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Amagnetohydrodynamic drive orMHD accelerator is a method for propelling vehicles using onlyelectric andmagnetic fields with nomoving parts, accelerating anelectrically conductivepropellant (liquid orgas) withmagnetohydrodynamics. Thefluid is directed to the rear and as areaction, the vehicle accelerates forward.[1][2]
Studies examining MHD in the field ofmarine propulsion began in the late 1950s.[3][4][5][6][7]
Few large-scale marine prototypes have been built, limited by the lowelectrical conductivity ofseawater. Increasingcurrent density is limited byJoule heating and waterelectrolysis in the vicinity ofelectrodes, and increasing the magnetic field strength is limited by the cost, size and weight (as well as technological limitations) ofelectromagnets and the power available to feed them.[8][9] In 2023DARPA launched the PUMP program to build a marine engine using superconducting magnets expected to reach a field strength of 20Tesla.[10]
Stronger technical limitations apply to air-breathing MHD propulsion (where ambientair is ionized) that is still limited to theoretical concepts and early experiments.[11][12][13]
Plasma propulsion engines using magnetohydrodynamics forspace exploration have also been actively studied as suchelectromagnetic propulsion offers highthrust and highspecific impulse at the same time, and the propellant would last much longer than inchemical rockets.[14]
The working principle involves the acceleration of an electrically conductivefluid (which can be aliquid or anionizedgas called aplasma) by theLorentz force, resulting from thecross product of anelectric current (motion ofcharge carriers accelerated by anelectric field applied between twoelectrodes) with aperpendicularmagnetic field. The Lorentz force accelerates allcharged 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 anelectric motor (more exactly alinear motor) except that in an MHD drive, the solid movingrotor is replaced by the fluid acting directly as thepropellant. As with allelectromagnetic devices, an MHD accelerator is reversible: if the ambientworking fluid is moving relatively to the magnetic field,charge separation induces anelectric potential difference that can be harnessed withelectrodes: the device then acts as apower source with no moving parts, transforming thekinetic energy of the incoming fluid intoelectricity, called anMHD generator.
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As the Lorentz force in an MHD converter does not act on a single isolated charged particle nor on electrons in a solidelectrical wire, but on a continuouscharge distribution in motion, it is a "volumetric" (body) force, a force per unit volume:
wheref is theforce density (force per unit volume),ρ thecharge density (charge per unit volume),E theelectric field,J thecurrent density (current per unit area) andB themagnetic field.[clarification needed]
MHD thrusters are classified in two categories according to the way the electromagnetic fields operate:
As induction MHD accelerators are electrodeless, they do not exhibit the common issues related to conduction systems (especially Joule heating, bubbles andredox 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 systems concentrate the MHD interaction in a limited volume, preservingstealth 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 decreasedrag, increasing the efficiency even further.[15]
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 thedrivetrain with a directly drivenpropeller by an engine. Problems with current technologies include expense and slow speed compared to a propeller driven by an engine.[8][9] 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 theUniversity of California, Santa Barbara. Way, on leave from his job atWestinghouse 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 ofSanta Barbara, California, in accordance with theoretical predictions.[16][17][18][15]
Later, a Japanese prototype, the 3.6-meter long "ST-500", achieved speeds of up to 0.6 m/s in 1979.[19]
In 1991, the world's first full-size prototypeYamato 1 was completed inJapan after six years ofresearch and development (R&D) by theShip & Ocean Foundation (later known as theOcean Policy Research Foundation). The ship successfully carried a crew of ten plus passengers at speeds of up to 15 km/h (8.1 kn) inKobe Harbour in June 1992.[2][20]
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.[8][9]
Military research about underwater MHD propulsion included high-speedtorpedoes,remotely operated underwater vehicles (ROV),autonomous underwater vehicles (AUV), up to larger ones such assubmarines.[21]
First studies of the interaction of plasmas withhypersonic flows around vehicles date back to the late 1950s, with the concept of a new kind ofthermal protection system forspace capsules during high-speedreentry. As low-pressure air is naturally ionized at such very high velocities and altitude, it was thought to use the effect of a magnetic field produced by an electromagnet to replacethermal ablative shields by a "magnetic shield". Hypersonic ionized flow interacts with the magnetic field, inducing eddy currents in the plasma. The current combines with the magnetic field to give Lorentz forces that oppose the flow and detach thebow shock wave further ahead of the vehicle, lowering theheat flux which is due to the brutal recompression of air behind thestagnation point. Such passiveflow control studies are still ongoing, but a large-scale demonstrator has yet to be built.[22][23]
Active flow control by MHD force fields on the contrary involves a direct and imperious action of forces to locally accelerate or slow down theairflow, modifying its velocity, direction, pressure, friction, heat flux parameters, in order to preserve materials and engines from stress, allowinghypersonic flight. It is a field of magnetohydrodynamics also calledmagnetogasdynamics,magnetoaerodynamics ormagnetoplasma aerodynamics, as the working fluid is the air (a gas instead of a liquid) ionized to become electrically conductive (a plasma).
