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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 fictional or theorized method of propulsion wherein thrust is generated without any need for an outside force or net momentum exchange to produce linear motion. The name comes from Newton's Third Law of Motion, which is usually expressed as, "[f]or every action, there is an equal and opposite reaction". Such a drive would necessarily violate the law of conservation of momentum, a fundamental principle of all current understandings of physics. In addition, it can be shown that the law of conservation of energy would be violated by a reactionless drive.
In spite of their physical impossibility, such devices are a staple of science fiction, particularly for space propulsion, and as with perpetual motion machines, have sometimes been proposed as working technologies.
Propulsion systems that react with some medium (e.g. interplanetary/interstellar medium) or cosmic bodies (planets, stars) do not expel reaction mass, but are not reactionless - even though they may be impossible due to other factors.
An oscillation thruster (also known as a stiction drive, an internal drive, or as a slip-stick drive) is not a reactionless drive as it requires outside masses in order to work. These thrusters include either vibrating or rotating masses, in which one portion of the cyclical motion is high-speed and the other low-speed, or alternately high-and-low impulse. The actual result is that, for some of the motion, a high force is generated, enough to overcome friction. However, on the "return stroke", the force is not high enough, and so any motion occurring in the first portion is not reset. In this way, the devices "steal" working mass from their supporting surface, a fact that may not be apparent to the casual observer.
Basically, an oscillation thruster works just like walking does - one mass is "thrown" backward, "thrusting" the device forward according to conservation of momentum (like a person taking a step forward), then the mass is more slowly brought forward to its initial position (like the person using their step to pull the rest of their body forward). The thruster is allowed to move forward during the first step because the mass is "thrown" back with large enough force to overcome static friction. The "thruster" doesn't move backward in the second step because static friction provides an outside force that overcomes the step (like the ground does when you're walking).
Although there have been many versions proposed, all oscillation thrusters have the following common components:
- Chassis (to support a system of masses)
- Conveyor (that moves the masses through an asymmetric cycle)
- Power source (for the conveyor)
A primary feature is that these internal masses go through some sort of cyclic motion where the motion in one direction is quicker than that in the return direction.
One of the most famous proposed reactionless drives was the Dean drive. Although Dean himself gave few indications of how his "reactionless drive" was supposed to work, it appears to be an attempt at an oscillation thruster. Other examples of oscillation thrusters are the Thornson Inertial Engine and Henry Bull’s Impulse Engine of 1935.
Magnetoelectric materials can be electrically polarized by magnetic fields - the quantum vacuum contains randomly fluctuating magnetic fields. A sufficiently small piece of magnetoelectric material can be placed in the quantum vacuum and then rotated 180 degrees. The random magnetic fields can induce a change in electric polarization in the block of material. It has been proposed that this change causes the combined electric and magnetic fields to push the block in one direction while they (the fields themselves - or more technically, the bosons of which these fields are composed) get pushed in the other direction. This mechanism was first suggested in December 2009, and has not yet been tested. The author of the original paper Dr. Alexander Feigel suggested that the mechanism could be made powerful enough to reorient satellites if a material with a magnetoelectric constant ten times higher than the highest known in such materials today could be found.
This would not strictly be a reactionless drive but a photon drive as the momentum of the bosons would balance the increased momentum of the item.
Although the basic tenet of reactionless propulsion is physically impossible, hypothetical cases have been put forward which would create a similar effect without (evidently) violating either Newton's Third Law or the law of conservation of energy.
- General relativity allows a hypothetical astronaut to "swim" in curved space without using reaction mass. In other words, if spacetime is flat, then an astronaut can change his or her orientation in space through certain body movements (in the same manner as a falling cat can orient itself so that it hits the ground feet first), but no amount of this sort of exertion will change the position of his center of mass. However, if local spacetime is curved, a similar trick can be used to take advantage of this curvature - mass held in the astronaut's outstretched hands moves in a slightly different path through curved spacetime than does mass at the astronaut's feet, and the resulting "force" on the astronaut can change his position. While this concept is allowed by the currently accepted laws of physics, it is not clear how (or if) this effect could provide a useful means of accelerating an actual space vehicle.
- 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.
- Solar sails do not expel reaction mass like a rocket. However, the photons carry energy and momentum. Therefore, the solar sail does not carry reaction mass but is not reactionless.
The Dean drive, invented by Norman L. Dean, is a device intended to be a reactionless thruster. According to Dean, his models were able to demonstrate this effect. He received two patents for related devices that are known to be unable to generate a unidirectional force, but he occasionally demonstrated devices that were different. Dean's claims of reactionless thrust generation have subsequently been shown to be in error, as the thrust generated is understood to be reliant on friction with the surface on which the device is resting.
Gyroscopic Inertial Thruster (GIT)
The Gyroscopic Inertial Thruster is a proposed reactionless engine which uses entirely mechanical principles. The concept involves various methods of leverage applied against the supports of a large gyroscope. 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 US Patent Number 5860317 for his theorized gyroscopic Propulsion System, which was purported to possibly create a linear thrust through gyroscopic/inertial forces.  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. Linear and coriolis accelerations are omitted from the analysis.
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