Another "Spinning the ship for artificial gravity" thread

[Dweller_Benthos]

I've read a good portion of the threads on getting artificial gravity from spinning the ship/station. My question is not how could it be done, what design would it take, but the practicality of it. Sort of. Let me explain.

Say you've got a large station, in the form of an O'Neill Cylinder, the whole thing is spinning enough to give close to 1g at the outer edge, the amount doesn't really matter. These have been used a lot in sci-fi shows, the one that comes to mind and actually brought on this train of thought is Babylon 5. If you're standing inside this thing, you can look "up" and see the land curving up over your head until you're looking across the diameter of the station to the land on the other side, where everyone there is "upside down" looking "up" at you.

Since the artificial gravity reduces the closer you get to the center of the cylinder, and is effectively zero at the exact center, climbing a ladder or some other means to get to the center reduces the gravity you're experiencing until it becomes zero, or at least very small, correct?

So, here's the question, since this type of gravity isn't "real" but is determined by you having a centripetal force applied to you from being in contact with the rotating portion of the ship, if you climb to the center of the thing, then let go, won't you effectively be in a zero gravity situation, or close to it, allowing you to then float down to the "ground" because you haven't touched the rotating part of the station in order to have the force applied to you again? Say you have a small thruster pack or MMU on you, could you then maneuver around just above the ground in essential zero g to do whatever task needed to be done that would normally require a ladder?

Seems to me that would be how it would work? Until you touch the outside, spinning, part of the station, you'll essentially be in zero gravity? Let's for the sake of argument say there are no other forces acting on you like air movement and such, or they are at least negligible.

I can imagine people in such a station climbing the center to achieve zero g and then float down to the ground to see just how close they can get without touching it.

Unless I'm completely wrong, which is quite possible.

[ZetaX]

Well, if you ignore the air you can do that on earth as well: just run with oribital velocity at ground height. Doing the same against the direction of rotation will give the same effect, to float you will have not rotate with the station at all.

[FenrirWolf]

I suppose someone who drops down from the middle like that would be "floating" relative to the rest of the environment. But at the same time the rest of the environment would then be rotating at quite a pace with respect to that person.

I can't think of any sci-fi station where that would be a possible scenario though. The rotating station concepts I've seen usually have a toroidal habitat instead of an open cylinder or something like that.

[PTNLemay]

The air resistance would be hard to completely ignore, but if the inside was a vacuum it would totally work. In fact that's what it would look like trying to approach from the outside. It would be damned tricky because the outer surface would be moving so fast relative to you, if there were any protuberances it could smash against you or your ship as you tried to get close.

There was a side strip in the web-comic FreeFall where Sam puts on a pair of roller-skates the escape a space station's security teams. I don't have it anymore, but he essentially slows his rotation relative to the station until he's almost floating. I don't know if that's how it would actually work, but it's the closest thing to what you're describing that I've ever seen in sci-fi.

[Xeldrak]

Yeah, it would surely work - you should read Rendezvous with Rama by Arthur C. Clarke. There is a section in the book that basically describes exactly what you imagined.

Main problem is, that the ground will move sideways with rather dangerous speed beneath you.

[Dweller_Benthos]

I have read Rendezvous with Rama, quite a long time ago, though, but it's possible the idea stuck with me while I forgot where it came from, heh. So, aside from the rotation of the "ground" once you got there, there would be no danger of "falling".

You see, the thought for this came from the Babylon 5 show, which I mentioned. In one episode, the commander has to escape a tram that runs down the center of the station, and jumps out. There follows a tense scene where he's "falling" toward the ground, or the outer edge of the station, which is completely bunk, because there's no real danger except for the momentum he gave himself jumping from the tram and the rotation of the station once he reached the outer edge, which would take a little while, I would imagine. It would take long enough for the people on the ground to get out a mattress or fireman's net for him to land in, I would think.

[lordlundar]

I have read Rendezvous with Rama, quite a long time ago, though, but it's possible the idea stuck with me while I forgot where it came from, heh. So, aside from the rotation of the "ground" once you got there, there would be no danger of "falling".

