Docking spaceships to ring habitats

[Strato Incendus]

As a result of one of our previous discussions, I had drastically cut down the distance between the six rings of my fictional generation ship Exodus to a mere 5 metres.

Only recently did it occur to me that the airlocks on each ring existed primarily to allow the first crew to board the ship. How do you squeeze a space shuttle (or any smaller spaceship, for that matter) in between two rings that are only 5 metres apart?

A space shuttle has a wingspan of 23.79 metres. I assume it would depend on the pilots skill in how narrow a gap they could "park" a shuttle in between two rings while docking to one of them. But my layman guess would be that there should be at least twice that distance in between the rings to avoid any collisions? So 50 metres minimum, to go with a round number. Alternatively, since my rings are 64 metres wide on the outside, I could simply put that same distance in between the rings, too.

Then again, that would only make room for a current-day space shuttle. Future spaceships may be much larger, especially such that would export colonists and all their belongings to a colony ship. The latter is the reason why I want the generation ship to allow smaller shuttles to dock to the individual rings to begin with. It would of course also be possible to dock to the ship's trunk (the "central pipe"), and then transport everything onto the individual rings using the lifts going through the spokes. But that would require the lifts to be much wider than humans are ever going to need it in their day-to-day life on the ship. And of course, it would also be highly impractical.

If you only have two rings (as many depictions of fictional space stations do), you don't face this problem yet, of course, because the airlocks would probably be on the outside of the rings, not facing each other. But as soon as you have a higher number of rings (4, 6, 8...), you run into this issue.

So, how wide would you suggest that the gap between two ring habitats be (regardless of whether it's on spaceships or space stations) to safely allow a smaller shuttle to dock to one of them in between them?

 

[.Scott]

So, as I understand it, you have multiple 64-meter rotating rings linked to a central "pipe" that is also rotating.
Docking to a rotating ring gets complicated. But if you really think that width of the lifts is an issue and docking ports on every ring would be better, here's an approach:

1) First decide whether you want to dock the entire ship or just a large removable payload segment (payload). I would suggest that only a payload segment dock. So the entire ship would be like a tractor-trailer combination, with the payload segment in the role of the trailer.
2) Keep a "pairing device" (PD) and a spare payload counter-balance (CB) handy.
3) Have the ship mate the payload to one end of the PD.
4) Adjust the mass of the CB to match the payload.
5) Have the CB mate with the other end of the PD.
6) Begin a rotation of the payload/PD/CB combination with a rotation axis parallel to the Generation.
7) Bring the rotation rate up to match the Generation spin.
8) Dock the payload/PD/CB to either end of the Generation hub. This will be the only step with any significant risk of collision with the Generation.
9) Have mechanical devices (Carriers) on the exterior of the Generation that are capable of crawling through the spokes and rings with a payload or CB.
10) Have one Carrier move the payload while another carrier mirrors the motion with the CB. This will keep the Generation from precessing in any way.
11) In this manner, the payload can be safely and precisely moved to any docking port.
12) Once docked, any major shift of mass while loading or unloading payload should be mimicked with the CB - minor changes could be accommodated with whatever stabilization system available in the Generation for this purpose.
13) Undock by reversing the procedure.

You would still need ring spacing, but this method would allow that spacing to be minimal. If you wish, the rings could touch with the docking ports on the inside ring surface - cargo coming in through the "ceilings".

 

[berkeman]

As a result of one of our previous discussions, I had drastically cut down the distance between the six rings of my fictional generation ship Exodus to a mere 5 metres.

Only recently did it occur to me that the airlocks on each ring existed primarily to allow the first crew to board the ship. How do you squeeze a space shuttle (or any smaller spaceship, for that matter) in between two rings that are only 5 metres apart?

So, how wide would you suggest that the gap between two ring habitats be (regardless of whether it's on spaceships or space stations) to safely allow a smaller shuttle to dock to one of them in between them?

Sorry, I have just stopped in because of the interesting thread title, and have no idea of the history of your post. Could you please post links to this previous discussion that you had, which hopefully includes some drawings/diagrams? Thanks.

 

[Rive]

Just dock it outside the ring. Hook up to a rail first (rail around the outer surface of the ring), then let it sync up with the rotation while 'hanging' on the rail (would look like attaching it below the floor if seen from 'inside').

You need to maintain the balance of the ring during the whole operation, of course. Docking on the ring means a shift in the axis.

