A reactionless drive (also known by many other names, including as an inertial propulsion engine, a reactionless thruster, a reactionless engine, a bootstrap drive or an inertia drive) is a device to generate motion without a propellant, presumably in contradiction to the law of conservation of momentum. 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 drive has ever been validated under properly controlled conditions.
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 cautionary tale for those making and reviewing similar claims.
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.
Several kinds of thrust generating methods are in use, that are sometimes described as reactionless, because these methods do not work like rockets and reaction mass is not carried nor expelled from the spacecraft during their application. However as such they are merely reaction-mass-less, but they all exchange momentum (react) with an outside agent instead.
- Electrodynamic tethers do not expel reaction mass like a rocket. However, as electromagnetic fields can carry energy and momentum, tethers do have a mechanism for momentum transfer, and hence are not reactionless. They react with the magnetic fields of for example a planet, and thus ultimately with the planet itself.
- Gravitational assist (sling-shot maneuver) is a field propulsion technique frequently used for interplanetary probes. The probe does not expel propellant but the interaction is not reactionless. Thrust is obtained by the spacecraft from the gravitational field of the planet when passing close by it; momentum is taken from the planet and is hence overall conserved.
- Solar sails (light sails) provide thrust by placing "sails" that reflect (or absorb) photons from a star, thus transferring momentum from the photons to the spacecraft.
- Magnetic sails and electric sails provide thrust by placing "sails" deflecting the flow of ionized particles of the solar wind by either magnetic or electric means, and thus transfer momentum from the particles to the spacecraft.
Although the laws of classical physics regard reactionless propulsion as impossible, hypothetical methods based on principles from quantum mechanics, electrodynamics, relativity and nuclear physics have been put forward that would create similar effects without, the authors claim, violating any laws of physics. So far none of these methods has been unambiguously demonstrated to work in free space.
- The EmDrive is an electromagnetic-radiation-based device. One experimental device design consists of a closed asymmetric resonant cavity that is flooded with microwave radiation during operation. It is claimed that it produces reactionless thrust from the differential in the radiation pressure on the interior walls of the closed resonant cavity.
- The micronewton electromagnetic thruster is an electromagnetically powered device. A 2012 experiment was reported to produce unidirectional motion.
- The Woodward effect is a hypothesis that predicts that an electromechanical device undergoing acceleration can generate a unidirectional force if certain assumptions regarding the nature of inertia are true. Experiments to conclusively demonstrate this effect have been ongoing since the 1990s.
- The quantum vacuum plasma thruster, or "Cannae drive", is a quantum-mechanics-based device. Its advocates claim it produces thrust by directing the charged particles produced by quantum vacuum fluctuations with electromagnetic fields. As such it wouldn't be truly reactionless, but its effect, should it be proven to work, would be similar to a reactionless device. An experimental device was tested by a NASA researcher in 2014, who claimed it produced anomalous readings inconsistent with standard physics.
Devices that do not generate 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.
- The Alcubierre drive is a hypothetical method of propulsion postulated from the theory of general relativity. Although this concept may be allowed by the currently accepted laws of physics, it remains unproven; implementation would require a negative energy density, and possibly a better understanding of quantum gravity. It is not clear how (or if) this effect could provide a useful means of accelerating an actual space vehicle and no practical designs have been proposed, but experiments are underway at NASA’s Johnson Spaceflight Center to attempt the first detection of an induced spacetime curvature, which could be the first step toward proving the validity of the concept.
- “Swimming in spacetime” is a geometrical motive principle that exploits the curved spacetime metric of the gravitational field to permit an extended body undergoing specific deformations in shape, to change position. In weak gravitational fields, like that of Earth, the change in position per deformation cycle would be far too small to detect, but the concept remains of interest as the only unambiguous example of reactionless motion in mainstream physics.
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