You see, the thought for this came from the Babylon 5 show, which I mentioned. In one episode, the commander has to escape a tram that runs down the center of the station, and jumps out. There follows a tense scene where he's "falling" toward the ground, or the outer edge of the station, which is completely bunk, because there's no real danger except for the momentum he gave himself jumping from the tram and the rotation of the station once he reached the outer edge, which would take a little while, I would imagine. It would take long enough for the people on the ground to get out a mattress or fireman's net for him to land in, I would think.

Well you do have a point, there's two things that would cause a problem:

1- The centripetal environment would cause an acceleration toward the outer edge, while it wouldn't match an earth free fall, would still be quite enough to cause a significant splat. The main reason is that while here on Earth air counteracts the force of gravity during a free fall, in a rotating station the air is doing both adding to the gravitational effect AND counteracting it. This creates a funky acceleration that can fluctuate based on air density which might not be even.

2- The rotational effect causes a curving effect on any ballistic trajectories. Trying to calculate that curve on the fly (so to speak) to get a pad out to catch is extremely difficult, especially considering you don't have much of a frame of reference.

The short of it is trying to pinpoint the landing in the time frame is unlikely, and even if you could, you'd still need something much more capable than a mattress or a fireman's net.

[K^2]

The air resistance would be hard to completely ignore, but if the inside was a vacuum it would totally work. In fact that's what it would look like trying to approach from the outside. It would be damned tricky because the outer surface would be moving so fast relative to you, if there were any protuberances it could smash against you or your ship as you tried to get close.

You can speed up to match station's velocity and then use your thrusters to "hover" in. You can try it out in Space Engineers. It's a bit tricky, but totally works. But yeah, if you are not moving with respect to station's center, the whole thing will be moving past you at quite some speed, regardless of whether you are in or out.

[PTNLemay]

@ K^2

The problem with rendez-vous-ing with the station exterior is that the outer edge follows a curve, what this means is that you would continuously have to adjust your speed and heading. Otherwise, if you approach it in a straight line as most spacecrafts will travel, you will experience a very quick singular rendez-vous (which will feel a lot like crashing into the side if you don't time it perfectly). The one place where this won't be an issue is encountering the station at it's ends, near the center of rotation. Which is what they did in Rendez-vous with Rama. It's the same problem you'd experience rendez-vous-ing with the interior edge if it was a vacuum and you were just floating by, close to the inner surface. To "catch" up to the spinning cylinder you wouldn't be able to speed up in a given direction. You'd have to speed up in an arc.

@Dweller

The reason why he still feels himself being thrown down when he jumps off the tram is that he's still got some of the side-ways momentum that he had when he was riding the tram. He would experience a strong corriolis effect (seeming to start falling sideways) but if the station had a proper 1G simulated gravity, he would fall respectably fast.

EDIT:

I just realized a good analogy for rendez-vous-ing with the exterior while travelling in a straight line. It's a lot like those old western films where the train flies by the station without stopping, but it picks up a bag of mail from a hook. That's pretty much the only type of interaction you could make with the space station's outer edge. A very quick, VERY carefully timed, pick up or drop off of cargo.

Edited December 6, 2013 by PTNLemay

[K^2]

@ K^2

The problem with rendez-vous-ing with the station exterior is that the outer edge follows a curve, what this means is that you would continuously have to adjust your speed and heading.

Not really. I mean, at the face value, yes. But if you match angular velocity of the station as you pass underneath, which is really easy to do, then all you have to do to keep constant position relative to station is thrust towards the center of the station.

Think about it from inside the station. You have a ship already sitting on the floor. You get in and tun on the lifting engines. They lift you off the floor. Did you suddenly start having control issues? No. From your perspective, you are just affected by gravity. The ship is already moving with the station, and keeping its orientation with respect to station. All you have to do is manage thrust to get hover. It's no harder than flying a VTOL jet on Earth. Ok, that takes training too, but presumably, we are talking about a trained pilot.

Same thing as you approach. From the Station's frame of reference, the approaching ship is moving "up", but in effective free-fall. So all the ship has to do is start hovering when at apex, and it's going to maintain position with respect to station.

Like I said, it's very easy to test out in Space Engineers. If you like KSP, you'd probably enjoy SE as well, so consider that a recommendation. In principle, you should be able to do this in KSP as well, but such a station seems like it'd be a major pain to launch.

[Dunbaratu]

One of two things has to happen, either one of which will be a major problem:

Scenario 1 - There isn't air, or if there is the air doesn't have time to push against you with friction long enough to make you match speeds with it.

