scientist with beakers

A magnetohydrodynamic unit (MHD) is a motor with no moving parts that creates thrust by accelerating a fluid charged with an electromagnetic field. This is known as the Lorentz force, the magnitude of which in newtons on any particular 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 teslas, and multiplying that product by the electrical charge of the particle in columbas.

When the strength of the electromagnetic field increases, both the impulse and the specific impulse of a magnetohydrodynamic impulse also increase. The Lorentz force can be exploited for propulsion in spacecraft, which use charged plasma as a fluid medium and are therefore called magnetoplasmadynamic (MPD) thrusters. Experimental prototypes were tested on Russian and Japanese satellites.

Magnetohydrodynamics in general is the scientific discipline that studies any electrically charged fluids. Explaining and predicting the behavior of electrically charged fluids requires combining the Navier-Stokes fluid dynamics equations with Maxwell’s electromagnetism equations. 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 prototypes for marine vessels that used magneto-hydrodynamic drives, but these reached speeds of only 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, silent and mechanically elegant. However, as their fuel source is electricity and we don’t yet have a cheap way to create high power density fuel cells, ships using the MHD drive must have a heavy onboard generator that burns diesel. If the cost of hydrogen fuel cells increases dramatically over the next few years, the MHD drive could be a viable alternative to the propeller.

In spacecraft, magnetoplasmadynamic thrusters require a fair amount of energy – in megawatts – to perform optimally. Today, even the most powerful spacecraft power generators provide only a few hundred kilowatts, which means that MPD thrusters remain primarily a technology of the future. However, the operating principles of MPD thrusters allow them to have extremely high specific impulses, more than 20 times the specific impulse of chemical rockets, with sufficient power.