> ...advances in rare-earth barium copper oxide (REBCO) magnets that have demonstrated large-scale magnetic fields as high as 20 Tesla that could potentially yield 90% efficiency in a magnetohydrodynamic drive, which is worth pursuing. Now that the glass ceiling in high magnetic field generation has been broken...
> There are multiple potential approaches to the MHD system including conductive and inductive approaches. The conductive approach involves a conductive current between a pair of electrodes within a magnetic field. The inductive approach uses a time-varying magnetic field and electric current.
IANAEE (Electrical Engineer)...but having a time-varying ~20 Tesla magnetic field in a drive system seems a bit at odd with their (presumed) requirement of stealth.
If I understand you correctly, you're concerned about detectable electromagnetic emissions.
My reading is that the peak magnetic field is 20 Tesla, not necessarily that the amplitude of the variations is 20 Tesla. Also, the reason they need to us ELF radio to communicate with submerged submarines is that seawater rapidly attenuates most radio frequencies.
Alternatively, if they're varying the magnetic field slowly enough... at the bottom of the ELF band, the wavelength (in a vacuum) is about 8 times the diameter of the Earth, so I'm guessing in seawater that's about 4 times the diameter of the Earth. Localizing such an EM source with enough precision (say, within the destructive radius of a 300 kt nuclear depth charge) to be tactically or strategically useful might prove very difficult if a quarter wavelength is the size of the Earth.
How far does that field travel, though, and how much does it affect the field that a submarine has by nature? Remember, a submarine has somewhere in the range of 6-12 inches or more of special types of steel for their inner pressure hulls, so that gives off a pretty freakin' big magnetic signature anyways. Generally, there aren't any super-reliable ways to track a sub from more than a few kilometers anyways, and those are mostly acoustic, so you want to reduce the acoustic signature way way more than the magnetic signature. A magnetic anomaly detector will generally just give you a "hot/cold" signal, as in the "you're getting hotter/colder" game you play as a kid, and "hot" here means "there might be a sub within the nearby few kilometers, but it could also be an unmapped underwater mountain full of iron or a cargo ship we can't see right now".
Yes - as heat, but, unlike in the case of airplanes, remote detection of submarines by infra-red is only possible insofar as the surface water temperature is increased, as infra-red is just another form of EM that is strongly attenuated in water.
Nevertheless, I believe the detection of submarines by their heat output is being used. One thing I had not thought of until writing this reply is that, so long as this method is inefficient, a submarine propelled by this method will require a larger powerplant, with a correspondingly larger heat output (and noise output from pumps, etc.) than comparable submarines driven by propellers.
Quoting the press release: "<...> could potentially yield 90% efficiency in a magnetohydrodynamic drive, which is worth pursuing". That's close to the efficiency of electric motor, which means the new drive will need roughly the same amount of power, as the existing ones.
Responses to this discuss EM emissions, I've no idea what 20 Tesla means in practice I initially read it as being a question about sound produced by mechanical distortion of strong and varying magnetic fields, rather like the banging and clanking you get inside an NMR scanner.
My impression of DARPA is that they found a lot of things without clear applications in mind. But even then, there is more to stealth than hiding from well equipped and organized groups.
Anyway, civilians could use a high-efficiency very low noise propulsion systems too.
DARPA finna be slipping past some sonar nets and listening to some rock and roll while conducting missile drills (or whatever the equivalent is for modern Russia)
Interesting. If I understand correctly, they need a electrode that's in physical contact with the sea water.
I'm not sure I understand it entirely, but there's a video on the Tech Ingredients channel that explains magnetohydrodynamic pumps: https://www.youtube.com/watch?v=LS3GQk9ETRU
Wouldn't this cause electrolysis? In seawater you would probably get some chlorine gas produced in addition to the usual hydrogen and oxygen. I wonder if the chemical signature could be detected somehow.
How "noisy" are Sweden's Sterling engine submarines really? When submerged, that is. I understand they run traditional combustion engines when skimming the surface, but are considered "near silent" when submerged.
Is there some sort of shielding inside the submarine ? 20 Tesla seem like it'll rip the iron off the blood of the personnel inside it. Or at least their glasses and computers.
Yup, the whole sub structure is a shield, as it's made from steel, which is ferromagnetic.
Also the 20T field is only in the "air gap" of the drive. I think one needs supercomductors to obtain that kind of magnetic field. The PUMP drive would use seawater as the conductive media, which also complicates things with electrolysis and Joule heating near the electrides. I don't know tge geometric configuration of the "air gap" but it could be a copule of milimetres thick in order to put a 20T field through it.
See this video explaining how it works on liquid metal. It uses the Lorentz force.
> There are multiple potential approaches to the MHD system including conductive and inductive approaches. The conductive approach involves a conductive current between a pair of electrodes within a magnetic field. The inductive approach uses a time-varying magnetic field and electric current.
IANAEE (Electrical Engineer)...but having a time-varying ~20 Tesla magnetic field in a drive system seems a bit at odd with their (presumed) requirement of stealth.