So I’m watching that sci-fi show Babylon 5 the other day. Apparently they have these ships (not the station itself) that don’t have artificial gravity in the Star Wars/Star Trek mystery field sense. Instead, they have these big rotating sections that provide the artifical gravity through centrifugal force.
Sounds like a great idea, right? Pretty believeable? Well, I remeber something in physics class where the professor had a student hold a rotating bike tire. The faster the tire spun, the harder it was to turn (some kind of gyroscope effect or something. Its why a bike doesn’t topple over while you are riding it).
So my question is this:
Wouldn’t a ship with rotating sections be extreamly dificult to turn?
First, that isn’t why your bike doesn’t fall over. The gyroscope effect from those lightweight wheels is no where near strong enough to do that.
Second, yes, a spinning section would make a ship harder to turn. But that may not be a major factor, if the maneuvering system is powerful in comparison to the angular momentum of the spinning section. If the turning part contains a significant part of the mass of the ship, it would indeed take some oomph to turn it.
On the other hand, there’s lots of space to travel in. If it takes you 20 miles to make your course adjustment, that won’t make a lot of difference if your whole trip is a couple million miles.
The gyroscope effect might actually help these ships navigate better. The difficulty in turning translates to stable flight. The attitude jets wouldn’t have to fire all the time to make adjustments.
Not really part of the answer to your question, but I’ve just thought of 2001 and 2010. If you recall, the Discovery had a rotating section for artificial gravity. Nine years after Bowman left the ship, they come back to it. The bearings of the big wheel have locked up. All that angular momentum had to go somewhere (it’s a conservation law), so the whole ship is spinning about its center of mass. They fix the bearings so they can transfer the spin back and stablize the ship. I like it when a movie gets the physics right.
They had the physics right (at least partially), but perhaps not the geometry. When they reach the Discovery in “2010”, it’s rotating about its pitch axis (if you visualized the ship as an aircraft, pitch is nose-up or nose-down). From the position of the pod bay and control deck windows, I don’t see how the centrifuge could contribute to the pitch angle. I have a book with a drawing of the interior layout of the Discovery, but I don’t know how official it is. According to that layout, if the centrifuge bearings had seized the ship would have rotated about its bank axis (one wing up and one wing down, like a plane doing an aileron roll at an airshow).
The speed of rotation doesn’t look right in the movie, either. Since the entire ship must have greater mass than just the centrifuge (and by several times, I would think), the ship would rotate at a fraction of the speed of the centrifuge.
This doesn’t seem like the kind of thing Clarke would get wrong, so we’ll blame Peter Hyams. End-over-end certainly looks better on film than just twisting in place. I’ll have to go back and read “2010” again, I seem to remember something about a flywheel seizing, rather than the whole centrifuge, and a flywheel could be small enough to have any orientation within the ship.
Actually, it is the gyroscopic action that keeps you upright. Have you ever tried to balance on an unmoving bike? It is very difficult. Or have you noticed how tippy a bike is at slow speeds? Though the weels are very light, they are morderately big and rotate moderately fast. That, and the fact that there are two of them, creat enough force to keep you upright.
In regards to the original question, not only would it make navigation easier, but many satelites currently in orbit have several gyrocopes in them to help with navigation. (Yes, they are a bit smaller, but the priciple is the same.)
Here’s an experiment you can perform on your own bicycle. Get going as fast as you can while moving in a straight line, and stand on the pedals. While maintaining a straight course, quickly tip the bike side to side. I found it’s not hard to make the plane of the bicycle rock ten or so degrees one way, then the other. Now go much slower, perhaps a tenth the speed of the first part, maybe less. The wheels now have a tenth the angular momentum of the first part. Explain how, even though you had no trouble overcoming the larger angular momentum to rock the bike back and forth when going fast, the lower angular momentum at the slower speed is still holding you up.
Here’s another thing to think about. The angular momentum of the wheels is not a restorative force; it doesn’t tend to make you stay upright, it tends to hold the bike wheels in their plane of rotation. If you’re tipped over a bit, the wheels’ angular momentum will tend to oppose you straightening out.
Um, I think it’s more difficult to balance a non-moving bike because you can’t balance by turning the handlebars and subtly changing the direction the bicycle is moving.
I don’t see a problem with this. The rotation axis doesn’t have to stay fixed relative to the ship, it just stays fixed in space. If you imagine a very thin and tall top spinning on a smooth surface, pretty soon it will topple and spin end-to-end. Angular momentum hasn’t changed much, since the rotation axis is still vertical. (Though of course it’d be slowed down somewhat by friction.)
In fact, spinning shapes tend to favor spinning around a short axis (end-to-end), like Discovery in 2010. This is because this mode of rotation has the least rotational energy for the given amount of angular momentum. Angular momentum is difficult to get rid of, but energy is easily lost - the major source of energy loss is friction within the shape/ship, from flexing and shaking. So left to itself, a long skinny spaceship will try to rotate in a way that has minimum energy for the given angular momentum. In the early days of the space program, someone (NASA, I believe?) designed satellites that spun around the long axis. It took a while to figure out why these satellites suddenly lost stability and started spinning end-to-end.
