Artifical gravity on spaceships

[russ_watters]

But the OP started with "constant acceleration is impractical" so he doesn't plan on butterflying all over the neighborhood. Presumably a vast majority of the trip will be in a straight line.

I was considering objecting to that, but since you brought it back up -- we don't really know what the OP is after here, whether this is intended to be science fiction or a plausible prediction of what intra-solar system travel may look like in a hundred years. I don't think it is too fanciful to speculate that higher thrust ion propulsion or a nuclear-pulse rocket could provide at least a few days of enough acceleration to matter. Or from the other direction, I think current chemical rocket technology is impractical for it, so we won't be seeing commercial trips to/colonization of Mars, for example, if we don't have rockets that can provide a few tenths of a g of thrust for a few days -- and at that point, there's no need for artificial gravity because the trip will only take a week or a few.

 

[russ_watters]

There's also the balance thing. I have a balanced spinning module and somebody goes through the center and ends up somewhere on the wheel where he wasn't before. Now the thing is out of balance and will put a continuous low-frequency vibration on the whole station. Somewhere there has to be a self-balancing mechanism that restores the center of gravity of the wheel back to its axis of rotation.

Does it? What happens if you don't use a self-balancing mechanism? Since the whole mechanism is floating in space, the balancing problem isn't as serious as a car tire or fixed wheel. Plus if you are only rotating at a few rpm, I'm not sure "oscillation" or "wobble" isn't a better word than "vibration". I don't think it hurts you, and the astronauts wouldn't feel it.

 

[GTOM]

I didnt say constant thrust is impractical, i said maintain high acceleration is very energy consuming.
With ion drives, i think sustainable acceleration is on the order of miliGs.

 

[supermath]

Constant Acceleration
Can Fission Vasmir provide constant acceleration for a couple of hours or days? Luna level gravity or less.

 

[member 656954]

The SpinCalc site allows you to play with different parameters to gauge how much apparent gravity different scenarios generate and gives a 'comfort factor' of the result.

But I'm a little nonplussed about your OP, @GTOM. What assumed technology level, mission duration, materials science, etc. are you thinking that puts context around your assertion?

 

[GTOM]

The SpinCalc site allows you to play with different parameters to gauge how much apparent gravity different scenarios generate and gives a 'comfort factor' of the result.

But I'm a little nonplussed about your OP, @GTOM. What assumed technology level, mission duration, materials science, etc. are you thinking that puts context around your assertion?

I assume no frictionless super materials for the joints if they rotate compared to ship, and mission time for months.

 

[russ_watters]

I assume no frictionless super materials for the joints if they rotate compared to ship, and mission time for months.

That's really not a good answer. Where are we traveling to? What overall technology level should we assume (answer with a year or relevant task like "advanced enough for manned missions to Jupiter")?

Your focus on the joints, a pretty trivial problem, is weird. E.g., why do you think friction matters?

 

[GTOM]

That's really not a good answer. Where are we traveling to? What overall technology level should we assume (answer with a year or relevant task like "advanced enough for manned missions to Jupiter")?

Your focus on the joints, a pretty trivial problem, is weird. E.g., why do you think friction matters?

A manned Jupiter mission is a good example.

Why friction matters? So rotating and nonrotating part joints. There will be friction. That friction can't erode parts significantly, otherwise air will leak.

 

[russ_watters]

Why friction matters? So rotating and nonrotating part joints. There will be friction. That friction can't erode parts significantly, otherwise air will leak.

While that's true, we've been building rotating machines, including air tight ones, for a hundred years. It doesn't really explain why you think it is a significant problem.

 

[GTOM]

While that's true, we've been building rotating machines, including air tight ones, for a hundred years. It doesn't really explain why you think it is a significant problem.

Ok it is good to know that actually it isn't that hard.

 

[member 656954]

mission time for months

Not to say zero gee isn't an issue, but a months-long mission has equally difficult issues to contend with such as radiation and physical systems resiliency. But if your example mission is to Jupiter (and back, I'm assuming) then we have invested a LOT of $$ into the ship, so adding some degree of gravity - most likely microgravity, probably not 1G - is feasible to ameliorate the physiological effects.

What's less feasible at the moment is a propulsion system that gets the astronauts there and back in a matter of months. Pioneer 10, Pioneer 11, and Voyager 1 were flybys and each took almost two years just to reach Jupiter. Galileo went into orbit, but that took 2,242 days - 6 years! - because it needed to be traveling slow enough not to just shoot on by. Similarly with Juno, which was faster, but not by much, it took a shade under 5 years.

Solve the fuel problem and you solve the gravity problem. Apply thrust all the way there and back, and voilà, job done

 

[GTOM]

Ion thruster and nuclear reactor. It can maintain acceleration for long time and achieve magnitude more delta-V than chemical rocket.
However rocket equations say you have to sacrifice thrust for fuel efficiency.
The ion drive could produce miliG order of acceleration on the long run.

