August 5, 2014

Air independent propulsion (AIP) Technologies and Selection

See a more generalised Submarine Matters discussion on why a country may or may not want AIP at Air Independent Propulsion (AIP) Issues of April 23, 2015.

If Australia selected an air independent propulsion (AIP) technology for conventional diesel-electric submarines it would be on a new-construction Future Submarine rather than being retrofitted to the current Collins subs.

Much of the following information draws on Australian Dr Carlo Kopp’s article originally published in Defence Today, December 2010. Much information is also drawn from Edward C. Whitman’s article with some major updating.

Some AIP Attributes When Selecting

In assessing the merits of any AIP system several factors, which include potential weaknesses, are important, including:

• Submerged endurance achievable with an Australian specified profile, comprising specific segments of submerged operation at specific speeds and depths, representative of real missions;
• Suitability of the AIP to Pacific, Indian and Southern Ocean operating environment as distinct from the North Atlantic-Baltic operating environments of most AIP builders;
• Acoustic signature contribution produced by the AIP system in specific operating regimes, but especially at varying speeds and depths;
• Vulnerability of the AIP systems (especially hydrogen or oxygen storage) to near miss explosive or implosive effects that otherwise not lethal to the submarine or its systems;
• Various failure modes of the AIP system and its oxidiser/fuel storage, and to what extent are these repairable if a failure or battle damage arise in a contested patrol area;
• Failover modes and internal redundancy in the AIP system, and what ‘casualty’ modes exist if a catastrophic failure arises to get the boat out of danger;
• Replenishment options (if any) of oxidiser and fuel from a tender when operating at large distances from a friendly port;
• Up-front costs of AIP systems in dollar amounts and as a portion of the total cost of the sub;
• Lifecycle cost of operating and maintaining the AIP system, at a representative operational-tempo.

In the final analysis, any AIP system will need to be subjected to some representative and tough testing before it even makes a shortlist (which may rule out the Russian and Spanish technologies which are only at a developmental stage). This is because AIP is becoming a mission critical single point of failure for the submarine in a combat environment. If the AIP system fails for whatever reason while the submarine is operating in a contested area, it may not have the option of snorkelling home.

The strength of AIP includes it being very quiet. More than two weeks submerged with a submarine immobile or moving slowly. AIP extends operating range but cannot drive the submarine at anything over low speed for medium to long ranges. Can AIP be seen as a form of backup if specific portions of the diesel-electric drive-train fail?

AIP Technologies

These are arranged by what I consider the likelihood that Australia might select a foreign company and/or submarine which utilises that technology.

Hydrogen-oxygen fuel cell system. (Diagram courtesy of )

See Type 212/214 Fuel cell PEM AIP on bottom submarine (diagram courtesy of this site)

Fuel cell systems – German PEM

Fuel cell - PEM AIP is likely if Germany’s TKMS is selected to build Australia’s Future Submarine.

Operation - Fuel cell based AIP systems typically employ a hydrogen oxygen fuel cell to generate electrical current, which then powers the boat’s systems. The principal issue in operating any fuel cell based system is the manner in which the oxygen and hydrogen are stored (physically and chemically) prior to introduction into the fuel cell. The available technology is the Polymer Electrolyte Membrane (PEM) cell used in the Siemens SINAVY AIP fuel cell modules.

The oxygen supply is stored as LOX in the PEM system. The hydrogen propellant supply in the PEM system is described as via “reformer gas” in some documents, or solid metal hydride in others. The former scheme typically involves the decomposition of a hydrocarbon fuel to generate hydrogen. The fuel cell can operate by recharging the subs batteries or in the case of the PEM directly feed the electric motor. The fuel cell produces distilled water as a waste product.

Strengths - A virtual absence of moving parts, except small pumps, which makes this AIP exceptionally quiet in terms of machinery noise and cheaper and easier to maintain compared to other AIP technologies. The PEM fuel cell only operates at 80 ° C reducing heat signature and cooling problems. PEM is relatively efficient 70% of energy (on what measure? Of LOX?), because the fuel cell directly feeding the electric motor. PEM has been successfully incorporated into many German Navy and export submarines.

