What is the best design for docking onto a rotating space-station?

Question

In the distant future, space-stations use centrifugal force to emulate gravity, effectively being a cylinder (or something similar like a wheel) spinning around a central axis at high speeds.

A spacecraft is coming up to dock, but how can it dock to such a fast, spinning object?

The possible approaches I could think of would be:

1. The dock being positioned on the side of the station, hooks (or tractor beams or what-have-you) are locked onto the craft, swinging it around at break-neck speeds while pulling it in to dock. (Doesn't sound so great for the passengers or the ship.)

2. The dock being positioned on the top/bottom of the station, the craft nears the dock, spinning around its own axis to match the speed of the station before closing in. (Sounds pretty plausible, since the speeds in the center would be slower. Perhaps some dizzy passengers, nothing more.)

3. Same as the previous idea, but the dock rotates instead of the ship. (No-one notices a thing.)

4. The dock being positioned anywhere, and the space station transfers its momentum to another body, causing weightlessness inside the station and allowing the craft to dock without using additional fuel. The momentum would then be transferred back onto the station, causing the apparent 'gravity' to return. (If done slowly, no-one should mind.)

5. There is a docking structure that doesn't move relative to spacecraft, perhaps a core around which the station rotates or additional rings rotating at various speeds.

I am particularly interested in the fourth idea. Perhaps it could be done using a sealed flywheel or something of the sorts, potentially reducing the complexity of the problem by reducing the skill required to synchronise the movement of the craft with that of the station, perhaps even reducing overall fuel consumption or other neat side-effects.

My question is one of engineering. Which of these solutions would be the most realistic or practical, based on engineering difficulty, crew safety, reliability and fuel consumption?

Should there be any inaccuracies or other approaches that come to mind, please leave them in the comments below.

Answer 1

5. The dock is static, the station spins. The cargo and passengers are spun up to station speed at an intermediate internal transition point on their way from ship to station.

This makes for a more complex station structure (though not as complex as 4.) but a much simpler docking procedure. Your priories here should be considered, the highest risk stage is during docking and hence that part of the process should be simplified as much as possible. This also allows for larger ships to dock with your station without their mass affecting the rotating mass of the station.

Answer 2

I think matching velocities by "landing" the ship on the inside part of a hubless ring station may be another solution.

Using landing gear and braking until full stop, then taxi towards closest docking port, at 1g.

This avoids engineering issues related to rotating seals.

Answer 3

#2 is the gold-standard method. Observe how it is done in 2001: A Space Odyssey.

Ship lining up on docking bay

In 2001 we see the simplest configuration. Getting 1968 audiences to accept rotating space station mechanics at all was a big enough leap. The station has both of the docks it can possibly have. (One is full, hence the red lights).

That seems awfully sparse, but you can have many, many more with an elevator. Gravity is outward, so decks are cylindrical -- you "lower" a ship to a "hangar deck" (could have many decks, supporting many, many craft). This scenario imagines ground hostlers with tugs on each level, but Babylon 5 takes it up a notch.

Babylon-5's "elevator" can move in 3 axes of motion like the Star Trek turbolifts: down to a hangar deck, then radially and fore-aft to a specific docking station. Meanwhile, back at the docking hub, another elevator platform has slid into place, to await the next ship. So it can cycle in a minute or two. This is in the rotating section only, I disregard any fixed docks on Babylon-5.

Your alternate proposal is to stop the station everytime you dock. You really don't want to stop a rotating station, ever. The reason is water. Unfettered by gravity, the ordinary water humans use in the station will go to places you couldn't even imagine water going to. Some of it may get there via condensation. This water and the corrosion that follows it will be a maintenance nightmare.

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The problem with #1 is that mass and balance calculations have to be very precise, and a lot of other things have to go right, or the snatch will go pear-shaped and the spacecraft will collide with the station. That’s never good. Even naval warships respond very badly to being collided with. Such a collision is certain to be fatal for the starship and potentially calamitous to the space station as well, depending on its construction standard. Lofting additional mass from earth simply to harden the station against docking collisions is sheer madness, it might make more sense if the materiél was mined in space.