 

[member 656954]

Ion thruster

Even a little gravity is helpful so do you know, @GTOM, if there are 'big' ion drive designs for a crewed ship of the size needed to get to Jupiter and back?

nuclear reactor

Would shielding be an issue? And propellant as well? Isn't a reactor (fission, I'm assuming or are we walking forward to fusion being solved?) a good "generate a large initial thrust then coast" approach?

 

[DaveC426913]

Would shielding be an issue?

Usually, you put the crew at the other end (which reduces radiation some). That's why the 'Discovery' in 2001 is so long and spindly.

And put your water storage in the middle (which reduces radiation a lot. And does so without adding dead-weight just for shielding.)

 

[.Scott]

BTW, there are other configurations for artificial gravity that you may not have examined.

It doesn't have to be just one section the ship. Tie two sections of the ship with a cable and have them spin about their CoM.

The nice thing about this is that you can make the cable of arbitrary length - say, a few hundred metres (assuming it's strong enough) and then you can get a nice high g value while minimizing the Coriolis force.

This would only be practical if your journey were mostly straight line, and not much maneuvering. You'd have to reel it in for maneuvering.

A similar configuration might be: L-----P---X
L: Living quarters for passengers and crew - kept at about 1G (or whatever the target).
P: Propulsion - kept at 0G.
C: Cargo used as a counter balance.
---: Cables.

There would be vibration/oscillation problems that would be resolved with shock absorbers on the cables and ramping propulsion up and down.

 

[mfb]

This would only be practical if your journey were mostly straight line, and not much maneuvering. You'd have to reel it in for maneuvering.

Not if both sides maneuver together. You don't maneuver much anyway on a realistic mission, it's just some course corrections on the way.

 

[trainman2001]

We can make frictionless bearing and already do. Magnetic levitated bearing in a vacuum would have almost zero friction. If the rotating portion is being driven by something on the core, the core would rotate in the opposite direction (the reason for a tail rotor on a single bladed helicopter). The force would have to be on the wheel itself. With the mass involved, the weight of the people inside wouldn't make much of a difference in the balance any more than the crew on a nuclear aircraft carrier would affect its list if they moved from one side to the other. Being weightless for years at a time would probably be physically untenable. I would be interested to know if some form of nuclear propulsion could generate a continuous 1/2g of acceleration. If would not have to be one g to be beneficial, although I don't know what the break point would be. Lockheed is working on a compact fusion system. They're predicting success much sooner than ITER seems to be going. It is planned to be a portable propulsion system. We need some new thinking to break the log jam.

 

[member 656954]

Lockheed is working on a compact fusion system

I wish them success, but fusion for energy is a hard problem and Lockheed Martin has been working on this for years with nothing evidently tangible, apart from a dark web rumor of a working fusion reactor last year

Still, a small power plant that generates constant electricity would be excellent for space missions, but can you actually generate thrust continuously? How much propellant do you need? Also, do fusion reactors generate heat? If so, can you dissipate it sufficiently that the reactor does not cook the crew?

 

[GTOM]

We can make frictionless bearing and already do. Magnetic levitated bearing in a vacuum would have almost zero friction. If the rotating portion is being driven by something on the core, the core would rotate in the opposite direction (the reason for a tail rotor on a single bladed helicopter). The force would have to be on the wheel itself. With the mass involved, the weight of the people inside wouldn't make much of a difference in the balance any more than the crew on a nuclear aircraft carrier would affect its list if they moved from one side to the other. Being weightless for years at a time would probably be physically untenable. I would be interested to know if some form of nuclear propulsion could generate a continuous 1/2g of acceleration. If would not have to be one g to be beneficial, although I don't know what the break point would be. Lockheed is working on a compact fusion system. They're predicting success much sooner than ITER seems to be going. It is planned to be a portable propulsion system. We need some new thinking to break the log jam.

With realistic ion drives and power sources, sustainable acceleration is rather on the order of miliGs.

 

[GTrax]

The designs of the science fiction artists and movie makers indicate they have thought this through. Whether it be gondolas on contra-rotating arms, or an entire torus, with all the really heavy stuff kept at the axis, once it is up to speed, I don't think there is much the incumbents can do to upset it much. Re-orientation of the entire structure does take some applied accelerations.

Keeping the body from deteriorating takes a significant, and permanent daily effort, as ISS folk will attest. Losing the body bone calcium was noted from the very beginnings of space travel missions. Perhaps, for really long journeys, the "gravity by large radius rotation" may be seen as worthwhile.

 

[some bloke]

I wonder if magnetism has any scope for artificial gravity.



