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A Magnetohydrodynamic drive or MHD propulsor, is a method proposed for propelling seagoing vessels. An electric current is passed through seawater in the presence of an intense magnetic field. Functionally, the seawater is the moving, conductive part of an electric motor. Pushing the water out the back accelerates the vehicle.
MHD is attractive to engineers because it has no moving parts, which means that a good design might be silent, reliable, efficient and inexpensive. Also known as a caterpillar drive for submarines, this was popularised in the movie The Hunt for Red October as being a "silent drive," an undetectable stealth superweapon in submarine warfare.
In the 1990s, Mitsubishi built several prototypes of ships propelled by a MHD system. These ships were able to reach speeds of 15 km/h.
The major problem with MHD is that with current technologies it is more expensive than a propellor 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.
When fuel cells become common, MHD propulsors may have lower costs in some applications than electric motors driving propellers.
NASA and other organizations are also sponsoring exploratory development of the magnetohydrodynamic drive for interplanetary craft.[1] (http://www.space.com/businesstechnology/technology/plasma_propulsion_000616.html),[2] (http://www.highway2space.com/sciresearch/adv_plas_prop.html) In this context, it is usually called "plasma propulsion".[3] (http://w3.pppl.gov/~fisch/proj_hallthruster.html)
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