Since the station is rotating, it itself has a great deal of stored energy, and the structure is constantly under strain to contain that energy. Significant structural damage to the station could cause knock-on effects that could tear the station completely apart.

On the other hand, "from the edge" is a great way to launch craft, particularly fighter craft, as seen in Babylon-5. The centripetal force assures a clean breakaway. Note the craft still land in the hub, and then elevators move them to launch position.

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Your options 3-5 have a serious problem: you have a rotating section in the space station. See all the above structural threats I just talked about. This is a very large and complex hinge, akin to the sophistication of the trackways used to move the Chernobyl arch in place, except in use 24x7 and presumably airtight in places.

Any serious problem with the rotating section has the risk of grinding the sections of the station against each other, and if the mutual structures start digging into each other, it would do ever-increasing damage. It would then be a footrace between whether a) the friction of the station parts wrecking themselves brings them to a relative halt, or whether the structure of the station is damaged to the point of tearing itself apart.

Or both: as the station grinds to a halt, both sections' rotation averages by mass. This makes the rotation off-axis and out-of-kilter, and centrifugal forces act in a different direction than they ever did on the station, including the non-spinning section that isn't built for centrifugal force at all. The spinning sections would list like a sinking ship, with all the floors now at an angle. But it would be pandemonium on the non-spinning section, where anything not tied down would be slammed into the walls of each compartment, again, see structural damage. I don't know what fuels your ships use, but they are now smashed against the walls of the docking area... If it’s the usual binary propellant, you now have busted propellent tanks, you now have a dock fire, and by "fire" I may mean "explosion".

Spinning sections are hard, and not that practical.

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A more practical approach would be a non-spinning space station that has a non-spinning "permanent dock" right next to a spinning one, with a cable between the two ships. Cable cars walk up and down the cable. Each station is able to immediately cut the cord and thruster away from each other in a prepared "breakaway maneuver". The cable car carries enough emergency thrusters to return to either station if it's cut loose.

Answer 4

Let's take the options one by one:

1. It's very easy to fly a ship into a dock that's weightless. It's very hard to land a rocket on something that's not weightless. This option requires the later, and gains precisely nothing. No space station designer would ever seriously consider this.

2. Sounds reasonable. However, it heavily restricts the amount of docks. You only get two docks easily, and you need some insane structures to provide any additional ones.

   This also requires, that the spaceships actually can rotate at a high enough speed. That's not a given: A cargo ship would likely be only a very thin shield against micrometeorites, and some extremely lightweight structure to hold the cargo in place. It would not be designed to take large forces from the cargo in any other direction than in the direction of its single main engine. Spinning such a ship at any serious rate would just have the cargo flying away in all directions...

3. The best solution imho. You can build your docks as big as you want, you can build as many docking rings between rotating station rings as you want, and, most importantly, you can dock any kind of ship, even the most frail ones.

   You only need to add transfer sections where you spin up the cargo before you pass it into the rotating part of the station, and despin it before you pass it into the docks.

4. Again, you are objecting docked ships to gravity which they may not be designed to take. You do want to allow any ship to dock and make trade with your space station, do you?

Answer 5

cmasters answer is good. But I have an extra argument for solution one: fuel efficiency. Say the station is 1000 m in radius. Since $a_{centripetal}=\frac{v^2}{r}$, in order to provide 1G gravity at its surface, it needs to spin with surface speed $v = \sqrt{1000\times10} = 100 m/s$

Depending on technology and orbital height, this might or might not be a significant deltaV. But if your station is really big, and orbiting far from its central body, its surface rotational speed can be similar to orbital speed of incoming and outgoing transfers. That means that by using side docks, you can spare a considerable amount of braking and reacceleration, deccelerating only to speed matching that of station spin, and smoothly dock to the hooks that appear momentarily stationary to you.

Vehicle structural integrity can be maintained if you orient it so that it recieves the loads in its thrust axis.

For fast passenger liners and military ships sporting high performance engines, the saving might not worth the extra complexity, but for bulk transport between two stations in high jovian orbit, it can mean that you can entirely leave out high thrust engines form your freighters.

