MHD Thruster
MHD stands for "Magneto Hydro Dynamic". In English, this translates to "the use of magnetic fields to push a liquid". This type of an engine has applications underwater, within an atmosphere and in the vacuum of space. The process is powered directly by electricity and requires the liquid entering the engine to be pre-magnetized. A great advantage is that this type of engine does not contain any moving parts.
The idea behind the MHD thruster originated in the 1960's when the US Navy was looking for a quiet way to power their submarines. Most submersible sailors would agree that the best way to spot other submarines is by listening to the noise of their propellers. The MHD thruster gets rid of the noisy propeller problem and instead magnetizes the ocean water and pushes it along to generate thrust. The underwater MHD thruster is often referred to as the "silent drive" since it produces no noise at all. The combination of an onboard nuclear reactor (a common power source for submarines) and an MHD thruster is lethal - just imagine a completely silent submarine that can remain submerged for decades. However, in 1991, the Japanese tested such a thruster (also known as a caterpillar drive) on their Yamato submarine and results indicated that the system was very inefficient - it consumed incredible amount of electrical power while producing minimal thrust.
Now for applications in the atmosphere, the air would have to be pre-ionized in order for it to interact with the magnetic field of the MHD thruster. This pre-ionization will require a lot of additional equipment making the engine very heavy (refer to the Electric Jet-Engine article in a previous edition). But even if the air is pre-ionized, there simply isn't enough suction power in the magnetic coils to push a reasonable amount of air through. Since air is significantly less dense than water, to produce similar thrust, much more air must be pushed along. The end result would be an over-weight and under-powered engine so atmospheric use of MHD thruster remains fictional for the time-being.
Lastly, for space-based applications in vacuum, an MHD thruster needs a source of liquid. The obvious choice is a plasma generator - a device that generates an ionized liquid and injects it into the MHD thruster. The plasma MHD thruster has potential to process large quantities of plasma thereby enabling both high-efficiency and high-thrust space propulsion. This is a unique capability since most electric engines like ion-thrusters are efficient but produce very little thrust. The US-based company Ad-Astra has been developing a vacuum-based MHD thruster (VASIMR engine) for several decades. VASIMR was supposed to debut several times on the International Space Station, but no such test has occurred yet. It is possible that they are running into serious issues, and we will just have to wait and see if these can be resolved. For the time being, Hall-Effect thrusters are the more popular and practical option for space missions.
In the end, one thing is clear, no matter where we operate an MHD thruster, water, air, or space, we run into the same issue - efficiency. It remains unclear whether there is a fundamental problem with the technology, or we just haven't learned to produce the right magnetic fields to make this practical. By itself, the MHD thruster may be a non-starter, but I am sure that eventually it can be combined with other technologies to produce truly groundbreaking propulsion.
Engineer at Choong Heng plastics
4 个月Do you know how a ballistic missiles thruster work? I am seeking developer or researcher to do a feasiblity study on using the magnetoplasma thruster system to drive exiting turbine generator to produce electricity. Since the world is moving toward using ammonia, hydrogen and moving away from fossil fuel, is the thruster able to replace existing lng combustion driver.
ALLIANCENTURE IMPORT EXPORT & PROJECT CO
1 年SIMPLER DESIGN TO NUCLEAR SUBMARINES CAN LOWER AND NOSE SEAWATER INLET DOUBLE JACKETED SEMI BALLOONED STRUCTURE WHICH SUCKS WATER AND PUMPS THEM VIA TAILED OUTLET
Chairman and Chief Executive Officer, Sabrewing Aircraft Company; Former Chief Technology Officer and Retired Test Pilot/FTE (NOT RESPONSIBLE FOR HR OR HIRING)
1 年The MHD is simply too heavy to be practical. While intriguing, the coils, power supply and other attendant components don't lend themselves very well to aviation. It occupies the same position in the engine pantheon as does the nuclear-powered engine: cool but not really feasible with modern technology.
Aviation SME. Instructor pilot, curriculum developer, and tech writer.
1 年Cpt. Ramius approves.
Senior Scientist at Ohio Aerospace Institute
1 年In the interest of being constructive, not critical, I'd point out that MHD, or MPD, propulsion is usually considered to use the Lorentz force for acceleration, which requires a current and a magnetic field. The figure you're showing (VASIMR) uses the guiding properties of magnetic fields on charged particles to convert thermal motion to kinetic; there are no applied currents in the plasma. Interestingly, Lorentz force acceleration is unique in that the thrust does NOT depend (first order) on the mass flow rate, or even the propellant species, or particle flux. This makes it fairly unique as a propulsion concept. The voltage does depend strongly on those things, however.