A reactionless drive is a hypothetical device producing

motion In physics, motion is when an object changes its position with respect to a reference point in a given time. Motion is mathematically described in terms of displacement, distance, velocity, acceleration, speed, and frame of reference to an o ...

without the exhaust of a

propellant A propellant (or propellent) is a mass that is expelled or expanded in such a way as to create a thrust or another motive force in accordance with Newton's third law of motion, and "propel" a vehicle, projectile, or fluid payload. In vehicle ...

. A propellantless drive is not necessarily reactionless when it constitutes an open system interacting with external

field Field may refer to: Expanses of open ground * Field (agriculture), an area of land used for agricultural purposes * Airfield, an aerodrome that lacks the infrastructure of an airport * Battlefield * Lawn, an area of mowed grass * Meadow, a grass ...

s; but a reactionless drive is a particular case of a propellantless drive that is a

closed system A closed system is a natural physical system that does not allow transfer of matter in or out of the system, althoughin the contexts of physics, chemistry, engineering, etc.the transfer of energy (e.g. as work or heat) is allowed. Physics In cl ...

, presumably in contradiction with the law of conservation of momentum. Reactionless drives are often considered similar to a

perpetual motion machine Perpetual motion is the motion of bodies that continues forever in an unperturbed system. A perpetual motion machine is a hypothetical machine that can do work indefinitely without an external energy source. This kind of machine is impossible ...

. The name comes from

Newton's third law Newton's laws of motion are three physical laws that describe the relationship between the motion of an object and the forces acting on it. These laws, which provide the basis for Newtonian mechanics, can be paraphrased as follows: # A body re ...

, often expressed as: "For every action, there is an equal and opposite reaction." Many infeasible reactionless drives are a staple of

science fiction Science fiction (often shortened to sci-fi or abbreviated SF) is a genre of speculative fiction that deals with imaginative and futuristic concepts. These concepts may include information technology and robotics, biological manipulations, space ...

for

space propulsion Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. In-space propulsion exclusively deals with propulsion systems used in the vacuum of space and should not be confused with space launch or atmospheric e ...

Closed systems

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.

Dean drive

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 Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. Types of friction include dry, fluid, lubricated, skin, and internal -- an incomplete list. The study of t ...

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 Eric Roberts Laithwaite (14 June 1921 – 27 November 1997) was an English electrical engineer, known as the " Father of Maglev" for his development of the linear induction motor and maglev rail system after Hermann Kemper. Biography Er ...

, the "Father of Maglev", received a US patent for his own propulsion system, which was claimed to create a linear thrust through gyroscopic and inertial forces. However, after years of theoretical analysis and laboratory testing of actual devices, no rotating (or any other) mechanical device has been found to produce unidirectional reactionless thrust in free space.

Helical engine

David M. Burns, formerly a

NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...

engineer at the

Marshall Space Flight Center Marshall Space Flight Center (officially the George C. Marshall Space Flight Center; MSFC), located in Redstone Arsenal, Alabama (Huntsville, Alabama, Huntsville postal address), is the Federal government of the United States, U.S. government's ...

in Alabama, theorized a potential spacecraft propulsion drive that could possibly exploit the known mass-altering effects that occur at near the

speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant exactly equal to ). It is exact because, by international agreement, a metre is defined as the length of the path travelled by light in vacuum during a time i ...

. He wrote a paper published in 2019 by

in which he describes it as ''"A new concept for in-space propulsion is proposed in which propellant is not ejected from the engine, but instead is captured to create a nearly infinite specific impulse".''

Open systems

Movement with thrust

Several kinds of thrust-generating methods are in use or have been proposed that are propellantless, as they do not work like rockets and reaction mass is not carried nor expelled from the device. However they are not reactionless, as they constitute open systems interacting with electromagnetic waves or various kinds of fields. Most famous propellantless methods are the gravity assist maneuver or gravitational slingshot of a spacecraft accelerating at the expense of the momentum of the planet it orbits, through the gravitational field, or

beam-powered propulsion Beam-powered propulsion, also known as directed energy propulsion, is a class of aircraft or spacecraft propulsion that uses energy beamed to the spacecraft from a remote power plant to provide energy. The beam is typically either a microwave or a ...

and

solar sailing Solar sails (also known as lightsails, light sails, and photon sails) are a method of spacecraft propulsion using radiation pressure exerted by sunlight on large surfaces. A number of spaceflight missions to test solar propulsion and navigatio ...

, using the

radiation pressure Radiation pressure (also known as light pressure) is mechanical pressure exerted upon a surface due to the exchange of momentum between the object and the electromagnetic field. This includes the momentum of light or electromagnetic radiation of ...

of electromagnetic waves from a distant source like a

laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word ''laser'' originated as an acronym for light amplification by stimulated emission of radi ...

or the sun. More speculative methods have also been proposed, like the Mach effect, the quantum vacuum plasma thruster or various hypotheses associated with resonant cavity thrusters.

Movement without thrust

Because there is no well-defined "center of mass" in

curved spacetime In physics, curved spacetime is the mathematical model in which, with Einstein's theory of general relativity, gravity naturally arises, as opposed to being described as a fundamental force in Isaac Newton, Newton's static Euclidean reference fra ...

general relativity General relativity, also known as the general theory of relativity, and as Einstein's theory of gravity, is the differential geometry, geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of grav ...

allows a stationary object to, in a sense, "change its position" in a counter-intuitive manner, without violating conservation of momentum. * The

Alcubierre drive The Alcubierre drive () is a speculative warp drive idea according to which a spacecraft could achieve apparent faster-than-light travel by contracting space in front of it and expanding space behind it, under the assumption that a configurabl ...

is a hypothetical method of apparent

faster-than-light Faster-than-light (superluminal or supercausal) travel and communication are the conjectural propagation of matter or information faster than the speed of light in vacuum (). The special theory of relativity implies that only particles with zero ...

propulsion for

interstellar travel Interstellar travel is the hypothetical travel of spacecraft between star systems. Due to the vast distances between the Solar System and nearby stars, interstellar travel is not practicable with current propulsion technologies. To travel between ...

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 Negative energy is a concept used in physics to explain the nature of certain fields, including the gravitational field and various quantum field effects. Gravitational energy Gravitational energy, or gravitational potential energy, is the po ...

density, and possibly a better understanding of

quantum gravity Quantum gravity (QG) is a field of theoretical physics that seeks to describe gravity according to the principles of quantum mechanics. It deals with environments in which neither gravitational nor quantum effects can be ignored, such as in the v ...

. It is not clear how (or whether) this effect could provide a useful means of accelerating an actual space vehicle and no practical designs have been proposed. * "Swimming in spacetime" is a general relativistic effect, where an extended body can change its position by using cyclic deformations in shape to exploit the curvature of space, such as due to a gravitational field. In weak gravitational fields, like that of Earth, the change in position per deformation cycle would be far too small to detect.

References

External links

"Breakthroughs" commonly submitted to NASA

at MathPages {{DEFAULTSORT:Reactionless Drive Spacecraft propulsion Pseudoscience Perpetual motion Hypothetical technology Propulsion Discovery and invention controversies Fringe physics