From: kasow@nutation.phys.columbia.edu (Steven K. Kasow)
Newsgroups: sci.space.tech
Subject: Re: Rolleron Ques.
Date: 13 May 1997 22:14:37 GMT

In article <19970511182700.OAA10287@ladder01.news.aol.com>,
Cb61 <cb61@aol.com> wrote:
>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
From: Henry Spencer <henry@zoo.toronto.edu>
Subject: Re: Rolleron Ques.
Date: Thu, 15 May 1997 00:26:23 GMT

In article <5lap4d$on2$1@lol.cs.columbia.edu>,
Steven K. Kasow <kasow@nutation.phys.columbia.edu> wrote:
>...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        |   henry@zoo.toronto.edu