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Why can't we tether the spaceship that carries the astronauts with a separate spaceship that carries the cargo (that will be only needed on mars) via a set of l

Why can't we tether the spaceship that carries the astronauts with a separate spaceship that carries the cargo (that will be only needed on mars) via a set of long, thin cables (to act as a counterweight) and spin the two around their joint center of mass? That solves the problem of needing a really large ship to make the rotation not uncomfortable, and allows for artificial gravity

3pt14159 5y ago
We can. It's been written about for decades. It's just expensive.
- londons_explore 5y ago
  But why is it hard or expensive. These spacecraft can already withstand earth gravity, because we built them here on earth and they didn't collapse during construction. They typically already have lifting points to lift them with a crane too. Cable is cheap and light. Getting the whole lot spinning can be done very slowly over many days with the same ion thrusters that are used for stationkeeping. Total delta-V isn't very high. Total fuel used isn't very high either. The only disadvantage really is that you lose most of the benefits of zero-G. for example, long running experiments requiring zero G. You also start to need walkways and paths. The ceiling of rooms becomes dead unusable space. etc. Docking new spacecraft requires stopping the spinning, which takes many days. Comms antennas and solar arrays get more expensive.
  - throwaway0a5e 5y ago
    >The only disadvantage really is that you lose most of the benefits of zero-G. for example, long running experiments requiring zero G. You also start to need walkways and paths. The ceiling of rooms becomes dead unusable space. etc. Docking new spacecraft requires stopping the spinning, which takes many days. For a one way trip to "elsewhere" none of that matters much. We'll probably see this approach investigated more fully when a one way trip to elsewhere seems more likely.
  - JoeAltmaier 5y ago
    TO dock you have to spin down and then spin back up again? It's a hard problem.
  - 3pt14159 5y ago
    I actually agree with your underlying urge: space travel should be pleasant! But what separates out engineering from science is essentially economics. In science we don't know the scale a discovery will make until we've made it. Before nuclear weapons how could we know that studying these tiny atoms would essentially end total war between major powers. With engineering smart people looked at the costs and said something like: > We're 99.9% sure this is going to be >2x the alternative and it's not worth it. Just off the top of my head costs that go up: - Instruments (lol, our cameras now need faster f-stops) - Every need now needs to be met during the spinning state and the non-spinning state. (lol, Frank you used the gravity toilet in the middle of the night but we stopped the spinning state yesterday) - Harder to maintain, since crew needs to lose angular momentum as they travel along the bridge and they can't toss things around as easily.
6gvONxR4sf7o 5y ago
The issue is probably the strength of the structures involved.
- kamray23 5y ago
  Starship weighs 1300+ metric tonnes. Accelerating to earth gravity, the cable will have a tension of 1.3 MN. On the other end the counterweight has to be of roughly equal mass by way of not being able to launch anything heavier (so prob. another starship), meaing that the barycentre will be roughly in the middle, and the counterweight applies a second 1.3 MN. The total tension the cable is under is going to be 2.6 MN, well outside any single cable today. Taking a comfortable velocity of 1 rpm, and calculating a radius, we get a cable 894 meters long. This is right in the comfort zone as far as difference in force over your body and disorientation from spinning are concerned. Our strongest cables, made of Aramid (which degrades too quickly because of radiation, but ignoring that) is going to be 58 cm and have a weight of 2.2 metric tonnes. Since we actually need twice the cable, it's actually 4.4 tonnes. A similar cable made of carbon fibre or another such element will give us a launch weight of 430+ tonnes (close to the mass of the ISS), which rules basically anything but polymer fibres out. The R&D funding that would have to go into developing the cabling and methods of shielding it and preventing MMOD, solar, or GCR degradation is astronomical. It's way cheaper to just make a special sleeping bag or two.
  - hexane360 5y ago
    I agree it's impractical, but I believe you're double counting the force on the cable. Each end of the cable pulls with 1.3 MN, which means that the cable itself can be in static equilibrium (in the rotating frame of reference). The same is true if you hang a 1.3 MN space station from the ceiling. The space station pulls down with a force of 1.3 MN, while the ceiling pulls up with a force of 1.3 MN.
    - kamray23 5y ago
      That's probably a reasonable conclusion. It does happen to be totally overshadowed by my bigger mistake, which is an order of magnitude. Repeating the calculations using my physics CAS today: Starship mass: 1.3·10³ kg Starship weight: 1.3·10⁷ Newtons Converting that into something reasonable, it's 13 MN, not 1.3. A Kevlar fibre cable will be 22 tonnes, a PE one 9000 tonnes, and a carbon one 2200 tonnes. I wouldn't like to even imagine steel. And these are just with a breaking strength of 13 MN. Not the most reasonable, though obviously we've held up things like Arecibo in the past. Using multiple smaller cables would be the optimal solution and what we would go with, since a single cable is >1 meter in diameter in any case. The mass doesn't get any smaller like that, though, so it's not a consideration we'd need to make.
