Reactionless drive

Not to be confused with Field propulsion, Propellantless propulsion, or Spacecraft electric propulsion.

Reactionless drives have been compared to perpetual motion machines. The October 1920 issue of Popular Science covered the topic.

Reactionless drive refers to hypothetical and unproven forms of spacecraft propulsion that would generate thrust without the expelling of propellant or otherwise exchanging momentum. A propellantless drive is not necessarily reactionless if it functions as an open system interacting with external fields, but a reactionless drive is generally conceived as a self-contained or closed-system device, which is why such claims are commonly treated as conflicting with Newton's third law and the conservation of momentum. Reactionless-drive claims are therefore often compared to perpetual motion machines or other impossible-energy schemes if their reported performance is taken at face value.

The expression has been in English-language newspaper use since at least 1963 and later became a recurring public label for alleged no-propellant engines. Concepts described this way have included the Dean drive, electrogravitics and lifters, gyroscopic and oscillation devices, microwave-cavity thrusters such as the EmDrive and Cannae drive, the helical engine, Mach-effect or MEGA drives, quantum drive claims, and the quantum vacuum thruster.

No reactionless drive has become an accepted propulsion technology, and reported effects from devices such as a thrusting antenna and the EmDrive have later been described as null results, below photon-thrust limits, or experimental artifacts rather than evidence of new physics. Speculative discussion continues in propulsion and space drive literature, including proposals involving Mach-principle coupling, inertial frame manipulation, distant-matter interaction, Heim-theory propulsion, and zero-point field vacuum-inertia models.