The “caterpillar drive” in The Hunt for Red October allowed the sub to travel virtually undetected through the ocean, but real examples of magnetohydrodynamic drives (MHDs) are rare. The US Navy’s recently announced Principles of Undersea Magnetohydrodynamic Pumps (PUMP) intends to jump-start the technology for a new era.
Dating back to the 1960s, research on MHDs has been stymied by lower efficiencies when compared with driving a propeller from the same power source. In 1992 the Japanese Yamato-1 prototype, pictured at the top of the page, was able to hit a blistering 6.6 knots (that’s 12 kph or 7.4 mph for you landlubbers) with a 4 Tesla liquid helium-cooled MHD. Recent advances courtesy of fusion research have resulted in magnets capable of generating fields up to 20 Telsa, which should provide a considerable performance boost.
The new PUMP program will endeavor to find solutions for more robust electrode materials that can survive the high currents, magnetic fields, and seawater in a marine environment. If successful, ships using the technology would be both sneakier and more environmentally friendly. While you just missed the Proposers Day, there is more information about getting involved in the project here.
I remember an article in Popular Science back in the 80s about using MHD as a more efficient way to generate electricity with lower emissions. Finely pulverized coal or natural gas would be ignited and accelerated down the MHD tunnel where a current would be induced in surrounding coils generating electricity. The hot gas would then be used in a secondary process to create steam to run turbines. This double generation method resulted in higher efficiency power production with lower emissions. In the case of using coal, the ash was easily captured and the coal was burned more efficiently. The only problem that was encountered was the erosion of the ceramic coating in the MHD tunnel. The test plant did report lower CO, CO2, NO, and SO2 compared to other power plants.
Yeah; anything that runs at a higher input temperature than the normal steam cycle can be used to extract a little extra energy before putting the heat to work in the same cycle it would have otherwise already gone into. A MHD using gas (or rather, plasma) really needs a lot of ionization and velocity, and keeping a potentially cryogenic magnet close enough to make a strong field is tricky, as is getting the vectors right while doing it, so that the energy doesn’t just get wasted in stray currents. The parameters are a bit different for liquids of course, and then different again for seawater in particular.
Thermionic converters were another high temperature device that never took off, despite being in many ways a higher efficiency, higher temperature alternative to semiconductor thermoelectric (peltier/seebeck) arrays. There’s a video that’s been around the internet where a small disc powers a motor when you aim a torch at it. Thermophotovoltaics is probably the new option there; we’re obsessed with semiconductors and it can do high efficiency and decent density.
Does anyone know what kind of field strength is needed for MHD linear induction motors to be effective in seawater? I’m guessing it’s more than 20T, given that electrode materials research seems to be a big part of this program.
Thrust density is just BxI (magnetic field * current) so if you can put more field (4T->20T means 5x the thrust density) you get more thrust for the same space, same with putting more current through. Problem is that more current ends up with more electrode erosion, more heating the working fluid, etc.
more electrolysis of the seawater… (which in the case of that working fluid contributes further to electrode erosion and makes the drive itself significantly less-stealthy)
Trading cavitation bubbles for different bubbles.
No doubt the large magnetic field is there to offset trying to keep the voltage lower than what’s needed to split the water.
bubbles are not the problem. It is when the bubbles quickly collapse do to the steam turning back to water, that causes the noise issue. In this case, most of the bubbles will not collapse until they hit the surface, so not an issue.
The water coming out is sterilized and toxic, having free chlorine. I would think there is some noise from gas generation down to some pressure. Maybe depending on the electric current (ionic current), which must be very high. There must be serious losses to heating and to the electrolysis and splitting the chlorine and the sodium ions and their recombination.
Sea water is not like a copper wire. There is no “sea of electrons” to push at one end of you push the other at nearly the speed of light. In sea water ions must travel from one electrode to the other. Collisions must take energy and cause heating, and don’t ions have to move from one electrode to the other in order to have a current? How long does that take? Or is it all about the electric field of the electrodes dragging the ions toward the plates? Curious people want to know.
With copper wires the signal carried by the EM field outside the wire is nearly the speed of light; inside the wire it’s about 10^8 m/s (1/3 lightspeed), or so I read a few minutes ago. The electrons themselves move at less than 1 mm/sec, which is much much slower. This is due to electrons traveling only short distances before colliding with copper atoms, and there not being much voltage difference from one collision to the next.
To get electrons to travel at a substantial portion of lightspeed, an accelerating voltage of over 10 kV between collisions is needed, such as inside of a CRT.
eV, electron volts, is useful here. It’s an adequate summary to say that if an electron is accelerated by X volts, then it will have X electron volts of kinetic energy and its velocity can be found from a formula for kinetic energy, since electrons have a known very small mass.
