleration? Shortening the tether would accelerate the objects at the end and we'd end up with a rotating drum filled with seasick astronauts. I sincerely doubt it's that easy.
  - simoncion 11y ago
    > so how do we stop the rotation while reeling in for acceleration? Thrust against the direction of travel, but angle the thrust appropriately to maintain tension on the cable or whatever? I mean, this part sounds like it's a low-level college physics problem.
    - Xylakant 11y ago
      Thrusters require energy and reaction mass (or chemical propellants). For every course change of the spinning construct you'd need to stop the rotation, reel in the drum - now you suddenly need thrust to keep the bodies apart while reeling in, change course, reel out (requires thrust), spin up. All of that requires reaction mass and energy (or chemical rockets). On top of that you can't do work on the outside of the space ship since everything (including the space walker) would fly away. You need a double tether at least for safety, etc. Storing the propellants for the thrusters will be an issue: solid boosters won't do because they can't be stopped. Storing oxygen/hydrogen over long periods is an issue in itself. It _is_ a low level physics problem, but the practical implications are massive.
      - simoncion 11y ago
        > Thrusters require energy and reaction mass... Yes, obviously. (Unless that microwave drive from earlier this year turns out to be really for reals). > For every course change of the spinning construct you'd need to stop the rotation, reel in the drum... I'm not convinced that this is entirely true. Doing so would simplify the math required to perform the course alteration, but I expect that altering the course of the craft while it is spinning is not impossible. Regardless, as pavel_lishin mentioned in the first comment you replied to in this subthread, you can reel the tether in when making craft course changes. > On top of that you can't do work on the outside of the space ship since everything (including the space walker) would fly away. Unless -as you go on to say- you're attached to the outside of the craft. > Storing oxygen/hydrogen over long periods is an issue in itself. ...if you're going to take a long journey in space, this is one of the problems you're gonna have to have solved, no? So, I'm a little confused. We go from: > so how do we stop the rotation while reeling in for acceleration? Shortening the tether would accelerate the objects at the end... to: > Thrusters require energy and reaction mass (or chemical propellants). ... It _is_ a low level physics problem, but the practical implications are massive. You're moving the goalposts. Your first question asked "How do we maintain the speed (and -thus- the g-forces in the craft) while altering the length of the tether?". Your follow-on comments to the answer to your question say -in a nutshell- "Gosh, solving that problem would make maneuvering too hard, and is thus kinda impractical.". :/
  - pavel_lishin 11y ago
    Thrusters mounted on the rotating components would be my first guess. Or two counter-rotating tethers that can be braked against each other; the Arenauts could move to a non-rotating part of the ship for the procedure.
    - Xylakant 11y ago
      If you want to use the energy stored in the rotation you'd need stiff tethers, not a slack line - more mass, shear forces acting on the tethers, can't be reeled in. Moving to a non-rotating part of the ship requires a hub and a way to move to the hub - that would either require a space-walk up-hill against the artificial gravity (yay, climbing about a quarter of a km in space with an eternal fall if you loose grip) or a tether that's massive enough to move through - but then we're talking a very massive space ship, not about a rocket stage as counterweight.
      - ethbro 11y ago
        Why not just a small vehicle attached to climb/drop down a small diameter tether? Similar to the space elevator idea?
        Pyxl101 11y ago
        Climbing the tether will change the center of mass and also increase the rate of rotation. It's going to be complicated.
        ethbro 11y ago
        Not if you have an equivalent mass climber on the other side mirroring your actions? Admittedly, this is going to take energy to climb "up", but I'm assuming energy_to_climb << energy_to_alter_rotation.
- dweinus 11y ago
  Aside from the manueverability problem mentioned in other comments, the tether has a materials problem: it needs to hold the combined ship and couter weights at approx 1g.

iwwr 11y ago

See my reply here: https://news.ycombinator.com/item?id=10649760 https://news.ycombinator.com/item?id=10649760

pavel_lishin 11y ago

Why would you need a full cylinder? Living quarters on one side, (rocket stage) counterweight on the other, tether in between. You'd probably want all that stuff reeled in together for acceleration and deceleration, but you can unspool the whole thing while you're coasting in between.

