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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. Studies examining MHD in the field of marine propulsion began in the late 1950s. Few large-scale marine prototypes have been built, 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. In 2023 DARPA launched the PUMP program to build a marine engine using superconducting magnets expected to reach a field strength of 20 Tesla. 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 in chemical rockets. Principle 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 opposite directions). If either positive or negative species dominate the vehicle is put in motion in the opposite direction from the net charge. 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: f = ρ E + J × B {\displaystyle \mathbf {f} =\rho \mathbf {E} +\mathbf {J} \times \mathbf {B} \,\!} 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. Typology MHD thrusters are classified in two categories according to the way the electromagnetic fields operate: Conduction devices when a direct current flows in the fluid due to an applied voltage between pairs of electrodes, the magnetic field being steady. Induction devices when alternating currents are induced by a rapidly varying magnetic field, as eddy currents. No electrodes are required in this case. As induction MHD accelerators are electrodeless, they do not exhibit the common issues related to conduction systems (especially Joule heating, bubbles and redox from electrolysis) but need much more intense peak magnetic fields to operate. Since one of the biggest issues with such thrusters is the limited energy available on-board, induction MHD drives have not been developed out of the laboratory. Both systems can put the working fluid in motion according to two main designs: Internal flow when the fluid is accelerated within and propelled back out of a nozzle of tubular or ring-shaped cross-section, the MHD interaction being concentrated within the pipe (similarly to rocket or jet engines). External flow when the fluid is accelerated around the whole wetted area of the vehicle, the electromagnetic fields extending around the body of the vehicle. The propulsion force results from the pressure distribution on the shell (as lift on a wing, or how ciliate microorganisms such as Paramecium move water around them). Internal flow systems concentrate the MHD interaction in a limited volume, preserving stealth characteristics. External field systems on the contrary have the ability to act on a very large expanse of surrounding water volume with higher efficiency and the ability to decrease drag, increasing the efficiency even further. Marine propulsion MHD has no moving parts, which means that a good design might be silent, reliable, and efficient. Additionally, the MHD design eliminates many of the wear and friction pieces of the drivetrain with a directly driven propeller by an engine. Problems with current technologies include expense and slow speed compared to 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. The first prototype, a 3-meter (10-feet) long submarine called EMS-1, was designed and tested in 1966 by Stewart Way, a professor of mechanical engineering at the University of California, Santa Barbara. Way, on leave from his job at Westinghouse Electric, assigned his senior year undergraduate students to build the operational unit. This MHD submarine operated on batteries delivering power to electrodes and electromagnets, which produced a magnetic field of 0.015 tesla. The cruise speed was about 0.4 meter per second (15 inches per second) during the test in the bay of Santa Barbara, California, in accordance with theoretical predictions. Later, a Japanese prototype, the 3.6-meter long "ST-500", achieved speeds of up to 0.6 m/s in 1979. In 1991, the world's first full-size prototype Yamato 1 was completed in Japan after 6 years of research and development (R&D) by the Ship & Ocean Foundation (later known as the Ocean Policy Research Foundation). The ship successfully carried a crew of ten plus passengers at speeds of up to 15 km/h (8.1 kn) in Kobe Harbour in June 1992. Small-scale ship models were later built and studied extensively in the laboratory, leading to successful comparisons between the measurements and the theoretical prediction of ship terminal speeds. Military research about underwater MHD propulsion included high-speed torpedoes.... Discover the Rich Froning David Thomas popular books. Find the top 100 most popular Rich Froning David Thomas books.