Space Opera Fans discussion

Since I've upped how many sci-fi books I read, I've noticed the majority of authors enjoy using a ship spinning to simulate gravity. Does anyone know why spinning would create the illusion of gravity, or does it actually create gravity?

Why would an object be pulled to the hard surface that is spinning? Why wouldn't a free floating object just float and be pushed by whatever hard spinning object it floats into?

Imagine you're standing on the inside of a cylindrical shape that is spinning around it’s axis. The spin will create a force perpendicular to your body. If the cylinder would disappear the moment the force was exerted, you would move down a straight line (sideways). But the cylinder is there. That means you are forced to move down the shape of the cylinder, which is circular. So your own inertia coupled with the physical constraint of the cylinder creates a radial force. It is not gravity, but you experience it the same way.

Don't know if I got all this right, science people let me know.

Oh, to get to the floating object part. The object has to be connected to the cylinder somehow to experience artificial gravity. Air would suffice. If there is a vacuum though the object would continue to float.

I know nothing about the real science, but I can't help but think of that ride, the Gravitron, that you often find at carnivals and county fairs. You lean up against a pad on the wall in a nearly-upright position, and then the ride starts spinning really fast and you basically become stuck to the wall. This is kind of an extreme example because the force is so strong that you can hardly move (I've honestly never really liked the ride) but it seems to me like it's the same basic concept.

https://xkcd.com/123/

Ya know, I had a feeling I was calling it wrong but I couldn't think of the name. I'm gonna change it when I get to a computer.

That's a funny comic.

I actually wouldn't change it! Centrifugal force is what a person actually feels when inside a centrifuge, so the term is correct.

And readers won't have heard of the centripetal force unless they took vector physics.

When you were a kid did you ever do the spin a glass of water trick in a circle all the way upside down?

http://www.oapt.ca/conference/2009/re...

My favorite carnival ride as a kid was the Turkish Twist, kind of like a gravitron, only it was indoors like a gigantic, deep washing machine tub so you could climb up the rubberized walls as it spun like Spiderman. Think it's like that.

[*I wonder if there's a sock-monster inside a space station?*]

Playgrounds use to have the round-and-rounds. Get those babies spinning fast enough and it felt like 2-Gs! Just before you flew off...

I used to love playing on the merry-go-round on our elementary school play ground especially in the summer when it was just the neighborhood kids and not the whole school trying to play on it. We would pretend it was a spaceship taking us to all the worlds we saw on Star Trek TOS. It was the 60's and early 70's afterall.

These days, the helicopter-parents won't even let their kids play. Everything has to be padded and nothing can risk injury. :-P No wonder kids don't have any imagination anymore except what the television tells them.

I was a total free range kid. 250 acre shade tree farm. Playing at the dump was a fav in those days.

I got a lot of cuts and bruises. Learned a lot of lessons and skills. Tetnous shots aren't so bad.

This is very simple. You're standing on the inside of a cylinder, the cylinder is spinning. Inertia is trying to make you go in a straight line, but the cylinder is making you go in a circle, so it has to keep pushing on you to keep changing your direction. That feels like gravity.

The bigger the cylinder the more it will feel like natural gravity.

Martin wrote: "The bigger the cylinder the more it will feel like natural gravity."

True. In "Islands in the sky" a character swims in a pool on a space station and notices that the water at the ends is above him.

R. wrote: "This is very simple. You're standing on the inside of a cylinder, the cylinder is spinning. Inertia is trying to make you go in a straight line, but the cylinder is making you go in a circle, so i..."

So the cylinder has to be of a specific size to match the gravity created by earth, because if the effects are off at all there will be adverse affects on the body which would ultimately kill anyone who lived in space for years at a time.

How big is that size? How fast does it have to move as well? You can't just build a cylinder and spin it up until someone says "yeah, that feel's about right". What kind of affects does it have on children, infants, the elderly, obese, all those not in optimum physical health like astronauts are?

Spinning around to simulate gravity sounds pretty nifty, but it's working opposite of how the gravity of celestial bodies works (i.e. Earth doesn't fling us off into space with its spin), so I'm not entirely sold on this spinning to simulate gravity is a great answer; maybe short months long jaunts it would work, but the adverse health affects are likely to be ridiculous because pressure is being asserted onto the body in a way that we were not evolved for.

