For nearly a century, diesel-electric submarines relied on constantly charging traditional lead-acid batteries by running their diesel engines either on the surface or later on, while
. This left them constantly vulnerable, as they could only hide deep beneath the surface for hours or a couple days at a time. Over the last few decades, Air Independent Propulsion (AIP) technology has revolutionized the traditional diesel-electric submarine Performance capabilities approaching the the domain of highly expensive and large nuclear submarines have became available for a fraction of the cost in the form of smaller, AIP equipped diesel-electric submarines. Now the diesel-electric submarine is on the verge of yet another revolution, one that will return to type to its original roots, relying on diesel engines and batteries alone to go about its clandestine business.
describes an idea more than one strict submarine configuration. The same general concept can be achieved via multiple methods. The most modern versions range from
, to the French MESMA (translated as autonomous submarine energy module) closed-cycle steam turbine system, to
to power the submarine while it is submerged for long periods of time. Each approach has its own advantages and disadvantages, with cost, complexity and technological risk being major factors beyond raw performance.
For instance, Sweden’s deadly but comparatively
uses Stirling Engines for AIP, and although the technology is well proven and affordable, it also requires the boat to lug around liquid oxygen oxidizer, which can have its own dangers, as well as inert gas to mix with it. The Stirling engines and other infrastructure needed to make the system work, much of a relatively small submarine’s bulk gets taken up by the system. Additionally, Stirling Engine based AIP has many moving parts, which can make noise even when a high-degree of soundproofing is designed into the submarine.
The French MESMA form of AIP, as
, is far more complex than the Stirling Engine AIP concept. It basically acts in a similar fashion as a nuclear reactor, although it uses ethanol and liquid oxygen combustion to generate steam, not a nuclear reaction, to spin a turbine and generate electricity. Once again, the boat has to lug around ethanol and volatile liquid oxygen as well as complex machinery—which produces noise—to make the system work, but it can produce a lot of power, which is good for high-speed operations. Cost is a major factor as MESMA is not a cheap technology to acquire or maintain.
Finally,
, although very high-tech and not capable of quickly ramping-up its power output like say a MESMA configuration can, is very quiet as there are few moving parts in the system. It also is a very efficient system for long endurance missions. So if you don’t have to sprint very fast but have to stay silent and stealthy for long periods of time, the technology has huge benefits. It is thought that
, of French origin, will use fuel cell AIP propulsion. These massive submarines will offer as close to nuclear propulsion capabilities as possible. Israel's
also use fuel cell AIP, which makes sense as they work as Israel's second-strike nuclear deterrent.
All sensors and weapons being equal, a Navy has to justify what type of diesel electric submarine to choose based not just on cost but also on what type of tactics they aim to employ and what type of combat environment they are most likely to fight in. For instance, if long-range patrols and ambush tactics are common, along with the need for maximum stealth, fuel cell AIP technology may be best. If bursts of high-speed during attack and evasion maneuvers are needed often, along with high endurance, MESMA may be most appropriate. For shorter-range littoral combat operations, the Stirling Engine-based AIP technology may make the most sense. The thing is that with the large leaps in battery technology realized over the last couple of decades, AIP technology may soon face serious competition in the world of submarine warfare.
High-tech diesel-electric submarines may begin to return to their simpler roots as Japan is pioneering a concept today that aims to eliminate AIP altogether. Their next generation
Soryu class diesel-electric attack submarines will be equipped with lithium-ion batteries, and just like submarines dating back before World War I, they will run undersea on battery power alone.
The
Soryu class is already a very modern submarine, having been introduced into service just over a decade ago. As an outgrowth of the previous
, these are not tiny boats, displacing 4,200 tons submerged and measuring 275 feet long, They are the largest submarines Japan has constructed since the end of World War II. They also feature an “X” tailplane configuration for extreme maneuverability in tight littoral environments.
Today seven boats are operational, all of which leverage Stirling Engine AIP technology licensed directly from Kockums—the same Swedish company that produces the
Gotland class. Japan, an island nation with long and complex coastlines, uses its submarines to patrol its territorial holdings and to protect its shores. As such, the proven and affordable Stirling Engine-based AIP technology, paired with the class’s large size and ample fuel reservoirs, was a good, balanced fit for their needs. But now Japan wants to eliminate AIP technology altogether without losing its benefits, and in doing so free up room for other capabilities while also simplifying design, construction and sustainment of their future submarines. Above all else, this new configuration should result in quieter operation than most existing AIP capable submarines.
The idea is to install thousands of lithium-ion batteries along with powerful diesel engines and generators, as well as large exhaust and intakes stacks to accommodate them, into a tweaked Soryu design. A new power handling system that can deal with high power loads and optimize efficiency is also included in the concept. Basically, the configuration is similar to a standard diesel-electric submarine, that uses diesel engines and batteries alone for propulsion, but infused with new technology.
Lithium-ion batteries have a ton of advantages over their old-school lead-acid cousins. They keep up their output even when their charge runs low, they are lighter than lead-acid batteries, they can be charged exceptionally fast (hence the more powerful diesel engine and generators), and they can store much more energy. Compared to the AIP system they aim to replace, endurance should be similar, while the overall boat’s propulsion system design will be less complex and bulky. Not just that, but lithium-ion batteries can provide large output on demand, allowing the boat to dash mush faster while dived compared to one running on an AIP system.
The main downside to lithium-ion batteries is very well publicized: they are known to “runaway” and combust—exactly what you don’t want on a submarine. When they do so they produce very high heat, give off toxic fumes and expel conductive dust. They are also hard to extinguish using traditional means. But because weight is not as much of an issue on a large sea-going vessel, new methods of abatement can be put in place to lower the risk of a fire and its potentially catastrophic results.