jerf 5y ago
This is one of several things I hope to see if SpaceX succeeds in crashing the cost/kg to orbit. I think it's difficult to overestimate how hamstrung our space program has been by the sheer expense of mass. New technology was certainly always going to be necessary, but there is so much that would be easier and/or possible if we could just get mass to orbit and weren't counting every gram. At current prices, it's hard to justify lifting dead weight just to spin.
kamray23 5y ago
Depends on the situation, though usually because of the force. Spinning a spaceship around a point with a long cable, you obviously produce a (fictitious if you're pedantic) outward force, that's kind of the point. For that fictitious force to exist though, that very force acting on the spaceship must be counteracted by an equal and opposite reaction force provided by your cable, creating the action-reaction pair required for tension. This much is obvious. The problem arises when you think about how large that force is. To generate an apparent artificial apparent gravitational acceleration of 1 G for the occupants, the entire ship must experience the same spin and thus the same acceleration. That's the source of the problem, the force you're counteracting is the same as the weight of the ship on Earth. What is being asked here is to hang a loaded spaceship from a building with cables. That might work for smaller spacecraft, whose mass is measured in metric tonnes, but it won't work for anything larger. You can get incredibly strong and light cables out there, but one capable of functioning in the space environment, light enough to launch, and strong enough to counter the tens of meganewtons of force required from it is going to be more difficult to find. Not only that, but you need to consider the counterweight as well. Because of the cable, the tension force total is also dependent on the acceleration of the counterweight. Since launching a heavier counterweight than an entire habitable ship is probably out of the question, you'll probably need twice as long a cable and experience roughly twice the force required just to lift the ship on earth. Needless to say, that's getting a bit out of the realm of current space capability. Not to mention the immense size you'd need to make a ship like this for the situation to not cause immense discomfort.
- throwaway0a5e 5y ago
  >That's the source of the problem, the force you're counteracting is the same as the weight of the ship on Earth. What is being asked here is to hang a loaded spaceship from a building with cables. We're talking about 10,000-100,000lb (i.e. just the crew module) depending on mission profile and the craft in question. You can handle that and more with commodity wire rope and hardware. Whoever we're sending to Mars will probably appreciate having a useful tow rope on Mars anyway so it's probably wise to just use a boring old steel cable rather than something that weighs 3lb but isn't up to the rigors of surface use. Remember, we're using "needs to go into space" margins here, not "overhead lifting on earth in a jurisdiction where OSHA matters" or "what makes Redditors sleep at night" margins here so you're not going to need a behemoth of a cable. Furthermore, you don't even need to generate 1g, just enough to not cause health problems and greatly simplify craft design.
rbanffy 5y ago
We could also use an inflatable module with a centrifuge inside it. The smaller diameter would make coriolis forces worse and walking would be tricky, but, at least, your body would spend some time under some gravity. Putting it inside a large inflatable makes it easier to pack and solves a problem with rotating seals and the need to stop rotation when a spaceship is docked (because an internal part is rotating, but the rest of the craft is static). One issue that it doesn't solve is that it'll doubtlessly pass some vibrations and some oscillations to the rest of the craft, so any microgravity experiments will need to account for that.
- GekkePrutser 5y ago
  But if you're in an inflatable you're no longer in a vacuum so you'll have air friction to deal with (and thus constant propulsion) right?
  - rbanffy 5y ago
    You'll need an electric motor to spin the centrifuge and a flywheel to store the momentum so the station doesn't rotate. Ideally, it'd be two separate motors and a clutch between them that acts in a catastrophic failure to prevent the whole station from rotating itself.
panick21_ 5y ago
Most people are giving technical explanations. But actually its very non-technical and deliberate. For some historical reason astronaut office space biology gained primacy in the NASA after Apollo. The reason they never seriously considered artificial gravity is because THEY WANT TO STUDY MICROGRAVITY. The whole goal of station is to solve micro-gravity research and human medical research is the most important of those. Its not a goal of the station to figure out how to most efficiently keep humans alive in space. Its simply a great reason to stay in LEO and do research for 50-100 years. There is a reason many space advocates since the 60s have pushed for artificial gravity research and almost nothing has been done. Its political. The technical problems are approachable and solvable but it has not political base unfortunately.