Xylakant 11y ago
so how do we stop the rotation while reeling in for acceleration? Shortening the tether would accelerate the objects at the end and we'd end up with a rotating drum filled with seasick astronauts. I sincerely doubt it's that easy.
- simoncion 11y ago
  > so how do we stop the rotation while reeling in for acceleration? Thrust against the direction of travel, but angle the thrust appropriately to maintain tension on the cable or whatever? I mean, this part sounds like it's a low-level college physics problem.
  - Xylakant 11y ago
    Thrusters require energy and reaction mass (or chemical propellants). For every course change of the spinning construct you'd need to stop the rotation, reel in the drum - now you suddenly need thrust to keep the bodies apart while reeling in, change course, reel out (requires thrust), spin up. All of that requires reaction mass and energy (or chemical rockets). On top of that you can't do work on the outside of the space ship since everything (including the space walker) would fly away. You need a double tether at least for safety, etc. Storing the propellants for the thrusters will be an issue: solid boosters won't do because they can't be stopped. Storing oxygen/hydrogen over long periods is an issue in itself. It _is_ a low level physics problem, but the practical implications are massive.
    - simoncion 11y ago
      > Thrusters require energy and reaction mass... Yes, obviously. (Unless that microwave drive from earlier this year turns out to be really for reals). > For every course change of the spinning construct you'd need to stop the rotation, reel in the drum... I'm not convinced that this is entirely true. Doing so would simplify the math required to perform the course alteration, but I expect that altering the course of the craft while it is spinning is not impossible. Regardless, as pavel_lishin mentioned in the first comment you replied to in this subthread, you can reel the tether in when making craft course changes. > On top of that you can't do work on the outside of the space ship since everything (including the space walker) would fly away. Unless -as you go on to say- you're attached to the outside of the craft. > Storing oxygen/hydrogen over long periods is an issue in itself. ...if you're going to take a long journey in space, this is one of the problems you're gonna have to have solved, no? So, I'm a little confused. We go from: > so how do we stop the rotation while reeling in for acceleration? Shortening the tether would accelerate the objects at the end... to: > Thrusters require energy and reaction mass (or chemical propellants). ... It _is_ a low level physics problem, but the practical implications are massive. You're moving the goalposts. Your first question asked "How do we maintain the speed (and -thus- the g-forces in the craft) while altering the length of the tether?". Your follow-on comments to the answer to your question say -in a nutshell- "Gosh, solving that problem would make maneuvering too hard, and is thus kinda impractical.". :/
- pavel_lishin 11y ago
  Thrusters mounted on the rotating components would be my first guess. Or two counter-rotating tethers that can be braked against each other; the Arenauts could move to a non-rotating part of the ship for the procedure.
  - Xylakant 11y ago
    If you want to use the energy stored in the rotation you'd need stiff tethers, not a slack line - more mass, shear forces acting on the tethers, can't be reeled in. Moving to a non-rotating part of the ship requires a hub and a way to move to the hub - that would either require a space-walk up-hill against the artificial gravity (yay, climbing about a quarter of a km in space with an eternal fall if you loose grip) or a tether that's massive enough to move through - but then we're talking a very massive space ship, not about a rocket stage as counterweight.
    - ethbro 11y ago
      Why not just a small vehicle attached to climb/drop down a small diameter tether? Similar to the space elevator idea?
      - Pyxl101 11y ago
        Climbing the tether will change the center of mass and also increase the rate of rotation. It's going to be complicated.
        ethbro 11y ago
        Not if you have an equivalent mass climber on the other side mirroring your actions? Admittedly, this is going to take energy to climb "up", but I'm assuming energy_to_climb << energy_to_alter_rotation.
dweinus 11y ago
Aside from the manueverability problem mentioned in other comments, the tether has a materials problem: it needs to hold the combined ship and couter weights at approx 1g.