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Field propulsion is the concept of spacecraft propulsion where no propellant is necessary but instead momentum of the spacecraft is changed by an interaction of the spacecraft with external force fields, such as gravitational and magnetic fields from stars and planets. Some methods such as the gravity assist have been in use for interplanetary spacecraft missions for decades. Others are speculative and have not yet been demonstrated to be of practical use or theoretically valid.

Practical methods
Although not presently in wide use for space, there exists proven terrestrial examples of "Field Propulsion", in which electromagentic fields act upon a conducting medium such as seawater or plasma for propulsion, is known as magnetohydrodynamics or MHD. MHD is similar in operation to electric motors, however rather than using moving parts or metal conductors, fluid or plasma conductors are employed. The EMS-1 and more recently the Yamato 1 are examples of such electromagnetic Field propulsion systems, first proposed in patent. There is definitely potential to apply MHD to the space environment and experiments such as the NASA's Electrodynamic Tether, Lorentz Actuated Orbits, the Wingless Electromagnetic Air Vehicle, and Magnetoplasmadynamic thruster (which does use propellant) lay a solid foundation for using "fields" to propel spacecraft without propellant and standard concepts of chemical thrust. Since electrodynamics is well proven science, electromagnetic fields themselves carry momentum (see the Nichols radiometer), and electromagnetic field propulsion is not limited to the ejection velocity of particle propellants these new concepts offer tremendous potential as a future space propulsion system. They represent a radical departure from current ideas of aeronautics and rocket propulsion, and as such are controversial, but field propulsion may offer the radical breakthroughs in performance capabilities required for deep space exploration. The main limiting factors appear to the generation of the significant amounts of electrical power required and a method of strongly coupling the fields to large volumes

Electrohydrodynamics is another method whereby electrically charged fluids are used for propulsion and boundary layer control such as Ion Propulsion

Other practical methods which could be loosely considered as field propulsion include: The gravity assist trajectory, which uses planetary gravity fields and orbital momentum; Solar sails and magnetic sails use the solar wind or radiation pressure for spacecraft thrust; Aerobraking uses the atmosphere of a planet to change relative velocity of a spacecraft. The last two actually involve the exchange of momentum with physical particles and are not usually expressed as an interaction with fields, but they are sometimes included as examples of field propulsion since no spacecraft propellant is required.

Speculative methods
Other concepts that have been proposed are speculative, using "frontier physics" and concepts from modern physics. So far none of these methods have been unambiguously demonstrated, much less, proven practical.

The Woodward effect is based on a controversial concept of inertia and certain solutions to the equations for General Relativity. Experiments attempting to conclusively demonstrate this effect have been conducted since the 1990s.

Although speculative, ideas such as coupling to the momentum flux of the zero-point electromagnetic wave field hypothesized in stochastic electrodynamics have a plausible basis for further investigation within the existing theoretical physics paradigm. Examples of proposals for field propulsion that rely on physics outside the present paradigms are various schemes for faster-than-light, warp drive and antigravity, and often amount to little more than catchy descriptive phrases, with no known physical basis. Any such schemes worthy of discussion must rely on energy and momentum transfer to the spacecraft from some external source such as a local force field, which in turn must obtain it from still other momentum and/or energy sources in the cosmos, in order to satisfy conservation of both energy and momentum.

Field propulsion based on physical structure of space
This concept is based on the General Relativity Theory and the Quantum Field Theory from which the idea that space has a physical structure can be proposed. The macroscopic structure is described by the General Relativity Theory and the microscopic structure by the Quantum Field Theory. The idea is to deform space around the space craft. By deforming the space it would be possible to create a region with higher pressure behind the space craft than before it. Due to the pressure gradient a force would be exerted on the space craft which in turn creates thrust for propulsion. Due to the purely theoretical nature of this propulsion concept it is hard to determine the amount of thrust and the maximum velocity that could be achieved. Currently there are two different concepts for such a field propulsion system one that is purely based on the General Relativity Theory and one based on the Quantum Filed Theory.

In the General relativistic field propulsion system space is considered to be a elastic field similar to rubber which means that space itself can be treated as an infinite elastic body. If the space-time curves, a normal inwards surface stress is generated which serves as a pressure field. By creating a great number of those curve surfaces behind the space craft it is possible to achieve a unidirectional surface force which can be use for the acceleration of the space craft.

For the Quantum field theoretical propulsion system it is assumed, as stated by the Quantum Field Theory and Quantum Electrodynamics, that the quantum vacuum consists out of a zero-radiating electromagnetic field in a non-radiating mode and at a zero-point energy state, the lowest possible energy state. It is also theorized that matter is composed out of elementary primary charged entities, partons, which are bound together as elementary oscillators. By applying a electromagnetic zero point field a Lorenz force is applied on the partons. Using this on a dielectric material could effect the inertia of the mass and that way create a acceleration of the material without creating stress or strain inside the material.

Advantages
The advantage of a field propulsion systems is that no propellant is needed only an energy source. This means that no propellant has to be stored and transported with the space craft which makes it attractive for long term interplanetary or even interstellar manned missions in which with current technology a large amount of fuel meant for the way back has to be brought to the destination which increases the payload of the overall space craft significantly.

Description
The principle of electro-osmosis or electroosmotic flow creates a flow of an electrolyte through a very small tube in the nano-meter range. To achieve this flow there is a cathode and an anode at the ends of the tube over which a voltage is applied. Due to this voltage the ions in the electrolyte stored in a reservoir directly connected to the tube can be accelerated and ejected. This way electrical energy is transformed into kinetic energy. The amount of thrust created by one nano thruster is in the micro newton range, however due to its size it makes sense to arrange a big amount in an array to achieve sufficient thrust. The thrust, exit velocity of the ions and the mass flow rate of the electrolyte are influenced by the applied voltage which makes it easy to regulate those parameters. The applied voltage and the pH-value of the electrolyte (amount of ions it contains) also vary the balance between thrust, efficiency and maximal exhaust velocity (determines the maximal achievable flight velocity). It is also theoretically possible to achieve a very high efficiency of nearly 100% as well as a high specific impulse and high thrust-to-power ratio. This system has not yet been build and experimentally tested because of difficulties with the production of the nano-tubes needed for it.

Advantages
Nano electrokinetic thrusters have a very high efficiency, specific impulse, exhaust velocity and thrust-to-power ratio which make them suitable for a wide variety of applications. Due to the fact that a thruster is made up out of an array of multiple nano thrusters it is easily possible to design thursters in all sizes and thrust ranges. These properties give the nano electrokinetic thrusters very good thrust control which makes it applicable for a wide range of spacecraft ranging from maneuvering thrusters for small spacecraft, such as satellites, to the primary propulsion system of interplanetary or interstellar spacecraft. This system also doesn't require additional heat or radiation shielding to protect the rest of the space craft which make the system (not including fuel compartment) light in comparison to other designs.

Disadvantages
The production of the required carbon nano tubes is very expensive and with current production methods the amount of surface defects in the produced carbon nano tubes is high which reduces the efficiency significantly and makes it unreliable. This design also requires a high potential difference in the range of 300 to 500 volts as well as a sufficient storage tank for the liquid electrolyte needed which increase the weight of the overall system.