Looking at this video of using an extremely powerful magnet to levitate a frog, the frog is reaching an equilibrium point in the middle of the tube, meaning that the forces on it from the magnetic field are pushing it in one direction. If this force is largely uniform across the frog, then it could exert a force in a similar manner to gravity. So perhaps an extremely powerful magnetic field (and a spaceship & crew utterly devoid of anything magnetic!) would be a way to go.

An alternative (which wouldn't be the same as gravity as it wouldn't affect the internals of the people) would be a magnetic suit which is then accelerated by a magnetic field, allowing a person to walk as if they were in gravity, even if their internal organs weren't being subjected to it.

 

[Janus]

I wonder if magnetism has any scope for artificial gravity.



Looking at this video of using an extremely powerful magnet to levitate a frog, the frog is reaching an equilibrium point in the middle of the tube, meaning that the forces on it from the magnetic field are pushing it in one direction. If this force is largely uniform across the frog, then it could exert a force in a similar manner to gravity. So perhaps an extremely powerful magnetic field (and a spaceship & crew utterly devoid of anything magnetic!) would be a way to go.

An alternative (which wouldn't be the same as gravity as it wouldn't affect the internals of the people) would be a magnetic suit which is then accelerated by a magnetic field, allowing a person to walk as if they were in gravity, even if their internal organs weren't being subjected to it.

The levitation of the frog in this video is done by diamagnetism. Basically, some material are repelled by magnetic fields. These are materials we generally consider as being non-magnetic.
The diamagnetic effect is very weak, so you need a strong magnetic field to cause a measurable effect.
Some problems:
Such a strong field would be difficult to maitain.
It would likely have negative effects on ship electronics
Diamagnetic materials are not equally diamagnetic. Different objects made from different materials will react differently. This includes parts of your body. The human body is not completely homogeneous, So different internal parts will "feel" a different diamagnetic effect. I'm not sure what the long term effects this would have.

 

[some bloke]

The levitation of the frog in this video is done by diamagnetism. Basically, some material are repelled by magnetic fields. These are materials we generally consider as being non-magnetic.
The diamagnetic effect is very weak, so you need a strong magnetic field to cause a measurable effect.
Some problems:
Such a strong field would be difficult to maitain.
It would likely have negative effects on ship electronics
Diamagnetic materials are not equally diamagnetic. Different objects made from different materials will react differently. This includes parts of your body. The human body is not completely homogeneous, So different internal parts will "feel" a different diamagnetic effect. I'm not sure what the long term effects this would have.

Yes, I did a fair bit of reading into it a while back. I do believe it's worth considering though. It would be difficult to maintain, but that's the sort of problem which is likely to be solvable - it's not like the technology can't exist, it just hasn't needed to yet. Electronics, I suspect that they can be designed to work in a magnetic field, and if the field is known then any effects it has on data signals can be accounted for. You certainly couldn't use off the shelf electronics in there! As for different parts of the body being affected to different degrees, this is no different from different parts of your body being more or less dense than one another in a gravitational field - it just might be different parts which become "heavier"! Long term effects, well, that could only be speculation!

 

[mfb]

Diamagnetic levitation is doing the exact opposite of what you want. It's a volume force that reduces internal stress and external forces on the body.

It's impractical, too: Scaling up the magnets would be extremely challenging, would come with a huge mass, and it's terrible if you want to move around in these strong fields.

 

[DaveC426913]

Diamagnetic levitation is doing the exact opposite of what you want. It's a volume force that reduces internal stress and external forces on the body.

Aside from the impracticality of using magnetism, surely the idea would be to offset the centre of the magnetic force within the ship, so that the levitative effect has a bias in the direction of the decks. Essentially "lifting" them downward.

 

[mfb]

You can't. Diamagnetic levitation is resisting motion relative to the magnet. A magnetic field that's bound to the ship will always reduce the effective g-force. You can try to make it time-dependent but then it won't last long and will make the astronauts sick.

 

[Vanadium 50]

Diamagnetic levitation is resisting motion relative to the magnet.

Is that what it is doing? Or is it providing a force in the direction of lower field? (That won't work either, of course.)

 

[mfb]

Ah right, that's limited to high temperature superconductors, and coming from flux pinning not their ideal diamagnetism. You still have some resistance from Lenz' law.
Getting a large field gradient and large field over a large volume is really hard. Wikipedia lists 1400 T²/m for water. A 50 T average field at a gradient of 30 T/m? What cools these magnets?

 

[Wasure]

Perhaps it would be better to have a larger ship with a lower acceleration?

 

[trainman2001]

It doesn’t have to be 1g, does it? How much below 1g would humans still be able to retain normal biological functions and musculoskeletal integrity. It would be considerably less energy to accelerate at 1/3 g than 1g, or to spin a habitat to that level. It all depends on what we need to thrive over the long haul. Living on Mars or Moon at their reduced gravities would tell us a lot.