# Is it believable to have a spaceship spin around a weight for artificial gravity?

In my narrative universe, there will be ships that use a 1g acceleration for artificial gravity. However, when they are not accelerating (whether on a longer journey at a constant velocity or simply "anchored" at some random location) is it feasible for the ships to extend a weighted anchor and create a spin for artificial gravity? I am aware this tether would need to be super strong and very long. Is this a reasonable concept, or should I find another way for the crews to have relative comfort in space?

• Your "gravity anchor" should have mass comparable to the rest of the ship. – Alexander Feb 25 '20 at 21:36
• @Alexander, won't the system rotate around the center of mass? Granted, the counterweight moving much faster than the ship might present problems with the tether strength... – Matthew Feb 25 '20 at 22:21
• Yes, they would rotate around the centre of mass, so if the anchor was much further away it should reduce the required mass. And yes, the anchor wouldn't be dead weight, it would be some part of the ship which could be disconnected, maybe an engine or storage bay or something along those lines. A scene that comes to mind is in the Serenity movie, during the space battle one of the ships slingshots another ship with a tether. – Markitect Feb 25 '20 at 22:25
• projectrho.com/public_html/rocket/… – Ville Niemi Feb 25 '20 at 23:37
• It's not realistic, unless you have some sort of handwavium propulsion system that ignores Newtonian physics. The extra mass is parasitic, taking extra fuel to accelerate. What would be realistic would be to design your ship as two or more modules that can be spun about each other, as in the illustration in @flox's answer. – jamesqf Feb 26 '20 at 5:03

Yes, but there are conditions.

If we look at the formula for acceleration simulating artificial gravity:

$$a = R {({2 \pi \over T})^2}$$,

• $$R$$ = Radius from center of rotation
• $$a$$ = Artificial gravity
• $$T$$ = Rotating spacecraft period

see Artificial Gravity for additional details,

We see no actual requirement for for a full circle. Any point on the path of rotation works, regardless of whether or not the rest of the structure is there, as long as structure exists to confine loose objects and transfer the force back to the centre of rotation.

The most classic form of this is the grand circle. This allows you to walk around in a larger space, and offers nice things like structural redundancy, but a single living capsule connected to a tether cable and spinning it around a given point will result in the same apparent gravity as an entire wheel.

• This is effectively using the simple 'bucket on a rope' demonstration that proves artificial gravity works at all.

We do however have to keep in mind some things such as the Dzhanibekov effect or Tennis racket theorem when designing our layout of masses.

• Failure to account for this may result in 'interesting rides' for any passengers.

We also probably want to be smart about what we use for the masses. Carrying a lump of steel around for no other reason than to be a counter weight is counter productive... It is probably a better idea to design something like a habitation module that is played out and spun around an engineering core.

Even splitting the craft into two sections and spinning them up with thrusters can work.

As for the needed strength of the cable: As long as you aren't placing large masses 'outside' of the habitat's gravity 'ring', then the cabling needed to tether the craft is the same as you would need to safely lift the module on earth.

• The entire module's mass, spun up to simulate roughly 1g effectively weighs roughly what it would if sitting still on a planet with 1g.
• That's pretty much exactly what I was thinking. I'm just trying to figure out what I would use as the weight. I'm hesitant to use the engines because of how integral they are to a starship. As for size of ship, in thinking of something along the lines of a mini-frigate. – Markitect Feb 25 '20 at 22:28
• Why not use the ship itself as the weight, and just the human hab facilities as the spinny bit? Add some thrusters in case the tether snaps, you'll have to do the math but I bet they can be more toward the maneuvering end of the spectrum than the main drive end. – Zwuwdz Feb 25 '20 at 23:20
• The engines, fuel, and other support structures (aside from life support and some power) could just be in one big block. And because that half will be much bigger than the life support section, it'll be closer, and will be under much less acceleration. You lock them together during a boost stage. – ltmauve Feb 26 '20 at 2:49
• You should also take into account the gravity differential with radius. It would be weird to have a different gravity for your head then your feet. – ignorance Feb 26 '20 at 21:43

## Yes - This is a very reasonable way to make artificial gravity, without requiring too much support structure.

There have been early studies of this form of spacecraft, however it does encounter the following issues that (although not insurmountable) need to be considered and may reduce their practicality/viability:

• The spacecraft would need to have 2 sections. Often this is no problem, however the two sections need to be separate, functional, and yield the objectives of the mission. An obvious one as in the above image is separating habitation and rocket / thrust modules, but this may not be practical depending on the purpose of the mission, length of time the mission lasts, and the additional engines and support structure required.
• The spacecraft if undergoing acceleration or any directional change it will lose equilibrium and become unstable. The spacecraft would need to be rejoined prior to any of these operations.
• As the spacecraft rejoins, the spin rate increases to preserve angular momentum. There would need to be spin acceleration and deceleration engines to ensure the spacecraft remains stable during this manoeuvre.
• Safety is a very strong priority in space missions: The tether needs to be robust enough to hold weight, and the spacecraft must withstand constant force throughout flight, which may be a significant structural overhead.

However it is an efficient way to produce gravity without the requirement of ring-like structures, and may enable the increase in diameter to allow non-Coriolis artefacts when in operation (ie. reducing the need for a large diameter ring and all the overhead associated with them to supply the same benefits).

I would imagine it would be a good way to do manned long distance missions in smaller spacecraft with not too many directional adjustments required.

• Tested on Gemini. Bonus points if you can find a description. – Joshua Feb 26 '20 at 4:04
• The tether gives a lot of potential for storybuilding when it inevitably snaps due to some plot reasons, and the two sections get separated - e.g. the habitation modules lose their rocket/thrust modules as in your example. – Peteris Feb 26 '20 at 22:45

If I understand your question correctly, when the ship is under flight he crew experiences 1g due to the ships acceleration. And when they are in orbit you want. them to have the same level of comfort. It seems like you don’t want the orientation of the crew space to change between traveling and orbiting. So, sure, if the ship had a mass tethered to the nose of the ship, then they could set up and orbit that would give something like 1g in same plane as when the ship was in orbit.

Unless the mass was 1000 to 1 the mass of the crew space, the orbit would be very wobbly, and require correction.

It seems a more realistic design to have the crew space realign itself so they have 1g in one plane while traveling, and then, align to a radial fashion when the ships spins on its center axis to generate artificial gravity in orbit.

• Unfortunately a long cylindrical spaceship will not spin on its longitudinal axis for long. Tiny tidal friction forces acting over time will dissipate its rotational energy at a faster rate than they dissipate its rotational inertia. To reconcile this the satellite instead spontaneously begins to tumble end over end, as most asteroids do. This happened to the United States first ever satellite, Explorer 1. – Level River St Feb 26 '20 at 22:23
• @LevelRiverSt, I guess my answer wasn’t clear. I tried to reword it to make my point that realigning the crew space to different axis seems a better solution to the proposed tether and orbit around the longitudinal axis – EDL Feb 26 '20 at 22:43