Can gyroscopic forces stop rotation?

[DyslexicHobo]

Before I ask the question, I'd like to let you know why I am asking it; feel free to put in any input on the situation, as well as an answer to my question. Skip to the bottom if you don't want to know about it. :P

My class is designing a "toy for space" in joint with Russian students-- hosted by NASA.

For our toy, we're planning on designing some sort of RC flier designed to work on the space station. This will be just like an RC car, but will also be able to utilize the microgravity on the ISS.

So far, we've thought the best approach would be a cylindrical body with our propulsion at the back end. Direction will be controlled by air displacement of remotely controlled fins.

We thought we were fine and dandy until we realized that there was no way to stop the flier once it started changing direction. The momentum of the direction change would keep the flier going in the direction we wanted to nudge it in, with no simple way to stop it. We decided the easiest solution to this problem would be to increase the air resistance by adding wings 90 degrees apart from each other.

The next problem we encountered was slightly harder to overcome, but we think a gyroscope (or two) will help the problem. We realized that when--in zero gravity-- the fins are tilted one way to change the flier's direction, due to rotational inertia the flier will continue to spin unless some force counteracts it.

Now for my question: Will a set of gyroscopes (two, so all planes of motion are covered: x,y,z) stop our flier from over-rotating?And if you were wondering... we don't HAVE to build a prototype, nor do we have to have this perfected-- it's just for fun. However, if we do a good job, it'd be really cool to see if NASA would take this on at least the vomit comet and try it out in zero-Gees. :D

 

[Andrew Mason]

If I understand your question correctly, you want to change the direction only while the steering force is being applied. As you have discovered, if the rocket is not spinning, a lateral steering force will impart rotational momentum to the rocket and the rocket will continue to rotate after the steering force ends.

The solution is exactly what you suggest. In order to stop the rocket from rotating after the steering force is applied, you need to give the rocket some angular momentum about its long axis - spin. When spinning (or a gyroscope in the rocket spinning along the rocket axis), the direction of the angular momentum vector is along the axis. A small steering force in a direction perpendicular to the rocket axis applies an angular impulse adding an angular momentum vector whose direction is perpendicular to the rocket direction and perpendicular to the steering direction. The sum of these vectors gives you the new angular momentum vector. Since there is no longer any torque when the steering impulse ends, that angular momentum vector does not change, so the direction of the rocket stabilises at that new direction.

AM

 

[DyslexicHobo]

Thank you for your answer, Andrew. That is exactly the answer I was looking for.

I'm unfamiliar with exactly how gyroscopes work, as I've never done any work with them. I have learned about angular momentum, though, but I'm not sure if I'm applying my knowledge correctly.

If I understand correctly, a gyroscope adds angular momentum-- this increases the amount of force required in order to change its direction. But what happens if the direction is changed, therefore changing the plane on which the gyroscope is acting? Will it want to go back to the plane of motion it was once on, or will it take its new orientation and stick with that? I'm pretty sure it's the latter, but not positive.

Also, in order for this to operate correctly, we would have to calculate how much the gyroscope would need to spin in order to correctly re-orient the craft after turning, correct? At first I thought we'd be able to have it constantly spinning, but due to conservation of angular momentum, then craft would continuously be rotating! This is not good!


If anyone can suggest any other solutions, please don't be scared to throw em at me. :)


Edit: By the way, we were probably going to build the body out of something like this: http://scientificsonline.com/product.asp_Q_pn_E_3108900" . We would have two tubes total, each containing momentum over two planes of motion.

 

[cesiumfrog]

I'd just suggest steering with either gyroscopes/flywheels or fins; using both seems inappropriately redundant. But then again, I'm hesitant to impose any logic when the design is specifically for a toy!

 

[AlephZero]

I would recommend you get a gyroscope and find out what it can do by playing with it. I suspect you are going to be disappointed in what you are trying to use it for, but you will learn more by first playing with a gyro and then finding out WHY it does what it does, than by just reading about one.

But then again, I'm hesitant to impose any logic when the design is specifically for a toy!

I don't agree with that. Of course you can be as creative and unconventional as you like with your design, but if you ignore the "logic" of Newtonian mechanics, it's unlikely to actually work!

 

[DyslexicHobo]

Well with the gyroscope idea, we were planning on having them constantly rotating. Because we would have them mounted perpendicular to each other, they would resist each other's desire to rotate the craft due to conservation of rotational momentum (at least that's how I was HOPING it is going to work).

This way, when the craft is going to turn (it will angle towards the way it is turning) it will not continue to turn after the propulsion returns to its normal position.

Please tell me if this will not work.

 

[D H]

Look into reaction wheels and control moment gyros. These do exactly what you are asking about.

 

[Andrew Mason]

Well with the gyroscope idea, we were planning on having them constantly rotating. Because we would have them mounted perpendicular to each other, they would resist each other's desire to rotate the craft due to conservation of rotational momentum (at least that's how I was HOPING it is going to work).

This way, when the craft is going to turn (it will angle towards the way it is turning) it will not continue to turn after the propulsion returns to its normal position.

Please tell me if this will not work.

Gyroscopes do not turn the rocket. They keep the rocket pointed in the same direction. A football "spiral" or a rifled bullet keeps directional stability due to the same principle.

The principle on which the gyroscope operates is "conservation of angular momentum". If no torque is applied to a gyroscope, the magnitude and direction of the angular momentum vector cannot change (Torque = rate of change of angular momentum (L) so if torque =0, no change in L). So the axis of spin, relative to the stars, of each gyroscope (ie. the direction of the angular momentum vector) will not change unless there is a torque applied. (NASA's rockets use gyroscopes on gimbals to measure direction changes of the rocket. If they are on gimbals, the rocket does not apply torque to the gyroscope. The rocket may turn using rocket steering but the gyroscopes' direction is fixed relative to the stars).

If you have a gyroscope mounted on a fixed axis inside the rocket, and if you apply a torque to the rocket with a blast of your steering rocket thrusters to the left, say, the rocket nose will not move left. The nose will dip down. So if you are going to steer this way, you will have to be careful.

AM

 

[AlephZero]

Using small gyros to provide a stable "direction indicator" as part of a control loop would certainly work - they have been used that way in ships and aircraft for many years. The actual stabilising force doesn't come from the gyro, it comes from something controlled by the gyro.

Trying to use a big gyro to stabilise the complete vehicle doesn't sound very practical. For example, when you spin the gyro up to speed, what is going to stop the vehicle rotating in the opposite direction to conserve angular momentum?

If you have a gyroscope mounted on a fixed axis inside the rocket, and if you apply a torque to the rocket with a blast of your steering rocket thrusters to the left, say, the rocket nose will not move left. The nose will dip down.

True - I think the OP was hoping to stop that by having another gyro at right angles to the first one, but (though I admit I don't have any practical experience of this) instinct says it won't work as intended. There will always be one axis of rotation where the two gyros will "cancel out" and not prevent rotation - and that's the axis the vehicle will rotate around.

Possibly with 3 gyros at right angles to each other you could make a vehicle so stable that it could not be turned easily in any direction at all, but that isn't very useful.