For the aspiring navies of certain developing nations that cannot afford, either politically or economically, the luxury of nuclear-powered submarines, air-independent propulsion (AIP) seems to be presenting an affordable alternative. Andres de Lionis weighs up the concept's pros and cons.
A few years ago the world of naval operations heard that Pakistan was to procure three advanced Agosta-90B class submarines from France. There were many remarkable elements within this news, including the fact that these boats will be the first exported units to be equipped with SM-39 anti-ship missiles and that Pakistan would be the first Third World country to operate submarine-launched missiles. Most notable, however, is that the third boat will be the first in the world to be fitted with air- independent propulsion (AIP) of the MESMA (Module de Energie Sous-Marine Autonome) class and the first AIP boat to be operated by a Third World nation. With the first boat delivered from France and the second one to be assembled locally in the new shipyard of Port Qasim (which is being built near Karachi), the last of the class will be built in Pakistan under license with French technical supervision. It is due to be laid down in 1999 and commissioned in 2002. The AIP module is intended to be retrofitted to the first two boats in due course.
Rival reaction
India protested strongly against the Pakistani deal and then responded with what could turn out to be a new arms race in the region: two Improved 'Kilo' class submarines (Type 636) have already been ordered from Russia and another two will probably be built in India under license. In addition, a refitting of India's current eight 'Kilo' class boats (employing the Type 877EM combat system) is being cautiously considered.
It is also possible that production under license of the German-designed Shishumar class (Type 1500) will be continued, although previous experiences with this project have just proved expensive and only partially satisfactory, highlighting as they did the problems related to indigenous shipyards. However, India's first naval priority is the development of a nuclear-powered submarine called the ATV (Advanced Technology Vessel), which is being developed with Russian technical assistance. This vessel currently has a projected in-service date of 2004.
Why AIP appeals
Many navies have high expectations of the new operational performance offered by AIP, even if there are many problems yet to be fully solved. The concept is considered attractive to many navies because it is perceived - rightly or wrongly - as an 'almost-nuclear' solution. Unlike the real nuclear solution, however, it is seen as politically, technologically and economically feasible. A country can therefore look to field a submarine with an underwater endurance far in excess of the average diesel-electric submarine (SSK) without having to face the political and technical difficulties associated with running a nuclear-powered submarine program.
The MESMA concept
The MESMA solution, chosen for the DCN- designed boats to be built for Pakistan, is perhaps the one most closely related to the very first AIP solution developed by Germany during the Second World War for the revolutionary Type XXVI boats. The German solution used Walter turbines with hydrogen peroxide and diesel fuel and provided high speeds (up to 25kts for six hours' endurance) that are still inconceivable for today's non-nuclear submarines. The MESMA system uses steam turbo-generators heated by ethanol and oxygen combustion. The first shore test plant was due to start work last year.
Other European solutions
Sweden has selected Stirling-cycle engines using diesel fuel, oxygen and helium for its AIP design. The resulting powerplant offers inherent low noise levels due to the fact that there is no explosion inside the engine but external combustion, with heated inert gas (helium) used as the working gas.
German U-212 type boats will be produced for Germany and Italy with fuel-cell AIP modules. Oxygen and hydrogen (the latter in the form of metallic hydrures) produce electric power through the electrolytic process. This system is also inherently discrete, and the power produced can be merged with that gained from normal batteries to supply surges of power.
Russian solutions
Closed-circuit diesels are the other AIP solution. The Soviet Navy deployed an entire class of over 30 submarines with closed-circuit diesel engines (Type A615, also known as the Quebec class) in the 1950s. These served until the mid-1970s despite a number of serious problems with oxygen leaks causing fires and explosions that led to extensive damage, casualties and even the loss of some boats. Furthermore, the closed-circuit diesels were rather noisy, reducing the potential benefits of such a solution.
Currently in Russia, the Rubin Design Bureau is cautiously offering a modified version of the Type 636 (Improved 'Kilo') with its hull lengthened by a 10-12 m section. Another submarine possibly fitted with AIP is the newest Amur (Type 1450) currently being built at Admiralty Yards in St Petersburg. In this case the AIP is contained in a plug-in section some 12m long. In both of the above cases fuel cells were the selected AIP solution. According to drawings released by the designers, Rubin has opted to install the liquid oxygen (LOX) storage tank in a vertically mounted cylinder while using horizontally mounted hydrogen accumulators and fuel cell modules, all placed inside the pressure hull (unlike the German solution).
