Magnetohydrodynamic drive
A Magnetohydrodynamic drive or MHD propulsor, is a method for propelling seagoing vessels using only electric and magnetic fields with no moving parts, using magnetohydrodynamics.
Principle
An electric current is passed through seawater in the presence of an intense magnetic field. Functionally, the seawater is then the moving, conductive part of an electric motor. Pushing the water out the back accelerates the vehicle.
MHD is attractive because it has no moving parts, which means that a good design might be silent, reliable, efficient, and inexpensive.
The movie The Hunt for Red October popularized the magnetohydrodynamic drive as a caterpillar drive for submarines, an undetectable "silent drive" intended to achieve stealth in submarine warfare. In reality, the current traveling through the water would create gases and noise, and the magnetic fields would induce a detectable magnetic signature. In the novel, of which the movie was an adaptation, the caterpillar was a pumpjet.
The major problem with MHD is that with current technologies it is more expensive and much slower than a propeller driven by an engine. The extra expense is from the large generator that must be driven by an engine. Such a large generator is not required when an engine directly drives a propeller.
If fuel cells become common, MHD propulsors may have lower costs in some applications than electric motors driving propellers.
A number of experimental methods of spacecraft propulsion are based on magnetohydrodynamic principles. In these the working fluid is usually a plasma or a thin cloud of ions. Some of the techniques include various kinds of ion thruster, the magnetoplasmadynamic thruster, and the variable specific impulse magnetoplasma rocket.
Prototypes
The first working prototype, the Yamato 1, was completed in Japan in 1991, by the Ship & Ocean Foundation (later known as the Ocean Policy Research Foundation). The ship was first successfully propelled in Kobe harbor in June 1992. Yamato 1 is propelled by two MHD thrusters that run without any moving parts.
In the 1990s, Mitsubishi built several prototypes of ships propelled by an MHD system. These ships were only able to reach speeds of 15 km/h, despite higher projections.
See also
- Lorentz force, relates electric and magnetic fields to propulsion force
- Magnetohydrodynamics
- Electrohydrodynamics
References
- Popular Science: Science Year by Year: Discoveries and Inventions from the Last Century that Shape our Lives (2001), New York: Scholastic, p. 208-209.
- Operation of the Thruster for Superconducting Electromagnetohydrodynamic propulsion Ship "YAMATO 1"*
- Development of a Super-Conducting Propulsion Ship (1985-1998)