Not sure if this should go here, IMHO or Great Debates.
I was a nuclear plant technician on a submarine for several years in the late 1980s. I recall one of the guys in my division talking about how, if super-efficient batteries (smaller/lighter/longer lasting) were to become a reality, a diesel-powered submarine could be developed that, when running on battery power, would be much quieter than nuclear-powered boats. The recent (and potential future) advancements in battery technology brought this concept to mind.
I assume the concept would involve replacing the reactor compartment and engine room components (a significant portion of the ship’s mass and interior volume) with appropriately sized diesel generator(s), primary propulsion motor(s), diesel fuel tanks, and banks of the aforementioned batteries. Since running the DGs to recharge the batteries requires air and is relatively noisy, the batteries would need to last for a significant time period between charges. I guess the this would be “make-or-break” design parameter. Maybe a week or two?
I’d be curious to hear technical opinions about the feasibility of such a design from the technical folks, particularly those with knowledge of the past, present and anticipated future of battery technology.
As I understand it, the only advantage a nuclear sub has is speed. Though in that case, stealth disappears. A nuclear sub can transit from the US coast to a combat area more quickly than a non-nuclear sub. Good for the US, not so necessary for most nations. They can make good use of electric subs.
There was a famous case where a Swedish sub penetrated a US Navy task force and “sank” the carriers and got away. So these are very capable subs.
And obviously, new battery tech will give them additional range. But they are quite capable today.
Here’s the thing- nuclear subs have to keep pumps running 24/7 to keep the reactor cool. This generates some unavoidable noise. Present day electric subs don’t have to do that- their electric motors are extremely quiet. But they’re generally limited by their endurance, mainly battery-related, in how long they can stay submerged without having to surface and recharge them using the diesels (which are NOISY!).
The concept you’re talking about would basically be to provide enough battery power through some sort of battery advancements to have considerably longer endurance than the current diesel-electric subs do, not to make them quieter than they already are (they’re very quiet already).
I suspect that the discussion has moved past better battery technology and into the realm of air-independent-power/propulsion (AIP) these days. A lot of higher-end non-nuclear subs are starting to use various technologies (Stirling-cycle engines, fuel cells, closed-cycle steam turbines) to increase endurance underwater into the roughly 3 weeks submerged realm.
Not at all. The main advantage is endurance. Nuclear submarines could stay at sea for months or years*, and can stay submerged for weeks on end. (That last was the most critical in WWII – subs had to come up for air (for the sailors, and for the engines) at least every few 10-12) hours. So a pursuing naval ship could just hang around until the sub was forced to surface. With unlimited nuclear power and modern air machinery, nuclear submarines have no such limitations.)
And the speed is just an artifact of fuel. It’s less efficient to go fast, so it uses up your fuel faster. With nuclear submarines having decades of fuel available, using up a lot ot go fast doesn’t matter.
Today, the time limits on submarine voyages are set by provisions. When the crew has eaten through all the food onboard, the sub has to put into port somewhere to restock (or meet a navy supply ship at sea).
Many (maybe most) do. The US Navy S8G reactor in the Ohio class boomers can operate “at a significant fraction of full power” using natural circulation. Which is why those submarines’ acoustic signature is colloquially referred to as “a hole in the water”
That’s true, but only the Ohios have natural circulation reactors that I’ve ever heard of- most nuclear subs are the old continuously pumped sort that emit a low background noise.
Anyway, my point upthread was that the diesel-electric boats are already quieter than most nuclear ones, when they’re running on batteries, and that the battery power issue was about endurance not stealth.
Diesel-electric boats are very quiet, and have been quieter than nukes for a long time, perhaps since nukes first arrived in 1954. (I’m not sure if Nautilus was quieter than her counterparts when launched but I’d guess not.)
But it is all about endurance. Once a submarine needs to surface or snorkel, it goes from hunter to hunted, and is at a distinct disadvantage. This was very apparent for the Russians during the Cuban Missile Crisis for example.
