Reactionless drive

A reactionless drive is a hypothetical 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 that is a closed system, presumably in contradiction with the law of conservation of momentum. Reactionless drives are often considered similar to a perpetual motion machine. The name comes from Newton's third law, often expressed as: "For every action, there is an equal and opposite reaction." Many infeasible reactionless drives are a staple of science fiction for space propulsion. 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 gyroscopic inertial thruster 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. More recently, the EmDrive was taken seriously enough to be tested by a handful of physics labs, but similarly proved to not produce a reactionless drive force. 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.