 

[Strato Incendus]

Thanks for your replies!

First, because @berkeman asked, here is the link to the latest previous discussion on the construction of the ship: https://www.physicsforums.com/threads/generation-ship-sfv-exodus-revised-designs.1013958/

So, as I understand it, you have multiple 64-meter rotating rings linked to a central "pipe" that is also rotating.

Sorry, I guess I should have made this clearer in the opening post: The central pipe itself does NOT rotate. It’s the hub of the rings that rotate. The parts of the pipe in between those ring hubs do not rotate. I came up with this design independently, but it is essentially the same as in Adam Oyebanji’s novel “Braking Day”.

Generally speaking, I guess I underestimated the difficulty of docking a shuttle to something that rotates to begin with. (It certainly wasn’t easy back when I was playing the original version of Elite on Atari ^^, but at least it was doable.) If that is so hard to do without a bunch of extra tools, perhaps having the ships dock to the central pipe only sounds like a more reasonable approach again.

Then of course I would need to explain why the rings still have airlocks. The obvious answers “for spacewalks” or “as emergency exits” doesn’t really apply, when there is no other system close enough which the crew could escape to, and spacewalks while traveling at 10 to 12.5% light speed are out of the question anyway, if only because of the amounts of radiation people would get exposed to. (Or would that be prevented by the fact that they are behind the massive water tank at the front of the ship?)

For the same reason - the fact that you cannot escape to any other system if the ship breaks down - lifeboats or rescue capsules on board don’t really make sense either. Unless of course for the eventual landing on the destination planet. And that would also be the only reason left to have airlocks on the rings - since the rings are designed to dismantle into smaller sub-sections and land on the surface, to the airlocks could serve as additional doors to the new “buildings” on the planet - but only after they have landed.

Just dock it outside the ring. Hook up to a rail first (rail around the outer surface of the ring), then let it sync up with the rotation while 'hanging' on the rail (would look like attaching it below the floor if seen from 'inside').

Going through the ring floor, i.e. having the airlocks in the ground, does sound pretty cool - a little more reminiscent of certain Star Wars ships, actually, as far as I recall. Though it would complicate one particularly relevant scene where one person is talking to another in the airlock, if all the visual angles change.

Also, in terms for delivering cargo to the ship, this would be somewhat impractical, since artificial gravity points tangentially to the floor of the ring. So all the cargo would have to be lifted through the “ceiling hole” of the shuttle. Unless of course the docking takes place before the rings even start rotating in the first place - then there is no gravity, and the cargo would be equally hard or easy to transport into any direction.

 

[DaveC426913]

I cannot imagine any rationale for letting a shuttle anywhere near the rings, let alone between them.

That's just straight up ship-o-cide. Full stop.Dock at the hub. Take advantage of the zero-G to effortlessly move your cargo to a ring. Then effortlessly lower it down a spoke. Easy-peasy. Why would you do it any other way??

 

[DaveC426913]

It certainly wasn’t easy back when I was playing the original version of Elite on Atari ^^

I played on my C=64.
Fifteen minutes holding my breath as I got closer and closer - till the docking bay's perimeter was so big is was off-screen.
And then blam! You hit the edge and exploded.
*sigh*

 

[Strato Incendus]

Dock at the hub. Take advantage of the zero-G to effortlessly move your cargo to a ring. Then effortlessly lower it down a spoke. Easy-peasy. Why would you do it any other way??

I assume by “hub”, you mean anywhere in the central pipe? ;) I’m asking since you were the one who suggested “Braking Day” to me, and in that story, “hub” refers specifically to the rotating hubs of the rings in the ship’s trunk (=central pipe). And landing on one of those would require the ship to squeeze itself between the spokes, which seems equally complicated.

Therefore, if they just dock to the central pipe anywhere else, fore or aft of the rings, that might indeed be the simplest solution.

That shifts the question from how wide the space needs to be to how long it needs to be. Based on the current design, there are 100 m of ship trunk in front of the rings, and 200 m behind it. Is that long enough? I’m just thinking of the comparison to aircraft carriers, which are around 350 m long.

 

[DaveC426913]

I assume by “hub”, you mean anywhere in the central pipe? ;)

Yes.

That shifts the question from how wide the space needs to be to how long it needs to be. Based on the current design, there are 100 m of ship trunk in front of the rings, and 200 m behind it. Is that long enough? I’m just thinking of the comparison to aircraft carriers, which are around 350 m long.

