Hubbry Logo
Magnetohydrodynamic driveMagnetohydrodynamic driveMain
Open search
Magnetohydrodynamic drive
Community hub
Magnetohydrodynamic drive
logo
8 pages, 0 posts
0 subscribers
Be the first to start a discussion here.
Be the first to start a discussion here.
Magnetohydrodynamic drive
Magnetohydrodynamic drive
Magnetohydrodynamic drive
View on Wikipedia
from Wikipedia
Yamato 1 on display in Kobe, Japan. The first working full-scale MHD ship.

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.[1][2]

Studies examining MHD in the field of marine propulsion began in the late 1950s.[3][4][5][6][7]

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.[8][9] In 2023 DARPA launched the PUMP program to build a marine engine using superconducting magnets expected to reach a field strength of 20 Tesla.[10]

Stronger technical limitations apply to air-breathing MHD propulsion (where ambient air is ionized) that is still limited to theoretical concepts and early experiments.[11][12][13]

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.[14]

Principle

[edit]
Main article: Lorentz force
See also: Magnetohydrodynamic converter
Illustration of the right-hand rule for the Lorentz force, cross product of an electric current with a magnetic field.

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.

Crossed-field magnetohydrodynamic converters (linear Faraday type with segmented electrodes). A: MHD generator mode. B: MHD accelerator mode.

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:

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.[clarification needed]

Typology

[edit]

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.[15]

Marine propulsion

[edit]
A view through a tube in the thruster of Yamato I, at the Ship Science Museum in Tokyo. The electrode plates are visible top and bottom.
A view of the end of the thruster unit from Yamato I, at the Ship Science Museum in Tokyo

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.[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 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.[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 prototype Yamato 1 was completed in Japan after six 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.[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-speed torpedoes, remotely operated underwater vehicles (ROV), autonomous underwater vehicles (AUV), up to larger ones such as submarines.[21]

Aircraft propulsion

[edit]
See also: Flow control (fluid)

Passive flow control

[edit]

First studies of the interaction of plasmas with hypersonic flows around vehicles date back to the late 1950s, with the concept of a new kind of thermal protection system for space capsules during high-speed reentry. 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 replace thermal 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 the bow shock wave further ahead of the vehicle, lowering the heat flux which is due to the brutal recompression of air behind the stagnation point. Such passive flow control studies are still ongoing, but a large-scale demonstrator has yet to be built.[22][23]

Active flow control

[edit]

Active flow control by MHD force fields on the contrary involves a direct and imperious action of forces to locally accelerate or slow down the airflow, modifying its velocity, direction, pressure, friction, heat flux parameters, in order to preserve materials and engines from stress, allowing hypersonic flight. It is a field of magnetohydrodynamics also called magnetogasdynamics, magnetoaerodynamics or magnetoplasma 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 to Paschen's law) using various techniques: high voltage electric arc discharge, RF (microwaves) electromagnetic glow discharge, laser, e-beam or betatron, radioactive source... with or without seeding of low ionization potential alkali substances (like caesium) into the flow.[24][25]

MHD studies applied to aeronautics try to extend the domain of hypersonic planes to higher Mach regimes:

  • Action on the boundary layer to prevent laminar flow from becoming turbulent.[26]
  • Shock wave mitigation for thermal control and reduction of the wave drag and form drag. Some theoretical studies suggest the flow velocity could be controlled everywhere on the wetted area of an aircraft, so shock waves could be totally cancelled when using enough power.[27][28][29]
  • Inlet flow control.[25][30][31]
  • Airflow velocity reduction upstream to feed a scramjet by the use of an MHD generator section combined with an MHD accelerator downstream at the exhaust nozzle, powered by the generator through an MHD bypass system.[32][33][34][35]

The Russian project Ayaks (Ajax) is an example of MHD-controlled hypersonic aircraft concept.[13] A US program also exists to design a hypersonic MHD bypass system, the Hypersonic Vehicle Electric Power System (HVEPS). A working prototype was completed in 2017 under development by General Atomics and the University of Tennessee Space Institute, sponsored by the US Air 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 a scramjet engine, but easier to design turbojets are also considered,[39][40][41] as well as subsonic ramjets.[42]

Such studies covers a field of resistive MHD with magnetic Reynolds number ≪ 1 using nonthermal weakly 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 the electrothermal instability occurring at a critical Hall parameter, which makes full-scale developments difficult.[43]

Prospects

[edit]

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 the gravity of Earth in water (due to buoyancy) nor in space (due to weightlessness), which is ruled out in the case of flight in the atmosphere.

