Artificial Gravity

Chimeran · Oct 24, 2013

Greetings, programs!

So, I'm writing a story that takes place on a relatively large space station that generates most of its 'gravity' via centripetal force (artificial gravity generators also exist in this universe, but those put their emphasis on the 'fiction' rather than the 'science').

I've hit a couple of walls with figuring out how exactly 'gravity' would work overall and in various within this station, and have being having trouble finding answers to my questions! So if anyone is willing to oblige, I'd like to have a conversation about artificial gravity.

Alright, so the station basically looks like this. It spins on its vertical axis, so 'down' is toward the outer edge of the disc, and 'up' is towards the center of the disc. I've been working under the assumption that the apparent gravity experienced would be strongest at the outer edge, gradually getting weaker as you approach the center. So my first question is, is this assumption correct?

King Neptune · Oct 25, 2013

Chimeran said:
Greetings, programs!

So, I'm writing a story that takes place on a relatively large space station that generates most of its 'gravity' via centripetal force (artificial gravity generators also exist in this universe, but those put their emphasis on the 'fiction' rather than the 'science').

I've hit a couple of walls with figuring out how exactly 'gravity' would work overall and in various within this station, and have being having trouble finding answers to my questions! So if anyone is willing to oblige, I'd like to have a conversation about artificial gravity.

Alright, so the station basically looks like this. It spins on its vertical axis, so 'down' is toward the outer edge of the disc, and 'up' is towards the center of the disc. I've been working under the assumption that the apparent gravity experienced would be strongest at the outer edge, gradually getting weaker as you approach the center. So my first question is, is this assumption correct?

First, you are talking about "centrifugal force", which is actually inertia as applied to a rotating thing. "Centripetal force" is the real force that keeps a rotating from flying away from the center.
http://staff.washington.edu/aganse/blog/files/centrip.html

But yes, the pseudo-gravity would be greatest at the edge of the disc. Using centrifugal force can be clumsy. Have you ever trier to walk across a merry-go-round while it was running. It is an accelerated frame of reference that would make any movement inaccurate. Always keep in mind that is it inertia that tends to make things fly straigh away at whatever speed. If the speed isn't very high, then it would feel like being in a low gravity are with gravity off-center.

Chimeran · Oct 25, 2013

Sorry, I got into the habit of not talking about centrifugal force after learning it's a pseudo-force. Whatever forces you refer to, it's the same principle in general of spinning things really fast to produce a sensation similar to gravity, anyway.

And apparently it might not be as awkward as it initially sounds! It's not all that similar to walking on a merry-go-round, since that would basically be like trying to walk on the 'wall' of a spinning space station. More similar to trying to walk inside a Gravitron! Which isn't to say that there isn't the possibility for disorientation but apparently the primary source for that is the Coriolis effect, which humans can apparently adapt to for rotations up to 25 rpm: http://chamberland.blogspot.ca/2006/07/dangers-of-artificial-gravity.html

According to this calculator, once you hit 1g at a radius of 1km, you're already below 1 rpm, so adapting to probably shouldn't be too much of an issue. Especially if you're a born spacer who may or may not have genetic or technological enhancements to help you live better in space.

I have some more questions about specific situations, but I think I might need to make up a few sketches to more clearly illustrate what I'm talking about.

robjvargas · Oct 25, 2013

Don't forget that rotation like that can cause Coriolis effects. If you throw straight down the axis of the station, the ball will *not* go straight. Rather, it won't appear to do so.

Kevin Nelson · Oct 27, 2013

Chimeran said:
According to this calculator, once you hit 1g at a radius of 1km, you're already below 1 rpm, so adapting to probably shouldn't be too much of an issue. Especially if you're a born spacer who may or may not have genetic or technological enhancements to help you live better in space.

Agreed. If you were trying to play a baseball game on board the space station, the Coriolis force might produce some strange effects; but I don't think it would be an issue for most everyday activities.