Answer 6

I think this is option #3.

There is a problem with centrifugal gravity, rotating things want to rotate around their center of gravity; so if the weight is not distributed quite evenly around the rim, then the center of rotation moves and the "rotation" becomes chaotic. i.e. it won't work, the structure will start tumbling (albeit in place).

For this reason you need automatic compensation, massive devices that can keep the ring balanced so as people and things move around inside your ring, they move in counter-point to keep the center of gravity (the balance point) in the center of the spin.

I think your best docking solution is to dock at the center point of the ring, around which everything is rotation. Imagine a bicycle tire with thick spokes. The rubber ring is the habitable area, the spokes are both travel tubes and "rails" carrying automatically moving counter weights. The hub is the docking station.

Let us add to the hub a transfer cylinder which can spin opposite the direction of the entire tire spinning. Now, to those inside the hub, this cylinder appears to be spinning, but to the ship outside the hub, the cylinder appears to be stationary while the station appears to be spinning.

So the ship approaches the stationary-for-it transfer cylinder, and can make an airlock with it. Objects (people and things) leave the ship to enter the hub; they are weightless inside it. They take seats or are otherwise secured to the walls. The airlock is closed. The transport ship can depart.

Then, from the space station's point of view, this spinning transfer cylinder is "slowed down" until it is fully spinning with the station, has centrifugal gravity (pointing out to its walls), and the passengers and cargo can exit the cylinder into the hub proper, where they can then take an elevator "down" one of the spokes to the habitable areas. The computers for the space station will automatically move counterweights up and down the outside of the spokes to maintain the balance with the new weights moving around.

All of this, to the passengers, would seem pretty similar to the modern experience of airlines. Waiting, strapping in, acceleration, waiting, deceleration for landing, waiting, departing the aircraft, collecting your luggage, navigating through your destination (finding the right spoke, floor, etc).

Answer 7

Space gangway.

https://thompsen13.artstation.com/projects/d9g3W?album_id=42421

This is not on the list, but closest to #1. It is less docking than mooring. Hooks extend radially out from the station - as far as several kilometers. * Ships are hooked on. Then they stay where they are. Flight control is ceded to the station which controls the ship speed to keep position at the end of its line. A gangway is extended down the line and used to transport materials and people back and forth.

Benefits

1: You do not need to pull ships in. Leave them at the end of the cable. Conservation of angular momentum means a ship pulled inwards towards the station will need to decelerate in order to maintain its position relative to the perimeter of the station.

2: You do not get the ships too close to the station, risking a collision. Or damage to the station if the ship explodes.

3: You have the ships at arms length, controlling access in case there is a threat aboard the ship.

4: If the ship loses control, is hit or departs unexpectedly it will damage the cable and gangway, not the station itself.

5. Ships might be large and there might be many of them. Mooring ships at the perimeter of a circle much larger than the perimeter of the station allows more space for ships and more / larger ships.

6. You can rearrange the position of moored ships according to need.

7. Ships depart by flying out in a straight line, on the tangent of the circle they were moored too.

*(bad geometry musings deleted here!)

Answer 8

All have their own merits and drawbacks and it would depend on exactly which devices the engineers manage to design better and various technology levels, materials available, and so on.

• For hooks to work, the cable spool could let out the cable so that the hook moves slower than the edge of the station. Once the ship is caught you can gradually slow down the letting out and start reeling it back in. Longer cables and faster rotating stations would need stronger cables. An electromagnet might also be better than a hook.
• The port will experience rotational shear stress. Ships will also waste fuel on rotating themselves every time (unless you use maneuvering gyros) - maybe it's better to have a mini hook instead. Two "arms" come out of the ship's nose and get caught by hooks from around the port. The passengers shouldn't get much dizzier than station crew unless you max out the roll jets. Just close the windows.
• Better, this way station provides the energy for matching angular velocities.
• Momentum transfer seems like it would only work for stations designed to be weightless sometimes. For a large station it would be impractical to shut down all gravity for every dock. Rotating dock is the most optimal version of this.
• Probably the easiest option, unless you want to not have a static section for some reason.

