Stopping Spin on a Video Camera Rocket

If you've watched some of the launch videos, you'll notice the rockets tend to rotate a bit too fast.
I'm thinking about adjustable spin tabs on the fins. Launch the
rocket, time the spin rate, launch again with an adjustment, time the spin, interpolate, adjust again, launch and hopefully have greatly reduced roll.
What am I missing? Besides requiring 3 successful launches before I get some decent video?
Zooty
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Try rollerons?
Or more simply, keep fins small, present a symmetric drag profile by mirroring protuberance such as launch lugs, optical windows, etc.
Most importantly, build fins straight and keep their airfoiling simple to minimize chances of inducing a roll moment.
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On Tue, 06 Jan 2004 01:58:31 GMT, "BB"
I've heard the term before, but had forgotten it. To be honest, I'm not sure how they work - the descriptions on the net aren't clicking for some reason. Anyone got a URL?
Has anyone actually used them on a rocket?

Already have that.

Built the fins as straight as I could, but there still seems to be a bit of misalignment. The airfoil is real simple. There is none. It's high power and getting to altitude is not exactly a problem with this 3 in. diameter rocket.
Thanks.
Zooty
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zoot wrote:

It's a pretty simple concept - each fin is hinged or pivoted (along a roughly radial axis relative to the airframe) so that it can twist and thereby act like the aileron of an airplane and apply a roll torque.
In the tip of the fin is mounted a small wheel, with its outer edge exposed to the airstream so that it is kept spinning by the dynamic pressure. If the rocket is performing a roll motion, the gyroscopic precession of the spinning wheels will twist the fins in a direction that will tend to counteract the roll motion.
The ASCII art graphic below may help clarify the concept: the trapezoidal shape represents the outline of a fin, and the vertical line to the left of it is the edge of the airframe at the tail of the rocket. The O and the line of dashes indicate the positions of the spinning wheel and the pivot hinge.
| | | ||\ || \ || \ || \ || | || | |--- O || | || | || / || / || / ||/
-dave w
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Zoot, a few things in addition to the aforementioned stuff.
Try to ensure that the mass distribution is also axially centered, in other words, so that you don't have a significant mass (like a camera body) creating an unbalanced condition, when viewed straight down the long axis of the rocket. Try laying out the camera components on a flat disk equal to the diameter of the rocket, to determine the right position for good balance. You may have to counterbalance the camera with some lead (or whatever) if the camera position can't be modified.
Also, if the camera is in the nose section, you could try building a couple of different booster sections to test either different designs, or just multiple copies of one design while trying to get the fins "just right".
Finally, remember if you sand a bevel into one side of a fin's trailing edge, it will act as an airfoil to "pull" the rotation of the rocket in the direction of the bevel. This might be an easier approach to final "fine tuning" of the roll rate, than messing with trim tabs.
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zoot wrote:

from http://yarchive.net/mil/rolleron.html
==From: snipped-for-privacy@nutation.phys.columbia.edu (Steven K. Kasow) Newsgroups: sci.space.tech Subject: Re: Rolleron Ques. Date: 13 May 1997 22:14:37 GMT
>I recently saw a drawing of a device called a rolleron. It mounts >to the fin of a missile and looks like a circular saw blade. What >is its purpose and principle of operation? I've seen them on >Sidewinders but never knew what they were. >Thanks
I have also seen them - on a Sidewinder mockup. The fins on the rear of the Sidewinder had metal disks resembling your description peeking out of the edge of the fin, with the 'teeth' pointed forward, as if the disks were intended to be spun up by the air stream.
There was one other thing I observed as well- the part of the fin the rolleron was in was subdivided from the rest of the fin and on a pivot: let me try and depict my (probably incorrect) recollection with some ascii graphics.
______________________________________________
        body of Sidewinder
______________________________________________ | / | / | / |------- / | /<- pivot here) / | / / ------------------------- ^^^^     rolleron here
If the rolleron is spun up by the air stream going by, it will be moving at a fair clip and have a decent amount of angular momentum in it since it is so massive. It seems to me that this could be a simple and effective passive guidance system. As the missile turns or spins, the spinning disk in the small fin will torque that fin out of line with the rest of the fin, much like an elevon or aileron.
I think the relevant phenomenon here is precession. Consider a gyroscope supported by a pivot:
| The ^ is a pivot, the --- is an axle, ----| and the | is a wheel spinning around the axle. ^ |
The angular momentum of the wheel is pointing along the axis of its rotation. If the top of the wheel is coming out of the screen, the angular momentum is pointing away from the pivot. The torque here is due to gravity pulling down on the wheel, and it points into the screen through the center of the wheel. We also know that T = dL/dt, e.g, the applied torque is the rate of change of the angular momentum. So if the torque is pointing into the screen, and L points to the right, it must change L so that L points a little into the screen. The wheel will start precessing about the pivot, rotating into the screen and coming back on the other side.
Consider a Sidewinder with some torque acting on it trying to rotate it about its long axis. We are looking at it from the rear, and some external force is trying to rotate it counterclockwise. That torque is pointing along the long axis of the missile, towards the rear of the missile, and out of the screen in the diagram below.
<----\ wind, etc, trying to rotate missile this way      _|_ \     / \ \ ---| x |---- \___/      |
Let us consider a rolleron on the right-hand fin. It is spinning so its outer edge is moving out of the screen, so its angular momentum is pointing down.
         _|_         / \      ---| |----      \___/ |          | | <--- angular momentum of rolleron      V
The angular momentum of the rolleron is pointing down, perpendicular to the fin. A quick sketch from the side: ______         | \         ________________
    <---- ======        ___|____________         | | /         ---|---      V
The arrow pointing left is the external torque trying to spin the Sidewinder. The arrow pointing down is the angular momentum of the roller. The roller will precess: it will try to tilt its angular momentum backwards, towards the applied torque, e.g,
______         | \         ________________
    <---- ======        ___/____________         | / /         -/-----      V
This will tilt the fin it is in downwards in front, which will deflect the airflow so that it will try and rotate the missile in the opposite direction. So it seems that the rollerons can serve to keep the Sidewinder from spinning along its long axis. They may also have other nice effects, of course.
That should all be taken with a large pinch of salt, since I'm a physics student and not an aerospace engineer and am speculating about the actual purpose of the thing without much hard evidence. I know there are more experienced and knowledgeable folks out there- am I at all close here?
        cheers,
            Steven
-- "There are three stages in the killing of an astrophysicist."     --Misner, C. W., K. S. Thorne, and J. A. Wheeler, 1973,         _Gravitation_, W.H. Freeman and Company, New York.
==Newsgroups: sci.space.tech
Subject: Re: Rolleron Ques. Date: Thu, 15 May 1997 00:26:23 GMT
>...So it seems that the rollerons can serve to keep the >Sidewinder from spinning along its long axis... I'm a >physics student and not an aerospace engineer and am speculating about >the actual purpose of the thing without much hard evidence. I know >there are more experienced and knowledgeable folks out there- am I at >all close here?
Spot on. The Sidewinder's IR seeker head controls pitch and yaw, pointing the missile toward the target. It can cope with a *slow* roll (rotation around the long axis) because it just tries to point the missile toward the target, regardless of which way that is. But if the missile is rolling too rapidly, the controls won't be able to keep up with the changing apparent direction of the target, and the missile will become confused. The rollerons are spun up by the slipstream and gyroscopically deflect the fins, just as Steve speculated, to keep the roll rate down. An elegant low-tech solution to a problem that others solved with much more complex hardware at much greater cost. -- Committees do harm merely by existing. | Henry Spencer -- Freeman Dyson | snipped-for-privacy@zoo.toronto.edu =
- iz
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Try rollerons?
Or more simply, keep fins small, present a symmetric drag profile by mirroring protuberance such as launch lugs, optical windows, etc.
Most importantly, build fins straight and keep their airfoiling simple to minimize chances of inducing a roll moment.
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I seem to remember a guy who made a gyroscopic platform for his camera to sit on. The rocket would still roll, but the camera base would not.
Mike Fisher
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On 06 Jan 2004 02:34:25 GMT, snipped-for-privacy@aol.com (Mfreptiles) wrote:

