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. 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 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.
Aircraft propulsion Passive flow control 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.
Active flow control 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. 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. • 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. • Inlet flow control. • 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. The Russian project
Ayaks (Ajax) is an example of MHD-controlled hypersonic aircraft concept. 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, as well as subsonic
ramjets. 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.
Prospects 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. 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, and
Subrata Roy with the
Wingless Electromagnetic Air Vehicle (WEAV). These futuristic visions have been advertised in the media although they still remain beyond the reach of modern technology.
Spacecraft propulsion 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.{{cite journal |last1=Rosciszewski |first1=Jan |title=Rocket motor with electric accelerationin tehthroat |date=March 1965 |journal=Journal of Spacecraft and Rockets |volume=2 |issue=2 |pages=278–280 |url=http://ayuba.fr/pdf/rosciszewski1965.pdf |doi=10.2514/3.28172 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 ==