I think you also forgot: 1. Ship vectors tangential velocity to station rim to match velocities at closest approach 2. Turns sideways and coasts 3. When at closest approach burn toward station to follow orbit-like path around station 4. Quickly attach to the docking port to avoid wasting fuel

The most realistic to me seems making the ship rotate. No extra devices needed, just service the port regularly.

Answer 9

I think a non-rotating hub along the axis of the station is the best option (IMHO it is likely the only option). The incoming and leaving ships don't usually have thrust to spare, and, as others pointed out, are not necessarily designed to withstand the stress of rotation. Also, this makes it possible to have several docking ports as they no longer need to be exactly on the axis as long as the only connection to the main parts of the station is via the hub.

The design challenge is then how to join the non-rotating axial hub with the rest of the rotating station. I'm not an engineer, but it may be difficult to have an airtight seam between two massive objects one rotating about the other. Therefore it may be simpler to keep the hub in zero pressure as well as zero gravity. So at some point along the axis we need a chamber where passengers can grab a choice of slowly revolving ladders and then climb down a bit (very low "gravity" here) into an airlock.

Answer 10

Another solution is that the rotating structure need not be the outer atmosphere-retaining wall. The outer station wall does not rotate. Any approaching ship can attach anywhere. Humans board a rotating superstructure within to experience rotational gravity . Cargo is stored and easily moved about the outer gravity-less shell.

Answer 11

A small, non-rotating docking gangway seems the best bet. If it is about 3 meters in diameter, this is not a tough engineering problem. A large space station does not need to rotate very fast, so the 3-4 meter "turntable" that the gangway extends from might make one revolution every few minutes. Put in some safety joints and an internal seal at each end to allow emergency mid-gangway rotation or to compensate for movement between craft and station, and you should be good.

It would probably be safest to only connect while transferring passengers and cargo, and otherwise park a few kilometers off. Alternatively, since docking might be a tricky maneuver, I would suggest that the station employ docking shuttles with highly specialized pilots and/or computerized flight controls.

Answer 12

I want to imagine myself docking to a gigantic space station, its about 20,000 km around the edge, shaped like a sphere, and it "rotates" at about one-one-hundredth of an RPM (Revolution per minute). There is a single "docking station" like situation 2, on one of the poles, called "McMurdo bay, Antarctica". Do people at McMurdo feel dizzy? No. They can't even tell they are "rotating". If I were landing a spaceship there would I feel dizzy? No. Even though at the edge of this giant space ship, people are "travelling" about 1670 km/hour. This space ship is the Earth and to everyone on it's surface, it appears still.

In other words, the idea of "rotation" is like the idea of "movement" - in space, at speeds much slower than the speed of light, it is all basically relative, and we only feel acceleration, not any kind of "absolute speed". Go back to Einstein's thought experiment - if you were inside an rocket ship accelerating so that your feet touched the floor at a nice comfortable earth-simulated-gravity of 1g, could you tell whether it was from gravity or from the rocket engine being turned on? assuming you ignore things like seeing the walls, etc? In other words, if you are at a stop light, and you see another car roll backwards out your window, without being able to see the background for some reason, then does your brain sometimes wonder if they really are rolling backwards or are you rolling forwards? If you are on a train going 245 MPH and you drop a ball out the window, did you throw a ball at 245 mph? No, you dropped it at 0mph, its only relative to the ground that its going 245 mph.

What I'm challenging is the idea that people will be dizzy from rotating at a constant rotational speed any more than astronauts can "feel" they are going a linear constant speed of 25,000mph. If a cosmonaut, astronaut, or taikonaut has no windows to see the distant stars, i do not believe they would even be able to tell that they were rotating at all, given a low enough rotation speed. We only really feel "acceleration", or, a quick change in speed, we dont feel the absolute speed. That is true of linear speed, and I believe it would also be true of rotational speed. Given low enough RPMs.