Weaknesses - Fuel cells have a relatively high up front cost. Fuel cells are relatively difficult and expensive to retrofit or cannot be retrofitted? Does PEM AIP generally produce less average power than Stirling or MESMA? Is there less “dash speed” in an emergency? The process of storing and replacing on board hydrogen and LOX involves significant safety hazards.

More Details - Fuel cells are employed in the German HDW built Type 212 (with nine 30-50 kW fuel cell units) and two 120 kW units are fitted on Dolphin class, on internationally marketed Type 214 subs and on some Type 209 mod subs.

It is unknown whether the two Type 218SG subs being built in Germany will utilise fuel cell PEM or Stirling engine AIP. If it is Stirling (an AIP Singapore is already familiar with in the two Archer class) then that might be one reason that the new submarines have been called "218" rather than "214".

Methanol defragmentor is the latest AIP being considered - German, France and Spain may be ahead in this.

Indian DRDO Fuel Cell - A March 2014 report indicated Naval Group (was DCNS) might install a DRDO developed AIP based on hydrogen fuel cell technology on one [or more] of the six Scorpenes beinge built at Mazagaon Dock under Indian Navy Project 75. This was in preference to an older version of French MESMA being offered for the Project 75 Scorpenes. The DRDO fuel cell might be also be built into the second line of Indian future conventional submarines under a different project, known as P75I.

Now Saab and FMV's Stirling AIP system.

Stirling-cycle heat engine with external combustion

 Stirling Engine

Stirling AIP is another strong contender for Australia’s Future Submarine if Japan’s Soryu drive-train or a direct purchase from Sweden (Saab) is selected.

In the Stirling cycle, heat from an outside source is transferred to an enclosed quantity of working fluid - generally an inert gas - and drives it through a repeating sequence of thermodynamic changes. By expanding the gas against a piston and then drawing it into a separate cooling chamber for subsequent compression, the heat from external combustion can be converted to mechanical work and then, in turn, to electricity. Like MESMA, this approach has an advantage over internal combustion systems, such as the closed cycle diesel (below) in that the combustion processes can be kept separate from those that actually convert heat to mechanical work. This provides significant flexibility in dealing with exhaust products and controlling noise.

The Swedish developed Stirling system employs LOX as the oxidiser and diesel as the fuel, which are combusted at a pressure higher than that of the surrounding water mass permitting the exhaust to be directly vented to sea. The Stirling engine is coupled to a generator that feeds into the boats’ primary electrical system.

Strengths - Stirling engine are relatively simple and less hazardess in that they just use diesel and oxygen with Stirling engines running on liquid oxygen and diesel oil to turn a generator to produce electricity to charge the sub's batteries. Stirling technology has been widely used by several navies including Sweden’s (current Södermanland and Gotland class), Singapore’s (two Archer class), Japan’s (Soryu class) and some Chinese PLAN SSKs.

Weaknesses – Its moving parts can contribute to noise. The Stirling engines operate at a pressure of 20 bars, which limits the submarine’s depth capability to 200 meters, unless an exhaust gas pressure intensifier mechanism is used. As with other AIP systems that burn LOX and diesel, the LOX supply is the principal constraint to achievable endurance.

More Details - On Sweden’s Gotland class subs there are two 75 kW Stirling engines for propulsion or charging batteries. The endurance of the 1,500-tonne boats is around 14 days at 5 knots (5.8 mph; 9.3 km/h). Japan’s Soryus (of the size and tonnage expected for Australia’s Future Submarine) utilise four 75 kW Kawasaki Kockums V4-275R Stirling engines

MESMA closed-cycle steam turbine

Closed cycle steam turbine - French "MESMA"

French "MESMA" closed cycle steam turbine systems could be compared to nuclear systems, in that heat is used to generate steam, which via a turbine or turbo generator charges the batteries that power the electric motor. Stored oxygen allows the fuel to burn creating the heat.

Naval Group (was DCNS) in France offer the MESMA (Module d’Energie Sous-Marine Autonome) system in lengthened Agosta or Scorpene class subs. Fitting MESMA requirers the insertion of a hull section about 8.3 metre long, weighing around 305 tonnes. The MESMA system burns ethanol, using stored liquid oxygen LOX as the oxidiser. The propellant mix is burned at 60 atmospheres at up to 700° C, which implies a need for seawater cooling. Naval Group claims MESMA triples the submerged endurance of non-MESMA subs.