Nonetheless, considering the current problem of the electric power source solved (for example with the availability of a still missing multi-megawatt compact fusion 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 several tonnes. 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 an axisymmetric body (shaped as a cylinder, a cone, a sphere...), the entire airframe being the engine. Lift and thrust would arise as a consequence of a pressure difference between the upper and lower surfaces, induced by the Coandă effect.[44][45] In order to maximize such pressure difference between the two opposite sides, and since the most efficient MHD converters (with a high Hall effect) are disk-shaped, such MHD aircraft would be preferably flattened to take the shape of a biconvex lens. Having no wings nor airbreathing jet engines, it would share no similarities with conventional aircraft, but it would behave like a helicopter whose rotor 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 the peer review literature from the mid 1970s mainly by physicists Leik Myrabo with the Lightcraft,[46][47][48][49][50] and Subrata Roy with the Wingless 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]

Spacecraft propulsion

[edit]
Main article: Plasma propulsion engine
See also: Magnetoplasmadynamic thruster and Pulsed inductive thruster

A number of experimental methods of spacecraft 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 or magnetoplasmadynamics.

In such electromagnetic thrusters, the working fluid is most of the time ionized hydrazine, xenon or lithium. Depending on the propellant used, it can be seeded with alkali such as potassium or caesium 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 with ion thrusters which rely on electrostatics to accelerate only positive ions using the Coulomb force along a high 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 greater thrust and higher specific impulse than conventional chemical 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 the magnetoplasmadynamic thruster sometimes referred to as the Lorentz force accelerator (LFA), and the electrodeless pulsed 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 enough energy density (such as hypothetical fusion reactors) to feed the power-greedy electromagnets, 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.

Fiction

[edit]

Oregon, a ship in the Oregon Files series of books by author Clive Cussler, has a magnetohydrodynamic drive. This allows the ship to turn very sharply and brake instantly, instead of gliding for a few miles. In Valhalla Rising, Clive Cussler writes the same drive into the powering of Captain Nemo's Nautilus.

The film adaptation of The Hunt for Red October popularized the magnetohydrodynamic drive as a "caterpillar drive" for submarines, a nearly 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 film, it was suggested that this sound could be confused with geological activity. In the novel from which the film was adapted, the caterpillar that Red October used was actually a pump-jet of the so-called "tunnel drive" type (the tunnels provided acoustic camouflage for the cavitation from the propellers).

In the Ben Bova novel The Precipice, the ship where some of the action took place, Starpower 1, built to prove that exploration and mining of the asteroid belt was feasible and potentially profitable, had a magnetohydrodynamic drive mated to a fusion power plant.

See also

[edit]

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Magnetohydrodynamic drive

View on Grokipedia
from Grokipedia
A magnetohydrodynamic drive (MHD drive) is a propulsion system that leverages the interaction between electric currents and magnetic fields to generate a Lorentz force, propelling electrically conductive fluids such as seawater or ionized gases without any mechanical moving parts. This technology exploits magnetohydrodynamics (MHD), the study of electrically conducting fluids in electromagnetic fields, to accelerate the fluid rearward and produce thrust forward. The fundamental principle underlying MHD drives is the , expressed as F = J × B, where J is the passing through the conductive fluid and B is the applied , oriented perpendicular to the current to maximize the force perpendicular to both. In marine applications, electrodes apply a voltage across saltwater (enhanced by for conductivity), while permanent magnets or electromagnets create the field, resulting in fluid at velocities up to approximately 1 m/s in small-scale experiments. For , MHD accelerators augment thermal rocket exhaust by seeding it with conductive materials like metals (e.g., 1.5% NaK by weight), achieving conductivity around 25 S/m and boosting exhaust by up to 80% through crossed electric and magnetic fields in configurations such as diagonal conducting walls. Theoretical efficiency can reach 50%, though practical systems often achieve 3-20% due to factors like , viscous losses, and field interactions. Notable applications include silent underwater propulsion for stealth vehicles, as demonstrated by Japan's experimental ship Yamato 1 in 1992, which used superconducting magnets to achieve 15% efficiency but proved commercially unviable due to high power demands. Recent interest as of 2023 includes DARPA's efforts to develop practical superconducting MHD drives for submarines. In , NASA's Magnetoplasmadynamic Augmented Propulsion Experiment (MAPX) has explored MHD for , using a 2-MW accelerator with a 2-Tesla field to enhance plasma flows at 1,312 m/s inlet velocity, potentially reducing fuel needs and vehicle mass. Electromagnetic pumps based on similar principles also drive liquid metals like sodium in nuclear reactors and have been proposed for in-space . Despite advantages like reduced noise and vibration, challenges persist, including the need for high electrical power exceeding thermal input and managing three-dimensional flow effects that degrade performance.