Something else to think about: the effective gravity wouldn't have to be exactly 1 g. I suspect it's more realistic that the designers of the space station would make it as low as they could get away with. For all we know, people might actually be healthier if they lived at 0.5 g.

Randy Lee · Oct 27, 2013

Chimeran said:
I've been working under the assumption that the apparent gravity experienced would be strongest at the outer edge, gradually getting weaker as you approach the center. So my first question is, is this assumption correct?

The assumption is correct. At the axis, the "gravity" is zero.

robjvargas · Oct 28, 2013

Kevin Nelson said:
Agreed. If you were trying to play a baseball game on board the space station, the Coriolis force might produce some strange effects; but I don't think it would be an issue for most everyday activities.

Well... that's a whole other issue. If you bat a ball, it's going to get caught on the other side of the station. Since there's no actual gravity to bring the ball "back to earth."

Randy Lee · Oct 28, 2013

robjvargas said:
Well... that's a whole other issue. If you bat a ball, it's going to get caught on the other side of the station. Since there's no actual gravity to bring the ball "back to earth."

In a frame of reference outside the space station, the baseball goes in a straight line while the floor of the space station travels in a curve. Therefore, the ball will hit the floor. In a frame of reference inside the space station, the baseball will appear to fall much as it would if were thrown on a planet's surface.

robjvargas · Oct 28, 2013

Randy Lee said:
In a frame of reference outside the space station, the baseball goes in a straight line while the floor of the space station travels in a curve. Therefore, the ball will hit the floor. In a frame of reference inside the space station, the baseball will appear to fall much as it would if were thrown on a planet's surface.

No, it wouldn't. Once that ball is hit, it's no longer being centrifugally accelerated by the inner surface of the station. The exact sequence of events varies depending on the design of the station. But I think we're both presuming a large tube. It would have a velocity that would cross the empty space far more quickly than the station would rotate. Thus, it would wind up a significant distance ante-spinward of where it was hit. The people on the field would not experience the "up and down" of a pop fly.

Randy Lee · Oct 28, 2013

robjvargas said:
No, it wouldn't. Once that ball is hit, it's no longer being centrifugally accelerated by the inner surface of the station. The exact sequence of events varies depending on the design of the station. But I think we're both presuming a large tube. It would have a velocity that would cross the empty space far more quickly than the station would rotate. Thus, it would wind up a significant distance ante-spinward of where it was hit. The people on the field would not experience the "up and down" of a pop fly.

Whatever velocity was already imparted by the rotation would be transferred to the ball in addition to whatever acceleration is imparted by the bat hitting the ball. After that, the ball travels in a straight line, its direction being determined by those factors. If I draw a picture of a circle, and then put a straight line inside the circle, sooner or later the line is either going to stop or it's going to hit the circle. If there's no forces acting on the ball, it's not going to stop. Therefore it's going to hit the circle, which is the floor. It falls.

Ergodic Mage · Nov 5, 2013

I like the idea of the station using both artificial gravity and centripetal acceleration, it gives the proper impression of an effective station design.
I am not sure the docking stations in your diagram will work as expected. Not sure of exact distances but it looks like your rotating portion is approx 1/4 km in radius and the docking stations 2 km long.
Assuming the 1g at the outer wall that gives:
Radius 0.25 km
Angular Velocity 1.9 rotations/minute
Tangential Velocity of 50 km/s
Centripetal Acceleration 1 g

Now expand that out to the edge of the docking stations would give:
Radius 2 km
Angular Velocity 1.9 rotations/minute
Tangential Velocity of 298 km/s
Centripetal Acceleration 8.1 g

Ships docked at the end would be under 8gs which is too high for humans and extremely difficult with loading and unloading of cargo. Also the diagram shows the shuttle in a landing position, this would mean the acceleration is to the back of he shuttle instead of the "bottom".
I would suggest smaller, individual docking stations on or near the hull to reduce the gravity. Check out docking the Newton in Rendevous with Rama for an idea. Or something similar to Babylon 5 with a central zero g entrance in the center and then loading/unloading on the inner hull.

blacbird · Nov 5, 2013

Do you actually have a story?

