The Orientation Ratchet:
Inspired by the way that a cat turns when it falls (it is able to land on its feet while continuously maintaining zero net angular momentum) Dr. Ojakangas and his students conceived of this robotic device, a variation of which could be used to turn satellites in outer space without the use of propellant thrusters or reaction wheels. The robot was built by Drury University students Daniel Ratchford, Allison Harris, James Stockton, and Jeremy Woolery. There WAS a link here at Drury University. The project originally garnered attention from around the world, including tech-news website Slashdot, Robot Haven, Space Daily, the Guardian, and EE Times. The physics of its motion is described in this document, including its behavior when attached to a torsional pendulum (a long, strong section of fishing line). The project was submitted as a proposal to the NASA Reduced Gravity Student Flight Opportunities Program, and it flew on the Weightless Wonder in the Spring of 2004.
Far left: Drury students discuss the construction of the unique robot. This was before 3D printing, so the device was constructed from PVC pipe, servos, a BrainStem microcontroller, and other parts.
Near left: the unique robot is suspended from a long fishing line to simulate behavior in weightlessness. The fishing line was approximated theoretically as a very weak torsional pendulum. This worked well, but we needed to study it in the weightless environment of the NASA aircraft.
In the video on the left, Drury students operate the device in the microgravity environment of the NASA Weightless Wonder aircraft.
The sketch on the right describes the motion of the robot. The robot is first placed motionless in the lab (aircraft) coordinate system. The rotational inertia of the upper section is increased by expanding two weighted arms. Then the upper section is rotated relative to the lower section. It does not rotate as far as the lower section, since its inertia is larger. Then the arms are contracted, and the reverse rotation is performed. The net result is that the robot turns by a prescribed angle in space, without ever possessing any net angular momentum.