A magnetohydrodynamic (MHD) drive uses an electromagnetic field to accelerate a charged fluid, creating thrust without moving parts. The Lorentz force can be harnessed for propulsion in spacecraft, but requires a lot of power. MHD motors are reliable, efficient and quiet, but require a heavy on-board generator that burns diesel. MPD thrusters have extremely high specific impulses, but remain primarily a future technology due to power requirements.
A magnetohydrodynamic (MHD) drive is a motor with no moving parts that creates thrust by accelerating a charged fluid with an electromagnetic field. This is known as the Lorentz force, the magnitude of which in newtons on a specific charged particle can be calculated by adding the electric field density in volts per meter to the particle’s instantaneous velocity in m/s, multiplying the sum by the magnetic field density in tesla, and multiplying that product by the particle’s electric charge in columbs.
As the electromagnetic field strength increases, both the thrust and the specific impulse of a magnetohydrodynamic drive increase. The Lorentz force can be harnessed for propulsion in spacecraft, which use charged plasma as the fluid medium, and are therefore called magnetoplasmadynamic thrusters (MPDs). Experimental prototypes have been tested on both Russian and Japanese satellites.
Magnetohydrodynamics in general is the scientific discipline that studies any fluid with an electric charge. Explaining and predicting the behavior of electrically charged fluids requires combining the Navier-Stokes equations of fluid dynamics with Maxwell’s equations of electromagnetism. This means that two sets of differential equations must be solved simultaneously, which means that the calculations are computationally intensive and often require supercomputers.
In the 1990s, Mitsubishi built prototype seagoing vessels that used magnetohydrodynamic motors, but these only achieved speeds of 15 km/h (9.3 mph), despite predictions of 200 km/h (124.3 mph). Due to the lack of moving parts, magnetohydrodynamic motors can in principle be reliable, economical, efficient, quiet, and mechanically elegant. However, since their fuel source is electricity and we don’t yet have a cheap means of creating power-dense fuel cells, ships using the MHD unit must have a heavy on-board generator that burns diesel. If the cost of hydrogen fuel cells rises dramatically in the coming years, the MHD unit could prove to be a viable alternative to the propeller.
In spacecraft, magnetoplasmadynamic thrusters require a fair amount of power, on the order of megawatts, to operate optimally. Today, even the most powerful spacecraft power generators deliver only a few hundred kilowatts, meaning MPD thrusters remain primarily a future technology. However, the operating principles of MPD thrusters allow them to possess extremely high specific impulses, more than 20 times the specific impulse of chemical rockets, given sufficient power.
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