C J Cherryh's Downbelow Station in KSP

[MerlinsMaster]

Hey Guys,

I'm currently reading Downbelow Station by C J Cherryh, and I thought it would be fun to recreate the titular space station in KSP.  There is, however, a problem I'm running into.  In the book, all of the docking ports on this space station are on the ring, not the hub.  Given the diameter of the station and how fast it would have to spin to generate 1g, the surface of the ring is moving by at about 73 meters per second, way too fast to even hope to perform a successful docking.

So to anyone here who has read the book or is even just familiar with the story, what am I missing here?  Cherryh's work in science fiction is for the most part pretty well grounded in scientific accuracy, so how is it she did not think of this?  

If anyone has any insight into this subject, I would really love to hear your thoughts.

Thanks.

[eddiew]

I'm not familiar with the story, but traditionally the plan is to dock on the central axis of a rotating space station because you just have to spin at the same speed. See 2001, Interstellar, and Elite I-IV for examples  

Whether you would want to recreate this in KSP is another matter, but you probably could match the rotation of a station as long as you have an axial docking port on the approaching vessel.

[MerlinsMaster]

1 minute ago, eddiew said:

traditionally the plan is to dock on the central axis of a rotating space station because you just have to spin at the same speed. 

That's true, which is why I decided to make this post.  It seemed silly to even ask, as I know already that there is no good way to dock on the ring, but I guess I wanted to make sure that I wasn't missing anything.

[eddiew]

I mean, you could do something a little weird like grapple onto the hub and move down to the ring via a robotic arm before actually 'docking'... but then the arm has to hold the full weight of the ship at full extension. Maybe have an elevator platform that rides down one of the spokes?

Or you can stop the station.

Or you can have the station be like a tyre with an inner tube; the inner tube is spinning, but the outer edge is stationary. You do have a problem of getting from the tyre to the tube though. Sooner or later someone's gonna lose an arm...

[Vanamonde]

She did think of it, but assumes that crews/autopilots are competent to perform the maneuver. 

And she's one of my favorite authors.

[MerlinsMaster]

3 hours ago, eddiew said:

I mean, you could do something a little weird like grapple onto the hub and move down to the ring via a robotic arm before actually 'docking'... but then the arm has to hold the full weight of the ship at full extension. Maybe have an elevator platform that rides down one of the spokes?

That's crazy, I was thinking that exact thing.

[MerlinsMaster]

2 minutes ago, Vanamonde said:

She did think of it, but assumes that crews/autopilots are competent to perform the maneuver. 

And she's one of my favorite authors.

I like her too.  I read the Chanur series first, and now I'm just starting the Union-Alliance books.

As for the ships being able to perform that maneuver, the problem is that the lateral movement of the ring is pretty fast, and the direction is constantly changing.  A smaller ship, like the Rocinante in the Expanse, for example, might be able to do it because it is so maneuverable.  But the Norway, as best as I can tell, is roughly the size of a medium container vessel, and I have considerable doubts it would be possible.

Also, I was just discussing this with Winchell Chung from Atomic Rockets, and he expressed a concern that the Norway docking with the station would put the station wildly off balance.  He said it would be like putting a cinder block in a dryer and running it.

[Gargamel]

Well, there's the interstellar method, which is to kinda guess where the moving docking port is and just slam into it and pray....

 

[MerlinsMaster]

2 minutes ago, Gargamel said:

Well, there's the interstellar method, which is to kinda guess where the moving docking port is and just slam into it and pray....

But in that situation, the docking port, while spinning, was only moving in one direction.  Coop only had to match that direction, and then match the spin.  In my scenario, the direction is continually changing.  It is changing in a predictable manner, however, so it's conceivable that with a good enough RCS system and sufficient manueverability, that it could be done.

[Gargamel]

39 minutes ago, MerlinsMaster said:

But in that situation, the docking port, while spinning, was only moving in one direction.  Coop only had to match that direction, and then match the spin.  In my scenario, the direction is continually changing.  It is changing in a predictable manner, however, so it's conceivable that with a good enough RCS system and sufficient manueverability, that it could be done.

I've had spinning stations, albeit much slower, that MJ was able to handle.  Took me a while to figure out what was giving it so much trouble, and taking manual control, which usually works, just made things worse, as I was expecting a stationary target.    I'd be curious to see if MJ would be able to handle such a target. 

[AVaughan]

You can simply plan a path that is tangent to the docking port, and time your arrival so the ship and the docking port both arrive at the point where tangent touches the circle, with the same velocity and at the same time.  In the real world you could aim for a 0.2 metre gap, and have the docking clamps reach out and grab the ship at the point of closest approach.  (You could start the ship rotating during it's approach, so that at the moment of docking the ship is motionless from the perspective of the docking port/clamps). 