Result: If that's the case then you float down nice and gentle toward the outer floor, but the floor is moving laterally very fast and you hit it like a cymbal strike when you touch it.

Scenario 2 - The air has enough time to push against you long enough to make you match velocity with it as you pass down toward the floor.

Result: You match motion with the floor's lateral motion, but in so doing you are no longer falling nice and gently and you're falling faster. You got rid of the lateral motion problem by starting to rotate with the ship, and therefore fall.

[Dweller_Benthos]

Thanks for the answers, as I expected, it's not all cut and dry. I was mostly joking about landing on a mattress or net, but how about a large body of water? In a very large station as described in the wiki here, how fast is the ground going to be rotating past you once you get close enough to it? That page mentions 40 revolutions per hour, with a diameter of five miles. That seems to me to be pretty slow, but I don't have the math to figure it out.

That also seems to bring another thought to mind, even accounting for air resistance, getting a good head of speed while on a bike or electric car would seem to let you counter act quite a bit of the force that's creating your gravity, and would let you do some pretty awesome tricks if you hit a ramp and go airborne.

I was also thinking about using the MMU to accelerate in the same direction as the rotation once you get close to the outer edge, but came up against the same linear vs circular vectors of motion problem. But if you timed it just right, and hit the same velocity as the outside of the station just as you landed, you could probably land without too much trouble, I would think.

[Karriz]

Thanks for the answers, as I expected, it's not all cut and dry. I was mostly joking about landing on a mattress or net, but how about a large body of water? In a very large station as described in the wiki here, how fast is the ground going to be rotating past you once you get close enough to it? That page mentions 40 revolutions per hour, with a diameter of five miles. That seems to me to be pretty slow, but I don't have the math to figure it out.

Circumference of the station would be 2*pi*5/2 = 15.7 miles. 40 * 15.7 = 628.3

So, the speed would be 628.3 miles per hour, that is 280 m/s.

Actually, that seems to be surprisingly fast. I wonder if I did something wrong here?

[ZetaX]

You probably didn't. It is strange that Dweller_Benthos thinks that almost a rotation per minute of something that is 10 miles wide is slow...

[Dweller_Benthos]

The other thread I was reading had a bit more complex way to figure it out, here it is, a bit beyond me.

But you are right, something five miles across spinning nearly once a minute? Yeah, that's fast. Oh well, no skateboarding into "orbit" then, lol.

... wouldn't the circumference just be pi*d ? d = diameter? Comes out the same.

[PTNLemay]

You get in and tun on the lifting engines. They lift you off the floor. Did you suddenly start having control issues? No.

Well... yeah, actually. It sort of would wouldn't it? Like I said earlier, it has to do with the Coriolis effect.

The red dot is a person standing on the inside edge of the space station. The black dot is a person falling from the center of the station. A person flying up using rockets or jets would experience similar such problems. It has to do with the conservation of angular momentum. As you fly up you are still going sideways very quickly, too quickly for air resistance to stop you. So you would have to thrust your engines to slow you down as you were flying upwards (or speed you up as you were falling).

I do agree with you though that if you were approaching from the outside (and you did time it very carefully) you could thrust towards the station's central axis as you made your close fly-by. Then ideally latch on with a solid physical connection and "hang" there off the station's outer edge as it continued spinning. It's bound to be pretty crazy, but it probably would be a lot faster and more fuel efficient than landing on the end-caps, so maybe it would actually be something for emergencies or rapid-response teams.

[peadar1987]

Presumably you'd get some pretty crazy air circulation going on as well, a lot of shear going on to get from 0 to 1000+ km/hr in such a short distance.

[K^2]

Well... yeah, actually. It sort of would wouldn't it? Like I said earlier, it has to do with the Coriolis effect.

Coriolis effect is proportional to velocity. It does not affect hover or any sufficiently slow motion on sufficiently large station. In fact, station has to be large enough so that the Coriolis effect on the person is pretty much negligible. This is why you can't build a station with artificial gravity 10m across.

Basically, any station large enough for you to be landing a shuttle on, you won't be needing to worry about Coriolis effect.

I do agree with you though that if you were approaching from the outside (and you did time it very carefully) you could thrust towards the station's central axis as you made your close fly-by. Then ideally latch on with a solid physical connection and "hang" there off the station's outer edge as it continued spinning. It's bound to be pretty crazy, but it probably would be a lot faster and more fuel efficient than landing on the end-caps, so maybe it would actually be something for emergencies or rapid-response teams.