I don't give a shite about the technology of "artificial gravity", nor will most readers. Star Trek, Star Wars, and any other number of space-faring stories employ some technology called "artificial gravity" on the spaceships and space stations, and nobody gives two lemur farts under a full moon in January that it's all fantasy bullshit.

What's your story?

caw

Albedo · Nov 5, 2013

blacbird said:
Do you actually have a story?

I don't give a shite about the technology of "artificial gravity", nor will most readers. Star Trek, Star Wars, and any other number of space-faring stories employ some technology called "artificial gravity" on the spaceships and space stations, and nobody gives two lemur farts under a full moon in January that it's all fantasy bullshit.

What's your story?

caw

I think your advice is extremely unhelpful. Did you read the OP's post? He's not talking about 'fantasy bullshit'. He's asking about real world physics. You may as well tell mystery writers to ignore all that bullshit about police procedure. Just make it up, noone cares whether it concords with how things function in the actual universe, right? As long as the story's good...

blacbird · Nov 6, 2013

Albedo said:
Just make it up, noone cares whether it concords with how things function in the actual universe, right? As long as the story's good...

A hell of a lot of SF works exactly in this manner. Maybe I'm reacting to other threads than this one, specifically, but we seem to have a lot of threads here that ask questions like "How does time travel really work?" and the like. It's not unreasonable to ask about the issue of actual story.

2001: A Space Odyssey featured a rotating space station and a spaceship, both providing "artificial" gravity via centrifugal force, and nobody was much concerned about the specific physics of how that would really work.

People were far more concerned about how the story really worked.

We also get a lot of questions here about "world-building", with no indication of any story involved. It all strikes me as a red-herring problem for many writers.
caw

Albedo · Nov 6, 2013

A general thread about the importance of verisimilitude in science fiction might be useful. But I don't know if that debate belongs here, where the OP has a specific issue he/she wants to solve.

Telergic · Nov 6, 2013

Yes, it's unhelpful to answer a research question with "don't do research". The OP isn't asking you for help with the story, and IMO it's insulting to suggest he or she may not have one in mind.

Anyway, one thing about rotating a station to fake gravity is the bigger the station the better it works, and the less severe the coriolis effects will seem for the inhabitants. The reason many classic space station designs are rings is to avoid wasting mass with a lot of low- or micro-gravity space, though there may be good reason to have some of this too.

James D. Macdonald · Nov 6, 2013

We have tons of threads here about story. This thread is about artificial gravity.

As it happens, just this summer I published an SF story which dealt with (among other things) artificial gravity. Also why a monastery might want to locate at a Lagrange point. (The monks use the microgravity to make perfectly-spherical candies and the vacuum to distill their brandies.)

I like to think it also has a story, but while I was working out the details of the space station the story wasn't the foremost thing in my mind.

For some (many?) SF readers, having the equations balance is part of the fun.

True, if I could figure out how to make a faster-than-light spacecraft I wouldn't be writing novels, I'd be in Stockholm picking up my Nobel Prize. Same for time travel. (I think I'll get that last year.) But still. Gravity and artificial gravity are well understood (for some values of "well understood"), and perpetrating howling stupidities on the reader will throw many of them right out of the story (however charming that story is).

amergina · Nov 6, 2013

blacbird said:
Do you actually have a story?

I don't give a shite about the technology of "artificial gravity", nor will most readers. Star Trek, Star Wars, and any other number of space-faring stories employ some technology called "artificial gravity" on the spaceships and space stations, and nobody gives two lemur farts under a full moon in January that it's all fantasy bullshit.

What's your story?

caw

blacbird said:
A hell of a lot of SF works exactly in this manner. Maybe I'm reacting to other threads than this one, specifically, but we seem to have a lot of threads here that ask questions like "How does time travel really work?" and the like. It's not unreasonable to ask about the issue of actual story.

