# 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 100m 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 100m 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." :)

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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.

Staff Emeritus
Or is it total nonsense?

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

sbrothy
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?

Staff Emeritus
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?"

Gold Member
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.

Gold Member
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.

Staff Emeritus
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?

Staff Emeritus
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?

Gold Member
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
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?

Staff Emeritus
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?

Gold Member
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.

Fervent Freyja and GTOM
Gold Member
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.

.Scott
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.

Mentor
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.

GTOM
Gold Member
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?

Gold Member
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.

GTOM
Gold Member
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.

Mentor
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.

Mentor
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.

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?

russ_watters