Air ionization is achieved at high altitude (electrical conductivity of air increases as atmospheric pressure reduces according toPaschen's law) using various techniques:high voltageelectric arc discharge,RF (microwaves) electromagneticglow discharge,laser,e-beam orbetatron,radioactive source... with or without seeding of lowionization potentialalkali substances (likecaesium) into the flow.[24][25]
MHD studies applied toaeronautics try to extend the domain of hypersonicplanes to higher Mach regimes:
The Russian projectAyaks (Ajax) is an example of MHD-controlled hypersonic aircraft concept.[13] A US program also exists to design a hypersonic MHD bypass system, theHypersonic Vehicle Electric Power System (HVEPS). A working prototype was completed in 2017 under development byGeneral Atomics and theUniversity of Tennessee Space Institute, sponsored by the USAir Force Research Laboratory.[36][37][38] These projects aim to develop MHD generators feeding MHD accelerators for a new generation of high-speed vehicles. Such MHD bypass systems are often designed around ascramjet engine, but easier to designturbojets are also considered,[39][40][41] as well as subsonicramjets.[42]
Such studies covers a field ofresistive MHD withmagnetic Reynolds number ≪ 1 usingnonthermalweakly ionized gases, making the development of demonstrators much more difficult to realize than for MHD in liquids. "Cold plasmas" with magnetic fields are subject to theelectrothermal instability occurring at a critical Hall parameter, which makes full-scale developments difficult.[43]
MHD propulsion has been considered as the main propulsion system for both marine and space ships since there is no need to produce lift to counter thegravity of Earth in water (due tobuoyancy) nor in space (due toweightlessness), which is ruled out in the case offlight in theatmosphere.
Nonetheless, considering the current problem of theelectric power source solved (for example with the availability of a still missing multi-megawatt compactfusion reactor), one could imagine future aircraft of a new kind silently powered by MHD accelerators, able to ionize and direct enough air downward to lift severaltonnes. As external flow systems can control the flow over the whole wetted area, limiting thermal issues at high speeds, ambient air would be ionized and radially accelerated by Lorentz forces around anaxisymmetric body (shaped as acylinder, acone, asphere...), the entireairframe being the engine. Lift and thrust would arise as a consequence of apressure difference between the upper and lower surfaces, induced by theCoandă effect.[44][45] In order to maximize such pressure difference between the two opposite sides, and since the most efficient MHD converters (with a highHall effect) are disk-shaped, such MHD aircraft would be preferably flattened to take the shape of abiconvex lens. Having nowings norairbreathing jet engines, it would share no similarities with conventional aircraft, but it would behave like ahelicopter whoserotor blades would have been replaced by a "purely electromagnetic rotor" with no moving part, sucking the air downward. Such concepts of flying MHD disks have been developed in thepeer review literature from the mid 1970s mainly by physicistsLeik Myrabo with theLightcraft,[46][47][48][49][50] andSubrata Roy with theWingless Electromagnetic Air Vehicle (WEAV).[51][52][53]
These futuristic visions have been advertised in the media although they still remain beyond the reach of modern technology.[54][11][55]
A number of experimental methods ofspacecraft propulsion are based on magnetohydrodynamics. As this kind of MHD propulsion involves compressible fluids in the form of plasmas (ionized gases) it is also referred to as magnetogasdynamics ormagnetoplasmadynamics.
In suchelectromagnetic thrusters, the working fluid is most of the time ionizedhydrazine,xenon orlithium. Depending on the propellant used, it can be seeded withalkali such aspotassium orcaesium to improve its electrical conductivity. All charged species within the plasma, from positive and negative ions to free electrons, as well as neutral atoms by the effect of collisions, are accelerated in the same direction by the Lorentz "body" force, which results from the combination of a magnetic field with an orthogonal electric field (hence the name of "cross-field accelerator"), these fields not being in the direction of the acceleration. This is a fundamental difference withion thrusters which rely onelectrostatics to accelerate only positive ions using theCoulomb force along ahigh voltage electric field.
First experimental studies involving cross-field plasma accelerators (square channels and rocket nozzles) date back to the late 1950s. Such systems provide greaterthrust and higherspecific impulse than conventionalchemical rockets and even modern ion drives, at the cost of a higher required energy density.[56][57][58][59][60][61]
Some devices also studied nowadays besides cross-field accelerators include themagnetoplasmadynamic thruster sometimes referred to as the Lorentz force accelerator (LFA), and the electrodelesspulsed inductive thruster (PIT).
Even today, these systems are not ready to be launched in space as they still lack a suitable compact power source offering enoughenergy density (such as hypotheticalfusion reactors) to feed the power-greedyelectromagnets, especially pulsed inductive ones. The rapid ablation of electrodes under the intense thermal flow is also a concern. For these reasons, studies remain largely theoretical and experiments are still conducted in the laboratory, although over 60 years have passed since the first research in this kind of thrusters.
Oregon, a ship in theOregon Files series of books by authorClive Cussler, has a magnetohydrodynamic drive. This allows the ship to turn very sharply and brake instantly, instead of gliding for a few miles. InValhalla Rising, Clive Cussler writes the same drive into the powering ofCaptain Nemo'sNautilus.
The film adaptation ofThe Hunt for Red October popularized the magnetohydrodynamic drive as a "caterpillar drive" forsubmarines, a nearly undetectable "silent drive" intended to achievestealth insubmarine 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 film, it was suggested that this sound could be confused with geological activity. Inthe novel from which the film was adapted, the caterpillar thatRed October used was actually apump-jet of the so-called "tunnel drive" type (the tunnels provided acoustic camouflage for the cavitation from the propellers).
In theBen Bova novelThe Precipice, the ship where some of the action took place,Starpower 1, built to prove that exploration and mining of theasteroid belt was feasible and potentially profitable, had a magnetohydrodynamic drive mated to afusion power plant.