Fundamentals

Principle of Operation

A magnetohydrodynamic (MHD) drive is a propulsion system that generates thrust without moving parts by accelerating electrically conductive fluids, such as seawater, plasma, or ionized gases, through electromagnetic interactions. The foundational physics lies in magnetohydrodynamics (MHD), the study of electrically conducting fluids in magnetic fields, where the fluid behaves as a single continuum and electromagnetic forces couple with mechanical motion. This framework modifies the classical fluid dynamics equations to account for these interactions, particularly in the momentum balance. The key equation is the Navier-Stokes momentum equation augmented with the electromagnetic term: ρ(∂u∂t+(u⋅∇)u)=−∇p+J×B,\rho \left( \frac{\partial \mathbf{u}}{\partial t} + (\mathbf{u} \cdot \nabla) \mathbf{u} \right) = -\nabla p + \mathbf{J} \times \mathbf{B}, where ρ\rho is the fluid density, u\mathbf{u} is the fluid velocity, pp is the pressure, J\mathbf{J} is the current density, and B\mathbf{B} is the magnetic field. The Lorentz force term J×B\mathbf{J} \times \mathbf{B} provides the body force per unit volume that drives the fluid acceleration. The Lorentz force originates from the motion of charged particles in crossed electric (E\mathbf{E}) and magnetic (B\mathbf{B}) fields. For an individual charged particle with charge qq and velocity v\mathbf{v}, the magnetic component of the force is q(v×B)q (\mathbf{v} \times \mathbf{B}); the electric field induces motion that contributes to the current. In a conducting fluid, the net effect on the ensemble of charges yields the macroscopic force density F=J×B\mathbf{F} = \mathbf{J} \times \mathbf{B}, where J=σ(E+u×B)\mathbf{J} = \sigma (\mathbf{E} + \mathbf{u} \times \mathbf{B}) from Ohm's law in the fluid frame (σ\sigma is electrical conductivity). This force is perpendicular to both J\mathbf{J} and B\mathbf{B}, directing the fluid flow orthogonally to the applied fields. In the typical MHD drive configuration, electrodes inject current into the conductive fluid to establish J\mathbf{J}, while electromagnets or superconducting coils generate a uniform B\mathbf{B} perpendicular to the current path. The resulting propels the fluid rearward, imparting forward to the vehicle via Newton's third law of motion. Often, superconducting magnets are employed to achieve the high field strengths (several tesla) required for practical levels. This setup is inherently reversible: when fluid motion precedes the fields, the device operates as an MHD generator, converting of the moving conductor into electrical power through the inverse Lorentz interaction.

Types of MHD Drives

Magnetohydrodynamic (MHD) drives are primarily classified into conduction and induction types based on how electric currents are generated within the conductive fluid to produce the for propulsion. Conduction MHD drives employ passed through s in direct contact with the fluid, creating an perpendicular to a steady that accelerates the fluid via the . This approach offers mechanical simplicity and has been widely studied for its straightforward implementation. However, it suffers from significant drawbacks, including due to chemical reactions with the fluid and electrolysis-induced gas bubbles that generate and reduce . These issues necessitate frequent maintenance and compromise stealth in applications like underwater vehicles. In contrast, induction MHD drives generate currents through using time-varying magnetic fields produced by coils, eliminating the need for electrodes and thus avoiding direct fluid contact. This enhances and reduces , while also minimizing from gas evolution, making it suitable for low-signature operations. Drawbacks include the requirement for more complex to manage alternating currents and potentially higher power demands to achieve comparable field strengths. Efficiencies can reach over 70% with optimized configurations, such as specific window functions for field shaping. MHD drives are further categorized by flow configuration: internal and external. Internal flow systems confine the conductive fluid within a nozzle or channel, where fields accelerate it directionally to produce thrust, offering higher efficiency due to controlled flow paths and reduced viscous drag. External flow systems apply fields across the vehicle's surface to interact with surrounding fluid, primarily for boundary layer control and drag reduction rather than direct thrust, though at the cost of lower overall efficiency from increased form drag. Some approaches use alternating currents in conduction setups to mitigate polarization effects. These designs aim to improve performance across fluids with varying conductivities like seawater or plasmas, though they require careful tuning of electrical parameters.