By what criteria? Plausibility?

That depends on its function. Your docking bay will only be used for cargo at the beginning and end of the journey.

Does Exodus have shuttle craft? Well, yes, it'll need them to make planetfall. I can see a case for keeping the shuttles compacted in storage for the trip. They'd probably take up a lot of room if they had to be operational during the journey.

And why would they? There's very little reason for needing a shuttle during the journey. If you needed to do repairs, you could have a much more compact and utilitarian work platform for ship repairs - and you would make it heavily shielded. it would act as both a shelter a shield for workers outside - little more than a box with attitude jets and heavy shielding.

 

[Strato Incendus]

Does Exodus have shuttle craft? Well, yes, it'll need them to make planetfall. I can see a case for keeping the shuttles compacted in storage for the trip. They'd probably take up a lot of room if they had to be operational during the journey.

Yes, they do have landing ships - which of course could technically be counted as lifeboats in a pinch, too. But they wouldn’t have enough fuel to get anywhere within a human lifetime if something happened to the main ship during the journey.

Since the central pipe has a diameter of 100 m, there should be more than enough room to contain these shuttles within the ship.

The landing ships do in fact present a plot-hole problem for how I plan to end the story, with one person having to stay behind on the ship, to steer the landing of the individual modules after the autopilot for this procedure has failed: If there are landing ships, that person could just follow the others after being done on the bridge.

But I’ve found that usually, if existing technology creates a plot hole, rather than claiming the ship designers had somehow forgotten about this very important thing, it’s better to include it in the ship construction, and then have some maniac destroy this particular piece of technology during the trip. The advantage of that approach is that, while you can include pretty much any kind of technology in the ship before it leaves, once it’s gone, it’s on its own, and some things that break might not be replaceable anymore.

In this case, somebody who absolutely wants to prevent the crew from landing on the surface has every reason to try and blow up the shuttles (or rather: Launch them too early and have them crash into one of the gas giants in the target system).

I’m considering a similar in-story explanation to get rid of the sperm bank that was included in the ship at the start, so that, once additional genetic material is gone, the issue of maintaining genetic diversity on board becomes more pressing. Here the idea is that the first generation born on the ship, some of whom were not happy about their parents having made the choice to board the ship for them, destroy the sperm bank, hoping that their sabotage of the mission will incentivise the ship to turn around and go back home while it still can (at this point, they’re only 25 years into the 125-year-long journey). Hence, these first rebels called themselves Turning Point.

If the ship has the technology for genetic printing, it would make sense that these same rebels destroy that, too. In that sense, this explanation is superior to the alternative I had considered, with the sperm bank simply being destroyed by radiation somehow. Also, readers tend to care more about deliberate human action, since it’s tied to character motives, than cosmic accidents. I just want to include a fair share of the latter, too, since I do think that people tend to overestimate the potential for human malevolence while underestimating the threat of “the universe’s indifference to life”.

And why would they? There's very little reason for needing a shuttle during the journey.

That’s exactly what I mean by there being no lifeboats in a strict sense, since they would be pointless during the duration of the trip.

it would act as both a shelter a shield for workers outside - little more than a box with attitude jets and heavy shielding.

I’m still conflicted about the idea of spacewalks during the ship’s coasting phase. It’s obvious that you can’t get blown away from the ship as easily as you could be if you were walking on the outside of a moving train or aircraft, since obviously, there is no air or wind around that could throw you off. As we’ve established before, to the people on the ship - even if they are standing outside on the hull of the ship - it would feel like they’re stationary in space.

However, a point that PBS Space Time made is the exposure to radiation. The Exodus was originally supposed to travel at 10% light speed. The latest and apparently most viable calculation from the “Emergency Braking in Space Thread”, which was one of the “overshooting the target destination” approaches (rather than an actual emergency braking), had the ship traveling at 12.5% light speed. PBS Space Time figured that spacewalks at such traveling speeds “are out of the question”.

In terms of shielding, we of course have the two big water tanks (with several sub-tanks) on the front and back, creating the iconic dumbbell design you proposed. ;) Presumably, spacewalkers would be safe from radiation from the front and back if they were only working in between the spheres. However, they would not be shielded from the side. Then again, I would expect the majority of, or certainly the strongest intensity of radiation, to come from the front.

The problem with spacewalks and radiation would probably be particularly severe if something on the fore tank had to be fixed - either “on top” or “on the bottom” of it, or at the actual front of the ship.