3pt14159 5y ago

We can. It's been written about for decades. It's just expensive.

londons_explore 5y ago
But why is it hard or expensive. These spacecraft can already withstand earth gravity, because we built them here on earth and they didn't collapse during construction. They typically already have lifting points to lift them with a crane too. Cable is cheap and light. Getting the whole lot spinning can be done very slowly over many days with the same ion thrusters that are used for stationkeeping. Total delta-V isn't very high. Total fuel used isn't very high either. The only disadvantage really is that you lose most of the benefits of zero-G. for example, long running experiments requiring zero G. You also start to need walkways and paths. The ceiling of rooms becomes dead unusable space. etc. Docking new spacecraft requires stopping the spinning, which takes many days. Comms antennas and solar arrays get more expensive.
- throwaway0a5e 5y ago
  >The only disadvantage really is that you lose most of the benefits of zero-G. for example, long running experiments requiring zero G. You also start to need walkways and paths. The ceiling of rooms becomes dead unusable space. etc. Docking new spacecraft requires stopping the spinning, which takes many days. For a one way trip to "elsewhere" none of that matters much. We'll probably see this approach investigated more fully when a one way trip to elsewhere seems more likely.
- JoeAltmaier 5y ago
  TO dock you have to spin down and then spin back up again? It's a hard problem.
- 3pt14159 5y ago
  I actually agree with your underlying urge: space travel should be pleasant! But what separates out engineering from science is essentially economics. In science we don't know the scale a discovery will make until we've made it. Before nuclear weapons how could we know that studying these tiny atoms would essentially end total war between major powers. With engineering smart people looked at the costs and said something like: > We're 99.9% sure this is going to be >2x the alternative and it's not worth it. Just off the top of my head costs that go up: - Instruments (lol, our cameras now need faster f-stops) - Every need now needs to be met during the spinning state and the non-spinning state. (lol, Frank you used the gravity toilet in the middle of the night but we stopped the spinning state yesterday) - Harder to maintain, since crew needs to lose angular momentum as they travel along the bridge and they can't toss things around as easily.

6gvONxR4sf7o 5y ago

The issue is probably the strength of the structures involved.

kamray23 5y ago
Starship weighs 1300+ metric tonnes. Accelerating to earth gravity, the cable will have a tension of 1.3 MN. On the other end the counterweight has to be of roughly equal mass by way of not being able to launch anything heavier (so prob. another starship), meaing that the barycentre will be roughly in the middle, and the counterweight applies a second 1.3 MN. The total tension the cable is under is going to be 2.6 MN, well outside any single cable today. Taking a comfortable velocity of 1 rpm, and calculating a radius, we get a cable 894 meters long. This is right in the comfort zone as far as difference in force over your body and disorientation from spinning are concerned. Our strongest cables, made of Aramid (which degrades too quickly because of radiation, but ignoring that) is going to be 58 cm and have a weight of 2.2 metric tonnes. Since we actually need twice the cable, it's actually 4.4 tonnes. A similar cable made of carbon fibre or another such element will give us a launch weight of 430+ tonnes (close to the mass of the ISS), which rules basically anything but polymer fibres out. The R&D funding that would have to go into developing the cabling and methods of shielding it and preventing MMOD, solar, or GCR degradation is astronomical. It's way cheaper to just make a special sleeping bag or two.
- hexane360 5y ago
  I agree it's impractical, but I believe you're double counting the force on the cable. Each end of the cable pulls with 1.3 MN, which means that the cable itself can be in static equilibrium (in the rotating frame of reference). The same is true if you hang a 1.3 MN space station from the ceiling. The space station pulls down with a force of 1.3 MN, while the ceiling pulls up with a force of 1.3 MN.
  - kamray23 5y ago
    That's probably a reasonable conclusion. It does happen to be totally overshadowed by my bigger mistake, which is an order of magnitude. Repeating the calculations using my physics CAS today: Starship mass: 1.3·10³ kg Starship weight: 1.3·10⁷ Newtons Converting that into something reasonable, it's 13 MN, not 1.3. A Kevlar fibre cable will be 22 tonnes, a PE one 9000 tonnes, and a carbon one 2200 tonnes. I wouldn't like to even imagine steel. And these are just with a breaking strength of 13 MN. Not the most reasonable, though obviously we've held up things like Arecibo in the past. Using multiple smaller cables would be the optimal solution and what we would go with, since a single cable is >1 meter in diameter in any case. The mass doesn't get any smaller like that, though, so it's not a consideration we'd need to make.

