Cost of typical AIP sub is 100 million USD to 250 million USD, compared to 1,6 – 3 billion USD for typical nuclear submarine; though estimates for possible US diesel subs were as high as 500 million to 1 billion USD.
Here is list of submarine costs:
T-96 class (Sweden): 100 million USD
212-type (Germany): 250 million USD
Moray class (Netherlands): 250 million USD
Dolphin class (Germany): 500 – 870 million USD
Scorpene class (France): 450 million USD
Los Angeles class: 1 billion USD
Seawolf class: 2,8 billion USD
Virginia class: 1,8 – 2,5 billion USD
Astute class: 1,17 – 1,82 billion USD
Further, while nuclear submarines are not really harmful for its crew, disposal of spent nuclear fuel is very costly. Operating costs are also lower for AIP subs – nuclear submarines can cost 21 million USD per year to operate, along with 200 million USD refuelling and modernizing at half-life of 15 years, which adds up to 830 million USD over nuclear sub’s lifetime. At the end of service life, it can be refuelled and overhauled for 410 million USD, giving it another 12 years of life, and adding 662 million USD to total lifetime operating cost.
AIP subs generally provide submerged (AIP) endurance of 14-30 days, and total endurance of 45 to 90 days, as AIP systems cannot yet replace oxygen-dependant diesel engine. Nuclear subs, on the other hand, typically have endurance – submerged or not – of 90-100 days, limited by the food storage for the crew. Gotland class has submerged endurance of 14 days at 5 knots, while Type 212 submarine has submerged endurance of over 30 days at 4 knots, and can cruise for cca 3 000 miles.
Main advantage of modern submarine is stealth. 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.
While Los Angeles class can dive to 450 meters regularly, German Type 214 (improved Type 212) can dive to 426 meters.
Nuclear submarines are typically faster than AIP subs, making them more suitable for open ocean. However, typical AIP sub is smaller and more maneuverable than typical nuclear submarine. This, combined with smaller acoustic signature, makes them far better suited for littoral waters, such as in North Sea, Mediterranean Sea or Western Pacific, where in some cases nuclear submarine is longer than the water is deep. As such, in islanded areas or choke points nuclear submarine can fall victim to smaller AIP subs, unable to detect and outmaneuver them. 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, however, and effectivelly deny access to enemy nuclear submarines. HDM and MESMA systems used in AIP subs (submarines using them typically cost 250 million USD) are also far quieter than nuclear plant.
In shallow water, AIP sub is just as dangerous to surface ships as it is to nuclear subs. As Capt. Tom Abernethy, who commands the sub-hunting Destroyer Squadron 22 based in Norfolk , Va. , said: “Shallow water, you get a lot of noise reverberation and additional traffic, and you’re fighting in somebody else’s back yard which they know pretty well …. [In that environment, even a diesel sub] is absolutely a real threat, a formidable threat …. ”. Furthermore, unlike nuclear submarine, diesel submarine can hide on the floor, completely silent and immobile, until something passes nearby. And even with usage of active sonar, it is not easy to discern submarine from its surroundings.
Walter hydrogen peroxide turbine allows for 26 knots of submerged speed, however, though it is likely a “sprint” speed (which is in 30 – 35 kt range for nuclear subs) and not cruise speed. Perhydron fuel used in WW2 and later Walter submarines is combustive, and fuel lines must not have any right angle turns, as it can pile up and spontaneously combust in such angles – and even without that, Russian Walter submarines were called “cigarette lighters” due to their tendency to flame up. Further, turbine is very fuel-thirsty, limiting the range.
Out of modern AIP technologies, closed cycle steam turbines offer highest short term power output, but have lowest efficiency and highest fuel consumption. Stirling engine is quiet and simple, but large compared to its power output. PEM fuel cells currently have very low power output, but like all other AIP technologies, there is a lot of room for improvement.
In exercises, AIP and diesel subs have proven their worth. While exercuses are usually scripted (sometimes to an extent of being completely unindicative of actual combat capabilities of different weapons – this is case with USAF exercises involving 5th generation fighters), it was known for submarine commanders to deviate from script, with deviations producing rather interesting results. In 1981 NATO exercise Ocean Venture, an unnamed 1960s vintage Canadian diesel submarine “sank” the carrier USS America without once being itself detected, and a second unidentified vintage sub “sank” the carrier USS Forrestal. In 1989 exercise Northern Star, Dutch diesel submarine Zwaardvis “sank” carrier USS America. In RIMPAC 1996, Chilean diesel submarine Simpson “sank” carrier USS Independence. In 1999 NATO exercise JTFEX/TMDI99 Dutch diesel submarine Walrus “sank” carrier USS Theodore Roosevelt, exercise command ship USS Mount Whitney, one cruiser, several destroyers and frigattes, and Los Angeles class nuclear fast attack submarine USS Boise. In RIMPAC 2000, Australian Collins class diesel submarine “sank” two US fast attack submarines, and almost “sank” carrier USS Abraham Lincoln. In 2001 Operation Tandem Thrust, HMAS Waller sank two US amphibious assault ships in water barely deeper than length of submarine itself, though it was later sank herself, and Chilean diesel sub took out Los Angeler class nuclear fast attack sub USS Montpelier twice during exercise runs. In October 2002, HMAS Sheehan hunted down and “killed” Los Angeles class USS Olympia. In September 2003, several Collins class submarines “sank” two US fast attack subs and a carrier. In 2005, Swedish Gotland-class submarine “sank” USS Ronald Reagan.
At least one similar occurence happened outside exercises: in 2006, Chinese Song-class diesel submarine reached striking distance of carrier USS Kitty Hawk undetected. While US are thinking about emulating diesel submarines with UUVs, most likely outcome will be platform just as, or less, capable than AIP submarine, while costing just as much as nuclear submarine, and being far more unreliable than either. As Robert Gates said, US spend more and more money for fewer and fewer platforms. I might add: and ones that are more vulnerable in many scenarios than what US are currently using.
AIP subs, while having disadvantages – mainly regarding speed and range – compared to nuclear submarines, also have many advantages that make than a must-have for any serious naval force. They can also be a nightmare for ASW and any other surface or submerged units when employed properly.