Air Independent Propulsion (AIP) Issues

MISSION

On the Comment thread https://www.blogger.com/comment.g?blogID=19245896&postID=5077212490045962874 Anonymous on April 9, 2015 asked:

"I have few questions to ask you about submarines, what is actually air independent propulsion system of non nuclear submarine. There are many types of AIPS technology in the world and few are under development too so can you tell me which is best among these. And is it possible for a submarine with AIPS system to perform same like a nuclear submarine. I heard recently in a article talking about future diesel submarine will be capable like nuclear submarine with support of latest AIPS system."

On Comment thread https://www.blogger.com/comment.g?blogID=19245896&postID=6049042900970745217&page=1&token=1429705517253 another Anonymous on April 21, 2015 asked: ""why would you want the AIP removed from TKMS Type 214 for Australia?"

"S" has provided useful comments, included in the text and Comments thread

RESPONSE

At some stages-usually a submarine will draw air/oxygen (while near the surface) through its snorkel to power its diesel engines. The submarine's batteries can then be recharged. For a limited time Air Independent Propulsion (AIP) technology makes snorkel air/oxygen drawing unnecessary.

AIP provides diesel-electric submarines with greater submerged endurance (several weeks as opposed to just several days (on existing lead-acid batteries)) and very quiet operation. This enhances a submarine's survivability and mission flexibility. AIP systems use limited amounts of stored (chemically, liquid or compressed) oxygen or hydrogen fuel.

AIP involves chemical changes and sometimes moving parts. My article Air Independent Propulsion (AIP) Technologies and Selection of August 5, 2014 which has diagrams, addresses questions on how AIP systems work. Basically the most developed AIP systems were/are made by 3 Western European countries centred around the North Atlantic and Baltic Seas. These are:

- German fuel cell AIP offered in German TKMS-HDW's latest submarine designs (214s, KSS-IIs, Dolphin 2s, probably 218s, offered for the future 216).
- Swedish Stirling AIP submarines (in Swedish Kockum's designed Gotland, Södermanland and Archer classes, Japan's Soryu class and China's Yuan class)
- French MESMA AIP (just on 3 French DCNS designed Agosta 90B submarines operated by Pakistan). Offered for France's newer Scorpene submarines (no takers?). MESMA and a second generation AIP are probably being offered to Australia within a offered SMX Ocean/conventional Barracuda. It is unclear whether Brazil's 4 future Scorpenes will have AIP.

Based on sales numbers and different countries buying them, Germany's fuel cell AIP and Sweden's Stirling AIP have been the most successful.

Countries that have recently announced plans to develop AIP include: Russia, Spain, India (DRDO) and France indicates it may develop a "second generation AIP". Possibly Japan and South Korea may develop indigenous AIP systems.

Different countries and different submarines builders will claim that their AIP is best. But these are  commercial claims. It depends what the customer country wants, what they actually need - all dependent on their typical mission profiles. Technological advances (such as Lithium-ion batteries and evolving anti-submarine sensors) also influence the value of AIP products. 

Any comparison/claim that AIP is like nuclear propulsion is a commercial or government/Navy sales pitch. It is like comparing a conventional high explosive bomb with a nuclear weapon. Nuclear propulsion can drive a submarine continuously at 30 knots for 3 months (only limited by food stocks for the crew). In comparison AIP technologies might only move an operational submarine (with full warload) at slow speed (4 knots?) for 3 weeks (?). Higher performance AIP systems may be possible but there might be major downsides eg. their stored explosive hydrogen and/or oxygen might make them too unsafe to be a usable weapon system.

Additional discussion of AIP issues comes by way of my response to Anonymous April 21, 2015 question "Why would you want the AIP removed from TKMS Type 214 for Australia?":

Australia might not want AIP because: 

-  AIP is not a standard inclusion for all new diesel submarines. Inclusion of AIP depends on a customer navy's typical mission profiles. Within the Type 214 a fuel cell AIP seems to be a compulsory sales inclusion that many countries don't need.

- AIP is ideal for countries with mission profiles that value 2 to 3 week submerged-slow or zero speed (sitting on the sea bottom). Some countries, like Sweden or Germany, when in the Baltic Sea, (or Singapore near its area) might have only 2 to 3 day missions when AIP alone may be sufficient. 

Australia's mission profile is very long distances, warm water, fast transit, then perhaps mainly medium speed patrolling. This places more value on efficient conventional diesel-electic operation which would not be at the expense of weighty AIP inclusion.

Also technological advances are trending toward submarines using Lithium-ion batteries (LIBs). Issues of relative energy efficiency AIP - batteries compared to LIBs - and LIBs compared to other batteries are very complex. See page 6 of this publication http://navyleague.org.au/wp-content/uploads/2012/06/The-Navy-Vol_70_No_4-Oct-2008.pdf for a brief discussion of the relative capacity. 

The newest batch of Soryus are believed not to have Stirling AIP but instead have LIB to cover slow-medium, prolonged, submerged operation. See S's calculations in the Comments thread. 

AIP's other possible downsides include:

- it is very expensive to incorporate into new build submarines or to retrofit - often amounting to one third of the submarine's price.

- non-replenishable during a mission

- very unsafe, flammable, even explosive, "S" reported, April 22, 2015 that "I agree about weak points of hydrogen fuel cell system for submarine. Submarines of this type are safe under the ordinary or non battle situation, but most important thing is safety under the battle situation with strong vibration or shock. Even if hull is not damaged, if slight hydrogen leakage is caused by tiny damages of piping system including valves, pressure gauges and joints, the submarine becomes perfectly dysfunctional. Because concentration of explosion limit for hydrogen is very low, you must avoid any kinds of stimuli as such heat or electrical ignition which cause explosion. And in the case of accident with hydrogen leakage, perhaps you cannot rescue the submariners by hull cutting with ignition."

- AIP involves weight tradeoffs. The weight taken up by AIP may in an Australian submarine be considered better used for extra batteries or diesel fuel.

- can break down (especially if it has moving parts like Stirling AIP)

- may be more efficient in cold water rather than Australia's mainly warm operating areas

For all these and other reasons AIP was never placed in the Collins sub - even though the Swedish-Kockums designers specialised in AIP subs.

Pete