Strengths – Relatively high output power is available. MESMA AIP permits faster reaction times for more rapid acceleration than Stirling of Fuel Cell AIP. Can more rapidly The design permits relatively easy? retrofitting into existing submarines by adding an extra hull section-plug.

Weaknesses - MESMA generates more detectable heat (and probably more carbon dioxide and monoxide) than Fuel cells and Stirling AIP. The hazards (like Fuel cells and Stirling) of storing and handling the liquid oxygen (LOX) and ethanol. While the MESMA may provide higher power output, its net efficiency might be the lowest (estimated at 25%) as its rate of oxygen consumption is higher. MESMA has significant moving parts, which may radiate detectable noise. Ultimately, the maintenance and crew training requirements of the MESMA steam turbine system are significant - adding to cost. The burning process yields exhaust carbon dioxide which needs to be expelled behind the sub at any depth perhaps making it vulnerable to advanced airborne and ASW ship sniffing sensors?

Offered by the French Naval Group. MESMA has a relatively small customer base having only been retrofitted into some Pakistani Agosta class subs.

See a French Naval Group announcement of a "second generation AIP" (more fuel cell type - not really a MESMA development) at Naval Group's new submerged SSK solution - Lithium-ion battery and 2 AIPs of November 6, 2014.

The Spanish S-80's AIP - known (in Spanish) as Propulsion Independiente de aire. 

Spanish Bio-ethanol (or maybe Methanol) (Closed cycle steam turbine?)

Though Spain’s Bio-ethanol AIP technology is not fully tested and of course hasn’t been used in practice Australia has a counter-intuitive purchasing history of untried military technology. This would be in the context of Australia selecting the not fully tested or even deployed Navantia S-80 Isaac Peral class submarine.

The S-80 AIP technology under development is purportedly “completely different” from MESMA. The S-80's AIP system is based on a bioethanol-processor consisting of a reaction chamber and several intermediate Coprox reactors, that will transform the BioEtOH into high purity hydrogen. The output feeds a series of fuel cells. The Reformator is fed with bioethanol as fuel, and oxygen (stored as LOX) generating hydrogen and carbon dioxide as subproducts. The produced hydrogen and more oxygen is fed to the fuel cells.

The bioethanol-processor also produces a stream of highly concentrated carbon dioxide and other trace gases that are not burned completely during combustion. This gas flow is mixed with sea water in one or more ejector venturi scrubber and then through a CO2 Removal System and whose purpose is to dissolve the "bubbles" of CO2 in water to undetectable levels.

The oxygen and fuel flow rates are directly determined by the demand for power. The AIP power in the S-80 submarine is at least 300 kW. A permanent-magnet electric motor moves a fixed propeller of a special design, that doesn't create cavitations at high speed.

Weakness – Sounds highly complex. Cost unknown and cannot currently be estimated. Undeveloped. Unused even by the Spanish Navy. This problematic formula of claims may increase the chances of Australia selecting it.

Closed-cycle diesel engine (Diagram courtesy of )

Closed cycle diesel (CCD) systems

Australia almost definitely won’t buy this AIP system. indicates “CCD systems have been developed by a number of firms in Germany, Britain, the Netherlands, and a few other countries. However, except for a 300-horsepower demonstration system refitted onto the German Navy's ex-U 1 in [1992-] 1993, no modern CCD systems have entered naval service. England's Marconi Marine recently acquired CCD pioneer Carlton Deep Sea Systems and is marketing a CCD retrofit package for existing conventional submarines, such as South Korea's nine Type 209s. Although one key advantage of CCD systems is their relatively easy backfit into existing submarine engineering plants, there have been no takers. Despite the additional supply complication of needing regular replenishment of cryogenic oxygen and inert gas, there are logistics advantages in retaining standard diesel engines and using normal diesel fuel.”

