Artifical gravity on spaceships

[GTOM]

The only plausible solution for this is rotation (maintain 1g acceleration for long time requires insane amount of energy).
What kind of problems caused by this? If living habitats rotate, the whole ship will rotate unless the habitats are detached somehow. It affects manuevering. Is this a very hard problem, or nothing that can't be solved by a good computer?
Is there any way to prevent the rotation of the axis of the ship without wasting much fuel and erode the joints?

 

[sbrothy]

Just off the top of my head you'd definitely have to deal with the Coriolis force. Maybe counter rotating cylinders would be a thing?

Although it's for purposes of scifi writing if you're designing your own rocket you might benefit from this site:

http://www.projectrho.com/public_html/rocket/basicdesign.php

 

[GTOM]

Just off the top of my head you'd definitely have to deal with the Coriolis force. Maybe counter rotating cylinders would be a thing?

Although it's for purposes of scifi writing if you're designing your own rocket you might benefit from this site:

http://www.projectrho.com/public_html/rocket/basicdesign.php

At this point, stress of the cargo bay with rotating axis isn't my main concern. The living ring should have at least 1`00m radius to avoid motion sickness. The cargo bay and reactor core can be much smaller in diameter.

 

[sbrothy]

At this point, stress of the cargo bay with rotating axis isn't my main concern. The living ring should have at least 1`00m radius to avoid motion sickness. The cargo bay and reactor core can be much smaller in diameter.

Here's quite an in-depth discussion of motion sickness in relation to spaceflight:

http://www.projectrho.com/public_html/rocket/humanfactor.php

And yes, I like this site. Both for the subject itself (space, science and sci-fi), but I also like the density of information per page as I'm sometimes offline for longer periods in which the cached pages provide lots of entertainment. :)

"Canned monkeys don't ship well." :)

 

[GTOM]

Otherwise i wonder, could a spin ship shift its axis without using thrusters?
Move a large weight left, then the rest of the ships back move right. After a half turn, do the opposite, weight moves to left, back of ship right again.
Or is it total nonsense?

If the axis can be positioned properly, acceleration and deceleration isn't affected by rotation.

 

[Vanadium 50]

Or is it total nonsense?

You mean "can I move the center of mass by only internal rearrangements?"

 

[GTOM]

You mean "can I move the center of mass by only internal rearrangements?"

I guess my phrases weren't the best, but no. I don't want to move center of mass, but the axis of the spin ship with the thrusters on the end. The center of mass don't change course until thrusters don't fire.
My problem was how to perform course corrections when whole axis rotates?

 

[GTOM]

Maybe i am still not clear enough.
So the whole ship is rotating but i don't want to simply speed up or slow down, but shift course. If there is a thruster on the side of the ship, it will rotate too. So that make manuevering difficult with side thrusters.

Can it be solved by shift main axis with internal arrangement, then use main thruster?

 

[Vanadium 50]

Can it be solved by shift main axis with internal arrangement,

Do you mean "can I move the angular momentum axis by only internal rearrangements without applying an external torque?"

 

[Halc]

You want to spin that parts that need gravity, which is typically where the people are. So you have a wheel (or two of them counter-rotating) that spin independently of the rest of the ship. There seems to be little reason to put cargo in high gravity.

As for turning the ship without thrusters, there is conservation of angular momentum to worry about. That's why you have two wheels. If angular mometum is then zero, the ship can be re-oriented with gyros, without using any thrusters. It cannot be done quickly (at least not practically), but it can be done with minimal energy expenditure.

 

[GTOM]

Do you mean "can I move the angular momentum axis by only internal rearrangements without applying an external torque?"

Yes.

 

[GTOM]

You want to spin that parts that need gravity, which is typically where the people are. So you have a wheel (or two of them counter-rotating) that spin independently of the rest of the ship. There seems to be little reason to put cargo in high gravity.

As for turning the ship without thrusters, there is conservation of angular momentum to worry about. That's why you have two wheels. If angular mometum is then zero, the ship can be re-oriented with gyros, without using any thrusters. It cannot be done quickly (at least not practically), but it can be done with minimal energy expenditure.

How can it be achieved that the wheels spin independently from the rest of the ship? They can be detached if ship don't accelerate. But ion thrusters should generate constant thrust.

 

[Halc]

How can it be achieved that the wheels spin independently from the rest of the ship? They can be detached if ship don't accelerate. But ion thrusters should generate constant thrust.

Using bearings. A car wheel spins independently of the car, and yet (most of the time) do not part company with the car when ion-thrusters accelerate the whole business.

Really, Ion thrusters? Efficient, but very low power. The people will all die of old age before the ship gets anywhere. Anyway, I'm just commenting on how a wheel can spin without spinning the whole ship.

 

[Vanadium 50]

Do you mean "can I move the angular momentum axis by only internal rearrangements without applying an external torque?"

Yes.

How does this conserve angular momentum?

 

[GTOM]

How does this conserve angular momentum?

So, does that imply that torque can be generated but the rotation of the living ring slows down in return?

 

[GTOM]

Using bearings. A car wheel spins independently of the car, and yet (most of the time) do not part company with the car when ion-thrusters accelerate the whole business.

Really, Ion thrusters? Efficient, but very low power. The people will all die of old age before the ship gets anywhere. Anyway, I'm just commenting on how a wheel can spin without spinning the whole ship.

So the basic design should be central part with thrusters and two large wheels rotating in opposite directions, their axis is perpendicular to central part.
Can they rotate compared to central part with air tight joints?

 

[Vanadium 50]

So, does that imply that torque can be generated but the rotation of the living ring slows down in return?

Where did you get the idea I said that?

Can the angular momentum change without an external torque applied?

If you are struggling with that, try the linear version:

Can the momentum change without an external force applied?

