I consider it a sign of poor quality writing when the article doesn't explain a technical concept early on. What is Air Independent Propulsion? The article never even makes it that clear. Paragraph six starts as if it is about to explain it, but then it tells you that Sweden uses subs powered by a Stirling engine. Is it the same thing? Do AIP submarines always have this type of engine, or just Swedish ones? The reader never knows.
AIP is an operational concept, not a specific technology. Several different types of power sources have been used. The most common approaches today are fuel cells and Stirling engines, both of which rely on fuel and liquid oxygen.
I certainly agree with this. "Air Independent" in this case means that the propulsion system doesn't need air from outside the submarine. Batteries are an "air independent" system but this is talking more about things like fuel cells or Stirling engines.
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.
And there are factual errors like the pump noise for nuclear plant cooling, that was a problem 40 years ago for some countries that were not able to design a convection-based system.
> While old diesel submarines needed to surface in a matter of hours or a few days at best to recharge batteries, new AIP powered vessels only need to surface every two to four weeks depending on type.
> 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.
If you're carrying LOX to power the Sterling engine or the fuel cells, then you could equally use the LOX to power the diesels (although obviously the diesels would be noisier).
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.
I don't think any current AIP subs can generate their own LOX. It's possible in theory, but the fractional distillation machinery is bulky and power hungry. So that's why range and endurance are severely limited.
>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.
Those Gotland class submarines are another tier entirely compared to other submarines, so I don't think it is fair to dismis this entirely as having been operational for decades.
The first Gotlands were launched in 1995, and I'm fairly certain that German-made diesel subs (Type 209s) were being retrofitted with Stirling engines well before the Gotlands begin construction in 1992.
You're not crazy. That's exactly what Japan chose to do for the latest block of Sōryū-class attack submarines. They removed AIP engines from the design and replaced them with a huge pack of lithium batteries. Underwater endurance is still limited, but there are significant operational advantages to the simpler battery based system for defensive submarines that will never deploy far from Japanese waters.
I'm wondering whether I'd prefer to not be stuck underwater with a huge pack of lithium cells, or to not be stuck underwater with a tankful of liquid oxygen and another tank full of diesel?
Probably 'stuck underwater' is the key phrase here.
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.
Propulsion isn't the gating factor for submarine adoption. The engineering and precision manufacturing is just extremely difficult all around. For one example there are only a few shipyards in the world with the right equipment and expertise to weld together hull sections. Most countries that want an attack submarine have to buy them used from an ally, or spend $100M+ with one of just a few European or Russian manufacturers. And even once you have a submarine it costs a fortune to maintain and operate safely. Most countries barely use the subs they have because they're in poor condition or lack trained crew.
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.
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?").
The US Navy has conducted multiple detailed studies on AIP submarines. They are effective for short-range coastal defense. However the Navy isn't really tasked with that mission anymore. 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. AIP submarines still can't do that. And even for the nuclear deterrent mission they lack the necessary submerged endurance.
> The US Navy has conducted multiple detailed studies on AIP submarines.
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.
I read the article and you're simply wrong. The critiques remain valid. Nuclear deterrent patrols last months, not weeks. And they need to make fairly fast transits to and from the patrol area.
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.
> AIPs are a big competitor to replace diesel submarines
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.
Using liquid oxygen doesn't seem to be a very imaginative way to become "Air Independent".
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.
I’ve never heard of AIP before, looks like this is using LoX and bunker oil to drive a fuel cell. What’s the reaction going on to create energy from oil without ignition?
They're not putting bunker oil into fuel cells. It's usually something like ethanol or pure hydrogen.
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.
"Killer" subs are designed to "kill" other subs. They're distinguished from nuclear missile subs, and from subs used mainly for scouting and transport.
> "Killer" subs are designed to "kill" other subs. They're distinguished from nuclear missile subs
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.
Perhaps one designed for tapping into undersea cables, stealthy deployment of Special Operations teams in hostile waters, and such? (Yes, if of any size, such a sub would probably have some torpedoes or other lethal weapons. But for military naming purposes, a few "hopefully it'll never actually need this thing..." weapons hardly qualify as it as a "killer".)
AIP isn't quieter than a batter powered submarine, it's quieter than a nuclear powered submarine.
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.
