# Tumbling (constant pitch) For Gravity Versus Rolling (constant roll) For Gravity

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Posted (edited)

According to kerbiloid you only need 100-200 meters.... If I quote him correctly, to get 1g rotation without messing up a human's equilibrium (sense of balance so they can walk without falling).

I know at a thousand meters you only need one rotation per minute (RPM), but at 100 or 200 meters I know it will be more than 1 RPM. So how would that look? A ship spinning fast like fan blades? A blur? Or more like a thrown rotating knife? Fast but not so fast you can't notice the rotation without having to focus on it.

Tumbling VS Rolling for gravity: Which you choose really decides how the spaceship is built and where crew will live inside it.

Tumbling: Crew would live in the frontal part of the spaceship, since when you tumble, g-force is felt at the farthest end under rotation, and the other end would have the engine and fuel tanks anyway so it's space is already taken up. The main advantage of tumbling is that you do not need a really big spaceship, you only need one that is at least 100-200 meters long to reap gravity from it. I can see a T-shaped cylindrical shape being quite useful for a tumbling spaceship, since the broadside frontal cylinder could have a rotating inner habitat cylinder in it which rotates and stops itself in the direction of the gravity coming from the tumbling, so that down is toward the nose of the ship. Such a shape would grant more room for the crew than if you only had a rocket or pure cylindrical shape, since the only space on the ship that would matter to the crew under tumbling rotation is anywhere inside near the nose. Tumbling for gravity is also arguably the easiest type of realistic gravity types if your vessel is an SSTO.

Rolling: To roll and get the same gravity from 100-200 meters you would need tethers deployed at or near that length. The alternative is to build a  spaceship 100-200 meters wide, which frankly would make reaching orbit with it a proper pain. Generally speaking, oblong shapes are more reasonable to reach oribit than fat ones because the less air resistance the easier it is to reach orbit in the first place. The main advantage of rolling is it can open up more space for gravity for the crew, yet such an internal structure would not favor SSTO's but rather pure orbiters.

Edited by Spacescifi
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The whole ship rotating end over end would cause problems for the engines in terms of the direction to thrust. You could mount them in the middle and rotate around them, or you could restrict the rotation only to the crew compartments part of the ship, like the Leonov from 2010.

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Posted (edited)

Interesting topic. My own writing featured a rotating spacecraft, so I've done a bit of research and done a bit of thinking about possible designs. With that said, I'm not an engineer, so there will be a ton of things that I haven't considered.

I found SpinCalc  to be a useful tool for establishing spacecraft dimensions. Basically, it lets you play around with four interlinked parameters:  radius, angular velocity, tangential velocity, and centripetal acceleration (i.e. artificial gravity) and see how they all affect each other. It also gives you an idea of whether a particular parameter is crew-tolerable or not, although I'm not sure how it calculates tolerability. That's not a particular problem though - if you want to be more conservative, you can just adjust the parameters to suit.

For example, plugging 1g centripetal acceleration and 100m radius into the calculator gives me an angular velocity of 3 rpm and a tangential velocity of a bit over 31 m/s. So, whilst the ship isn't spinning particularly quickly, the crew would definitely notice the view outside whipping past at 31 m/s. Incidentally, the calculator marks up that tangential velocity with a yellow flag, meaning that it would be too fast for immediate crew comfort (although some authors disagree) and would require them to acclimatize.

A rotating body will rotate about its centre of mass.  So, taking your T-shaped ship and assuming that both short 'arms' of the T have equal mass (for simplicity), the ship will tumble about a point somewhere along the long body of the T. If all the mass is up at the crew compartment, then the centre of mass will be further up towards that end of the ship - which means that the crew compartment isn't as far from the centre of mass, giving a shorter effective tumbling radius for creating artificial gravity. Conversely, if all the mass is at the other end of the ship, near the engines, propellant tanks etc.,  then the crew compartment will be further from the centre of mass, giving a longer effective radius for creating artificial gravity.

Centre of mass in any ship is likely to be strongly affected by the location of the propellant tanks - and will change as the propellant tanks empty out.