Neat trick - I'm not sure I could get it to work with this design.
Zooty
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Mfreptiles wrote:

now there's an idea
I can imagine the camera payload body a clear cylinder (polycarbonate?), a mirrored disk with a center cut for the tube to fit through, capped with a shroud for aerodynamics
the camera, positioned on a gyroscopic platform that remains fixed relative to the horizon while ascending, with the camera facing at an angle up and out to the mirror
the camera will always see through the polycarbonate tube to the mirror and down regardless of the roll, and although it would be seeing through different areas of the polycarbonate at different roll attitudes, it would always be oriented the same relative to the horizon
a single axis gyro could provide the signal to a servo which rotates the platform on which the camera sits.
The Robot Marketplace http://www.robotcombat.com/marketplace_rc.html
- iz
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Assuming you've got a camera or mirror sticking out on one side, try putting something of similar shape on the other side. It seemed to work for me.

Bill Spadafora http://home.att.net/~billspad or http://www.billsplumbing.com or NARTS http://www.nar.org/NARTS/cat.html snipped-for-privacy@comcast.net snipped-for-privacy@billsplumbing.com
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On Tue, 06 Jan 2004 04:05:20 GMT, Bill Spadafora

The rocket I'm using has a slight spin to it. I was wondering if there was an easy way to knock down the rate of spin.
Ah darn, we all know I want to build a bigger rocket, anyway - hybrid ready (or maybe hybrid only).
Zooty
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You can CA a small dowel to the trailing edge of one or two fins as a "spin tab" in the reverse direction of the spin. Cut it shorter for less spin braking, replace it with a larger one for more spin braking. You don't need much. 1/8" x 3" to start.
Jerry
--
Jerry Irvine, Box 1242, Claremont, California 91711 USA
Opinion, the whole thing. <mail to: snipped-for-privacy@gte.net>
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zoot wrote:

Try adjusting the fin airfoil to compensate.
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I'd just use adjustable spin tabs. However, since you are going big and heavy, you could also go active and use a heading hold gyro system to keep the roll angle constant.
Alan
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I've had good luck with just being very careful and putting the fins on very straight. I've got one video where the rocket only rotates once in getting to 2000 feet.
Hap Griffin

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<< What am I missing? >>
Make sure fins and mirror shroud are a straight as possible. Whenever I've had problems with too much spin, it's been due to misaligned fins or (less frequently) a mirror shroud.
Speed is another factor though. A rocket that exhibits very little spin on an J125 might have significantly more spin when flown on a J450.
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On 06 Jan 2004 07:45:32 GMT, snipped-for-privacy@aol.com (RayDunakin) wrote:

That should have been obvious. Didn't think about that. I wonder what the chances are of me getting at least two motors of the same kind at a launch?
If I can go to hybrid, that won't be as much of a problem.
Zooty
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(RayDunakin) wrote:

on an

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Closer than any two motors I've ever fired at the same launch, at least.... <sigh>.
Zooty
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