US Navy is not Buying the Cheaper and Deadlier AIP Stealth Submarines

The threat of super-stealthy diesel submarines being deployed around the world has been present for decades. Still, newer boats are coming armed with advanced anti-ship weapons and are being combined with new air-independent propulsion systems (AIP) making them near impossible to find in the ocean's depths.

In 2005, The HMS Gotland, a modern AIP submarine serving in the Swedish Navy created havoc in war games exercise. The Gotland virtually ‘sunk’ many U.S. nuclear fast attack subs, destroyers, frigates, cruisers and even made it into the 'red zone' beyond the last ring of anti-submarine defenses within a carrier strike group. Although it was rumored she got many simulated shots off on various U.S. super-carriers, one large-scale training exercise in particular with the then brand new USS Ronald Reagan ended with the little sub making multiple attack runs on the super-carrier, before slithering away without ever being detected.

The 1600 ton displacement Gotland Class was the first operational Air Independent Propulsion (AIP) submarines in the world.

Gotland Class Submarine
Gotland Class Submarine


AIP system can utilize advanced batteries that are charged by 75kw generators. The Gotland has generators run by a pair of diesel and liquid oxygen fueled Stirling Engines. The result of this unique, yet remarkably simple system is two weeks of submerged air independent propulsion while traveling at about 6mph. Kockums' AIP system is virtually silent, even in comparison to multi-billion dollar nuclear powered boats that still have to pump high-volumes coolant to their reactors.

The mall and quiet sub is even more deadly with a state of the art combat management system. It features an incredible user friendly interface. During a single attack, the system can guide multiple torpedoes at once, which can result in more than a mission kill for even very large naval combatants like aircraft carriers, with each torpedo striking in a different section of the hull if ordered to do so.

Maneuverability was a key factor in the Gotland Class design and this manifests itself in the boat's "X" shaped tailplane structure. This unique design provides four independent maneuvering surfaces at its stern and is tied to another two planes mounted on the boat's sail. These control surfaces, combined with the sub's advanced and highly automated control system, allows for incredibly tight turns, dives and ascensions even in very close quarters, such as in shallow littoral environments. Due to the boat's size, automation and maneuverability, the Gotland Class has been described as the F-16 Viper of the undersea combat world.



The Gotland Class hull was specifically designed for high efficiency while producing a very low noise signature and it is coated with sonar deadening materials. She also carries a series of electromagnets to counteract her magnetic signature and can short circuit very low frequency fields on command. Her sail is also covered with radar absorbent material and designers are said to have gone through great lengths masking the boat's infrared signature even when surfaced. On her interior, every piece of machinery is mounted on a series of rubber acoustic and vibration deadening buffers so as to minimize the accumulation of noise emanating from the craft's various mechanical subsystems.

The Gotland Class boats then participated in open-ocean exercises in the Atlantic where they trounced much more advanced Spanish, French and US players, including a French nuclear fast attack sub and the American Los Angeles Class SSN, the USS Houston.

Other exercises soon followed and the little Gotland Class boats continued to be a very lethal force to reckon with. By the mid 2000s, other countries were starting to field or develop AIP capable diesel-electric submarines, including Russia and especially China. Since the US Navy had retired its last diesel-electric (non-AIP) attack submarine in 1990, the USS Blueback (now a local resident here at Oregon's Museum of Science and Industry) there was no indigenous force to practice hunting down diesel-electric subs, yet alone ones with advanced AIP capabilities. Thus the US Navy went to Sweden hat in hand in hopes of leasing one of their ninja-like Gotland Class boats, and its crew, for a year. The Swedish sub would be playing the adversary to America's massive constellation of anti-submarine surface combatants, helicopters, fixed wing aircraft, and especially nuclear submarines.

In 2006, a Chinese Song-class attack submarine, created at least partially by Russian and Western technology and likely not nearly as advanced as the Gutland (the Song-class does not have AIP technology, for example) tailed the Japan-based U.S. Navy aircraft carrier USS Kitty Hawk in the East China Sea near Okinawa without being identified. While such a shadowing operation is quite normal, the sub “surfaced within five miles of the carrier, in deep waters off Okinawa, and only then was it spotted, by one of the carrier's planes on a routine surveillance flight.” Such submarines are armed with advanced anti-ship missile and wake-homing torpedoes.

Russia and China Pushing ahead with research and development to make AIP submarines stealthier and deadlier

Russia has new AIP submarine designs, the active Lada Class and the upcoming Amur Class. And that is just the problem, not only are AIP boats so hard to detect, but they are also relatively cheap, and will give friendly and enemy states alike a resurgent clandestine subsurface capability. The USA does not have any diesel electric AIP submarines. The USA has been sticking with nuclear submarines that are at least 5 times more expensive.

Russian Lada submarines are able to maintain such a low profile thanks to a clever implementation of a next-generation anti-reflective acoustic coating and a new improved hydro-acoustic system.