Applications

Marine Propulsion

Magnetohydrodynamic (MHD) drives for marine propulsion are particularly adapted for operation in seawater, which has a relatively low electrical conductivity of approximately 5 S/m compared to metals. This necessitates the use of strong magnetic fields, typically in the range of 1-20 Tesla, along with large electrode surfaces to facilitate sufficient current flow and generate adequate Lorentz forces without excessive voltage drops. A primary advantage of MHD drives in marine applications, especially for underwater vehicles like , is their silent operation due to the absence of mechanical propellers or rotating parts, which eliminates traditional sources and significantly reduces the for enhanced stealth capabilities. Conduction-type MHD systems predominate in settings because they enable passage through the conductive for steady . The U.S. initiated the Principles of Undersea Magnetohydrodynamic Pumps (PUMP) program in 2023 to address challenges in MHD . This effort focuses on developing superconducting magnets capable of generating fields up to 20 Tesla, which aim to boost thrust by compensating for 's low conductivity and minimizing ohmic losses in the electrodes and fluid. As of 2025, the program has progressed with contracts to companies like Energy for high-temperature superconducting magnets and for prototypes achieving up to 70% . From an environmental perspective, MHD marine propulsion offers benefits such as minimal wake generation, as the distributed electromagnetic acceleration of produces less than propeller-based systems. Additionally, the lack of rotating components prevents noise and bubble formation, reducing hydrodynamic disturbances and potential ecological impacts on sensitive to acoustic .

Aircraft Propulsion

Magnetohydrodynamic (MHD) drives have been explored for propulsion primarily to enhance flow control in high-speed atmospheric flight, leveraging the interaction between and ionized air to manage shock waves, reduce drag, and augment in hypersonic regimes. In hypersonic , the ambient air becomes partially ionized due to high temperatures, enabling Lorentz forces to influence the flow without mechanical components. This approach is particularly suited for compressible gaseous media, distinguishing it from liquid-based systems, and focuses on aerodynamic enhancements for sustained atmospheric operation. Passive MHD flow control utilizes onboard magnets to create magnetic barriers that interact with the naturally ionized air plasma, deflecting the plasma sheath and shock layer away from the vehicle surface during reentry or . This deflection reduces aerodynamic drag and thermal loads by increasing shock standoff distance and mitigating on the vehicle's leading edges. For instance, in reentry scenarios, the pushes the ionized shock layer outward, allowing for adjustments that prolong high-altitude deceleration in low-density atmospheres. Studies have shown this method can optimize peak heating by balancing field strength with vehicle attitude, though its effectiveness diminishes at lower velocities where is insufficient. Active MHD flow control employs to further ionize the air and generate currents that, combined with , accelerate the flow for augmentation, often integrated with engines operating at Mach 5 and beyond. By applying Lorentz forces to manipulate the , this technique enhances inlet compression and combustion efficiency, enabling better performance in off-design conditions. In applications, MHD accelerators can increase by redistributing energy along the flow path, with electron beam achieving 10-20% reduction in flow for improved net . Electrode-less induction-type MHD systems facilitate this without physical electrodes, minimizing in high-temperature air environments. The Russian (AJAX) project, developed from the 1990s to the 2000s by the Leninets design bureau, demonstrated MHD-accelerated airflow in hypersonic prototypes using bypass concepts. In this design, MHD generators at the scramjet inlet extracted from incoming air to slow it to subsonic speeds for efficient , while accelerators at the re-injected to boost exhaust velocity, targeting Mach 10-16 cruise with potential gains of up to 15%. This approach relied on weakly ionized plasmas seeded with alkali metals to enhance conductivity, addressing challenges in maintaining flow control at extreme speeds. In the United States, the Hypersonic Vehicle Electric Power System (HVEPS) program, sponsored by the Air Force and culminating in ground tests of a working prototype in 2006, applied MHD for boundary layer control in scramjet-driven hypersonic vehicles. The system used MHD interactions to generate power from the engine exhaust while simultaneously controlling the boundary layer to reduce drag by 10-20% through flow acceleration and shock mitigation. Ground tests with scramjet flows confirmed the feasibility of this dual-role MHD setup, harvesting electrical power for onboard systems while enhancing aerodynamic efficiency in Mach 5+ environments.