Seriously, it's not hard to approach a spinning station and hover to a landing. I have no idea what you are basing your conjectures on. It's not physics or experience. Physics says it's going to be almost identical to hovering in Earth's gravity, and experience with simulations confirms that.

Edited December 6, 2013 by K^2

[peadar1987]

Coriolis effect is proportional to velocity. It does not affect hover or any sufficiently slow motion on sufficiently large station. In fact, station has to be large enough so that the Coriolis effect on the person is pretty much negligible. This is why you can't build a station with artificial gravity 10m across.

Basically, any station large enough for you to be landing a shuttle on, you won't be needing to worry about Coriolis effect.

Seriously, it's not hard to approach a spinning station and hover to a landing. I have no idea what you are basing your conjectures on. It's not physics or experience. Physics says it's going to be almost identical to hovering in Earth's gravity, and experience with simulations confirms that.

My masters' dissertation was on a capture and release mechanism for a tether system for boosting payloads in orbit (and doing research for that was how I discovered KSP). You're going to have a tough time doing a normal docking procedure, as your window of opportunity is miniscule, the best option is a net placed so it intersects your outbound path, but not your inbound one.

Alternatively, have a stationary ring and reaction wheel at one end of the station. Dock to the edge of the stationary wheel, then spin it up to the same speed as the rest of the station using the reaction wheel, that way you can use solar power instead of burning fuel to match the rotation of the station, and you extend your docking window considerably.

[Dunbaratu]

I do agree with you though that if you were approaching from the outside (and you did time it very carefully) you could thrust towards the station's central axis as you made your close fly-by. Then ideally latch on with a solid physical connection and "hang" there off the station's outer edge as it continued spinning. It's bound to be pretty crazy, but it probably would be a lot faster and more fuel efficient than landing on the end-caps, so maybe it would actually be something for emergencies or rapid-response teams.

If I recall (and it has been a while now) one of the things that Babylon 5 got right about the physics is that if you are going to have a large rotating cylinder space station like that, you have to put the spaceport dock on the axis of rotation. I seem to recall that they did that - ships would approach the end of the station right on its center axis and drift inside through a giant set of blast doors. They never showed how the ships moved after that, but I imagined that the hangar bay inside was a big space with berths around the edge of it that you drifted out to and then had to match velocities with. As long as the berths are much closer to the axis than the outer floor of the station is, they'd have lighter "gravity" and slower sideways velocity to contend with.

A couple of other things they sort of "got right" - ish were that the fighters (okay, yes even HAVING some sort of manned fighter is a bit silly, I'll grant you) were launched from berths in the outer wall of the station, mounted in clamps that aimed them downward facing the "floor", so that when the clamps let go the fighters would be naturally flung outward from the station's rotation. (I note they never tried to explain how the fighters came back and got re-installed in those launch bays after use.)

They also had the notion that housing was not all at the same radius - that there were several floors of residential decks and the aliens lived in whichever deck was at the right radius to give them a fake gravity closer to what they were used to on their home planet. (And that one of the problems with the homeless living "downbelow" underneath the main floor is that they had to endure higher G forces because of where they were.)

[K^2]

You're going to have a tough time doing a normal docking procedure, as your window of opportunity is miniscule

Your window of opportunity is almost as big as you want to make it. First, we agree that we can set up a pass at an almost arbitrary distance from the station to have ship at rest at closest approach, right? There is question of precision, but we aren't trying to grab a dumb cargo drone with a station. We don't need it to pass within inches or even meters of the station. If you can give me +/- 50m, I'd shoot for a 100m approach and it'd be fine. Rest can be covered under power. Ideally, you need to go from free-fall to accelerating at 1g precisely as you pass under the station, at which point you are hovering and the rest of approach is trivial. Realistically, you can't do that, but you don't need to, either. What you do is start throttling up a few seconds in advance, which will bring up the line of closest approach closer to the station, curving in your trajectory. So long as you watch the time to closest approach, closest approach distance, and your velocity relative to the station, it's a pretty straight forward maneuver with a window in tens of seconds at 1G.

I'm sure that what you were working on required a perfect approach precision and an instant transition from inertial to accelerated movement. This is not the case with a ship being flown by an experienced pilot. It's just not the same problem.

