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Spin Stabilization vs "Spin-Can"

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MattJL

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I want to clear up what I believe is my own misunderstanding.

I'm very familiar with the concept of spin-stabilization, and my understanding of it revolves around giving the vehicle a very high spin rate in order to resist aberrations in thrust, aerodynamics, etc and average out forces during the flight.

However, discussions with a friend have brought the concept of the "spin-can" to my attention, which allows only the fin can to spin, keeping the body tube stationary. It has, apparently, been flown successfully, and is described in-depth in this video:


Now here's where my understanding breaks down. He's told me that doing something like this is more or less just as good as a full spin-stabilized setup, as the rest of the rocket is aerodynamically symmetrical, and any minor mass imbalances don't spiral into full-on precession.

I'm inclined to believe him, but I'm somewhat baffled as to why. Surely the moment of inertia on the spin-can is significantly lower than that of the entire rocket, and that can't be good for stability. Additionally, as far as I'm aware, the spin-can was invented to handle a control issue rather than a passive stability one, so I'm not sure if it's beneficial for most applications.

Where is my logic going awry?
 

OverTheTop

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I'm very familiar with the concept of spin-stabilization, and my understanding of it revolves around giving the vehicle a very high spin rate in order to resist aberrations in thrust, aerodynamics, etc and average out forces during the flight.
Correct. Does not have to be "very" high though, just high enough.

Jim used the spin can to get around one of the issues with his active stabilisation system. His canards at the front of the rocket produce a steering moment to restore the rocket to vertical, but in doing so they disturb the nice even airflow along the rocket. This disturbed airflow changes the apparent wind (to use a sailing analogy) and can thus produce a different steering moment at the rear. Depending on how the disturbed airflow lands on the rear fins it can result in a steering moment that adds to, cancels, or even reverses the intended steering actions. By allowing the rear can to spin it still provides the overall intent of keeping the pointy end forward (check out the maths of a cruciform fin tail in missile aerodynamics) but, because it is somewhat decoupled, without deleteriously interacting with the intention of the canards.

Physics says a rocket will degenerate into a tumble around the axis of maximum moment of inertia once the fins lose effect (assuming the absence of any other control force). How quick that happens depends on how well the rocket conserves its energy. Since any losses encourage the tumbling to occur, I suspect bearing losses (if the can is still spinning) would encourage tumbling slightly more quickly than a fixed can in the air-less region of flight. The extra mass that far out also makes the long axis moment of inertia larger (by the square of the distance from CG) than the other two axes so I guess it is more inclined to tumble due to that as well. Any physicists out there can chime in if I have got this wrong please.

Yes, the moment of inertia of the spin can is relatively small, but the spin does not actually come into the basic mathematics of the fins when in the atmospheric flight region.
 

MattJL

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Correct. Does not have to be "very" high though, just high enough.

Jim used the spin can to get around one of the issues with his active stabilisation system. His canards at the front of the rocket produce a steering moment to restore the rocket to vertical, but in doing so they disturb the nice even airflow along the rocket. This disturbed airflow changes the apparent wind (to use a sailing analogy) and can thus produce a different steering moment at the rear. Depending on how the disturbed airflow lands on the rear fins it can result in a steering moment that adds to, cancels, or even reverses the intended steering actions. By allowing the rear can to spin it still provides the overall intent of keeping the pointy end forward (check out the maths of a cruciform fin tail in missile aerodynamics) but, because it is somewhat decoupled, without deleteriously interacting with the intention of the canards.
That's what I suspected. I've been reading over his build thread for ThreeCarbYen - fascinating and inspiring work. A real testament to what tenacity and great engineering can do.

Physics says a rocket will degenerate into a tumble around the axis of maximum moment of inertia once the fins lose effect (assuming the absence of any other control force). How quick that happens depends on how well the rocket conserves its energy. Since any losses encourage the tumbling to occur, I suspect bearing losses (if the can is still spinning) would encourage tumbling slightly more quickly than a fixed can in the air-less region of flight. The extra mass that far out also makes the long axis moment of inertia larger (by the square of the distance from CG) than the other two axes so I guess it is more inclined to tumble due to that as well. Any physicists out there can chime in if I have got this wrong please.
I'm not a physicist, but are you visualizing a rocket at altitude tumbling along one of the short axes (i.e., in pitch or yaw) rather than in roll? I'd assume that the highest moment of inertia is along the long axis, not the pitch or yaw axis, but the equations for both imply otherwise (I = 1/2 * m * r^2 for roll, and I = (1/12) * m * (3r^2+L^2) for pitch/yaw). Guess it's one of those weird physics things.

Encouraging tumbling would be great for booster recovery, but not so much for upper stages, especially if you're vying for an altitude record. That too adds up with the use of the spin-can in ThreeCarbYen... which I think was on the bottom of the first stage. (Still reading over that thread!)

Yes, the moment of inertia of the spin can is relatively small, but the spin does not actually come into the basic mathematics of the fins when in the atmospheric flight region.
So, unless you have an active stabilization system like ThreeCarbYen's on board, there's not really much of a reason to use a spin-can? That makes sense.
 

OverTheTop

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I'm not a physicist, but are you visualizing a rocket at altitude tumbling along one of the short axes (i.e., in pitch or yaw) rather than in roll?
Roll is usually the lowest moment of inertia, due to the mass being closes to the rotational axis. The other two are waaay bigger.
 

MattJL

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Roll is usually the lowest moment of inertia, due to the mass being closes to the rotational axis. The other two are waaay bigger.
Makes sense to me! Certainly makes recovery interesting for long, thin darts.
 

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The spinning wheels on a bike help keep you upright and balanced, but the weight of the wheels are typically a small fraction of the overall total weight of the bike and rider combined. Even less of a fraction for a unicycle, but I think those things defy all physics.
 

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If you ride a carbon-fiber mountain bike (I do) after riding a steel-framed version you will be stunned by the lack of inertia in the frame (flicking side to side) and even in the effort needed to turn the handlebars. It is surprisingly noticeable what a difference making things lighter makes, and also keeping the mass closer to the CG. It actually took a couple of weeks to get my technique aligned with the new bike.

I actually found something similar with CF ski poles. Not noticeable greatly when I got them, but when going back to standard poles it is quite apparent. I think it is mainly windage in this case, with a small inertia effect.
 

jlabrasca

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I'm very familiar with the concept of spin-stabilization, and my understanding of it revolves around giving the vehicle a very high spin rate in order to resist aberrations in thrust, aerodynamics, etc and average out forces during the flight.
funny.

Guess it's one of those weird physics things.
There is rather a lot that needs to be understood on the way to understanding conservation of angular momentum. When successfully taught, it is the topic that can finally shake a student loose from the pernicious delusions of "conceptual understanding" and "intuition". It only makes sense when you treat it mathematically.



 
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