Here the question remains whether the ship still has an additional ice shield in front of the fore tank. @John Strickland originally suggested this - however, this was before we introduced the dumbbell design. To my layman understanding, the water tanks provide ample shielding against radiation, but none against the barrage of interstellar dust particles. I only have the good old computer-controlled deflector lasers for that part so far.

The main problem about having an ice shield in front of the ship, at least as far as I can tell, is: What to do with the ice shield once the ship turns around for braking? Not only would the ice shield have to be moved around the entire ship, since the shield must still face forward, into the direction of travelling; but also, it would now be in front of the exhaust / thrusters. So in all likelihood, it would simply melt away, wouldn’t it?

 

[Melbourne Guy]

What to do with the ice shield once the ship turns around for braking?

Your storytelling is already starting to head butt real-world physics / engineering, @Strato Incendus, and you're ship - and novel - has barely started it journey. Maybe it's time to throw the ice shield idea away and just handwavium an electromagnetic shield? Or something else technobabble worthy, because your ship clearly can't decelerate with a giant block of ice in the way of the engines!

 

[DaveC426913]

Your storytelling is already starting to head butt real-world physics / engineering, @Strato Incendus, and you're ship - and novel - has barely started it journey. Maybe it's time to throw the ice shield idea away and just handwavium an electromagnetic shield? Or something else technobabble worthy, because your ship clearly can't decelerate with a giant block of ice in the way of the engines!

The OP has previously expressed a strong desire to avoid techno-magical solutions.
And I got to hand it to him: if you look at his thread history, you'll see he's certainly not being lazy with his research efforts.

 

[Melbourne Guy]

The OP has previously expressed a strong desire to avoid techno-magical solutions.
And I got to hand it to him: if you look at his thread history, you'll see he's certainly not being lazy with his research efforts.

Yep, I get that and applaud it as well, but at some point the impossibility of colony ships needs to accommodated. Even Alastair Reynolds, known for eschewing FTL solutions and keeping his physics hard, handwaves his lighthuggers with their techno-magical Cojoiner engines. And his colony ships in Blue Remembered Earth are similarly handled.

 

[Strato Incendus]

Your storytelling is already starting to head butt real-world physics / engineering, @Strato Incendus, and you're ship - and novel - has barely started it journey.

Barely started the journey? It's over 250,000 words in already. I do agree it's nowhere near being finished, though...

And I got to hand it to him: if you look at his thread history, you'll see he's certainly not being lazy with his research efforts.

I appreciate you appreciating that!

Yep, I get that and applaud it as well, but at some point the impossibility of colony ships needs to accommodated.

That is actually the thing: In reality, I'm pretty pessimistic about manned interstellar travel. Even when somebody says we may never even get to send people to Mars, I'm not one to accuse them of being unrealistic.

But the generation ship is the opposite of that: It's a so-called "steelman" of a position I don't actually endorse myself. So I want to give it the highest possible chance of being functional.

I could easily have the mission fail for a bunch of mundane reasons that would most likely make such an endeavour difficult to impossible in real life. But if I did that, I would be making it easy on myself, with regards to my attempt of disproving the concept.

And in fact, I've given that same motivation to the character who started Project Exodus within the story: He's a scientist trying to falsify his own hypothesis. He doesn't believe the mission will succeed - he knows the odds are stacked against the crew's favour. But a part of him still hopes to be proven wrong.

 

[DaveC426913]

TIL: "Steelman". For all my argument/debate predilections, I've never encountered that before.

 

[Nik_2213]

IMHO, you need to revisit the classic sequence in '2001' where AC Clarke clinically put a 'thumb in eye' of his critics who claimed you could never, ever dock to a rotating space-station...

That dock had on-axis spin, so a trivial solution if you switch co-ordinate systems. Any de-spun / non-rotating dock complex would be even easier.

IMHO, your mega-ship needs a 'barbell' arrangement, with massive shields fore & aft, significantly overlapping and sheltering the axial-tethered cargo. That includes non-rotating components, such as multiple big docks, propulsion, tankage etc. Spun habitats' drums may be fitted around the central axis, or parallel in sort of multi-hull configuration, or both, providing redundancy & modularity. Parallel also provides multiple local non-rotating docking ports for eg shuttles at the drums' mountings...

'Spun Sickness' ? Dramamine and its ilk, plus 'getting your sea-legs'...