2001: A Space Odyssey featured a rotating space station and a spaceship, both providing "artificial" gravity via centrifugal force, and nobody was much concerned about the specific physics of how that would really work.

People were far more concerned about how the story really worked.

We also get a lot of questions here about "world-building", with no indication of any story involved. It all strikes me as a red-herring problem for many writers.
caw

*Puts on Mod Hat*

Blacbird, this is the Science Fact sub board of the greater SF/F section.

As Pthom said:

Pthom said:
This special sub-forum is specifically for the discussion of science and technology: research, new discoveries, and those things that astound, overwhelm, and just can't be kept to one's self (or between the covers of a magazine).

This sub-forum is not for discussions of speculative nature. That's what the main Science Fiction and Fantasy forum is for. Please keep discussions on a factual basis.

Feel free to begin new threads for each new topic. Feel free to offer additional information to any topic. And, feel free to remain interested, innovative and above all, polite.

You might not give a shite about technology, but other people do. My suggestion is to not post in this forum if you're not interested in how the science works. There are plenty of other posts in the SF/F forum to talk about story.

zanzjan · Nov 6, 2013

blacbird said:
Do you actually have a story?

I don't give a shite about the technology of "artificial gravity", nor will most readers. Star Trek, Star Wars, and any other number of space-faring stories employ some technology called "artificial gravity" on the spaceships and space stations, and nobody gives two lemur farts under a full moon in January that it's all fantasy bullshit.

blacbird,

While I think you've got a valid point about the relative importance of story to the majority of readers, and in a thread elsewhere on "how much science vs. how much story" or something like that this would be a valid single point of input, in a room dedicated to science fact, your last couple of responses misse the mark by a wide margin, and are indelicately put to boot.

As writers, for some of us, the science is a critical part of our worldbuilding process; whether or not the entirety of it ends up IN the story (or AS the story) we need it to help us lay out the story. And sometimes it's a lot of fun to speculate on realistic ways we might justify what otherwise could be handwavy "fantasy bullshit".

And some readers care a lot that we put that effort in.

Let's try to be a bit more RYFWy here, and move on.

Thanks!

Chimeran · Nov 6, 2013

Goodness! I didn't expect my thread to kick up a fuss like that, haha.

I do have a story. Hopefully it's a decent one. However, as it's going to be told in graphic novel format, my co-writer/artist and I wanted to try and be as accurate as possible. Especially since sometimes the harder science approach can be more visually fantastical that just making it up as you go along.

@ErgodicMage
Yeah, I was figuring the docking stations might need to be tweaked, but as they don't actually play much role in the story, their workings aren't that important, haha. Possibly the docking spars can temporarily detach from the axis and cancel their spin for docking, then accelerate back up to be in sync with the station again or something.

Though the docking spars should be at much less than 8g. The center of the station is very low gravity, as it lines up with the rich sector where artificial gravity generators are in full use, and a patrons can enjoy exactly as much gravity as they choose to.

Currently I'm thinking that the gravity at the maximum radius of ~8km is 4-5g. The viewing walkway would use artificial gravity generators to cancel that out enough to be comfortable for humans. The outermost decks of the main ring are inhabited by alien workers who are more tolerant to higher gravity than humans. It's probably not that pleasant, but they're kind of an oppressed working class in human space. Then you get the poor human sectors, where everybody moves very carefully and padded clothing is a fashion statement because serious injury just from falls is an everyday worry. Then the middle class rings are at +/- 1g.

Thank you all for your replies! The next part of my question is actually specifically related to events in the story, wherein a woman is tossed out of an 'aircraft' in the Park (<1g overall) and ends up caught on a transport spoke running from from the floor of the Park up to the rich sector.

I can't for the life of me figure out if she hangs down toward the ground (though without particularly much weight), as if there was gravity, as the result of the contact between her clothing caught on the spoke accelerating her, or if the resistance of the air as the spoke spins causes her to kind of trail behind it, like a flag. Or something else?