Now, there is a problem if you were rotating too fast... but how fast? A quick google search shows that the RPM for a space station thats rotating to simulate earth gravity will be somewhere around 1 to 3 RPMs, depending on the radius of the station. Now how fast is 1 RPM, for a human mind?

Stand in the middle of your floor. Now get a stop watch and set it for one minute. Now, turn around so that you will make a complete circle in one minute. That is one RPM. Did you get dizzy? Could your brain even tell you were rotating? Close your eyes and do it. Could you tell you were rotating at all?

Its the same principle almost that Virtual Reality uses. If you close your eyes and walk in a straight line, most of the time you wont be able to do it, because your brain cant even tell what a straight line motion is. It doesnt care that much. Thats how VR can simulate walking on a path even though you are in a tiny room - it tricks you into walking in circles without your brain even knowing, because it shows you walking a relatively straight path using distorted optics. Our brains are not that sensitive to motion as we like to think.

In other words, In my opinion, option 2 is not really a problem. Other than the problem of getting two ships to spin at a very close rate of spin. If we watch a movie like Interstellar this seems quite dramatic - but only because Chris Nolan's team was showing us the background stars, planet, etc, pumped the music up, had dramatic character moments at the same time.

In reality, imagine docking against some huge rotating ship. At some distance the view screen will only show the ship. No background. Now assuming RPM is low enough, you wont even feel the rotation, just like you cant feel the rotation in your room turning around at 1 RPM. It will essentially be just like a normal docking procedure against a 'stationary' space station, except that you have to "correct" your "rotation"... which is what we do already every time Soyuz docks with the ISS. It has to make itself steady in regards to the other ship, and it has roll thrusters to accomplish that, should there be some inadvertent roll for some reason.

As you get close, and the station looms large in your view, the rotation itself becomes an illusion - you and the station are actually standing still, it is the rest of the universe that is spinning. But since you cant see that universe, you don't even notice.

This is like the illusion we have on Earth every day. I do not feel myself rotating on an axis at 1600 km/h. I see the moon and stars spinning around me if I look up and wait long enough. I do not feel myself rotating around though. I feel still. The ground feels still. The roads and buildings and mountains feel still and immovable to me. I do not feel myself rotating around the Sun either. Nor do I feel the rotation of our spiral arm aroundst the center of our Galaxy, nor do I feel the movement of our galaxy cluster in relation to others. It is relative.

And so in my humble opinion, the realistic option is number 2.

The other options will introduce unbalanced mass to the station and cause wobble, which is a much much bigger problem. As for stationary piece of the station, that involves some kind of super complicated air-tight slip ring system to mate the stationary portion to the main portion, again more problems.

And fuel required to rotate is very small. Remember essentially there's no friction in space. What if the ship has run out of fuel or thrusters are inoperable? Then either the people can get out and spacewalk or the station can send a rescue pod, like a little tugboat, that can attach to the ship and force it to rotate

Once it is rotating, there will be almost zero additional propulsion needed, it will keep spinning by itself. Just like the planets spin - even though they dont have rockets strapped to them.

Now ... I realize there is a big question here. How can people feel 1g on the space station if it's rotating, but they feel nothing at the center? What's the point of all the rotation if nobody can feel anything from it?

That goes to the crucial factor about artificial gravity, which perhaps we can alternately call artificial simulated gravitational acceleration through centripetal motion. The artificial gravity here is dependent, entirely, on the distance one is from the center of the object rotating. Just like on a merry go round, or on a ride at a fair, or even like an ice skater - when her hands are tucked close to her body, there is not much feeling on them, but when she lets them branch out as she widens her arms to a wide pose, she can feel her hands getting "heavier". In other words, at the center of the rotation, the artificial gravity is essentially zero. Only at the edge of the ship is there any feeling of artificial gravity - and that is only because your body is accelerated by being in contact with the ship, as you make your way out to the ring. You will most definitely feel the weight coming on you as you travel through the Jeffries Tubes or whatever from the central docking bay to the outer ring where everyone lives. But that does not mean there will be artifical gravity at the center of the station.

Of course until someone actually builds it.... I wouldnt say im 100% sure!!! But thanks for reading if you made it through all this.