Or maybe I'm really wrong, and I lack the necessary knowledge of physics to understand why spinning to simulate gravity is a great and "realistic" sci-fi idea.

Anyone know of any websites I can use to drill further into this?

Martin wrote: "Playgrounds use to have the round-and-rounds. Get those babies spinning fast enough and it felt like 2-Gs! Just before you flew off..."

I loved doing that. Somedays I was picking mulch out of my hair the next morning.

There really is no difference between spinning inside a cylinder and walking on a planet as long as you get the speed right. Most negative health effects would come from the lack of shielding from cosmic radiation.

I see (just found an article on this) how the math is there to create artificial gravity using spin, but I do wonder about the adverse health affects. I suppose we won't know until a spinning chamber is created in outer space and people sent up to test it.

The major problem as I see it is that when using spin gravity it's exceedingly difficult to dock another ship to the spinning one. Also optical instruments and high-gain aerials have to be rotated relative to the ship to stay on target.

That's why I use a bit of handwavium called "pseudograv" and don't bother with the spin.

R. That's probably why most usages of the centrifuge idea on ships and space stations to simulate gravity in most science fiction and even some modern ship/station designs involve the combination of a spinning living quarters and then another section of the ship that is without gravity. Other cheaper ship designs include the idea of spinning the entire ship to simulate that grav, but then you would run into that docking problem, and just the general faux pas of turning yourself into a human boomerang.

Jonathan, long term problems of hypergravity would involve motion sickness due to the "semicircular canals of your inner ear become "confused.""(1) and cardiovascular problems among other problems relating to unequal distribution of force. As you said though it will take some actual testing to see how long term exposure would effect humans. And it looks like someone at NASA is on it.

http://www.nasa.gov/missions/science/... (1)

This article also seems to answer the how big question. "radius of several kilometers, large enough to generate high artificial gravity without rotating fast enough to trigger the tumbling illusion."(1)

Rion wrote: "R. That's probably why most usages of the centrifuge idea on ships and space stations to simulate gravity in most science fiction and even some modern ship/station designs involve the combination o..."

that's great. thank you very much.

That article gave voice to what I've been wondering since I read Leviathan Wakes. What happens if I'm being spun to hold myself to floor (air pressure holding me there, thanks Packi) in a reasonably sized spaceship.

Before I read that article I figured blood would pool in the feet simply because of the amount of pressures involved with the spin. The scientist didn't mention that, so maybe I was wrong about that, but small cylinders would have an adverse effect. (is it effect or affect?)

I guess that's why the term "fiction" is in science-fiction.

The smaller the cylinder the greater the dg/dh (rate of change of gravity with height off the floor) and hence the worse the problems. Remember that on the axis of spin gravity is zero. There are also gyroscopic effects, turning a corner when carrying a heavy object could be startling.

There are always tradeoffs. The longer you want the crew to function the more earthlike the environment has to be. That's why my little spaceships which could last ten weeks without touching a planet for resupply have a nitrogen/oxygen atmosphere at close to 15psi, even though the human body can function at much lower pressures for short periods. This means accepting the weight of a thicker hull to keep the pressure in and a more complex life support system to do the recirculation.

You could make a lighter ship by dropping the pressure, but here would be long-term consequences for the crew.

Incidentally airliners are only pressurised to about 12psi for the same reason.

Jonathan wrote: " The scientist didn't mention that, so maybe I was wrong about that, but small cylinders would have an adverse effect. (is it effect or affect?)"

Effect.

Rion wrote: "R. That's probably why most usages of the centrifuge idea on ships and space stations to simulate gravity in most science fiction and even some modern ship/station designs involve the combination o..."

I don't have the source in front of me, but I wrote a story with a orbiting space station and it was 750 feet in diameter and per some calculations I got from Wikipedia-I think, it rotated at about 2.8 revolutions per minute. PRETTY FAST, in order to create 1G environment.

From my brief forays into reading about this it seems like that is a major reason ship designers are hoping humans can do just fine in 1/3rd G, you don't need nearly the rotational speeds.

For work I had to write an article a few months ago about the physiological effects of microgravity, its interesting stuff, and very hard to test since so few humans have experienced significant time in low-G.

EDIT: other amusing things about being spun in a centrifuge: balls thrown straight down will bounce at an angle, you will lean over when standing straight up, gravity is zero in the centre of the "room".

