Seems that battery power density might be getting to the point where we once again can operate an all-electric submarine and only charge it at the pier when required. Only this time, for weeks of patrol, not hours.
One wonders if it might be practical to build underwater "charging stations." Connect said stations to "water turbines" or some other means of extracting electrical power from the flow of water. These would be both obvious and easy targets, but... maybe of some value. Preventing sabotage would be vital.
The subs themselves might be able to self-power in some circumstances... lock themselves to the ocean floor and deploy "water turbines" or some such. Probably not good enough to make the subs truly self sufficient, but maybe good enough for backup power.
How about this: the sub anchors somewhere with a high water velocity... near a river outlet or some such. Then it releases a parachute. The water pulls the parachute away from the sub, unreeling a kilometer or so of high strength cable. As the cable is unreeled, it spins a generator. When the cable reached the end, the parachute is reefed to reduce drag and pulled back in, rinse and repeat. Badguys appear during the chargin process, the cable is cut loose. All that's lost is a parachute and a cable, not the expensive charging gear.
One wonders if it might be practical to build underwater "charging stations." Connect said stations to "water turbines" or some other means of extracting electrical power from the flow of water. These would be both obvious and easy targets, but... maybe of some value. Preventing sabotage would be vital.
The subs themselves might be able to self-power in some circumstances... lock themselves to the ocean floor and deploy "water turbines" or some such. Probably not good enough to make the subs truly self sufficient, but maybe good enough for backup power.
How about this: the sub anchors somewhere with a high water velocity... near a river outlet or some such. Then it releases a parachute. The water pulls the parachute away from the sub, unreeling a kilometer or so of high strength cable. As the cable is unreeled, it spins a generator. When the cable reached the end, the parachute is reefed to reduce drag and pulled back in, rinse and repeat. Badguys appear during the chargin process, the cable is cut loose. All that's lost is a parachute and a cable, not the expensive charging gear.
For a long range design, maybe in range 2300 Ton class and endurance of Nuclear submarine of 3 months and underwater speed of 10.7 Knot. One might need like 4-10 times of current power density of Li-Ion battery (about 250-260 Watt/pound) Without recharging and range of around the world.
That however still limit the Hotel load to about 200 KW which quarter is for life support (50 KW). The upgrade to electronics appears to be constrained by the battery capacity, more electronics = more power = more battery = more volume+weight needs to be allocated, the submarine will go bigger and may lose speed.
Still, despite being potentially attractive to smaller nation, i dont think diesel generator or other backup means of power generation can be dropped. Maybe it can use much smaller diesel but not entirely drop it.
The best batteries available today have an energy density about 10 times that of lead-acid batteries. If we take the 60 hours of underwater endurance (at 9 km/h) of a Type XXI as a starting point, you'd expect a current submarine using the best available batteries to be able to run for 25 days at slow speed. Double the speed and you'll divide radius of action by 4. Still marginal for most navies. Plus, Li-ion batteries are a major fire hazard.
Nevertheless, Japan already has a sub in operation (Ōryū) with Li-ion batteries replacing the AIP engine of the earlier boats in its class. It still carries diesel main propulsion though.
One wonders if it might be practical to build underwater "charging stations." Connect said stations to "water turbines" or some other means of extracting electrical power from the flow of water.
An RTG would be simple, but an RTG large enough to drive a sub would be 100x larger than the RTGs we use today.
The reactor on the NR-1 was about the size we're talking about (100 kW), and that was a steam plant, with much of the complexity of a large reactor, just scaled down to a smaller package. Control rods, water and steam, fission products, radiation all present and complicated.
A small nuclear pile would seem to be much less complex than a fully integrated power source similar to the units intended for battlefield deployment in larger hq areas. Would it be enough at the size needed to be fitted into the smaller sub's?
Well what's that "pile" for first ? If we take it for following purpose e.g :
1.To charge battery underwater
2.To handle hotel load (Modern Diesel Electric Submarine can have about 200 KW of Hotel load, not including propulsion).
We can perhaps use value of 250 KW (some 350 Hp). Then we can compare it to typical power density of nuclear reactor. PWR is about 38 Kg/Hp (thus 13 Metric Tonne) for the entire plant, Liquid Metal Reactor is 25 Kg/Hp (8.9 Metric tonne) and Nuclear gas turbine 21 Kg/Hp (7.5 Metric tonne). Clearly the Nuclear Gas Turbine offers the lowest weight and therefore lower amount of volume to be accommodated in the submarine. However technology is rather immature for this despite repeatedly being proposed since 1960's.
Putting 100 rtgs in a sub would not seem to be a major problem given how small and simple they are. Probably run better than space rtgs since shedding heat is a whole lot easier in water than vacuum.
The cost might be a problem though. Current Pu-238 RTGs cost in the region of $30M each (for a few hundred W) because Pu-238 is difficult to make. You could use different materials (a shorter half life than 88 years wouldn't be a problem for a sub), but the nice thing about Pu-238 is that its entire decay chain is alpha particles, which means you only need minimal shielding.
Solution: work to produce Pu-238 in industrial quantities. Lots of benefits from doing so, using various processes... not only do you get to scrape the old pits out of your antiquated nukes and replace them with shiny new ones, you also get breeder reactors and enough Pu-238 to put RTGs into series production. One Pu-238 production method uses thorium reactors, which are worth doing on their own. And now you can fill the skies and the surfaces of worlds with hundreds and thousands of probes while also emplying a bagrillion shiny new nuclear engineers and scientists. You know, the kind of people who might actually make *fusion* work. And you get to toodle around the ocean for years at a time in itty bitty coastal subs that don;t emit any of that eeeeevvvviiiilllll carbon dioxide and aren't filled to overflowing with explodey lithium batteries.
Just recently looking at some cost curves with regard to EVs, and if the >60% cost reduction 1.5x+ performance increase materialize by 2030s (not implausible with dozens of attempts using different approaches going on) the economics of submarines can just flip. Just throw more battery at the problem and it goes away.
Now, ever improving SSK is one thing. I think low cost pure electric enables things like long range low cost USV (pure battery is very simple) and short range transports. I'm staring at the 180km gap between Taiwan and China that can be crammed with USV of all kinds if batteries are cheap and pressure hulls can be avoided. (items designed to survive pressure) Numbers that approach land vehicles as opposed to ships might be on the table...I'm not sure if anyone has any good idea how to fight a war in that kind of environment.
The ideas that the nuclear reactor is in another boat. A single SSN can probably service two dozen or more SSK in the battle area (if it is its main mission) and this does not require major changes to the SSN, only the provision of electricity transmission equipment. This force multiplication enables far more hulls in the combat area without SSK constantly moving off to snorkel. (or to base, if pure electric)
A SSN with reactor scaled for 30kt+ fast dashes is wasting potential energy output almost all the time: It would be a good idea to make use of this energy. Just need someone to figure out underwater underway replenishment. Imagine, SSNs can powering SSKs at say 20kts to keep up with surface formations and detach at the battle area, all without surfacing. There are lots of times where two Subs at 20knot is better than one sub at 30knot.
It is not clear to me that given the constraints of submarine communication what kind of electrical transmission system requirements (underway speed, multiple ship charging, power transmission rate, etc) would be most useful, but the potential is there.
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Another idea that recently hit me is the use of Hydrogen Peroxide/Monopropellant/etc powered USV as a battery recharging point. Such fuels has been extremely dangerous and thus considered unsuitable for submarine use, however it shouldn't be a very big problem if a cheap USV with a big tank blows up once a while as long as crews, expensive sensors, pressure hulls, propulsion and likes are protected by being in another hull some safe distance away, connected only by wires.
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