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Yes, this is a missile. Sorry about that. But it turns out the AIM-9 Sidewinder is the only well-documented example I can find, on the web, of a machine that employs these interesting little widgets called “rollerons.” See the little metal pinwheels at the trailing corners of the fins? The rolleron is basically an air-driven gyroscope, as Tom Harris explains over on HowStuffWorks:

[A] spinning wheel resists lateral forces acting on it. In this case, the gyroscopic motion counteracts the missile’s tendency to roll — to rotate about its central axis. The simple, cheap rollerons steady the missile as it zips through the air, which keeps the seeker assembly from spinning at top speed. This makes it a lot easier to track the target…

Cool, neh? And there could certainly be nonlethal applications for all you hobby rocketeers out there. [Thanks, Lewis!]

Sean Michael Ragan

I am descended from 5,000 generations of tool-using primates. Also, I went to college and stuff. I write for MAKE, serve as Technical Editor for MAKE magazine, and develop original DIY content for Make: Projects.


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Comments

  1. My Pet Fly says:

    I’m a volunteer docent at the USS Midway Museum in San Diego, and several of our aircraft have training Sidewinders mounted on them. Someone explained a long time ago what these roller were for, but I’d forgotten, and I kept forgetting to ask fellow volunteers that would know. This will let me sleep now.

  2. inventorjack.myopenid.com says:

    Cool stuff. I love learning about somewhat-obscure mechanisms. I’m fortunate to work in a technology-related field of the military, so I get to see some pretty amazing, and sometimes unusual, circuitry, parts, and mechanisms. I’ll have to see if I can get a tour of the flight line, though, so I can see these cool rollerons up close.

  3. kn says:

    Are there any physics majors who can unconfuse this for me:

    It looks to me like the gyroscopes on opposite fins are spinning in the same plane but opposite directions. I thought this meant their angular momentums would cancel-out? And if the vector sum of their momentums is zero then there can’t be any precession. So how do these things work?

    I’m trying to remember this from my college physics, which is pretty rusty…

    1. RocketGuy says:

      The wheel provides precession forces that tilt the fin it’s riding in. This bends the tab that it sits in like an aileron cancelling the roll that produced the precession.

      Very elegant negative feedback loop, works like a charm although roll isn’t entirely eliminated.

      I know a model rocketeer who did this on a much smaller model rocket, he discovered that it was necessary to blow a stream of air at the rollerons before launch to get them spun up (since it was ground launched, not in an already moving air stream like the AIM-9 is usually. I think he used an air compressor/tank and some tubing to direct a narrow stream at the wheels from above (and to the side, since otherwise it’d hang up).

      Helps with onboard video, keeps it from getting totally spinny.

  4. FlatTop says:

    The wheels aren’t meant to stabilize the whole missile, just the ailerons they are mounted in. In the last picture you can see that they are hinged at the front (the brass rectangle at a 45* angle). Imagine you are watching the missile from behind as it flies along. If it starts to rotate clockwise, the precession would cause the top aileron to move to the right, toward the direction of rotation. All of them would move in the direction of rotation, actually, creating an aerodynamic force to spin the missile the other way. If it rotates too far counterclockwise, the ailerons would move the other way, rotating the missile back again.
    At least I think that’s how they work.

    1. Man, that launcher is wicked-looking. Belongs on the shoulder of a Gundam.

    2. RocketGuy says:

      Now the control electronics and algorithms are so fast and sophisticated that the roll motion is compensateable (I presume, unless they’ve floated the sensor head).

  5. MSeto says:

    Excellent article on the Sidewinder from Invention&Tech magazine. This is the quintessential maker story…

    http://www.americanheritage.com/articles/magazine/it/1989/2/1989_2_56.shtml

    The problem of preventing the rocket from rolling too rapidly was solved by a technician named Sidney Crockett. He suggested that solid gyroscope wheels be mounted on flaps on each of the four rear wings. These “rollerons” had notches cut into their outer rims so that the airstream flowing past the missile would make them rotate; should the missile start to roll, the rollerons would automatically respond by forcing the flaps out into the airstream to oppose the roll.