Spacecraft Propulsion

Magnetohydrodynamic (MHD) drives for spacecraft propulsion rely on plasma-based systems that operate effectively in the vacuum of space, generating thrust without mechanical components. These systems use seeded gases such as argon or xenon as propellants, which are ionized to achieve high electrical conductivity, enabling the interaction between electric currents and magnetic fields. The thrust is produced by accelerating the resulting plasma through the Lorentz force, where charged particles experience a force perpendicular to both the current density and the applied magnetic field, expelling the plasma at high velocities from the thruster. Performance characteristics of these MHD drives highlight their suitability for extended operations, with specific impulses ranging from 1000 to 5000 seconds, far exceeding those of chemical systems. However, they deliver moderate levels, typically between 0.1 and 10 N, making them ideal for sustained rather than rapid maneuvers. Viable systems require power inputs exceeding 100 kW, often in the range of 100–500 kW, to achieve efficient plasma generation and , with higher powers enabling better performance metrics. Internal flow designs in MHD drives commonly feature configurations, such as those in magnetoplasmadynamic (MPD) thrusters, which incorporate an expanding to convert the plasma's swirl energy into directed axial thrust. Plasma is created through arc discharge between electrodes in a conduction-type setup, ionizing the injected gas within a central cathode-anode assembly surrounded by magnetic coils. This configuration allows for quasi-neutral plasma acceleration in vacuum, with applied axial magnetic fields enhancing the interaction. Compared to chemical rockets, MHD drives offer significant advantages in efficiency for long-duration missions, as their high reduces propellant mass needs, enabling deeper with limited resources. Furthermore, their high power demands align well with integration of nuclear systems, such as fission reactors, which can provide the megawatt-scale required for sustained operation without frequent refueling.

History and Development

Early Concepts and Experiments

The foundational concepts of magnetohydrodynamic (MHD) propulsion emerged from Hannes Alfvén's pioneering work in plasma physics during the early 1940s. In 1942, Alfvén published his theory on the existence of electromagnetic-hydrodynamic waves—now known as Alfvén waves—which described how magnetic fields interact with conducting fluids like plasmas to propagate wave-like disturbances. This Nobel Prize-winning contribution in 1970 established the theoretical basis for MHD phenomena, including the potential for propulsion through the manipulation of charged fluids via magnetic fields, laying the groundwork for later engineering applications in conductive media such as seawater. In the United States, initial experimental efforts in the 1950s focused on electromagnetic pumps and flow control for applications, conducted by organizations like Argonne and Oak Ridge National Laboratories. These early tests utilized as a conductive in controlled channels to demonstrate basic MHD effects, such as flow acceleration under transverse , often in open trough setups with Reynolds numbers around 50,000 to study turbulence reduction. By the mid-1960s, the U.S. Navy supported more directed propulsion trials, including the EMS-1 prototype—a 3-meter model developed by Westinghouse and tested at the —tested in July 1966 in California's waters using , electrodes, and conventional magnets to generate thrust via the on ionized fluid. This small-scale demonstration achieved a speed of approximately 0.4 meters per second, validating the principle of silent, propeller-less propulsion in saline environments but highlighting the need for stronger fields. Soviet researchers in the advanced MHD technology primarily through generator development, which directly inspired reversible concepts due to the inherent duality of MHD systems—where generators convert to electrical, accelerators do the reverse. At facilities like the , early experiments with liquid-metal and plasma-based MHD generators, such as the U-02 pilot plant operational by 1965, explored seeded gas flows and ionization stability, achieving short bursts of 60 kW output and informing designs for compact marine ers. These efforts emphasized two-phase flows and closed-cycle systems, recognizing the potential for in naval contexts by inverting generator configurations to produce from electrical input. Early experiments across both nations revealed significant challenges, particularly low efficiency in (DC) setups, often below 10% due to suboptimal strengths (typically 0.02–0.1 tesla) and corrosion in saline media, which limited and required unoptimized power inputs for even modest velocities. Turbulent interactions under magnetic fields further complicated scalability, as non-uniform fields in finite channels reduced flow predictability and overall energy conversion.

Key Prototypes and Milestones

One of the earliest practical prototypes was the EMS-1, developed in 1966 by Westinghouse and tested at the . This 3-meter-long, 900-pound external-field model submarine validated MHD generation in controlled aqueous channels, achieving a sustained speed of 0.4 m/s for 20 minutes using conventional electromagnets and a 30 V . In 1979, constructed the ST-500, a small-scale demonstrator that served as a precursor to larger marine MHD efforts. The 3.6-meter wooden model utilized a 2.0 T superconducting to reach speeds of 0.6 m/s, generating 20 N of and demonstrating the feasibility of electromagnetic in . The Yamato 1, completed in 1991 by , marked the first full-scale MHD-propelled ship. Measuring 30 meters in length with a displacement of approximately 185 tons, it employed superconducting NbTi coils producing a 4 T field to achieve speeds up to 15 km/h during sea trials in Harbor in 1992, operating in conduction mode by accelerating seawater ions. A key milestone in the 1980s was the widespread adoption of superconducting magnets, which reduced power requirements for generating strong magnetic fields—exemplified by the ST-500's use of liquid helium-cooled systems at -269°C to enable compact, high-field thrusters. In the , concepts emerged for integrating MHD drives with plants in , leveraging existing 20-40 MW electric outputs from reactors like those in Los Angeles-class vessels to support cryogenic cooling and high-current electrodes for stealthy propulsion.