One interesting comment, you have to define 'straight down' the object will move in a straight line from the point it leaves your hand until it hits a surface. From the observer's perspective the object will move slightly 'up rotation' then a spin will be imparted to it from the moving surface. In my story about the space station, the crew likes to move quickly against the rotation because they'll be moving at less than 1 g and they can move easier. When they move with the rotation their acceleration is increased and it takes more energy. So they like to run in one direction and they tend to run into each other because they can jump long distances.

The problem with having a station 750 feet in diameter is that for a given internal air pressure the tension in the skin goes up linearly with radius. A torus works better.

I did design one ship that was a ring of cylindrical sections to sidestep this one.

True. In my story the structure is a turus with spokes and a docking station at the juncture of the rings. The structure is held together by carbon fiber strands.
Your statement about the forces increasing linearly, I think is understating the real nasty forces. I suggest the forces are dependent on the mass and its distance from the center of rotation which increases by R Squared. Regardless the forces are HUGE. You are correct about the air pressure causing a linear increase in skin forces.
On a separate webpage I had a lengthy discussion about the plausibility of a 'flat' planet. Too be honest we got way too far into the minutia of it all. Fun discussion though; I loved it.

If you've read Hollow Moon by Steph Bennion, the hollowed-out asteroid they use as a worldship uses centrifugal force to generate gravity, but with uneven (and sometimes amusing) effects depending where you are inside the ship. Some places in the ship are almost zero-G, while others light-gravity, and some places heavy.

R. wrote: "The problem with having a station 750 feet in diameter is that for a given internal air pressure the tension in the skin goes up linearly with radius. A torus works better.

I did design one ship t..."

So if I'm understanding the torus correctly, a ring inside the donut (torus) would be spun in two directions at the same time. Is that correct?

In my use of the torus (Donut) I have just one ring spinning at about 2.8 RPM providing the gravity and it is held together with spokes connected to a central docking port. It's a pretty standard configuration. I'm also curious how R. used the concept. Just throwing a quick thought out, two counter rotating rings could help with the torque issue which would be huge with a giant rotating structure. In my story the station loses most of the crew due to a virus and the systems start to break down. The station commander considers changing the axis of rotation to manage the sun's heating and comments about how changing the axis could rip the the station apart. They elect to take their lifeboats back to Earth where almost everyone has died and then they have to face other challenges.

Anna wrote: "If you've read Hollow Moon by Steph Bennion, the hollowed-out asteroid they use as a worldship uses centrifugal force to generate gravity, but with uneven (and some..."

That would be fun to work in. A rotating sphere would present a ton interesting forces on the inside surface.

Just getting around would be a challenge. As long as a worker stayed on the inner surface his centrifugal force would be like gravity. As he moved along the surface towards the poles the gravity would decrease until he was weightless at the pole. If he went through the center of the moon to the other inner surface he would have to be attached to something. If he wasn't he would lose his momentum passing through center then when he got to the other side it would be rotating and he would have to grab on and then stabilize for a minute before the gravity oriented him.

Anna wrote: "If you've read Hollow Moon by Steph Bennion, the hollowed-out asteroid they use as a worldship uses centrifugal force to generate gravity..."

Thanks for the mention! The formula to calculate the effect is:

a = (r(sπ/30)²)/g

a = pseudo-gravity due to centrifugal force (Earth-like = 1);
r = radius of habitation space (m);
s = rotation speed (rpm);
g = acceleration due to Earth's gravity (9.8 m/s²).

The Dandridge Cole, the 'hollow moon' (asteroid colony ship) of my books, has a central cavern a kilometre in diameter which spins once a minute, producing a centrifugal force something like half Earth's gravity against the inside wall.

Excellent information. Let me point out a key variable within the equation "r". The full force of g will occur only at the equator. The radius of rotation is the perpendicular radius at the latitude of point of interest. At the poles r is zero so g is zero. Regardless of the boring math details, its a great idea, I have to check it out. Without reading your book "yet", the different g values at the increasing latitudes could create zones where different creatures, activities or environments could be interesting.

I'm sure it shows up lots of other places as well, but the "hollow asteroids spun to simulate gravity" idea also comes up in Kim Stanley Robinson's 2312, one of my favourite books. The hollowed asteroids serve as wildlife refuges.