jerf 5y ago

This is one of several things I hope to see if SpaceX succeeds in crashing the cost/kg to orbit. I think it's difficult to overestimate how hamstrung our space program has been by the sheer expense of mass. New technology was certainly always going to be necessary, but there is so much that would be easier and/or possible if we could just get mass to orbit and weren't counting every gram. At current prices, it's hard to justify lifting dead weight just to spin.

kamray23 5y ago

Depends on the situation, though usually because of the force. Spinning a spaceship around a point with a long cable, you obviously produce a (fictitious if you're pedantic) outward force, that's kind of the point. For that fictitious force to exist though, that very force acting on the spaceship must be counteracted by an equal and opposite reaction force provided by your cable, creating the action-reaction pair required for tension. This much is obvious. The problem arises when you think about how large that force is. To generate an apparent artificial apparent gravitational acceleration of 1 G for the occupants, the entire ship must experience the same spin and thus the same acceleration. That's the source of the problem, the force you're counteracting is the same as the weight of the ship on Earth. What is being asked here is to hang a loaded spaceship from a building with cables. That might work for smaller spacecraft, whose mass is measured in metric tonnes, but it won't work for anything larger. You can get incredibly strong and light cables out there, but one capable of functioning in the space environment, light enough to launch, and strong enough to counter the tens of meganewtons of force required from it is going to be more difficult to find. Not only that, but you need to consider the counterweight as well. Because of the cable, the tension force total is also dependent on the acceleration of the counterweight. Since launching a heavier counterweight than an entire habitable ship is probably out of the question, you'll probably need twice as long a cable and experience roughly twice the force required just to lift the ship on earth. Needless to say, that's getting a bit out of the realm of current space capability. Not to mention the immense size you'd need to make a ship like this for the situation to not cause immense discomfort.

throwaway0a5e 5y ago
>That's the source of the problem, the force you're counteracting is the same as the weight of the ship on Earth. What is being asked here is to hang a loaded spaceship from a building with cables. We're talking about 10,000-100,000lb (i.e. just the crew module) depending on mission profile and the craft in question. You can handle that and more with commodity wire rope and hardware. Whoever we're sending to Mars will probably appreciate having a useful tow rope on Mars anyway so it's probably wise to just use a boring old steel cable rather than something that weighs 3lb but isn't up to the rigors of surface use. Remember, we're using "needs to go into space" margins here, not "overhead lifting on earth in a jurisdiction where OSHA matters" or "what makes Redditors sleep at night" margins here so you're not going to need a behemoth of a cable. Furthermore, you don't even need to generate 1g, just enough to not cause health problems and greatly simplify craft design.

rbanffy 5y ago

We could also use an inflatable module with a centrifuge inside it. The smaller diameter would make coriolis forces worse and walking would be tricky, but, at least, your body would spend some time under some gravity. Putting it inside a large inflatable makes it easier to pack and solves a problem with rotating seals and the need to stop rotation when a spaceship is docked (because an internal part is rotating, but the rest of the craft is static). One issue that it doesn't solve is that it'll doubtlessly pass some vibrations and some oscillations to the rest of the craft, so any microgravity experiments will need to account for that.

GekkePrutser 5y ago
But if you're in an inflatable you're no longer in a vacuum so you'll have air friction to deal with (and thus constant propulsion) right?
- rbanffy 5y ago
  You'll need an electric motor to spin the centrifuge and a flywheel to store the momentum so the station doesn't rotate. Ideally, it'd be two separate motors and a clutch between them that acts in a catastrophic failure to prevent the whole station from rotating itself.

panick21_ 5y ago

Most people are giving technical explanations. But actually its very non-technical and deliberate. For some historical reason astronaut office space biology gained primacy in the NASA after Apollo. The reason they never seriously considered artificial gravity is because THEY WANT TO STUDY MICROGRAVITY. The whole goal of station is to solve micro-gravity research and human medical research is the most important of those. Its not a goal of the station to figure out how to most efficiently keep humans alive in space. Its simply a great reason to stay in LEO and do research for 50-100 years. There is a reason many space advocates since the 60s have pushed for artificial gravity research and almost nothing has been done. Its political. The technical problems are approachable and solvable but it has not political base unfortunately.