Taking a new look at fundamental tech for quiet undersea propulsion

> but having a time-varying ~20 Tesla magnetic field in a drive system seems a bit at odd with their (presumed) requirement of stealth.

.. unless it's underwater that shields all EMI very well.

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".

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.

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.

Dang you beat me to it :-) Found the book last week when packing for a move, that can’t be a coincidence!

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)

Archimedean Dynasty and the Entrox drive is surely your first thought? That awesome underwater game made by BlueByte software.

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.

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.

https://youtu.be/nakeHnWVUjQ