[2] There is potential to apply MHD to the space environment such as in experiments like NASA's electrodynamic tether, Lorentz Actuated Orbits,[3] the wingless electromagnetic air vehicle, and magnetoplasmadynamic thruster (which does use propellant).
Electrohydrodynamics is another method whereby electrically charged fluids are used for propulsion and boundary layer control such as ion propulsion[citation needed] 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 respectively the radiation pressure and solar wind for spacecraft thrust; aerobraking uses the atmosphere of a planet to change relative velocity of a spacecraft.
The Woodward effect is based on a controversial concept of inertia and certain solutions to the equations for General Relativity.
Due to the pressure gradient a force would be exerted on the space craft which in turn creates thrust for propulsion.
[9] 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.
If the space-time curves, a normal inwards surface stress is generated which serves as a pressure field.
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 crewed missions.
[10] 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.
The energy required to reach the high speeds involved begins to be non-neglectable for interstellar travel.
For example, a 1-tonne spaceship traveling at 1/10 of the speed of light carries a kinetic energy of 4.5 × 1017 joules, equal to 5 kg according to the mass–energy equivalence.
This means that for accelerating to such speed, no matter how this is achieved, the spaceship must have converted at least 5 kg of mass/energy into momentum, imagining 100% efficiency.