How easy it is to get a ship rotating about an axis will depend on its moment of inertia about that axis. Moment of inertia is a function of mass and distance from the axis of rotation. So, consider two equally massed spacecraft attached by a tether. The centre of mass for that system will be at the mid point of the tether and the moment of inertia will depend on the length of the tether. Long tether = masses are further away from the centre of mass = higher moment of inertia. On the other hand, consider two balloons tied to opposite ends of a brick. Here, most of the mass is going to be the brick itself (unless you have really heavy balloons  ), so most of the mass is a short distance from the centre of mass, so the moment of inertia for the system is going to be fairly low.

If you're relying on tethers, you need to consider how the tethered compartment interacts with the rest of the ship and how the crew get in and out of it.

Edited by KSK
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Posted (edited)
1 hour ago, Vanamonde said:

The whole ship rotating end over end would cause problems for the engines in terms of the direction to thrust. You could mount them in the middle and rotate around them, or you could restrict the rotation only to the crew compartments part of the ship, like the Leonov from 2010.

How would it cause problems for the engine's direction of thrust?

Main engine would not do the tumbling, RCS thrusters would.... which would also stop the tumble when necessary.

And in real life would not the rotation of the crew compartment cause the rest of the vessel to counter spin?

Just like would happen if you tried slinging objects by spinning them with tethers from a spaceship?

Edited by Spacescifi
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3 hours ago, KSK said:

Centre of mass in any ship is likely to be strongly affected by the location of the propellant tanks - and will change as the propellant tanks empty out.

This is a huge issue in real life. A 2003 NASA study of a crewed mission to Callisto examined having a bi-modal NTR spacecraft turn end over end for artificial gravity, but at a certain point there wouldn’t be enough fuel to maintain the CoM for it- so the crew would have to spend a full year or so in microgravity on the return voyage.

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Posted (edited)
1 hour ago, Spacescifi said:

And in real life would not the rotation of the crew compartment cause the rest of the vessel to counter spin?

I don’t quite understand why you want this if you’re creating artificial gravity by having the whole vessel tumble.

If your T-shaped spacecraft is tumbling end over end, then the centrifugal force will be acting along the long axis of the ship towards the cross-cylinder (and also towards the other end of the ship but we don’t care so much about that in this scenario). In other words ‘down’ will be towards the nose of the ship anyway - which is what you said you wanted. There’s no need for a centrifuge.

Edited by KSK
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12 minutes ago, SunlitZelkova said:

This is a huge issue in real life. A 2003 NASA study of a crewed mission to Callisto examined having a bi-modal NTR spacecraft turn end over end for artificial gravity, but at a certain point there wouldn’t be enough fuel to maintain the CoM for it- so the crew would have to spend a full year or so in microgravity on the return voyage.

If I recall correctly project Orion was supposed to tumble for gravity on a planned mars trip.

And Orion would have lots of extra fuel anyway for it's size so it should not run out merely by tumbling.

Just now, KSK said:

I don’t quite understand why you want this if you’re creating artificial gravity by having the whole vessel tumble.

If your T-shaped spacecraft is tumbling end over end, then the centrifugal force will be acting along the long axis of the ship towards the cross-cylinder. In other words ‘down’ will be towards the nose of the ship anyway - which is what you said you wanted. There’s no need for a centrifuge.

I don't.

I was merely playing devols advocate with Vanemonde's idea.

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Posted (edited)
11 hours ago, Spacescifi said:

I can see a T-shaped cylindrical shape being quite useful for a tumbling spaceship, since the broadside frontal cylinder could have a rotating inner habitat cylinder in it which rotates and stops itself in the direction of the gravity coming from the tumbling, so that down is toward the nose of the ship.

As far as I can tell, the rotating inner habitat cylinder is not required to make ‘down’ be towards the nose of the ship.

Edited by KSK
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13 minutes ago, Spacescifi said:

And Orion would have lots of extra fuel anyway for it's size so it should not run out merely by tumbling.

You’ve got that backwards. It’s not that tumbling makes the fuel run out, it’s that the fuel running out causes the tumbling behaviour to change.

I haven’t read the study that @SunlitZelkova refers to but it sounds like once enough propellant had been used, the vessel’s centre of mass was shifted sufficiently close to the crew compartment (or may even have been within that compartment) such that spinning the vessel was no longer useful for creating artificial gravity. Happy to be corrected.

This is all quite general - it doesn’t matter whether the propellant is a tank of liquid methane or a rack of nuclear pulse propulsion units.