Ray wrote: "In my use of the torus (Donut) I have just one ring spinning at about 2.8 RPM providing the gravity and it is held together with spokes connected to a central docking port. It's a pretty standard c..."

I didn't have counter-rotating rings. The point about the torus is that because air pressure on the inside of the ring is trying to squash it inwards, and on the outside trying to blow it out, the skin tension in the torus is less than that for a cylinder of the same outside diameter.

What I did do was the infamous Ninevite "Five Sided Thing". Curiously this only works with odd numbers of sides, and five is optimal.

Take five cylinders and make them into a pentagon by joining them to a pressurised sphere at each corner. Each cylinder is quite stable, there is tension in the skin but all the forces go in sensible directions. The corners are the problem, because of the cylinders joining at an odd angle the spheres want to fly outwards. Now add tension cables making a pentagram with the points at the spheres. Now we have got what we want, pure tension in the skin with no bending forces involved, so we can make a light skin.

However the pentagram has a convenient hole in the middle. Add an ion drive connected by tension cables to the spheres, with its exhaust pointing through the hole and you have propulsion, spin the whole thing and you have gravity.

Very interesting, but to be honest I've got to see a picture. I don't disagree with the concept, I'm dwelling a little on the terms 'Blow it out' and 'squash it inwards'. I suspect your referencing a structure built by a space structure engineer. The forces will be huge for any large space structure and that is why I'm using composites which are very strong in tension. By making the outer circumference very strong it will help to balance the loads caused by the inside. But to save the other readers from engineering overload, I'll back off on the details. Can you give me a reference for the structure you're referencing?

Ray wrote: "Excellent information. Let me point out a key variable within the equation "r". The full force of g will occur only at the equator..."

Exactly. In the book, the spacecraft airlock is at the zero-gravity point. Spinning hollow asteroids are obviously not my invention; I named my colony ship after Dandridge MacFarlane Cole, a rocket scientist who proposed the concept back in the 1960s.

P.S. I've edited my earlier post to clarify the definition of 'a'.

Ray wrote: " Can you give me a reference for the structure you're referencing? "

Sorry, no. It only exists in my notes.

Imagine you come in to the space station with a 1000m radius, rotating 1x per minute to give it ~1g on the surface(I think I got that right). You dock at the center (0g) and you let yourself float "down" (out) to the surface. Ignoring the air pressure for the moment you are still weightless all the way "down" to surface. The problem is that once you get to the surface it is whipping by pretty fast, circumference is 2piR, so 6.28km every minute, or what my people call 234 miles an hour. Not a faster than a speeding bullet, but on par with an arrow. Definitely faster than you want to smack into a tree.

But of course, you can't ignore the air pressure. If the air at the surface is "still" it is blowing 234 mile an hour to the floating visitor, but only at the surface, at the hub it is not moving much. As they float down it starts blowing faster and faster. It's not going to get them up to surface speed but it should get the visitor up to terminal velocity, which for human is ~120mph, in the direction of spin. That acceleration is going to translate into an outward vector or a pseudo gravity. So you would "fall"towards the "ground" but not as fast as earthgrav would pull you. You would gradually drift toward the ground... And then get smacked by a tree going 120mph faster then you.

So long story short, don't drift down, slide down a hauser so the hab can gradually pull you up to speed or the speeding trees will smack you like a looney toon.

R. Michael wrote: "...Ignoring the air pressure for the moment you are still weightless all the way "down" to surface..."

Actually, you're not. It's all to do with 'frame dragging' gravitational effects. Once you move away from the central axis you'll start to fall (even in a vacuum), though slowly at first.

An elevator from the centre to the 'ground' is the answer; rather like a space elevator in reverse.

Steph: rotational frame dragging (the warping of underlying space time due to rotational acceleration of the spinning structure) may exist (I don't think it has actually been unambiguously detected in the real world yet) but like all relativistic effects in normal every day interactions it is so small it would be negligible. I mean we are talking REALLY small, trillionths of a G. So small that ordinary mass based gravity of nearby (astronomically speaking) planets/stars would have a much bigger effect.

R. Michael wrote: "Imagine you come in to the space station with a 1000m radius, rotating 1x per minute to give it ~1g on the surface(I think I got that right). You dock at the center (0g) and you let yourself float ..."

That explanation makes the water scene in Hull Zero Three make sense now. I thought it was interesting at the time of reading, but had no clue what was going on.