It isn’t about how fast they go, it is about how many there are. Electron flow in a wire can be extremely slow because the density of free electrons is so high. The number of them crossing a line per amp is 6.24 x 10^18 per second. Ions in water are a different matter. And they are being shoved downstream.
So it must just be the motion of the ions produced by the electric field between the electrodes. Magnetic fields can not do work (you can get this effect with a permanent magnet) so the work must be done by the electric field through the ion current. Huh. The Na+ and Cl- ions have opposite charge. But they are going in opposite directions and therefor all move in the same direction.
Well that was why I was asking about induction motors, specifically where the seawater is the motor armature–no need for electrodes.
7.4 mph -> 11.91km/h
The beauty of Freedom-units…
Seems they may have gone to some slightly excessive effort in the above-water aerodynamics of that very fancy hull in the picture at the top.
Pretty quick, considering the weight of water in front of the cross section.
By eyeball it looks like it would only be doing displacement speed (Wikipedia article gives higher speed 8knots) which is because they need the tunnel/intake submerged, it’s not gonna be a planing craft when you have to do that.
I would be designing a surface craft that had a “torpedo” submerged drive pod between hydrofoils if I seriously wanted to make it look like the propulsion of the future, i.e. not outpaced by a sporty trailer sailer.
Come off it, that’s clearly a photograph of Thunderbird 2.
No doubt it’s carrying Pod 4, which is where the MHD is really found.
An ASW magnetometer would detect a submarine from 1000 km away if they need at least >20T of magnetic field.
That’s what I was thinking. Enemy torpedo guidance would consist of a two axis compass.
Just make the torpedo out of steel and push it over the side, with the MHD magnetic field the torpedo will just be pulled in.
No, I don’t think so. You are perhaps imagining the 20T field generated by a straight wire. The MHD drive would likely have a helical wound construction, like a solenoid, and the field outside a solenoid is quite a bit less than the concentrated field inside the solenoid. https://physics.stackexchange.com/questions/70452/magnetic-field-outside-a-solenoid
MAD detects submarines just from their ferrous body’s effect on the geomagnetic field, adding a 20T field (even a ‘closed’ field) is going to light them up light a Christmas tree to those exquisitely sensitive detectors.
Solenoid will make ions travel in circles. Magnets can not do work. Only change direction. Cross product of current and mag field. https://www.youtube.com/watch?v=F1PWnu01IQg
while that is true on a basic level (and is an issue that must be fixed for military subs), the magnetic fields can be directed down, or to the sides, instead of radiating out in a sphere. This would greatly lower the chance of finding the sub that way.
Would not need a magnetometer; just follow the tin cans bobbing along the surface.
I mean aside from the obvious EM signature, the cost, introducing inefficiencies by converting steam to electricity first rather than directly to propulsion, multiplying the power requirements of the reactor, making a system not serviceable by the crew (what could go wrong with that, especially on a submarine), and requiring an entire redesign of a well tested efficient and effective fleet, think of the benefits. Look at the success of the F-35, we could spend like $5T on this and end up with like 2 subs that might never leave the shipyard, if you are a defense contractor you can tap an infinite revenue stream, of course its a good idea.
Don’t worry about the cost, stupid Australians will gladly throw money at the USA for submarines that may or may not ever be delivered. https://www.abc.net.au/news/2023-03-15/paul-keating-anthony-albanese-penny-wong-aukus-nuclear-china/102098142
So you might not hear it coming but you’ll be able to see it with a multimeter and a compass.
And it also attracts sharks for many kilometers around it. :P
Not quite, sharks detect electric field, not magnetic field
Moving magnetic field = electric field.
You could harvest the hydrogen and oxygen being electrolysed in the tunnel, then burn that to generate electricity to drive it. Perpetual motion!
big oil hates this trick
Instant death to all life going through it all the way down to microbes and plankton which pass propellers.
The size of the submarine engine compared to the ocean around it makes the death of microbes irrelevant…
Likely not. You and the microbes in your body can easily survive an MRI, which is up to 10T field strength.
Nope: https://www.ru.nl/hfml/research/levitation-explained/diamagnetic-levitation/
That is not an american image… its japanese
https://en.wikipedia.org/wiki/Yamato_1
Bill Fraser has been doing some fun tinkering with LiPOs and tiny models down in S. Africa. Hardly defense-contract work but it bubbles right along.
https://youtu.be/-CYdpcceVT0
The Yamato-1 prototype would make a really cool looking model… Anyone know if anything like that exists?
Those things would NOT be more environmentally friendly. The whole point of a submarine is stealth, so current subs do not actually make that much noise, as I am sure that is a major engineering focus.