5 ms·

How do you propose this would work actually? The last article I read on the subject suggested that AG isn't viable near term as the space craft would have to be

by deelowe 11y ago

How do you propose this would work actually? The last article I read on the subject suggested that AG isn't viable near term as the space craft would have to be prohibitively large. I recall the primary issue being that living quarters have to be a certain distance from the axis of rotation. [EDIT] Here you go. The relevant info is on wikipedia: https://en.wikipedia.org/wiki/Artificial_gravity#Rotation https://en.wikipedia.org/wiki/Artificial_gravity#Rotation The radius of rotation would have to be a minumum of 224 meters.

slfnflctd 11y ago
...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.". :/
  - 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.
exDM69 11y ago
A radius of 224 meters is 1 g acceleration at 2 revolutions per minute. It is directly proportional, so half the acceleration is half the radius. 0.5g or less might be good enough. Now this is a pretty huge space craft, but compare that with the ISS which is already 108 meters wide by 73 meters long. It's a huge undertaking but in the realm of possible. Of course, there's still the issue with mass and thrust required to propel a huge craft to an interplanetary trajectory. The question is whether artificial gravity is a requirement for interplanetary human expeditions.
- iwwr 11y ago
  That assumes the ship's radius is a limiting factor. It isn't if you spin the ship against a counterweight with a tether (the spent third stage which is going with you on the same trajectory more or less).
  - imglorp 11y ago
    Yes, especially if you keep half your fuel/water/supplies in that tethered section. Consume at differing rates to shift CG where you want it.
    - iwwr 11y ago
      Well, there's no "consume" in terms of matter, stuff isn't thrown overboard. Besides, the food and water (or what they become) is still valuable radiation shielding.
      - imglorp 11y ago
        Well, I think the whole point of fuel is you want to throw it overboard :-) ... at high velocity and on fire.
        iwwr 11y ago
        The tethering happens after the upper lift stage is exhausted. It's then spun from a tether for the duration of the trip, then disconnected before the deceleration burn to orbit Mars. There's no mass exchange while the tether is connected.
dkuntz2 11y ago
If you're going to mars, why strive for 1g when you can have .38g (Martian gravity), meaning you'd only need an 85 meter radius.
- ant6n 11y ago
  Plus, you can pick some less sensitive humans who can deal with larger RPM (rotations per minute).
  - creshal 11y ago
    Surgery at 10 RPM is going to be entertaining, at least.
    - ant6n 11y ago
      Why? As long as the system is dynamically stable. The only issues are that the g is slightly different at different heights, and the Coriolis force, which should be relatively constant. If you want 0.3g at 6RPM, it means a radius of 30m.

by deelowe 11y ago

slfnflctd 11y ago

...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.". :/
- 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.

exDM69 11y ago

A radius of 224 meters is 1 g acceleration at 2 revolutions per minute. It is directly proportional, so half the acceleration is half the radius. 0.5g or less might be good enough. Now this is a pretty huge space craft, but compare that with the ISS which is already 108 meters wide by 73 meters long. It's a huge undertaking but in the realm of possible. Of course, there's still the issue with mass and thrust required to propel a huge craft to an interplanetary trajectory. The question is whether artificial gravity is a requirement for interplanetary human expeditions.

iwwr 11y ago
That assumes the ship's radius is a limiting factor. It isn't if you spin the ship against a counterweight with a tether (the spent third stage which is going with you on the same trajectory more or less).
- imglorp 11y ago
  Yes, especially if you keep half your fuel/water/supplies in that tethered section. Consume at differing rates to shift CG where you want it.
  - iwwr 11y ago
    Well, there's no "consume" in terms of matter, stuff isn't thrown overboard. Besides, the food and water (or what they become) is still valuable radiation shielding.
    - imglorp 11y ago
      Well, I think the whole point of fuel is you want to throw it overboard :-) ... at high velocity and on fire.
      - iwwr 11y ago
        The tethering happens after the upper lift stage is exhausted. It's then spun from a tether for the duration of the trip, then disconnected before the deceleration burn to orbit Mars. There's no mass exchange while the tether is connected.

dkuntz2 11y ago

If you're going to mars, why strive for 1g when you can have .38g (Martian gravity), meaning you'd only need an 85 meter radius.

ant6n 11y ago
Plus, you can pick some less sensitive humans who can deal with larger RPM (rotations per minute).
- creshal 11y ago
  Surgery at 10 RPM is going to be entertaining, at least.
  - ant6n 11y ago
    Why? As long as the system is dynamically stable. The only issues are that the g is slightly different at different heights, and the Coriolis force, which should be relatively constant. If you want 0.3g at 6RPM, it means a radius of 30m.