I agree that the Discovery in 2010 had a lot more angular momentum than you’d expect from centrifuge. Artistic licence, I suppose. It’s been a long time since I read the book, but I think in the book it wasn’t spinning fast enough to make entry difficult.
That’s probably what he/she is saying, and I agree with it. Have you ever tried to ride a bike with the wheels stuck in a narrow groove? It’s impossible to do this, because you can’t steer the bike to keep balanced. On the other hand, someone made a bike with a counter-rotating flywheel to cancels the gyroscopic effect of the wheels, and reported that such a bike is relatively easy to ride.
Of course it doesn’t require a conscious correction; the whole point of “learning to ride a bike” is to learn the reflex of turning the handlebar to the right when you start to fall to the right, and vice versa. Ever seen the carnival trick bike modified to reverse the handlebar motion? That is, when you turn the handlebar to the right, the wheel turns left. It’s impossible to ride one unless you acquire the new reflexes. If gyroscopic action is what balances a bike, these trick bikes should be as easy to ride as normal bikes.
You might also try riding a long-wheelbase recumbent bike at a very low speed (i.e. walking pace). They respond much slower to these constant corrections, so the weaving motion is very visible. If it was gyroscopic effect balancing the bike, these bikes should be as table as upright bikes at same speeds. (These bikes handle much better at high speeds, by the way.)
We’re not saying. Angular momentum is a known and measurable law of physics. In the experiemnt I mentioned in the OP, there is an actual force that prevents the spinning bike tire from being turned. The same force makes a bicycle or motorcycle stable at higher speeds.
If you had to constantly make slight course corrections to balance on a motorcylce going 100 mph, it would become unstable. Have you ever ridden a skateboard (the wheels on a skateboard are too small to provide significant angular momentum) straight down a hill where it build up too much speed? It starts to oscillate or “wobble”. The more you try to correct for the oscillation, the bigger the wobbling becomes. Eventually, you either reach the bottom of the hill and reduce speed or the oscillations becomes so great the board throws you.
Of course it is. As long as the axis of the wheels are parallel, the angular momentum forces will act in such a way as to keep the bike stable.
Right, but you only need to do this at slow speeds. Once a kid has the confidence to take the bike to a faster speed, it becomes easier to ride. As a small child, I found I crashed a lot more often at lower speeds.
You just proved our point. A long wheelbase bike is more unstable (remeber the car comercial, wider is better), therefore you have to expand more effort at low speed to control it. That weaving or wobling only happens at low speeds because you are are the only thing keeping that bike (or skateboard) balanced. Once you build up enough speed, the wheels will tend to stay upright on their own.
Have you ever ridden a bike or seen someone ride a bike with no hands? They usually don’t fall unless they hit a bump or something.
The thing to remember is that angular momentum is a function of the radial velocity and the mass of the tire. You can overcome it by turning the bike or leaning to one side. It isn’t a magical shield that prevents the bike from being knocked over, but it does help.
I guess that also answers my spaceship with rotating sections question.
An excerpt from this article confirms what I was taught about bicycle stability back in first technical physics:
The tires turn in response to leaning because of the changes in drag as one side of the tire rises up off the ground. Naturally, this effect becomes more pronounced as the RPM of the wheels increases. It’s not conscious, or even unconscious, action on the part of the bicyclist that provides this stability, although the cyclist must contribute to control of the bike with weight shifts.
A simple experiment you can perform in regards to bycicle stability: You’ll need a bicycle of your choice, and a puddle. Get the bike up to whatever you consider to be a sufficient speed, and then ride through the puddle. Ride as straight as you can. Then, get off the bike and look at the tracks. The tire tracks, especially from the front tire, will be weaving back and forth. QED
I have rode a long wheel-base recumbent. If you will notice, the front wheel is much smaller than that of a normal bike. This means that it has much less rotational stability at low speeds. However, when you get to high speeds this becomes less of an issue as you said. This is important with the front wheel because the lesser stability will cause it to turn more easily so that the bike is more twitchy.
Wheel size does play a role, and gyroscopic effects are not negligible. But it’s not the primary effect keeping a bike upright. I own a long wheelbase recumbent, as well as a short wheelbase recumbent and a folding upright bike. Of these bikes the long wheelbase recumbent has the largest front wheel, yet it has by far the most weaving motion. Because of the long wheelbase, the front wheel must move a long distance to move the center of gravity by a small amount. At high speeds, even a small turn of the handlebar moves the front wheel quickly to the side so it’s more stable.
Riding a bike, and especially riding hands-off, is an acquired skill. That alone strongly suggests that it’s the rider making constant corrections to keep the bike upright, rather than the bike keeping itself upright. I think gyroscopic force does help you by slowing down your fall, but without the rider’s input (either handlebar motion or balance shift) a bike will tip over sooner or later.