Boomers stay submerged generally from 60 to 80 days. Diesel-electric isn’t getting you anywhere near that.
New advances in battery technology like the Solid State Batteries that John Goodenough’s team developed (same guy who invented lithium-ion batteries) could change the equation somewhat and make diesel/battery subs more enduring.
Ever heard of the fuel-cell German submarines? Type 212A. The first unit was launched in 2002, technology can only have gotten better, but the stats way back then were already impressive: 3 weeks without snorkeling, 8000 miles range, much cheaper than a nuclear submarine. Here is an article from 2017 about the 212A and its later succesor, the 214. Sounds convincing.
Mentioned upthread. A question I have is, can the hydrogen to run the AIP system be regenerated at sea? By some diesel plant or the like? Or is it, when the hydrogen tanks are depleted, the party ends?
One thing the wiki for the 212 mentions is that the hull is non-magnetic? Does that mean no joy from a MAD search? If so, is that even possible with a nuke boat, given the need for a pressure vessel for the teakettle out back? (I know the, e.g., Alfa class was made of titanium, but what was the reactor made out of? Or did the weird Pb-Bi coolant obviate the need for a tough steel reactor vessel?
It’s both. The nuclear sub’s advantage is endurance at high speed, as compared to the speeds non-nuclear submarines can sustain for long periods.
Air Independent Propulsion (AIP) non-nuclear subs (the German Type 212 was given as examples, but numerous countries not including the US have similar designs) can stay submerged for weeks. Their limitation relative to nuclear boats is the speed they can sustain over those weeks, several knots. Modern nuclear boats can sustain speeds in the range of 20 kts* without radiating significantly more noise than when travelling slowly, even if not quite as quiet as conventional subs on battery.
Back on non-nuclear side, Japan decided to adopt lithium-ion batteries and no AIP system in its latest Soryu class subs (previous Japanese subs had an AIP system similar to that pioneered by Sweden, different than the German fuel cell system). The idea is that more hours of endurance at high speed which the Li batteries can allow compared to old style lead-acid batteries might be more tactically useful than the weeks of slow speed operation AIP systems allow.
Non-nuclear subs, ie diesel subs, can also snorkel on their diesels near the surface for basically indefinite periods, but emitting significantly more noise, and the snorkel mast may also be detected by radar, etc.
*note this is not their top speed which is higher, but the sub would be radiating significant noise at top speed. The USN defines a speed at which the submarine can still operate with ‘discretion’. Both speeds are classified so there’s no point in debating the exact numbers based on internet links. The point is just that the nuclear sub can operate indefinitely at a speed a multiple that of the AIP boat’s weeks’ long speed, yet still at a very low noise level. The AIP/lead-acid battery boat can also dash at a 20kt-ish speed, on its battery, but only for an hour or few.
I am not a ship/submarine guy but have experience with the production of hydrogen.
NASA used to truck the hydrogen for launches from Louisiana. And NASA didn’t even need Hydrogen to the purity needed by fuel cells.
So yes, hydrogen can be regenerated at sea (not under but on a surface vessel). Fuel cells are extremely sensitive to Carbon monoxide or the like left behind in Hydrogen during manufacture and this hydrogen fuel supply system will have to sit on a barge.
It is much easier though to carry the hydrogen either pressurized or liquefied in tankers.
nitpick. Though a common misunderstanding, no launches were ever conducted from the NASA facilities in Louisiana or Mississippi. The LH NASA used was for testing at the very large engine test stands at Stennis Space Center in Mississippi. I live there and when the engine(s) were being tested you could hear them 15 miles away. They very rarely tested more than one at a time. But when they did, and when they tested 3 F-1 engines for Apollo at once, Everyone heard that! But no launches there.
It’s not just endurance at speed or even endurance while submerged (a major advantage of both nuclear and AIP over diesel-electric with batteries), but also endurance in the patrol area (a major advantage of nuclear over even AIP).