Russian Kristall-27E AIP as it may appear on a yet to be developed Russian Amur-Lada-Kalina conventional submarine. (Sourced from here

Russian Kristall-27E AIP

Russia hopes that it could develop the 4th generation Lada-Amur class have been discontinued because Russia has been unable to develop satisfactory air independent propulsion (AIP). This makes prospects for an Amur class from dim to nil.

January 2016 reports from Russia are that Russian AIP (essential to market Lada or Amurs) has not been developed hence additional Ladas (after L2 and L3) and any export Amurs will no longer be built. Prospective Amur customers have probably been offered the existing Improved Kilo (636) class instead. This is assuming customers (other than China) would not accept a Russian offer of a future "fifth generation" Kalina class SSK - with the Kalinas only likely to be operational (with AIP) in the Russian and maybe Chinese Navy in the mid 2020s.

Russia is far behind Western countries in fully developing and actually deploying AIP systems. Russia has been talking about its untested Kristall-27E AIP for at least 12 years. 

Kristall is described as a system with alkali matrix electrolyte, intermetallid storage of hydrogen, cryogenic storage of oxygen and a low-temperature electrochemical generator.  See Kristall-27E AIP described in great detail here and here.

Strengths – None verifiable as yet.

Weaknesses – Its Russian - unpopular with the Australian military. All of this Russian originated description appears to be marketing claims in place of full development and deployment. It appears that Russia is not developing AIP for its own submarine corps but instead may develop it if foreign customers places orders for AIP submarines. The risk appears to lie with customers during a lengthy development phase. This may explain why customers have restricted their purchases to non-AIP Kilos - that are less expensive and less technically risky than AIP Amurs-Ladas. It will be interesting if the Kalinas sell - though we are talking years of development.

Nuclear reactor ("AIP") on submarine (Source - Excellent explanation of nuclear technology)

Nuclear Propulsion

While politically unpopular with Australia’s previous (Labor) Government nuclear power is the ultimate AIP as it presents no restrictions on submerged time, range or operation at a sub’s maximum speed.

Arguments like “we couldn’t train Australian nuclear technicians-engineers (or hire US-UK personnel) within 20 years to operate or maintain submarine reactors” appear to be poorly argued, but current, conventional wisdom.

For the usual safety reasons such Australian SSNs could be based at Fleet Base West, Rockingham, Western Australia which US SSGNs and SSNs already regularly visit.


Using any of the AIP technologies would involve lengthy selection processes. Extensive foreign and local training would be required for the technicians, engineers, officers, crew and officials required to maintain, use, administer and repair these technologies. All this takes years and in many cases decades. AIP technologies need to interact with the complete submarine weapons system - so can't only be seen or assessed in isolation.

Sources Used Include

Edward C. Whitman’s article 

The Submarine Research Center reference on AIP and U31 at



A said...

Hi Pete,
As usual very informative article Pete.Regarding ur comment about that the MESMA being built into the Indian Navy last 3 Scorpène -class its a verly unlikely scenario. DRDO-NMRL have developed a Fuel-Cell based AIP System for Indian Navy that could go to the Scorpéne though.
Regarding Australian Navy's dilemma for AIP,my personal choice will be a SSN. That would serve RAN's very nicely considering the Area Of Opertaion they have to cover.

Pete said...

Hi A

Thanks for the advice.

I've deleted mention of the Indian Scorpenes from the MESMA section and added mention under the Fuel Cell section accordingly.



Pete said...

As at November 2017 the main AIP for Submarine manufacturers iclude:

- SAAB AB (Sweden)
- Siemens AG (Germany)
- DCNS SA [now Naval Group?] (France)
- China Shipbuilding Industry Co (China), and
- UTC Aerospace Systems (US)

Pete said...

Interesting comparison of AIP technologies by Robert Farlay, The National Interest, January 28, 2018 at Notable comment that:

"The United States is a global leader in the development of fuel-cell technology, so it is likely that American sub builders would go that route."

"...Before investing in AIP boats, the U.S. Navy should also take care to rigorously game out future submarine warfare scenarios that involve Undersea Unmanned Vehicles. Autonomous and semi-autonomous drone submarines potentially have many of the advantages of AIP boats, without requiring investment in new submarine designs."