A reactionless drive or inertial propulsion engine (also reactionless thruster, reactionless engine, and inertia drive) is any form of propulsion not based around expulsion of fuel or reaction mass - the name comes from Newton's Third Law of Motion, usually expressed as, "For every action, there is an equal and opposite reaction." Such a drive would use a hypothetical form of thrust that does not require any outside force or net momentum exchange to produce linear motion. While such a device may not necessarily violate the law of conservation of energy, it would appear to violate conservation of momentum, a fundamental principle of all current understandings of physics, and is therefore considered by most physicists impossible to construct outside of science fiction.
Invariably the devices fail to operate under controlled conditions, and are found to rely on a non-linear effect of the supports they sit on. Some examples include devices that inch along a tabletop or when floating in water, but stop working once they are suspended or in vacuum.
The classic example was the patented Dean drive, named after Norman L. Dean, which gained considerable publicity in the 1950s and 1960s, particularly in the columns of John W. Campbell, editor of Astounding Science Fiction magazine. Authors in the science-fiction genre have continued to make considerable use of the concept. For example, the Basestar in Battlestar Galactica, impeller-driven ships of the Honorverse and some ships in Larry Niven's Ringworld all use reactionless drives.
There are hundreds of such devices, but the most common is the oscillation thruster, which uses friction to transfer momentum to the device. Misconceptions as to how these devices work have led to people believing they are building reactionless drives - when in reality they are not. Another class of similar devices uses interaction with the air in a similar manner. As such they are not economic breakthroughs, as wheels and propellors are far more efficient ways of moving a vehicle in reaction to air or ground.
In most cases the devices in question are supported by a single inventor. Often, some people involved in the creation and promotion of the device (namely the inventor) blame "Big Science" for the failure of the idea to take off.
An oscillation thruster (also known as a stiction drive, internal drive or slip-stick drive) uses the motion of internal masses to create a net thrust. These thrusters include either vibrational 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 result is that for some of the motion there is a high force being generated, enough to overcome friction. However on the "return stroke" the force is not high enough, and 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 casual observation.
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 in 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:
Inventors of oscillation thrusters extrapolate its behavior to mean that it can work in a vacuum in zero gravity. However, since there is no friction in space, this doesn't work.
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:
Electrostatic anti-gravity relies on oddly placed capacitors and a high-voltage source. Claims of electrostatic anti-gravity are explained by the Biefeld–Brown effect, which is distinctly not anti-gravity. Believers of this form of anti-gravity agree that the Biefeld–Brown effect is in action, but say that it does not account for all the upward force. This area of research has not been fully exhausted, but all verified experiments fail to show any new physics. Supposed "Electrostatic anti-gravity"-machines have been tested in vacuums, and it has been observed that they do not generate any force when placed in a vacuum.
It has been claimed that a device called the electromagnetic tether constitutes a reactionless drive but this is technically inaccurate; see for example the description offered at the Special Projects Group, Harvard-Smithsonian Observatory. The key point is that electromagnetic fields can carry energy and momentum.
The fundamental scientific problem is one of momentum transfer. If there is no momentum transfer, the postulated device is classified as a "reactionless" drive and labelled a fraud. If there is a mechanism for momentum transfer, then the device is classed as a reaction drive and is therefore by definition not a "reactionless" drive. Tethers do have a mechanism for momentum transfer and although they do not expel reaction mass like a rocket, they do transfer momentum and hence can not be a true reactionless drive.