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3 hours ago, KSK said:

You’ve got that backwards. It’s not that tumbling makes the fuel run out, it’s that the fuel running out causes the tumbling behaviour to change.

I haven’t read the study that @SunlitZelkova refers to but it sounds like once enough propellant had been used, the vessel’s centre of mass was shifted sufficiently close to the crew compartment (or may even have been within that compartment) such that spinning the vessel was no longer useful for creating artificial gravity. Happy to be corrected.

This is all quite general - it doesn’t matter whether the propellant is a tank of liquid methane or a rack of nuclear pulse propulsion units.

Yup!

Except I misremembered. It was an MPD thruster vehicle, not BNTR.

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Posted (edited)

I’d also recommend it to @Spacescifi as a very nice and accessible reference article for near-future deep space missions based on a variety of propulsion systems.

Edited by KSK
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i think the choice really depends on how good your engines are. if your engines are going to be on most of the time and providing low thrust, id go with roll. if you are mostly doing intermittent burns with chemical engines, you are probibly better off with the tumbler. the difference is that the tumbler needs to be reeled in for each maneuver, barring some really complicated and failure prone control system. if you got a torch drive, you don't need either.

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Posted (edited)
23 hours ago, Spacescifi said:

How would it cause problems for the engine's direction of thrust?

Main engine would not do the tumbling, RCS thrusters would.... which would also stop the tumble when necessary.

And in real life would not the rotation of the crew compartment cause the rest of the vessel to counter spin?

Just like would happen if you tried slinging objects by spinning them with tethers from a spaceship?

doing maneuvers on a rotating space craft is pretty straight forward mathematically. im pretty sure nasa already uses it on spin stabilized space probes. rcs thrusters perform differently while in a rotating frame. say you want to do a pitch moment, and you get a yaw instead.  its sort of like how burning normal/antinormal rotates your orbital plane along the axis from where you perform the burn, to the center of the thing you are orbiting. and its pretty much the same thing, just replace gravity with structural binding and also are constrained to circular orbits by structure. there is some vector math for this kind of thing but it escapes my memory.

Edited by Nuke
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A T shape is not necessarily stable in rotation.

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Maintaining a full 1g isn't really necessary: Earth has the highest gravity of anything you could feasibly be landing on in our solar system and (after discounting Venus for obvious reasons) nothing else comes close to that; Mars and Mercury are both less than 0.4g, Ganymede and Luna (Earth's Moon) are less than 0.2g and it only goes down from there. Assuming that this hypothetical spaceship is going to Mars, why maintain a full 1g when reducing it to Mars gravity will help the crew adapt to the conditions they'll find on the surface, reduce the mechanical loads on the ship's structure and will also help to negate the disorientation that can arise when your senses disagree on whether you're standing still or moving.

As for a T-shaped ship, it would be vulnerable to Dzhanibekov effects which would be very unpleasant for anyone on board. See the EVA experiments kit added in KSP 1.11 and at 22:20 in this video, for example:

Not something you want to have to put up with for years in space.

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9 hours ago, Nuke said:

i think the choice really depends on how good your engines are. if your engines are going to be on most of the time and providing low thrust, id go with roll. if you are mostly doing intermittent burns with chemical engines, you are probibly better off with the tumbler. the difference is that the tumbler needs to be reeled in for each maneuver, barring some really complicated and failure prone control system. if you got a torch drive, you don't need either.

This isn’t entirely correct. A tumbler doesn’t necessarily have to have tethers. The NASA IPP Mars mission (von Braun’s from 1969) was elongated enough so that if the two craft that would fly it were docked at the front, it could tumble to generate meaningful artificial gravity.

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7 minutes ago, SunlitZelkova said:

This isn’t entirely correct. A tumbler doesn’t necessarily have to have tethers. The NASA IPP Mars mission (von Braun’s from 1969) was elongated enough so that if the two craft that would fly it were docked at the front, it could tumble to generate meaningful artificial gravity.

Agree, on lots of the nuclear powered designs you had the engines, shadow shield, fuel, an tower with some systems midway and the crew quarter at the front.
This works if the trust period is short as in some days who it would be with NTR.