With its new air-independent propulsion plant, a Lada submarine can remain submerged for as many as 25 days. With its vast array of weapon systems, the Lada is also world’s first non-nuclear submarine to be equipped with specialized launchers for cruise missiles.

The bulk of China’s conventional sub armada consists of 13 Song-class (Type 039) diesel-attack subs and 13 Yuan-class (Type 039A) air independent-powered (AIP) attack submarines with an additional 20 Yuan-class vessels planned for production.

The Song- and Yuan-class attack submarines are equipped with German-made state-of-the-art diesel engines — the 396 SE84 series — designed by MTU Friedrichshafen GmbH of Friedrichshafen, Germany.

“They are the world’s leading submarine diesel engines,” according to an experienced submarine engineer. Each Song- and Yuan-class vessel is equipped with three such engines, which have been built under license by Chinese defense contractors since 1986. The Yuan-class is also said to have incorporated quieting technology from Russian-designed subs and to be equipped with Stirling air-independent propulsion technology.

China has also been experimenting with lithium-ion (Li-Ion) batteries, power sources that offer much higher energy density and longer dive times. “Chinese researchers clearly see Li-Ion batteries as the wave of the future for conventional submarine propulsion. They’re not there yet, but they are determined to get there,” Erickson noted. Erickson said China was discussing putting Li-Ion batteries “on a new generation of conventional subs sometime between now and 2020, but there is no indicator as yet of the type of submarine that might be.”

US Navy Procurement Bias to nuclear submarines. The USA has a procurement bias to nuclear submarines.
Admirals and officers have had decades of believing that nuclear submarines were the best option and they were Not since 1990 has the U.S. Navy operated conventionally powered boats. Over 25 years
Admirals and officers get into coalitions in Navy internal politics.The companies receiving tens of billions of dollars for more expensive nuclear submarines have a lot of influence.


Nuclear submarines had certain advantages over diesel submarines. Nuclear submarines had greater submerged endurance of 90-100 days versus 3 days for conventional submarines. This was limited by the amount of food that could be carried. Nuclear submarines also had higher speed.

Air independent propulsion (AIP) submarines use fuel cells, stirling engines, batteries or liquid oxygen storage for closed cycle diesel engines or close cycle steam.

AIP submarines are quieter

While nuclear submarines have measures to reduce sound and magnetic signatures, nature of nuclear propulsion (steam turbine) makes them far more noisy than AIP submarine of same size. They also tend to be larger on a whole, making them even more detectable through either acoustic, infrared or magnetic sensors. Further weakness of nuclear submarine is that it has to cool down nuclear reactor, with hot water being dumped into ocean, leaving long trail behind the submarine; as such, it is even more detectable by IR sensors than just size difference suggests.

Nuclear submarines have cruise speeds of 20 – 25 knots, compared to 10 – 15 knots for AIP subs. Combining slower cruise speed with bursts of high speed can allow AIP subs to cover relatively large area. They can deny access to enemy nuclear submarines. HDM and MESMA systems used in AIP subs (submarines using them typically cost 250 million USD) are far quieter than nuclear plant.

Using traditional diesel engines, a fuel cell, large lithium-ion battery pack, and liquid oxygen to replace the air normally used in combustion engines, the SMX-Ocean sub concept could stay off-shore in deepwater operations for 90 days at a time. The AIP system also allows the sub to remain submerged for up to 21 days at a time, and gives it a range of about 18,000 miles at an average speed of about 10 knots.

Here is details of AIP technology and capabilities.

Submarine snorkeling for oxygen can be detected with new radar

Batteries continue to improve

The Lithium ion batteries for the Soryu and SMX-ocean are getting higher energy densities and lower costs because of the success of electric cars like the Tesla Model S. These will further increase the advantages of the AIP submarines.

AIP submarines cost $100 million to 900 million. The AIP models with the greatest market success are the Japanese Soryu ($600 million), Russian Kilo ($350 million), Swedish Gotland ($365 million), Spanish Scorpene ($500-800 million). They can be submerged for about 14-21 days.

If you see Tesla and other electric cars with new batteries that double and triple range then there will be AIP submarines that can stay submerged for 40-80 days. It will be easy to upgrade old AIP submarines with a new set of batteries for about ten million dollars.

The nuclear submarines are $1.3 billion to 3 billion. The equivalent AIP submarines are three to five times cheaper.

Nuclear submarines may see a revival in cost effectiveness in 15-20 years when a new generation of molten salt reactors become available. However, for now there is little reason economically or militarily for nuclear submarines.

Nuclear submarines have 20 megawatts of power compared to 3 megawatts for diesel submarines. There is no need for high power for any new laser or other weapon systems.

The US is looking to add unmanned submarines for about $40 million each. Capable unmanned submarines seem to be 5-10 years away. The unmanned systems could track other submarines or operate as part of pack working with a primary manned mothership submarine.

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