Recent Advancements

In 2023, the initiated the Principles of Undersea Magnetohydrodynamic Pumps () program to address key materials challenges in MHD drives, focusing on developing novel materials suitable for operation and prototyping scalable systems for military applications. As of 2025, the program continues to focus on developing materials and prototyping scalable MHD systems for military applications, with goals to achieve practical efficiency and lifetime for undersea vehicles. In October 2025, Tokamak Energy secured a contract with to supply high-temperature superconducting (HTS) magnets for the U.S. Navy's next-generation program, integrating with the initiative to enable compact MHD systems. These HTS magnets, derived from fusion research, produce strong magnetic fields in smaller volumes, improving and reducing the size of drive components for stealthy underwater operations. The collaboration involves simulation, design, and fabrication of HTS magnets by Tokamak Energy, with handling system integration and coordination with for electrode development. HRL Laboratories achieved a milestone in April 2025 with a proof-of-concept demonstration of a silent undersea pumping system under DARPA funding, utilizing MHD principles to generate thrust via electromagnetic forces on seawater without moving parts, achieving up to 70% efficiency and a projected lifespan exceeding 5 years, with reduced gas bubble formation for minimal acoustic signature. This innovation replaces traditional propellers, significantly reducing noise and maintenance requirements for marine applications, thereby enhancing stealth capabilities in underwater environments. Advancements in were highlighted in a 2025 AIAA paper on external plasma-breathing magnetohydrodynamic , proposing an architecture that integrates MHD thrusters with concepts for efficient orbital transfers. This approach leverages conductive plasma interactions for low-thrust , offering sustainable maneuvers by combining active MHD acceleration with passive deceleration, potentially yielding efficiency improvements over conventional electric systems. Market trends in North America reflect growing adoption of MHD drives, propelled by innovations in HTS magnets for defense and marine sectors, with increasing investments in stealth technologies and sustainable propulsion.

Challenges and Limitations

Technical Hurdles

One major technical hurdle in implementing magnetohydrodynamic (MHD) drives is electrode degradation, particularly in conduction-based systems where seawater or conductive fluids interact with electrodes. In such setups, electrolysis induces corrosion, leading to material pitting and erosion; for instance, stainless steel electrodes can corrode completely within minutes under high electric fields of 3 kV/m and current densities of 20,000–80,000 A/m². To mitigate this, inert materials such as platinum or carbon-based electrodes like graphite are required, as they exhibit minimal corrosion—graphite, for example, shows only 1 mil thickness reduction after exposure, compared to severe degradation in platinum-plated copper where peeling and pits up to 30 mils deep occur. In plasma-based MHD systems, electrodes face additional ablation challenges, where high-temperature plasma causes erosive wear; graphite electrodes, for instance, erode by forming CO₂ and CO in oxygen-rich plasmas, accumulating losses primarily at the channel entrance and necessitating frequent refurbishment after several runs. Generating the necessary magnetic fields for effective MHD presents another significant barrier, as fields exceeding 10 Tesla are typically required to produce substantial Lorentz forces without prohibitive energy demands. Achieving these strengths relies on cryogenically cooled superconducting magnets, such as high-temperature superconductors (HTS) like REBCO tape, which operate at 4.2 K to maintain zero-resistance states and enable fields up to 20 T. However, this cryogenic operation demands sophisticated thermal management, including coolers that consume 400–475 W for maintenance and initial cooldown periods of up to 20 days, while joint resistances introduce additional power losses on the order of 100 nΩ, complicating integration into compact systems. Without such superconductors, conventional magnets cannot reach the required intensities efficiently, as fields limited to 6–10 T with current technology already strain power budgets. Fluid conductivity limitations further constrain MHD drive performance, as the Lorentz force depends critically on the electrical conductivity (σ) of the working medium. In marine applications, seawater's inherently low σ of approximately 4 S/m requires accelerating large fluid volumes to generate sufficient , as low conductivity increases the necessary current densities and duct sizes—often exceeding 300 m³ for viable —making systems bulky and impractical for smaller vessels. For spacecraft propulsion using rarefied space plasmas, which have even lower natural conductivity, seeding agents such as 0.1% cesium or are essential to enhance σ to levels like 1 S/m, lowering the ionization temperature to around 2500 K and enabling practical operation; without seeding, the plasma's poor conductivity renders acceleration ineffective. Effective heat management remains a persistent challenge in MHD drives, primarily due to from I²R losses in the conductive fluid, which dissipates as and undermines overall system performance. These losses, representing the fraction (1 - η_e) of input power where η_e is , can dominate in low-conductivity regimes, with baseline cases showing up to 4.59 × 10⁶ dissipated as . In current designs, such heating reduces net to below 30%, as seen in configurations with of 2–10 T where frictional and thermal losses compound, for example, 37% at 10 m/s, decreasing to 24% at higher speeds like 20 m/s. Managing this excess requires advanced cooling strategies, but the distributed nature of Joule dissipation within the fluid volume complicates containment and recovery, often leading to thermal gradients that further degrade material integrity.