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3 hours ago, jimmymcgoochie said:

Maintaining a full 1g isn't really necessary: Earth has the highest gravity of anything you could feasibly be landing on in our solar system and (after discounting Venus for obvious reasons) nothing else comes close to that; Mars and Mercury are both less than 0.4g, Ganymede and Luna (Earth's Moon) are less than 0.2g and it only goes down from there. Assuming that this hypothetical spaceship is going to Mars, why maintain a full 1g when reducing it to Mars gravity will help the crew adapt to the conditions they'll find on the surface, reduce the mechanical loads on the ship's structure and will also help to negate the disorientation that can arise when your senses disagree on whether you're standing still or moving.

As for a T-shaped ship, it would be vulnerable to Dzhanibekov effects which would be very unpleasant for anyone on board. See the EVA experiments kit added in KSP 1.11 and at 22:20 in this video, for example:

Not something you want to have to put up with for years in space.

So in other words, T-shaped vessel may have to exhaust more RCS propellant for stabilization than a more balanced shape?

On the other hand one could exploit the instability.

For example a T-shaped space fighter could use it to flip around suddenly to turn toward a target while drifting or to be unpredictable in it's movements.

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On 6/10/2022 at 12:32 AM, Spacescifi said:

How would it cause problems for the engine's direction of thrust?

Make a KSP ship, start it rotating end over end, then fire up the engines and watch what happens. It's going to follow a spiral or corkscrew or other whonky path, unless you figure out a way to keep the engines always pointing in the same direction while the rest of the ship spins.

Quote

And in real life would not the rotation of the crew compartment cause the rest of the vessel to counter spin?

Yes, it would need a counter-weight rotating the other way. Presumably the Leonov from the movie has internal fly wheels or some such.

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Posted (edited)
26 minutes ago, Vanamonde said:

Make a KSP ship, start it rotating end over end, then fire up the engines and watch what happens. It's going to follow a spiral or corkscrew or other whonky path, unless you figure out a way to keep the engines always pointing in the same direction while the rest of the ship spins.

Yes, it would need a counter-weight rotating the other way. Presumably the Leonov from the movie has internal fly wheels or some such.

I never intended to rotate under thrust, rotate while drifting was the plan.

Also the thing about CMG's and reaction wheel counterweights is that they will become saturated over time. Forcing a vessel to use RCS to counter the failure of the CMG's.

Unless you are saying that two counter rotating wheeld cancel each other put from rotating the entire spaceship?

If that is the case then I would expect a manned orbiter transfer vessel to be a long rocket with an array of counter rotating arms with habitat pods at their ends.

At the very least I would say it would need to counter rotating arms with habitat pods.

Even better is something with two counter rotating rings... held to the main engine rocket by 100 meter arms.

Edited by Spacescifi
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Posted (edited)
20 hours ago, Vanamonde said:

Yes, it would need a counter-weight rotating the other way. Presumably the Leonov from the movie has internal fly wheels or some such.

im not sure large counter torque wheels are even required anymore from building various large centrifuge ships in ksp. counter torque could be provided by rcs. once the centrifuge is spun you only need to counter any friction loss at the bearings (which can be very small with some bearing types) and losses from maneuvers. there is a point where the extra propellant requirements will make the high angular velocity rotor a viable alternative, but thats all a matter of the mission design. it may be just as viable to leave it at the space dock to save the mass. ive built some with rotors (usually an equipment bay with a motor and some ore tanks inside spinning at high rpm), some without and it can work either way.

Edited by Nuke
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Sadly an experiment with small centrifuges is claimed to have had zero effect on preventing intraocular presssure on eyes.

Not sure if scaled up rotational designs will make any difference.

If not then that means that means that even rotational gravity is not enough for crew health.

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The centrifuge in that vid did not help with the eyes because it was so small that the subject's eyes were nearly on the axis and thus experiencing almost no acceleration. It doesn't disprove the concept.

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46 minutes ago, Vanamonde said:

The centrifuge in that vid did not help with the eyes because it was so small that the subject's eyes were nearly on the axis and thus experiencing almost no acceleration. It doesn't disprove the concept.

I thought the small size may be the cause.

That's good. Means there is still hope for manned space travel.

It hinges most on effective gravity via rotation, since 1g via torchship is not realistic and who knows if it ever will be.

Even those would need rotating arms since in orbit you are not under thrust.

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