Magnetohydrodynamic drive

A magnetohydrodynamic drive or MHD accelerator is a method for propelling vehicles using only electric and magnetic fields with no moving parts, accelerating an electrically conductive propellant (liquid or gas) with magnetohydrodynamics. The fluid is directed to the rear and as a reaction, the vehicle accelerates forward. A magnetohydrodynamic drive or MHD accelerator is a method for propelling vehicles using only electric and magnetic fields with no moving parts, accelerating an electrically conductive propellant (liquid or gas) with magnetohydrodynamics. The fluid is directed to the rear and as a reaction, the vehicle accelerates forward. The first studies examining MHD in the field of marine propulsion date back to the early 1960s. Few large-scale working prototypes have been built, as marine MHD propulsion remains impractical due to its low efficiency, limited by the low electrical conductivity of seawater. Increasing current density is limited by Joule heating and water electrolysis in the vicinity of electrodes, and increasing the magnetic field strength is limited by the cost, size and weight (as well as technological limitations) of electromagnets and the power available to feed them. Stronger technical limitations apply to air-breathing MHD propulsion (where ambient air is ionized) that is still limited to theoretical concepts and early experiments. Plasma propulsion engines using magnetohydrodynamics for space exploration have also been actively studied as such electromagnetic propulsion offers high thrust and high specific impulse at the same time, and the propellant would last much longer than chemical rockets. The working principle involves the acceleration of an electrically conductive fluid (which can be a liquid or an ionized gas called a plasma) by the Lorentz force, resulting from the cross product of an electric current (motion of charge carriers accelerated by an electric field applied between two electrodes) with a perpendicular magnetic field. The Lorentz force accelerates all charged particles (positive and negative species) in the same direction whatever their sign, and the whole fluid is dragged through collisions. As a reaction, the vehicle is put in motion in the opposite direction. This is the same working principle as an electric motor (more exactly a linear motor) except that in an MHD drive, the solid moving rotor is replaced by the fluid acting directly as the propellant. As with all electromagnetic devices, an MHD accelerator is reversible: if the ambient working fluid is moving relatively to the magnetic field, charge separation induces an electric potential difference that can be harnessed with electrodes: the device then acts as a power source with no moving parts, transforming the kinetic energy of the incoming fluid into electricity, called an MHD generator. As the Lorentz force in an MHD converter does not act on a single isolated charged particle nor on electrons in a solid electrical wire, but on a continuous charge distribution in motion, it is a 'volumetric' (body) force, a force per unit volume: where f is the force density (force per unit volume), ρ the charge density (charge per unit volume), E the electric field, J the current density (current per unit area) and B the magnetic field.