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A reactionless drive is a device producing motion without the exhaust of a propellant. A propellantless drive is not necessarily reactionless when it constitutes an open system interacting with external fields; but a reactionless drive is a particular case of a propellantless drive as it is a closed system presumably in contradiction with the law of conservation of momentum and often considered similar to a perpetual motion machine. The name comes from Newton's third law, which is usually expressed as, "for every action, there is an equal and opposite reaction." A large number of infeasible devices, such as the Dean drive, are a staple of science fiction particularly for space propulsion. To date, no reactionless device has ever been validated under properly controlled conditions.
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Closed systems
Through the years there have been numerous claims for functional reactionless drive designs using ordinary mechanics (i.e. devices not said to be based on quantum mechanics, relativity or atomic forces or effects). Two of these represent their general classes: The "Dean drive" is perhaps the best known example of a "linear oscillating mechanism" reactionless drive; The "GIT" is perhaps the best known example of a "rotating mechanism" reactionless drive. These two also stand out as they both received much publicity from their promoters and the popular press in their day and both were eventually rejected when proven to not produce any reactionless drive forces. The rise and fall of these devices now serves as a cautionary tale for those making and reviewing similar claims.
Dean drive
The Dean drive was a mechanical device concept promoted by inventor Norman L. Dean. Dean claimed that his device was a "reactionless thruster" and that his working models could demonstrate this effect. He held several private demonstrations but never revealed the exact design of the models nor allowed independent analysis of them. Dean's claims of reactionless thrust generation were subsequently shown to be in error and the "thrust" producing the directional motion was likely to be caused by friction between the device and the surface on which the device was resting and would not work in free space.
Gyroscopic Inertial Thruster (GIT)
The Gyroscopic Inertial Thruster is a proposed reactionless drive based on the mechanical principles of a rotating mechanism. The concept involves various methods of leverage applied against the supports of a large gyroscope. The supposed operating principle of a GIT is a mass traveling around a circular trajectory at a variable speed. The high-speed part of the trajectory allegedly generates greater centrifugal force than the low, so that there is a greater thrust in one direction than the other. Scottish inventor Sandy Kidd, a former RAF radar technician, investigated the possibility (without success) in the 1980s. He posited that a gyroscope set at various angles could provide a lifting force, defying gravity. In the 1990s, several people sent suggestions to the Space Exploration Outreach Program (SEOP) at NASA recommending that NASA study a gyroscopic inertial drive, especially the developments attributed to the American inventor Robert Cook and the Canadian inventor Roy Thornson. In the 1990s and 2000s, enthusiasts attempted the building and testing of GIT machines. Eric Laithwaite, the "Father of Maglev", received a US patent for his "Propulsion System", which was claimed to create a linear thrust through gyroscopic and inertial forces. After years of theoretical analysis and laboratory testing of actual devices, no rotating (or any other) mechanical device has ever been found to produce unidirectional reactionless thrust in free space.
Movement with thrust
Several kinds of thrust generating methods are in use or have been proposed that are propellantless, as they do not work like rockets and reaction mass is not carried nor expelled from the device. However they are not reactionless, as they constitute open systems interacting with electromagnetic waves or various kinds of fields.
Most famous propellantless methods are the gravity assist maneuver or gravitational slingshot of a spacecraft accelerating at the expense of the momentum of the planet it orbits, through the gravitational field, or beam-powered propulsion using the radiation pressure of electromagnetic waves from a distant source like a laser.
More speculative methods have also been proposed, like the Woodward effect, the quantum vacuum plasma thruster or various hypotheses trying to explain the thrust apparently produced by the EmDrive.
Movement without thrust
Because there is no well-defined "center of mass" in curved spacetime, general relativity allows a stationary object to, in a sense, "change its position" in a counter-intuitive manner, without violating conservation of momentum.