Big idea: Building a rotating space station with artificial gravity isn’t a far-out sci-fi idea. The idea has its roots in firm, realistic engineering & science.
Let’s start here Rotating space stations in fact and science fiction. This is an image of a Rotating O’Neill cylinder space station, see the animation here.
Next we see a fictional space ship from the classic sci-fi TV series, Babylon 5. This is an Omega class destroyer. As you can see, the main engines are in back as one would imagine in a typical spaceship.
When the main engines are not on there is no acceleration, so everyone inside would be in zero-gee. That is why there is a rotating section. People living in the rotating elements far from the axis would feel something like gravity (depending on the diameter of the ship, likely only a small fraction, such as 025 gee.
Babylon 5 – EAS Charon Omega Class Destroyer
The rotating sections simulate gravity. This would be useful for people spending long periods of time in a spaceship or space stations
Again, above we see an Omega class destroyer from Babylon 5: There is a problem with the spaceship shown above: My friend Albert points out
That ship wouldn’t work unless it had jets on the main hull to stabilize it. There’s nothing holding the rest of the ship “in place”. It would start spinning because of conservation of angular momentum. Same reason you can’t have a helicopter without the back rotor generating thrust.
That’s a great point. I found a discussion of this physics from the people making the show. They knew about this issue. They took time to get most of the science, measurements, and scale correct. But they were producing this show on a weekly basis in the 1990s. Cost and time constraints led them to avoid discussing or showing such details in-show. Be that as it may, the science on Babylon 5 was leaps and bounds ahead of most science fiction TV shows.
But let’s expand on Albert’s point. Are such rotating ships or stations controllable? They have counter-intuitive physics. They can become unstable easily
Consider the tennis racket theorem, aka Dzhanibekov Effect, aka intermediate axis theorem.
See it demonstrated here, with a (you guessed it) tennis racket! Tennis racket theorem GIF demo
It is named after Soviet cosmonaut Vladimir Dzhanibekov who noticed one of the theorem’s logical consequences while in space in 1985 – although the effect was already known for at least 150 years before that.
The theorem describes the following effect: rotation of an object around its first and third principal axes is stable, while rotation around its second principal axis (or intermediate axis) is not.
Dzhanibekov effect, Intermediate axis theorem, Tennis racket effect (NASA)
In many situations spaceships or space stations would demonstrate such behavior.
In theory such motion is completely predictable and deterministic; the motions follow from a standard analysis of classical mechanics. But in practice, a human making the station or ship move wouldn’t be able to do this physics in their head. We might want the ship to speed up and move right, but when thrust is applied this counter-intuitive motion would likely occur.
There are two solutions:
One solution is to use a layer of computer control between the navigator/pilot and the actual thrusters. The pilot inputs the path desired, and the computer works backward from the desired path, figuring out which directions thrust should be applied (and which ways gyroscopes should be spun)
Another solution is to have two separate rotating sections, aka counter rotation. This reduces the likelihood of counterintuitive motion and makes it easier for a human to engage in manual control. It increases the stability of the design overall.
This is the same reason that helicopters have two counterrotating rotors.
This does introduce some engineering costs. Torsional stresses exist at every place where sections rotate. So two such sections would double the maintenance and associated costs.
Here is an example of a hypothetical rotating space station with counterrotating rings. This GIF from a Reddit user at Reddit Kerbal Space Program
One of the science folks I spoke with pointed out:
Now, one would think that such a centrifuge would act as a titanic gyroscope, doing its best to prevent the ship from changing its orientation.
The obvious solution is to have two counter-rotating centrifuges, so their torque cancels out. Just like contra-rotating propellers on an airplane.
Alternatively you can use one centrifuge plus a monstrous counter-rotating flywheel with the same mass.
Aerospace Engineer Bill Kuelbs Jr points out that if the centrifuge is a sufficiently large percentage of the ship’s total mass, it will not prevent turning. What it will do is alter the axis of any turning force by ninety degrees. The technical term is gyroscopic precession. Rev up a toy gyroscope and try to turn it and you’ll see what I mean.
The solution to that is fairly simple. The turning thrusters will have to be effectively at ninety degrees to where you’d expect.
In reality, this means that when the centrifuge is spinning, the “pitch the nose downward” control button will actually fire the “yaw to the left” thruster.
Very few helicopters have two counterrotating rotors. It’s much easier to manage the problem through adding a thruster – a tail rotor – than to build the complex mechanics and the blade angle control required from double rotors (in a smaller helicopter – if it needs two rotors anyway things change).
Example: The Mote In God’s Eye
The Mote in God’s Eye is a science fiction novel by American writers Larry Niven and Jerry Pournelle, first published in 1974. The story is set in the distant future of Pournelle’s CoDominium universe, and charts the first contact between humanity and an alien species.
Here is a visualization of one of the spaceships in the novel, the I.N.S.S. MacArthur, made by William Black.
Jonathan Cresswell-Jones writes
To get some Niven overlap, the design of INSS Macarthur in MOTE spun the whole ship for simulated gravity on long periods on station (using flywheels to create the spin), then stopped the spin to accelerate for shorter periods of time.
I assume the flywheels would be stationary during acceleration mode, since the gyroscopic effects would be undesirable in battle and a damaged flywheel could explode if spinning.
Nicholas Bretagna II expands on this idea
The flywheel would be the equivalent of the counter-rotating cylinder.
Consumption of angular momentum means to spin it one way, something has to spin in the other (alternately, eject reaction mass to convert linear motion to angular momentum, but that’s not part of this idea, but IS how things are mostly done right now).
So — assume the flywheel spun ONE way, and it was along the central (“spin”) axis of the ship. Then the ship would spin the OPPOSITE way, with a clear mass-rotational speed relationship (e.g., the flywheel mass related to the ship mass, the flywheel speed related to the ship “spin” rate.
To slow the spin “for gravity”, you slow the flywheel back down. The “gravity” spin would slow in a proportional manner. When the flywheel stopped, the spin gravity would be gone, because that is where the whole system started (I’m ignoring losses to to friction, entropy, etc., of course).
Hence your “I assume the flywheel would be stationary during acceleration mode”, is unnecessary. It’s inherent in the overall design concept. You spin it up to get the ship rotating in the opposite direction, you spin it down to stop the spin, both reaching zero at nominally the same instant.
External Links
The Bizarre Behavior of Rotating Bodies – Veritasium – The Dzhanibekov Effect or Tennis Racket Theorem
Artificial gravity/Atomic rockets
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