Magneto­hydro­dynamic Propulsion

What is MHD?

  1. What is magnetohydrodynamics?
    • From Wikipedia: Magnetohydrodynamics (MHD; also called magneto-fluid dynamics or hydromagnetics) is the study of the magnetic properties and behaviour of electrically-conducting fluids.
    • While magnetohydrodynamics technically refers only to water, hence ‘hydro’, the meaning has evolved over time, and has come to refer generally to all electrically-conducting fluids.
  2. What is magnetohydrodynamic (MHD) propulsion?
    • It is a form of propulsion that utilises electromagnetic forces to move fluids—such as ionised air, plasmas, seawater, etc.—without the need for propellers or any other kind of airfoil, in fact, without the need for any moving parts whatsoever.
    • Specifically, the working principle of MHD propulsion involves the acceleration of an electrically-conductive fluid by the Lorentz force, which is the repulsive magnetic force, appearing wherever an electric current is perpendicular to a magnetic field.
  3. What is the difference between ion wind propulsion and MHD propulsion?
    • Ion wind propulsion, or more generally, electrohydrodynamic propulsion, works by accelerating ionised air or another electrically conductive fluid, utilising only electric fields instead of magnetic fields.
    • Speaking generally, EHD/ion-wind propulsion has a lower thrust given the same volume compared to MHD propulsion.
    • While there is a distinct difference between EHD and MHD propulsion, in many cases the two can overlap and may even be used together in the same vehicle.
  4. What is ion wind?
    • Ion wind, or as it is sometimes called ‘electric wind’, is produced when an intense electric field causes a breakdown of air, forming a cloud of like-charged ions or electrons, depending on the polarity of the field. The ions/electrons are repelled by the electric field and each other, expanding/moving outward or towards an electric field of opposite polarity. In the process, the ions/electrons collide with neutral air molecules, creating a wind away from the initial electric field, either outward or toward another electric field of the opposite polarity.

MHD propulsion fundamentals

  1. To begin with, let's remind ourselves how wings work.
    • If you look deep enough into this subject, you will find that there is no one single answer to explain how wings work. But you will find that all explanations revolve around the concept of creating a pressure differential between the top and bottom of the wing, with the aim being to increase the relative pressure on the underside of the wing.
    • To clarify, the exact mechanism of lift really depends on the specific circumstances, flight environment, wing design, etc. Different mechanisms must be at work in different situations, or even multiple mechanisms may be active at the same time, or for the same wing or airfoil at different times, lower or higher speeds, etc.
  2. An explanation of the Lorentz force.
    • The Lorentz force is the repulsive force between electromagnets, or more specifically, it is a force that arises when electric current is at a right angle to a magnetic field.
    • A fluid can be accelerated, decelerated, or its pressure increased or decreased via the Lorentz force.
    • In order for the Lorentz force to act on a fluid, the fluid must either be conductive already, or made conductive in the moment (such as by ionisation or the addition of electrolytes). Oppositely charged electrodes are used to create an electric current through the fluid, a magnetic field is then passed at a right angle to it, giving rise to the Lorentz force, accelerating or decelerating the fluid.
    • If the fluid is already moving, a Lorentz force can be used to either accelerate the fluid further and thereby decrease its pressure, or to decelerate the fluid and so increase its pressure.
    • A more detailed description of the Lorentz force can be found on Wikipedia: Lorentz force - Wikipedia, the free encyclopedia
    • The Silence Barrier scientific comic book (available as an e-book in the PDF file format) by Jean Pierre Petit provides an interesting, entertaining and quite detailed explanation of MHD propulsion: The Silence Barrier - Association Savoir Sans Frontieres (Association Knowledge without Borders)

An outline of how MHD propulsion functions in an atmosphere

The following is a relatively brief step-by-step outline of how magnetohydrodynamic (MHD) propulsion functions in an atmosphere.

  1. Electric and magnetic fields can only act on a conductive medium. Therefore to operate in an atmosphere, an MHD propelled vehicle must first ionise the air.
    • This can be done in one of several ways, but the most efficient method is known as ‘high-frequency pulse discharge’ (HF pulse discharge), this is where an alternating current at radio to microwave frequency (or higher) produces electromagnetic radiation and fields that ionises air or other gasses, very efficiently.
    • The ionised air forms a conductive sheath just off the surface, which absorbs all or most of the emitted radiation.
  2. Once the air is ionised it can be moved via electromagnetic forces, such as the Lorentz force or via an ion wind effect.
    • This is where ion wind propulsion and MHD propulsion can sometimes be confused with each other, because in some vehicle designs both forms of propulsion can be implemented on the same vehicle to maximise efficiency. Also, different versions or iterations of a vehicle might use only ion wind propulsion, only MHD propulsion, or both.
  3. The airflow created by MHD acceleration (or ion wind) is then directed over an airfoil, creating lift.
    • This can be airfoils in the form of more-or-less conventional wings.
    • A saucer-shaped Coandă effect airfoil provides greater lift than a conventional wing, and is well suited to MHD (or ion wind) that creates a ‘wind’ without the need to move the entire vehicle, nor even the airfoil. Think of the saucer airfoil as a wing wrapped around the vehicle.
    • MHD propulsion can also come in the form of a MHD plasma ‘jet engine’, with a very high power density, that is, a small device will produce very high thrust. This is in sharp contrast to ion wind thrusters, which has a very low power density, and therefore very low thrust unless used with an airfoil.

MHD propulsion in space

  1. The simplest and easiest form of MHD space propulsion is MHD jet engines, or rather, MHD rocket engines.
    • These can utilise the Lorentz force to directly accelerate a cold plasma.
    • Alternatively, radio frequency discharge or microwaves can be used to superheat a gas into a hot plasma state, which is then kept off the walls via magnetic fields (which involves Lorentz forces). The hot plasma is then allowed to escape as with a conventional chemical rocket engine.
    • There are advantages and disadvantages to both, both can be high thrust and high efficiency. Though using hot plasma will generally give a higher thrust but lower efficiency unless the plasma is extremely hot (fusion temperatures).
  2. In the not so distant future we will utilise MHD space propulsion that pushes off of the quantum vacuum directly.
    • Empty space is not actually empty, it is filled with extremely short lived particles usually called ‘virtual particles’, the name is a misnomer (and historical misnaming) as these particles are no different than ordinary particles.
    • The only difference between the so-called virtual particles and ordinary particles is lifetime, virtual particles are exceedingly short lived, most things happen over very long periods of time in comparison, so the activity of the ‘virtual’ particles cancels out and can be safely ignored. However, for very high energy, very high frequency, or both high energy and high frequency electromagnetic fields these virtual particles cannot be ignored.
    • Therefore, ultrashort and intense electromagnetic pulses can interact with these abundant and ubiquitous particles, particularly the short-lived electrons and antielectrons (also known as positrons, a kind of antimatter). These ultrashort and intense electromagnetic pulses can then be used to create Lorentz forces on the electron-positron plasma, creating thrust.