It's not that important in the scheme of things, but sometimes it's getting those little details right that can really wow readers with an eye for such things, y'know?

King Neptune · Nov 6, 2013

The old-fashioned way to dock to a space station is to have the docking in the center with no pseudo-gravity.

Randy Lee · Nov 6, 2013

Chimeran said:
The viewing walkway would use artificial gravity generators to cancel that out enough to be comfortable for humans.

So if you're going to have artificial gravity generators, may I ask what the point is of rotating the spacecraft? Artificial gravity generators are like FTL and time travel, as in, they doesn't exist, and no one has any idea of how to make them work. I have no problem with using artificial gravity generators in SF, but if that capability exists it seems unnecessary to use rotation to generate the effect. Anyway, it's none of my business, but that question stood out to me, and would if I were reading your story.

Chimeran · Nov 6, 2013

Randy Lee said:
So if you're going to have artificial gravity generators, may I ask what the point is of rotating the spacecraft?

It's a good question! The general idea is that the artificial gravity generators require a constant input of energy to maintain a sense of gravity. So while they're willing to expend the energy to maintain reasonable gravity for the rich and the tourists over relatively narrow areas, they don't want to expend the energy to cover the entire station with gravity generators.

Secondarily, the rotational 'gravity' is more reliable. If the station experiences some sort of power issue, it'll keep on spinning, and the people who have to fix the issue before people start dying don't have to worry about everything suddenly floating around on them.

Since the artificial gravity generators are completely fictional, they don't necessarily have to have the limitations I'm giving them, I prefer not to make up technologies without limitations. Thematically, it's also part of the socioeconomic divide on the station. Lower class, high gravity. Having control over your gravity is a luxury (and actually a technology that humans have borrowed from other species, and fairly rare. Oneiros is a very... opulent station, for those who can afford it).

Randy Lee · Nov 6, 2013

Chimeran said:
It's a good question! The general idea is that the artificial gravity generators require a constant input of energy to maintain a sense of gravity. So while they're willing to expend the energy to maintain reasonable gravity for the rich and the tourists over relatively narrow areas, they don't want to expend the energy to cover the entire station with gravity generators.

Secondarily, the rotational 'gravity' is more reliable. If the station experiences some sort of power issue, it'll keep on spinning, and the people who have to fix the issue before people start dying don't have to worry about everything suddenly floating around on them.

Since the artificial gravity generators are completely fictional, they don't necessarily have to have the limitations I'm giving them, I prefer not to make up technologies without limitations. Thematically, it's also part of the socioeconomic divide on the station. Lower class, high gravity. Having control over your gravity is a luxury (and actually a technology that humans have borrowed from other species, and fairly rare. Oneiros is a very... opulent station, for those who can afford it).

Thank you for clearing that up.

TomW22 · Dec 18, 2013

King Neptune said:
The old-fashioned way to dock to a space station is to have the docking in the center with no pseudo-gravity.

Babylon 5 is a good example of this.

You do of course have a problem using rotation if the station/ship has any parts that don't rotate. Then how do people move between these areas? That was always an issue with me in Babylon 5 and something I try to address when I use rotation (fortunately my current projects are far future enough that I can just make up a ''well generator'' and move on).

Artificial Gravity

Chimeran

King Neptune

Chimeran

robjvargas

Kevin Nelson

Aspiring to authorship since 1975

Randy Lee

knew the job was dangerous

robjvargas

Randy Lee

knew the job was dangerous

robjvargas

Randy Lee

knew the job was dangerous

Ergodic Mage

Neophyte Writer

blacbird

Albedo

Alex

blacbird

Albedo

Alex

Telergic

Overwriting

James D. Macdonald

Your Genial Uncle

amergina

Pittsburgh Strong

zanzjan

killin' all teh werds

Chimeran

King Neptune

Randy Lee

knew the job was dangerous

Chimeran

Randy Lee

knew the job was dangerous

TomW22