[HN Gopher] Taking a new look at fundamental tech for quiet unde...
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       Taking a new look at fundamental tech for quiet undersea propulsion
        
       Author : Tomte
       Score  : 18 points
       Date   : 2023-05-31 18:13 UTC (2 days ago)
        
 (HTM) web link (www.darpa.mil)
 (TXT) w3m dump (www.darpa.mil)
        
       | loloquwowndueo wrote:
       | Yay red October here we go?
        
         | napoleongl wrote:
         | Dang you beat me to it :-) Found the book last week when
         | packing for a move, that can't be a coincidence!
        
       | bell-cot wrote:
       | > ...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.
        
         | krasin wrote:
         | > 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.
        
         | KMag wrote:
         | 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.
        
           | twic wrote:
           | Doesn't need to be a wave. Sub hunters are already flying
           | these things around:
           | 
           | https://en.wikipedia.org/wiki/Magnetic_anomaly_detector
        
         | ok_dad wrote:
         | 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".
        
       | elihu wrote:
       | 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
        
         | HPsquared wrote:
         | 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.
        
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       (page generated 2023-06-02 23:00 UTC)