Air Independent Propulsion Could Create Silent Killer Submarines
nationalinterest.org
nationalinterest.org
The cool thing about Stirling engines is that there isn't a "bang" associated with the power stroke, that makes them quite (unlike diesel engines). Fuel cells obviously generate power from the catalyzed combination of chemicals, and so are also silent.
As the article points out they are quiet but their range is limited so they are good for patrolling your coast, not great for going across the ocean stealthily.
I was impressed that the Gothard class was getting 75kW from their Stirling engine. I had not known that there were production engines greater than 50kW. So that was new to me.
Submarines and anti-sub destroyers aren't using their sonar to listen for the sound of the motor. They're listening for cavitation from the screws. Motors are already silent-enough. The problem for stealth is how to make a silent mechanism for moving water.
> a Gotland class submarine is reduced to just 6 miles per hour if it wishes to remain submerged at maximum endurance
two to four weeks submerged while idling? hardly seems like an improvement over diesel boats. also, what is the limiting factor? are they literally out of fuel, since they cant just snorkel to recharge batteries? or do they carry some kind of liquid oxygen maker? if they can make LOX, why dont we compare how long before diesel refueling is required?
weird amount of detail in the article. too much to be clickbait, not enough to be an examination.
Becoming "Air independent" by just loading a lot of compressed air onto the boat doesn't seem particularly clever, to me. It smells like a crude hack.
>In the 1990s, submarines powered by Air Independent Propulsion (AIP) technology entered operational use. Though the concept dated back to the 19th century and had been tested in a few prototype vessels, it was left to Sweden to deploy the first operational AIP-powered submarine, the Gotland-class, which proved to be stealthy and relatively long enduring. The 60-meter long Gotlands are powered by a Stirling-cycle engine, a heat engine consuming a combination of liquid oxygen and diesel fuel.
AIP is not new and has been operational for decades.
https://jalopnik.com/sweden-has-a-sub-thats-so-deadly-the-us...
This https://www.aspistrategist.org.au/future-submarine-coming-up...
gives me 4.5 MJ/L density for fuel
This, just chosen randomly https://www.cei.washington.edu/education/science-of-solar/ba...
Gives me 670 Wh/L as an upper value...and a MJ 277 WH...so 2.4 MJ/L
Unless I'm utterly wrong, I'd just wait for batteries to get better. New Musk investment, 'The Unterseebat Company'
I'm attracted to the idea of battery submarines mostly because you'd be latching onto a huge (and growing) commercial development world. Much less need for highly specialized tech.
Thinking it through, assuming that propulsion is the gating factor for submarine adoption (maybe it isn't), what happens when pretty much any nation can afford a silent undersea navy?
The next thing I'll hear about is recharging station surface ships.
https://www.thedrive.com/the-war-zone/41075/canadian-submari...
Full size submarines can recharge their own batteries on the surface using diesel engines. Only some small, short-range submarines rely on other vessels for recharging.
(and the batteries can always be upgraded)
https://www.youtube.com/watch?v=8rBL54Hdixw
AIPs are a big competitor to replace diesel submarines for the reasons the article sets out. Nuclear has superior performance, but the costs aren't in the same ballpark as either AIP or diesel (particularly cost to do nuclear safely).
I could see the US retiring out a handful of nuclear subs and replacing them with AIPs simply as a cost saving measure (e.g. SSBN mission). It would reduce the cost of the submarine fleet while saving face in terms of sub count and on-paper capabilities (although then we go down the rabbit-hole of "is the US sub fleet oversized?").
Indeed, and the technology has continued to improve. Thus, the linked article and related discussions.
> They're an expeditionary force and they need fast, high endurance submarines which can cross the Pacific Ocean in a matter of days or escort a nuclear-powered carrier.
Nobody was suggesting they could replace their hunter-killer fleet with AIPs. What was specifically discussed in-name was using them in their nuclear ballistic missile role.
> And even for the nuclear deterrent mission they lack the necessary submerged endurance.
I'd suggest you read the linked article before responding using outdated information. They now have four week endurance up from under a week many years ago. This was a valid critique, this isn't currently a valid critique.
The National Interest is basically a site written by dilettantes for fanboys. It's not a reliable source for deep analysis of defense procurement policy.
Then by all means link to whatever better source you're using to base your arguments.
AIP is a supplementary system used by diesel submarines, not a replacement for them. They don't have sufficient power for anything but slow quiet cruising. AIP equipped subs will retain diesel generators for faster sprints, and greater endurance while transiting on the surface or via snorkel.
The two basic technologies for AIP are fuel cells and sterling engines. Both are deployed successfully at the moment. The video above is a neat summary of a 2nd generation fuel cell system that uses an onboard reformer to generate H2 from diesel, avoiding the downsides of H2 storage.
And what's described isn't a propulsion system; whether you use a Stirling engine or a fuel cell is neither here nor there from a propulsion point-of-view. The propulsion system is an electric motor driving a propeller.
I'd be very interested to hear about a propulsion system that uses electricity directly (without moving parts) to move water through a duct. Such a system would be quieter than a system using screws and motors, and might have the potential to be more efficient.
I imagine such a system must have been invented; I further imagine that it would have been instantly classified.
I think you'd need electric current through the water in the duct. Then you could push water through the duct using a coil, -> silent propulsion. It wouldn't even be that hard to prototype. But I have no idea what technical and efficiency problems might crop up.
Those subs generally have separate diesel fuel tanks for the surface engines. Although in theory the hydrogen for fuel cells can also be generated by reforming diesel fuel.
The 'largely around coastal waters' (littoral) should hint at this.
Isn't that what I said?
> and from subs used mainly for scouting and transport
For actual navies (as opposed to for example drug smugglers) scouting and transport (which is super-niche) is done by hunter killer submarines - they aren't a separate thing.
I imagine if you’re studying life in the deep ocean, a silent submarine is nice.
Unless you're doing ridiculously long research dives (in which case batteries don't suffice), or you care about noise on the surface (where you would use a traditional air burning diesel engine to recharge) it doesn't seem like this brings any advantages.