From memory they pump water around to adjust the center of mass to compensate when things dock/undock.  But stations have gone unstable in that universe (typically stations that aren't being properly managed).  But in the real world you probably couldn't transfer enough water fast enough to handle something as massive as Norway docking to the rim of a station at 1g, but Cherryh's worlds are more space opera rather than hard science fiction, so we aren't dealing with the constraints of the real world.  (In the real world you could probably thrust inwards for a few minutes and maintain a powered orbit around the rotation point of the station, which would give enough time for the docking ports/clamps to engage and latch, and allow the station time to transfer fluid to maintain its center of rotation).

Edited June 24, 2018 by AVaughan

[MerlinsMaster]

44 minutes ago, AVaughan said:

You can simply plan a path that is tangent to the docking port, and time your arrival so the ship and the docking port both arrive at the point where tangent touches the circle, with the same velocity and at the same time.  In the real world you could aim for a 0.2 metre gap, and have the docking clamps reach out and grab the ship at the point of closest approach.  (You could start the ship rotating during it's approach, so that at the moment of docking the ship is motionless from the perspective of the docking port/clamps). 

I had thought of that, but my concern was that they wouldn't be at the point of closest approach long enough (a couple of seconds) for the docking clamps to take hold (keep in mind, they would probably be pretty huge).  But that's probably the best option, barring tractor beams or the earlier mentioned elevator option.

[Vanamonde]

Cherryh is oddly vague about many specifics of her fictional worlds, but her idea of a station is really big. As in tens or hundreds of thousands of people can live on them on food largely produced on-station. Even a very large ship wouldn't swing the barycenter all that far. 

[MerlinsMaster]

3 hours ago, Vanamonde said:

Cherryh is oddly vague about many specifics of her fictional worlds, but her idea of a station is really big. As in tens or hundreds of thousands of people can live on them on food largely produced on-station. Even a very large ship wouldn't swing the barycenter all that far. 

Well, before you say that, check this out.

According to my research, Pell Station is about 3600 feet in diameter.  I'm not sure exactly what size the Norway is.  I'm guessing about the size of a container vessel.  It probably doesn't matter that much, as I'm sure in the 24th century, they have sufficiently advanced reaction control technology that a large ship docked with the station wouldn't be a serious issue.

[flantomas]

Also, you could put your ports to scoop up the ship as it passes near the circle, but it will be a violent docking.

The pro is that in needs less manouvering, since one you aligned, you just need to almost match the speed at wich the station is rotating.

I have ilustrated what I understood you wanted first, and what I proposed in the second graph.

https://imgur.com/a/ujvTP7w

Reference:

black = rotating station
Blue = a ship
Red = Port direction
Green = Manouver?

First time posting pictures in this forum, so no idea if it will work.

Edited June 26, 2018 by flantomas
Will never learn how to post pics, sorry

[MechBFP]

Even if the manevour was possible, it would destroy the space station (in real life anyway). You are suddenly adding a large amount of mass in a non-symmetrical way to a spinning ring. It will no longer be balanced. 

[FleshJeb]

That's a great book--I love how she presented the Downers. If you ever get into Cyteen and the Regenesis, beware that they're very dense reads.

[MerlinsMaster]

6 hours ago, flantomas said:

Also, you could put your ports to scoop up the ship as it passes near the circle, but it will be a violent docking.

The pro is that in needs less manouvering, since one you aligned, you just need to almost match the speed at wich the station is rotating.

I have ilustrated what I understood you wanted first, and what I proposed in the second graph.

https://imgur.com/a/ujvTP7w

Reference:

black = rotating station
Blue = a ship
Red = Port direction
Green = Manouver?

First time posting pictures in this forum, so no idea if it will work.

That is an interesting idea, which would be fine for a small ship, but the ship in question is huge, so like you said, it would be a violent docking to be sure.

5 hours ago, MechBFP said:

Even if the manevour was possible, it would destroy the space station (in real life anyway). You are suddenly adding a large amount of mass in a non-symmetrical way to a spinning ring. It will no longer be balanced. 

Yeah, Winchell Chung from the Atomic Rockets website had expressed the same concern with me on Twitter, which I agreed with.  Of course, this is set in the 24th century, and it's not unreasonable to assume that they have much more advance reaction control technology.

10 minutes ago, FleshJeb said:

That's a great book--I love how she presented the Downers. If you ever get into Cyteen and the Regenesis, beware that they're very dense reads.

Yeah, I've read a few of her other books, so I'm more or less expecting that to be the case.

[flantomas]

the only way then, would be docking it inwards instead of outward, if the ship passes through the center of the station, and the mass of the ship is closest to the center of the station (and it doesnt brreak your ship apart) it might work.  regretably I dont have ships/stations with those stats to test anything.