Russian experience with AIP was probably gathered from the Beluga (Type 776A) experimental submarine during the late 1980s/early 1990s. The approach of Russian shipbuilders to AIP is very cautious and thus in contrast with the aggressive marketing of other European shipyards. Rubin is stressing that, if required by the customer, the solution is technically available, yet the bureau is discouraging the use of AIP, holding it to be a potential source of trouble greater than the benefits gathered. It is hard to say whether this attitude is based on economic considerations (i.e., it is better if the customer orders more traditionally designed boats instead of a smaller number with AIP modules) or on lack of trust or reliability of the fuel cells already tested. More simple tactical and technical considerations concerning the Third World may also have shaped this attitude.
The AIP market takes off
Meanwhile, Italian and German builders of midget submarines and underwater work craft are already marketing AIP variants of their products. Italian firms Maritalia and Cosmos have produced versions of their midget submarines powered by closed-circuit diesels fed with LOX. Bruker of Germany has built Seahorse-II KD, a small AIP submarine officially earmarked for underwater work and offshore applications. This has an argon closed-circuit diesel engine. Seahorse-I, a standard diesel-electric boat, was delivered under the same official cover to North Korea via a commercial firm in the Far East. German authorities stopped delivery of the AIP version after strong international pressure.
Reliability required
All the possible systems adopted for AIP require the onboard presence of highly aggressive LOX or highly concentrated peroxide, both of which are inherently dangerous. Moving parts (such as in closed-circuit-diesels or MESMA plants) and the presence of lubricants can easily create the conditions for dangerous accidental reactions. The LOX tanks and pipes need to be highly reliable to accommodate the low temperature (-180C) required for LOX storage. In some cases the designers have preferred to put the LOX and the other reactant tanks outside the pressure hull for safety reasons in case of minor losses. It goes without saying that in these cases the tanks are exposed to a more aggressive environment and that the shock induced by a nearby explosion (depth charges, for example) could provoke creeks and ruptures, leading to the likely loss of the boat.
Careful maintenance is strongly required for all components of an AIP plant to avoid the dangers of fire, explosions and even increased noise due to additional moving parts. Thermal and chemical stressed items have to be checked frequently (including the storage tanks, pipes, reaction sections and heat exchangers). It is difficult to see how the navies of most developing countries would be able to face this challenge.
License-building's poor track record
It is perhaps useful to remember that the licensed production of submarines by Third World nations with German assistance faced frequent failure, despite the supervision and technical packages provided by German shipyards. India, which undoubtedly has a significant industrial base compared to most other emerging countries, had to bring the local construction of four Type 1500 boats to an end after just two units were built due to increasing costs and delays. Brazil had a program to locally build four Type
1400 boats, but this was terminated after two boats, then resurrected for a third and last-vessel after some delay. In 1977 Argentina signed a contract to build four TR 1700 boats; in 1996, with the first unit still only half way to completion, the shipyard was sold and the incomplete hull scrapped.
Similarly, the technology package given by Italy's Cosmos to the Pakistani Navy to maintain its MG-110 midget submarines and possibly start indigenous production under license without Italian supervision was also a total failure for lack of a credible industrial base. It remains to be seen if the program of building the first Agosta-90 B boat in France, assembling the second in Pakistan with the third being built completely in Pakistan - with the additional challenge of AIP - will be successful.
The tactical pros and cons
What is most appealing in the use of AIP from a tactical point of view is the possibility of extending the submerged range of a submarine for covert operations without the need to surface or snorkel to recharge batteries. In this case there are indications that covert duration of up to 15 days at low speed are possible. By the same token the use of AIP does not allow higher speed and unlimited endurance as with nuclear powered boats, making the tactical use of AIP boats very similar to that of standard SSKs. The most an AIP section can achieve is to allow batteries to be saved to keep their power untouched for possible surges of higher speed.
At any rate, to maintain fully discrete operations an AIP submarine should be able to detect and classify targets without exposing its periscope or other sensors, which seems an operational capability still beyond the reach of most developing countries.
The problems related to the onboard storage of LOX and other chemicals (mainly hydrogen and metallic hydrures) also applies to shore facilities, including cryogenic production sites, storage sites, charging facilities and transportation lines.
These are all sensitive items, needing careful design, maintenance, operation and security. Whether all this is possible in countries where the industrial base is often still strongly hampered is yet to be proven.
A future market
Theoretically, and according to some shipbuilders, AIP kits are now available for retrofit in a great number of existing submarines in the form of 'simple' plug-in sections. The navies of Germany and Sweden have, in fact, evaluated these new solutions, having installed AIP modules in SSKs and operated them for some years. Although this could sound appealing to many ambitious medium-sized navies of emerging countries, it is hard to define the AIP solution as a total gain when one has to compare the tactical advantages with the increased cost and technical risk.
For the time being, naval regional power balances will probably be more influenced by the procurement of conventional SSKs with advanced combat systems than by AIP-powered boats. Technological improvements in the future, however, along with the growth of developing nations' industrial bases, could present a different picture.