 

[Halc]

So the basic design should be central part with thrusters and two large wheels rotating in opposite directions, their axis is perpendicular to central part.
Can they rotate compared to central part with air tight joints?

Almost every design I've seen has one wheel, and the axis parallel to the direction of thrust. Otherwise the gravity goes up and down in the wheel, which will very much cause motion sickness.
The two wheels is there to cancel angular momentum. This part is often omitted because they don't have intentions of reorienting the ship during the trip, but the pair of wheels makes it easier to face it whichever way you want, without thrusters.

Engines can spin with the ship. There's just no reason why they wouldn't function that way. Many rockets/missiles spin as the ascend, which increases the stability of the thing. The center portions of a rotating ship will be lower gravity than further out. If the main ship is not to rotate, or there are two counter-rotating wheels, then yes, you need to design some joints that are air-tight.

Notice that nobody has ever made anything like this. The ISS is up there and they leave those guys weightless for months rather than all the extra engineering that would go into putting up a spinning module. The problems are real.

 

[GTOM]

Where did you get the idea I said that?

Can the angular momentum change without an external torque applied?

If you are struggling with that, try the linear version:

Can the momentum change without an external force applied?

Sorry but i really don't see why they are the same. I can still spin the wheels of a bicycle if i sit on it, and we levitate in zero G.

Or another examaple. I kick a box on the space station. Dont i change my angular momentum? What if i tie the box to my belt with a long rope. Will it cancel my spin?

 

[Vanadium 50]

I can still spin the wheels of a bicycle

By applying an external torque to them.

I kick a box on the space station. Dont i change my angular momentum?

The box applies an external torque to you.

Before we get further, we should figure out where the difficulty is. Do you not believe \tau = \frac{dL}{dt} or are you having difficulty applying it?

 

[DaveC426913]

Sorry, is there some some reason the OP isn't simply using a gyroscope? or am I missing it?
A massive flywheel at the center of mass will do fine to impart ship rotation.

 

[DaveC426913]

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.

 

[GTOM]

By applying an external torque to them.
The box applies an external torque to you.

Before we get further, we should figure out where the difficulty is. Do you not believe \tau = \frac{dL}{dt} or are you having difficulty applying it?

I am having pretty much difficulty with handle the different directions, radiuses.
I guess it is wrong how i wrote that torque generation (maybe i am so accustomed to having at least air as an outside force), but is it total unachievable without thrusters?
In case of bicycle i can apply external.force while i am still on the bike.

 

[GTOM]

Sorry, is there some some reason the OP isn't simply using a gyroscope? or am I missing it?
A massive flywheel at the center of mass will do fine to impart ship rotation.

If axis doesn't rotate but living thing does, their joints has to be airtight while they rotate.

 

[russ_watters]

If axis doesn't rotate but living thing does, their joints has to be airtight while they rotate.

That is not a significant technical challenge.

Note, this topic was also discussed a month ago, and NASA had proposed/considered including a module like this in the ISS. There really aren't any significant technical challenges to it.

 

[DaveC426913]

If axis doesn't rotate but living thing does, their joints has to be airtight while they rotate.

Sorry. I'm not sure what that has to do with a large gyroscope at the CoM of the ship to enable it to rotate. It's entirely internal.

 

[GTOM]

That is not a significant technical challenge.

Note, this topic was also discussed a month ago, and NASA had proposed/considered including a module like this in the ISS. There really aren't any significant technical challenges to it.

Interesting. Is there any further information, description about it?

 

[Halc]

Sorry but i really don't see why they are the same. I can still spin the wheels of a bicycle if i sit on it, and we levitate in zero G.

If you spin the bicycle wheel in zero G, you spin the other way. Angular momentum (zero in this case) is thus preserved.

In case of bicycle i can apply external.force while i am still on the bike.

You cannot apply external force on the bicycle-with-you system. At best you can throw the bicycle away from yourself to get it to go somewhere and you somewhere else. Otherwise you stay put and maintain zero net momentum of both kinds.

Anyway, I'm glad NASA has engineered the air-tight bearing thing to the point where they're considering putting one up there. 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.

Sorry, is there some some reason the OP isn't simply using a gyroscope? or am I missing it?
A massive flywheel at the center of mass will do fine to impart ship rotation.

That's what my 2nd counter-rotating wheel did, adding useful mass (more living space) instead of wasting it on a gyro. Yes, either way it keeps the angular momentum at zero, allowing ship rotation (slowly) without expenditure of thrust, although it is unclear why such rotation is part of the flight plan.

I'm not sure what that has to do with a large gyroscope at the CoM of the ship to enable it to rotate.

The large gyroscope can be anywhere. Surely there is something more useful to have at the CoM point. It's job is to absorb angular momentum, and it can do that anywhere, including on the edge of spinning wheel, an improbable but not impossible place to put it.

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.
View attachment 269073

You're right that a wheel shape is totally unnecessary. But how do you apply thrust to the setup you have pictured there?

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.

Reeling it in will make the tangential velocity of the end-modules stupid fast, raising the g-forces on the cables to the point where they'll break, not to mention killing everybody. Reeling it in doesn't dump the angular momentum.

 

[DaveC426913]

You're right that a wheel shape is totally unnecessary. But how do you apply thrust to the setup you have pictured there?

As I said, you don't.

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.

Reeling it in will make the tangential velocity of the end-modules stupid fast, raising the g-forces on the cables to the point where they'll break, not to mention killing everybody. Reeling it in doesn't dump the angular momentum.

Of course not. You can store the energy in a flywheel.

 

[russ_watters]

Interesting. Is there any further information, description about it?

It was in the other thread:
https://en.wikipedia.org/wiki/Nautilus-X

 

[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.