 

[DaveC426913]

That dock had on-axis spin, so a trivial solution if you switch co-ordinate systems.

The problem with that design is that:

• it's not scalable. You can only dock one vessel at a time.
• there's no margin for error, so not really "trivial". You need to be bang on. And yet:
• once your shuttle has spun up, you're going to have a real tough time making precision corrections.

 

[Nik_2213]

Sorry, IMHO, is readily scalable if dock big enough to enter. Mantis/HiAb cranes grapple craft, move it aside.
Rotating reference fields are not a problem. Especially if on-axis dock has guide lasers and such.
Shuttle control is trivial to auto-pilot cued to dock's rotating reference frame...
Worst case, shuttle crew slap their 'Big Red Button' to fire bow attitude-thrusters and back away...

Besides, this is for a small station. The mega-ship configuration I described has multiple non-rotating ports, for both dock onto and into...
YMMV...

 

[Strato Incendus]

Perhaps we're approaching this from the wrong angle: In principle, I'm completely fine with the shuttles only docking to the central pipe. Among others, because it allows me to keep my rings as closely together as they are right now (5 metres apart).

The problem is more about 2-3 major implications of this:

1) The width of the lift shafts. If everything that ends up in a room on any of the rotating rings has to be transported there through the lifts, rather than going directly through airlocks on the rings themselves, the lifts probably have to be a lot wider. I figured they had to be at least wide enough to allow for the transport of patient beds in an emergency (that is, the width of hospital elevators).
There is also no particular reason to make the spokes as thin as possible, so that the lifts could only carry a few people - aside from saving on overall ship mass, of course. That argument is true for pretty much anything else on board, too, though. So it's not really an argument to leave out something crucial: If the ship needs it, it needs it, and the extra mass simply has to be taken into account.
At the same time, the lift shafts won't be as wide as the rings themselves, since that would mean the lifts would cover (=block) the entire corridor down on the ring. They're supposed to go in the middle of the corridor, obviously, with people being able to pass by the lift shaft left and right, if they wish to keep walking along the ring.

2) Depending on the shaft width: How many lifts can fit into the same spoke?
So far, I was assuming that each spoke would only contain one lift each - especially given the ship's modest crew size of 1,500 people. (That is less about how much space is available for living, and more about how much space is needed to feed all those people; much like on Earth, space is more ample than resources.)
However, I could of course also make the spokes, say, 8 metres wide on the inside, but have it contain 3-4 lifts next to each other.
This will affect the plot at one point minimum, perhaps at several. Since it will influence whether characters will have to wait for the only left in a given spoke, or have to walk / run onwards to the lift in the next section.
Given the circumference of the rings being about 1,600 m, with each section only having one lift (going through the spoke in the middle of that section), that means the lifts are 400 metres apart. If I wanted to cut that distance in half, I would need my rings to have 8 spokes, rather than 4.
Considering the small crew size, several lifts per spoke are probably rarely needed (especially when that single lift is a particularly wide one). On the other hand, when they are needed, having to wait for the only lift in the range of 400 metres can be crucial. And planning the ship without such a level of redundancy in this regard therefore feels rather irresponsible.

3) Regarding the airlocks on the rings: If they are not needed for boarding (since no shuttle can dock on them), what else are they needed for? Only for after the landing (considering that my rings dismantle and become "buildings" on the surface, so having additional exits on the lowest deck would be helpful / required)?
Spacewalks while coasting at a speed of 10% to 12.5% light speed are a bad idea - at least according to what I as a layman got from PBS Space Time. However, that's mainly true with regards to radiation - that is, for humans. It's not like anything on the outside of the shipe can get "blown away by the wind", like you would when walking on the outside of a moving train or aeroplane, because there is obviously no air resistance in space.
Hence, the airlocks could be used to send robots out into space, e.g. to make repairs on the outer ship hull. Depending on the size of those robots, the airlocks could be too small for humans to fit through. That would be a problem, since I have one crucial scene with a human inside such an airlock. And I want it to take place on one of the rings - not in an airlock in the central pipe, where all the characters are floating around. So here we have another case where the ship design affects the events: Are the characters standing inside / in front of the airlock, or are they floating?
I do have at least one human-sized android on board, though - he just happens to be deactivated during the time of the first book, but is re-activated towards the end. So if that android has been sent out into space in the past, having airlocks in place for him (and other, smaller droids) might be one justification.