But you know what? I'm prepared to bring it down to a simulation. I have a VTOL simulation I've written a while ago. With a few modifications, I can get it to simulate something space-worthy. All I have to do is get rid of all of the aerodynamics code, modify thrusters a bit, change the control scheme, and add a bit more info on the HUD. Oh, and I'd have to model and simulate a simple station, I guess. That will require a few extra bits of collision code, but it's no trouble.

What I need from you are parameters you are willing to accept. Specifically, with what precision you would allow me to measure my velocity and position relative to the station? I'd also take accelerometer data and combine it with tracking data via Kalman filter. So I need error on acceleration data as well. And what about the Shuttle? For simplicity, lets say maximum linear and angular accelerations you'd allow. Naturally, I need something in excess of 1G along the vertical axis. Everything else will be pretty gentle, but I'm worried if I just make up numbers you'll accuse me of rigging it.

As for the station, lets say 40 RPH as mentioned earlier? That would put the station's diameter at about 4km to provide 1G at the outer edge. That's a nice big target to aim for. I can do either a wheel-type station or a tether. Your call.

(I note they never tried to explain how the fighters came back and got re-installed in those launch bays after use.)

Yeah, they did. Fighters returned through the same docking bays as general cargo. They show it a few times. The point is that launch needs to be instant. If it takes a while for fighters to come back, get loaded onto lifts, transported to the launch bay, and re-mounted, that's fine.

Edited December 7, 2013 by K^2

[Dunbaratu]

That also seems to bring another thought to mind, even accounting for air resistance, getting a good head of speed while on a bike or electric car would seem to let you counter act quite a bit of the force that's creating your gravity, and would let you do some pretty awesome tricks if you hit a ramp and go airborne.

Yes one of the ways in which rotating for "gravity" creates some weird effects is that if you run east you "weigh" more than when you're running west. (if you define "east" to be "in the direction the floor is moving". Actually the same effect happens on Earth (but reversed because we're on the outside of Earth, not the inside - so east makes you lighter and west makes you heavier). It's just that the amount by which it occurs is so minor that you won't notice it. But on a station with a radius far smaller, rotating far faster than Earth does, it would be noticeable.

If you live on a rotating space station and have a daily workout routine, then if you want a lighter workout for your morning jog, jog west. For a heavier workout, turn around and jog east instead.

[Dunbaratu]

Yeah, they did. Fighters returned through the same docking bays as general cargo. They show it a few times. The point is that launch needs to be instant. If it takes a while for fighters to come back, get loaded onto lifts, transported to the launch bay, and re-mounted, that's fine.

I remember seeing them come back to the main hangar bay, but not how they moved from there back to the launching bays. There didn't seem to be an internal mechanism for moving them through the ship back to the bays.

[K^2]

They never specifically shown it, but it was implied that there are elevators going every which way from the cargo bay. For example, if you recall, arrivals/departures were in normal gravity section. Passenger shuttles would be carried down from the cargo bay before they'd be allowed to disembark. They've shown that in a few places. The fact that something similar happens to fighters is merely implied, but given how thought-through everything else was, I wouldn't doubt that creators thought about it.

[magnemoe]

The air resistance would be hard to completely ignore, but if the inside was a vacuum it would totally work. In fact that's what it would look like trying to approach from the outside. It would be damned tricky because the outer surface would be moving so fast relative to you, if there were any protuberances it could smash against you or your ship as you tried to get close.

There was a side strip in the web-comic FreeFall where Sam puts on a pair of roller-skates the escape a space station's security teams. I don't have it anymore, but he essentially slows his rotation relative to the station until he's almost floating. I don't know if that's how it would actually work, but it's the closest thing to what you're describing that I've ever seen in sci-fi.

Yes I posted the Freefall strip here once, it was bugged as he did not accelerate to rotation speed. Yes even skating to escape speed anyway as your relative weight would approach zero giving you no traction.

In vacuum you would be free to hover just over the floor, with air you would be dragged in the rotation direction, this would push you outward so you end up hitting the floor with some speed, downward speed would probably be low while sideway speed would be far higher.

Note you can land on Minmus flat areas in orbital speeds, yes its hard but possible, more realistic is to use an railway style linear accelerator to launch thing from moon into orbit.