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Comment by gpm | Hacker News Reader

If they meet their 90% efficiency target, that's exceeding the efficiency of a normal boat. Just the propeller losses are higher than that (at ~80% efficient [1]).

If it's just some REBCO super-conducting magnets and conductive surfaces it might be reasonably inexpensive too. And without moving parts it could be a maintenance win as well.

I'm no sailor, but at first glance this seems like it would potentially replace all propulsion on boats/ships big enough to justify cooling REBCO. Obviously conditioned on actually reaching that fairly outrageous target.

[1] https://www.britannica.com/technology/naval-architecture/Act...

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MHD is nothing new though, so if it has not yet replaced propellers I'm guessing it is because of practical issues like corrosion, electrolysis and so on alluded to in the article that DARPA seems to have found solutions for but which are not going to be available for the general public for obvious reasons.

Also it's all electric so it poses the usual problems like the added complexity of an on-board electric generator or the added weight and cost of a battery system.

> because of practical issues

Like fitting supercooled tesla-power magnets to a moving vehicle, perhaps.

> the added complexity of an on-board electric generator

But you don't need that if the vessel is electric-powered, like a nuclear-powered sub; and I believe some modern warships use a traditional motor to drive a generator, which drives an electric motor for propulsion (i.e. an electric ship powered by a fossil-fuel generator).

> But you don't need that if the vessel is electric-powered, like a nuclear-powered sub

Yep.

> I believe some modern warships use a traditional motor to drive a generator, which drives an electric motor for propulsion

Sure, like a diesel-electric locomotive.

It turns out in some situations it's better to eat the conversion loss from electric generation rather than using a direct mechanical linkage, because then you can use flexible cables and electrical control systems rather than a bunch of mechanical folderol

Per the article the previous generation was only roughly 30% efficient, which makes it just not competitive with propellers.

Swapping out your diesel motor with rotating shafts going through your hull and an alternator to generate electricity for a diesel generator that just generates electricity is reducing complexity, not adding it.

the alternator is already there, too, since modern diesel submarines are actually diesel-electric submarines, able to charge batteries with the diesel engine and then shut it off and run from the batteries to go quiet

so, the difference is between props vs. no props, which is big.

In context I'm arguing for use on commercial surface boats and ships.

But they too have alternators (and/or dedicated generators)... because it turns out electricity is really useful.

So, the USN actually did park that Soviet sub captained by James Bond in a creek somewhere. I knew it!

I agree that an innovation of this magnitude would have a huge impact on naval propulsion if it were publicly available though I’d imagine the tech would be locked down to a ridiculous level. It’s likely destined to only appear in nuclear submarines.

Yeah, no. Once it's known that it's possible to achieve, some enterprising company will quickly replicate it.

Unless, of course, it needs unobtainable levels of power.

Well, people are talking about 90% efficiency up-thread. You can't need unobtainable levels of power and still have 90% efficiency, as people move boats around every day.

Also, about the need for a coolant, well, the fixed costs for it are on the order of a few $100/m on most cities nowadays. If people start to use lots of it, the fixed costs will fall to zero. (The variable cost isn't high, but it also doesn't dictate the boat size.)

You can absolutely have an energy requirement cliff that has to be exceeded for a thing to work before you can even measure it's efficiency. Trying to push something from standstill vs. keeping it moving.

Yes, there exist stuff that behaves like this.

MHD isn't one.

If it needs unobtainable levels of power, that means it's not efficient. If it's efficient you can use a diesel generator in place of your diesel engine and obtain the same amount of power.

Not quite. It might be efficient, but only if you need megawatt-level propulsion power.

This is not uncommon in engineering in general. There are many, many examples of devices that are highly efficient but only at higher power levels/speeds.

The government can and does lock down intellectual property for national security reasons.

The US government effectively locked down Juliet Marine Systems' IP, and gave the inventor no compensation for doing so. Took about more than 5 years for the inventor get any relief.

https://www.boatblurb.com/post/the-us-military-ghost-ship-is...

The US government won't be able to lock down a company in Sweden or Turkey. It's not like the principles of MHD propulsion are in any way secret. It's just that we now have much better superconducting magnets.

There's no reason to assume that. A ton of things DARPA pathfinds are commercialized, including the globe spanning network we're using to converse atm.

I'm not sure it would be restricted so much as just way too expensive to be practical. There is also the practical considerations of generating 20T magnetic fields out in the real world and around people.

Yeah, that seems to be the way it always goes. It's not actually all that difficult to build an AESA radar in your garage, it's just very hard to get enough of the types of parts needed for a useful one without nation state level funding. The FPGAs they use are on Digikey....for 5-10k ea.

Or on the vessel itself it would have to be shielded for the people and for any equipment.

I've seen experiments where a device using a paddle was put against a person's head and a strong magnetic field emitted. It was a TV reporter for Discovery Channel Canada and her jaw convulsed, she did not look pleased.

I think the experiment was about spirituality of all things. How to induce a feeling of a presence to a person. I believe it was called the God helmet but in the video I saw it was a paddle.

"Trans-cranial magnetic stimulation" — it has both been called "the god helmet" and has been used in that way.

IIRC it's a pulsed field to induce electric fields in your brain, and also has an extremely short range.

Somewhat more significant to human health is that magnetic material can be rapidly drawn towards the magnets, with the force on a steel office chair can exceed its own structural capacity in standard medical magnetic equipment: https://youtu.be/6BBx8BwLhqg

At very high field strengths, you also need to account for "your blood is electrically conductive, and electrical conductors resist motion when in a magnetic field due to induced electrical currents". According to test subjects, this "tingles".

At the most extreme fields strengths we can currently make, you also have to account for the water in your body being diamagnetic and repelling the magnets, but we can't ask any of the test subjects what that feels like as they were a frog and a rat because that's all that would fit in the tube.

The majority of DARPA research is public and you can get copies of the software or specs via an email typically.

I suspect the Navy actually does have room temperature superconductors[1] ... which could make this a practical technology.

[1] https://patents.google.com/patent/US20190058105A1/en

Google scholar shows that that patent 'author' has three patents 1. Piezoelectricity-induced High Temperature Superconductor 2. Craft using an inertial mass reduction device 3. High frequency gravitational wave generator

https://patents.google.com/?inventor=Salvatore+Cezar+Pais

And I think with those three "inventions" one could construct a space-ship that could travel to Alpha Centauri at a meaningful fraction of 'c'.

Extraordinary claims require extraordinary evidence. Whatever is going on here, it's hard to take this document as evidence for a working room temperature superconductor...

>Extraordinary claims require extraordinary evidence

The granting of a United States patent should be sufficient evidence to at least consider it possible that such things have been achieved.

I'm taking that 90% efficiency figure with a grain of salt since that is the paper calculation in a technology that is still very experimental.

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