As already mentioned upthread, endurance while submerged was of great importance for WWII subs (and would be still today) because (1) subs did eventually have to come up or at least snorkel for air and (2) the faster a sub went while submerged, the faster it depleted batteries (being much less efficient at high speed) meaning that, taken together, when they did surface, they could only get so far from the latest action. A brief interruption in sonar contact (when that was a thing) could more easily be made up for by anti-submarine assets because they knew the sub couldn’t get very far very fast from where contact was lost and all the while it had a metaphorical ticking time bomb on board that would force them to surface or snorkel.
But as to the other kind of endurance, in the patrol area… diesel-electric subs have always been quiet (or at least countries that knew what they were doing could make them quiet). The trouble is, any ship that isn’t nuclear powered is going to have to refuel every few weeks (or even days depending on how fast they’re going). That’s easy enough to do if you don’t mind being seen (surface ships routinely refuel while out to sea with tanker support), but not so great if your goal is to hang out within site of a major port and watch what goes on, what goes in, and what comes out. Or if your mission is to trail a nuclear submarine carrying ballistic missiles that could wipe out a good chunk of your nation’s population on its own.
One of the reasons that the US does not have ANY non-nuclear subs (apart from a handful of small research vessels) is because of that last problem (endurance on patrol). The US submarine force is routinely tasked to conduct operations on the other side of the world, across oceans. It’s no good trying to do that if every couple of weeks you need to pull back over the horizon and return to port (even to an allied port) or meet up with a tanker loitering in the vicinity and refuel.
In short, diesel-electric and AIP work great for coastal defense or regional power projection (like a foray into the nearest ocean and then back), but not so much for global power projection.
The performance difference between modern non-nuclear and nuclear subs is that the nuclear sub can operate at relatively high speed for an indefinite period, the AIP sub can operate at low speed for a practically similar period. Everything else is a ramification of that difference. That’s clearer if one avoids making comparisons where a big size difference not required by nuclear power is being mixed in*.
The reason nuclear subs could spend more time on station is if the threat level allows them to transit to patrol station at high speed, more of their total total endurance time. limited by crew endurance/provisions, can be spent on station. Big non-nuclear subs can operate over 1,000’s of miles. What they can’t do is transit to station at high speed, they have to use up more of their total endurance time transiting. And once on station they also cannot operate tactically at ‘high discrete’ speeds in 20 kt ballpark like nuclear subs can.
Why the USN has non-nuclear subs though is not a strict matter of relative capabilities and mission. It’s also the interaction of uniformed USN and civilian politics. It’s a much longer discussion beyond the scope here, and also involves opinion.
But the technical performance difference between nuclear and non-nuclear subs including AIP subs is simpler and factual: nuclear propulsion gives much longer submerged endurance at relatively high speed.
*for example comparing 7,800 tonne submerged US Virginia class nuclear powered attack subs to 1,840 tonne submerged German Type 212 includes a lot of difference in capability, and cost, due to size not propulsion per se. If instead you compared the French Amethyste type nuclear attack sub, 2,680 tonne submerged, to Japanese Soryu class (earlier boats of class are diesel/lead acid battery/AIP, later ones dieselLithium ion battery no AIP) at 4,200 tonnes submerged, the non-nuclear sub is actually bigger. But the basic performance difference due to nuclear remains the same: Amethyst can probably travel reasonably close to its published 23.5 kt sustained speed without high noise for long periods; the AIP Soryu’s can only go single digit knots when snorkeling or on AIP for practically comparable periods to the nuclear sub, but can only go fast, 20kts, for a matter of an hour or few on battery. The Litium-ion Soryu’s will trade off the ability to operate fully submerged at relatively slow speed for long periods on AIP for the ability to operate for much longer periods at relatively high speed on battery, though still not comparable to nuclear sub endurance at high speed. And would transit to patrol areas snorkeling. To take another navy, Australia envisions operating diesel subs on very long range patrols, as long as typical USN ones might be: those diesel/AIP subs just have to burn up more of their total practical endurance time snorkeling to the patrol area at relatively low speed than a nuclear sub would have to.