Efficiency and Scalability Issues

Magnetohydrodynamic (MHD) drives in marine applications typically exhibit overall efficiencies below 20%, primarily due to significant ohmic losses arising from the electrical resistance of seawater as the working fluid. These losses manifest as Joule heating, where a substantial portion of the input electrical power dissipates as heat rather than contributing to thrust, with experimental rectangular thrusters achieving around 20% efficiency under optimal conditions of 226 kW input yielding 44 kW output thrust power. The power efficiency can be approximated by the ratio of thrust power to input electrical power: η=PtPe=VJBVJB+J2σ\eta = \frac{P_t}{P_e} = \frac{V J B}{V J B + \frac{J^2}{\sigma}} where VV is the fluid velocity, JJ is the current density, BB is the magnetic field strength, and σ\sigma is the electrical conductivity of seawater; the term J2σ\frac{J^2}{\sigma} represents the ohmic loss density. This formulation highlights how low seawater conductivity (approximately 5 S/m) exacerbates losses, limiting practical efficiencies to 20.7% in submarine-scale designs compared to over 50% for conventional propellers. Scalability of MHD drives faces inherent power density constraints, particularly for high-thrust applications like , where achieving speeds above 30 knots demands integration with gigawatt-scale nuclear reactors to supply the required 100-140 MW electrical . Current MHD thruster designs yield volume power densities on the order of kW/L, insufficient without advanced compact reactors like high-temperature gas-cooled or fast breeder types to meet needs while maintaining stealth. for smaller platforms, such as underwater drones, proves unfeasible due to the bulky size of superconducting magnets needed for sufficient field strengths (8-10 T), which dominate volume and preclude integration into compact, low-power systems. Economic barriers further hinder widespread adoption, with superconducting magnets essential for viable MHD performance costing $150-200 per kA-m of wire, translating to over $10 million per full-scale unit when accounting for fabrication, cooling, and integration. This results in MHD systems being 5-10 times more expensive than conventional setups, which achieve comparable at a fraction of the capital outlay without specialized cryogenic . Technical hurdles like can indirectly exacerbate these efficiency declines by accelerating material degradation.

Cultural Impact

Representations in Fiction

In fiction, magnetohydrodynamic (MHD) drives are often portrayed as advanced, silent propulsion systems ideal for stealthy or travel, frequently appearing in marine settings where they enable covert operations without mechanical noise or . Tom Clancy's 1984 novel features the Soviet submarine Red October equipped with a "caterpillar drive," an MHD system that propels the vessel noiselessly through seawater by generating electromagnetic forces, allowing it to evade detection during a high-stakes . This depiction inspired real-world interest in MHD technology and emphasizes its potential for undetectable underwater maneuvers. Similarly, in Clive Cussler's Oregon Files series, starting with Golden Buddha in 2003, the covert ship utilizes an MHD drive for silent, high-maneuverability , facilitating the crew's clandestine missions against global threats. The technology allows the vessel to execute sharp turns and operate undetected, enhancing its role as a disguised high-tech . Common tropes in these portrayals idealize MHD drives as highly efficient with unlimited stealth capabilities, often overlooking practical constraints like immense power requirements or conductivity issues, presenting them as near-perfect solutions for and exploration narratives.

References

  1. https://digitalcommons.csbsju.edu/context/physics_students/article/1003/viewcontent/Magnetohydrodynamic_Salt_Water_Drive.pdf
  2. http://www.pmaweb.caltech.edu/Courses/ph136/yr2011/1018.1.K.pdf
  3. https://ntrs.nasa.gov/api/citations/20110008067/downloads/20110008067.pdf
  4. https://www.darpa.mil/news/2023/undersea-propulsion
  5. https://www.darpa.mil/program/principles-of-undersea-magnetohydrodynamic-pumps
  6. https://doi.org/10.1371/journal.pone.0178599
  7. https://www2.mps.mpg.de/solar-system-school/lectures/space_plasma_physics_2007/Lecture_6.pdf
  8. https://ntrs.nasa.gov/api/citations/19960000249/downloads/19960000249.pdf
  9. https://www.sciencedirect.com/topics/earth-and-planetary-sciences/magnetohydrodynamic-generator
  10. https://dspace.mit.edu/bitstream/handle/1721.1/163449/Scali-wscali-ne-me-2025-thesis.pdf?sequence=1&isAllowed=y
  11. https://apps.dtic.mil/sti/tr/pdf/ADA231623.pdf
  12. https://dspace.mit.edu/bitstream/handle/1721.1/155879/daus-daus-ne-me-2024-thesis.pdf?sequence=1&isAllowed=y
  13. https://hal.science/hal-04908736v1/document
  14. https://www.naval-technology.com/features/darpa-silent-mhd-magnetic-drives-for-replacing-naval-propellers/
  15. https://www.osti.gov/servlets/purl/10165830
  16. https://tokamakenergy.com/2025/10/21/tokamak-energy-contracted-by-general-atomics-to-advance-next-generation-submarine-program/
  17. https://www.hrl.com/news/2025/04/02/hrl-advances-quiet-undersea-propulsion-innovation
  18. https://www.sciencedirect.com/science/article/pii/S1110016816300126
  19. https://www.esa.int/gsp/ACT/projects/mhd_reentry/
  20. https://ntrs.nasa.gov/api/citations/20040000981/downloads/20040000981.pdf
  21. https://www.researchgate.net/publication/270820169_MHD_hypersonic_flow_control_for_aerospace_applications
  22. https://arc.aiaa.org/doi/10.2514/6.1996-4609
  23. https://apps.dtic.mil/sti/tr/pdf/ADA471429.pdf
  24. https://arc.aiaa.org/doi/10.2514/6.2006-8010
  25. https://www.japcc.org/essays/applied-field-magnetoplasmadynamic-thrusters/
  26. https://svs.gsfc.nasa.gov/4560
  27. https://apps.dtic.mil/sti/tr/pdf/AD0706643.pdf
  28. https://core.ac.uk/download/pdf/36718968.pdf
  29. https://apps.dtic.mil/sti/tr/pdf/ADA004614.pdf
  30. https://ieeexplore.ieee.org/document/606542
  31. https://inldigitallibrary.inl.gov/Reports/ANL-92-35.pdf
  32. https://www.darpa.mil/research/programs/principles-of-undersea-magnetohydrodynamic-pumps
  33. https://magneticsmag.com/seeking-the-ultimate-in-marine-stealth-darpa-pursues-quest-to-finally-make-magnetohydrodynamic-drive-a-practical-reality/
  34. https://interestingengineering.com/military/stealthy-submarines-no-rotating-components-pump-tech
  35. https://finance.yahoo.com/news/hrl-advances-quiet-undersea-propulsion-160000839.html
  36. https://arc.aiaa.org/doi/pdfplus/10.2514/6.2025-2038
  37. https://www.researchgate.net/publication/387724707_External_Plasma-Breathing_Magnetohydrodynamic_Spacecraft_Propulsion
  38. https://www.linkedin.com/pulse/north-america-magnetohydrodynamic-drive-market-aydic
  39. https://www.linkedin.com/pulse/north-america-high-temperature-superconducting-hts-magnet-daxyc
  40. https://ntrs.nasa.gov/api/citations/20080033025/downloads/20080033025.pdf
  41. https://ntrs.nasa.gov/api/citations/20240015387/downloads/Magnet_Systems_Analysis_FINAL.pdf
  42. https://apps.dtic.mil/sti/tr/pdf/ADA221088.pdf
  43. https://electricrocket.org/IEPC/IEPC-2007-357.pdf
  44. https://www.researchgate.net/publication/368765005_Review_of_Marine_Magnetohydrodynamic_Motor_Development
  45. https://apps.dtic.mil/sti/tr/pdf/ADA213401.pdf
  46. https://transducer-research-foundation.org/technical_digests/HiltonHead_2022/hh2022_0202.pdf
  47. https://www.nextbigfuture.com/2023/09/markets-and-future-costs-of-superconducting-magnets-and-superconducting-wire.html
  48. https://pubs.aip.org/aapt/pte/article/55/8/460/277847/The-Hunt-for-Red-October-II-A-magnetohydrodynamic
  49. https://www.kjmagnetics.com/blog/magnetohydrodynamic-drives
  50. http://odlsystem2.utm.my/2911/1/GlennRogelio2012_MagneticPropulsionDevicesandTechnologies.pdf
Add your contribution
Related Hubs
User Avatar
No comments yet.