Air Independent Propulsion (AIP) technology has arguably been the holy grail of submarine design ever since German engineer Hellmuth Walter first began experimenting with his high purity hydrogen peroxide powered engine in the early 1930s.
Although this avenue of research was to largely dominate AIP until well into the Cold War era, its technical limitations eventually saw it abandoned in favour of other approaches, and less volatile fuels, but not before it had comprehensively established the desirability of the whole concept.
Today, there are four basic technologies - closed cycle diesel engine (thought the last submarine to use this system was scrapped in the 1970s), closed cycle steam turbines, Stirling engines and fuel cells - and each has its own peculiarities.
Close cycle steam turbines, such as the French 'Module d'Energie Sous-Marine Autonome' (MESMA) system, offer the highest short term power output of any AIP, but have the lowest efficiency and highest fuel consumption.
The typical beta-type Sirling engine, as used on the Swedish Gotland class submarines for instance, is simple and quiet, both of which are plusses for submarine use, but is large relative to its power output - which clearly is not.
Polymer electrolyte membrane (PEM) fuel cells, like the ones fitted to Howaldtswerke-Deutsche Werft's revolutionary Class 212A, have the greatest potential for extending submerged endurance and their primary exhaust product is water, but in their current form, their output is low.
Clearly, no AIP technology currently has all the answers, but the appeal is obvious. Allowing a submarine to remain submerged for long periods, without the need to surface, brings increased range and improved underwater endurance, without the high cost of nuclear propulsion, while retaining the advantages of conventional diesel electric power.
In addition, the size of AIP boats allows them to operate effectively in littoral waters, where access is often a problem for their nuclear counterparts, making the technology particularly attractive to smaller regional navies and nations which lack the expertise, the budget, or the desire, to pursue the nuclear-powered route.
Such technology will not allow a submarine to remain underwater indefinitely, but it does represent a significant enhancement to its operational capability in comparison with a standard diesel-electric power plant.
Unsurprisingly, in May, the ICD Research report 'The Global Submarine Market 2011-2021' concluded that mature AIP systems are increasingly seen as a 'must-have' capability, especially since the varying mission profiles of modern deployments now prioritise submarines as multirole platforms.
A growing tactical advantage
AIP is, consequently, a developing field. Back in 2009 US Submarine Veteran, David Schueler, warned in a presentation to the USSVI's Seattle Base, that although it primarily offered a low-speed, long-endurance adjunct to underwater performance, it would be unwise to underestimate the technology or ignore the tactical threat it posed. Two years on, he observes that while currently the primary role of AIP systems still remains to extend submerged operating time and supplement battery charging capability, their potential advantage is growing.
"There continue to be advancements in AIP technology," he says, "but so far none of the technologies seem powerful enough to run a submarine on its own. I expect this to remain the case for at least the next five to ten years. That said, even the currently available systems make diesel-electric submarines much more dangerous than in the past. Being able to extend the time on station and time between using the diesel are important advances." Moreover, enhanced endurance and the attendant increased survivability it confers, has led some to express concerns regarding the uptake of this technology by states with nuclear arsenals, in case it leads to the mounting of these warheads on submarine-borne delivery systems.
Six years ago, the James Martin Center for Nonproliferation Studies publication 'NIS Export Control Observer', highlighted the dangers of the burgeoning number of submarines able to launch cruise missiles appearing on defence inventories in Asia and the Middle East.
Today, as India and Pakistan explore fitting AIP upgrades to their fleets, and Israel allegedly deploys nuclear-armed cruise missiles aboard its German-built Dolphin-class submarines, those warnings seem somewhat prophetic.
Putting aside this potential boost to regional arms races, the growing global interest in AIP is beginning to have a positive effect on technology development, as Schueler explains, "I think one change is the increase in the number of countries that are seriously looking to purchase or build AIP equipped submarines, especially in Asia. The competition between companies - and countries - to sell to this market should help drive innovation."
He remains unsure, however, just how the current woes of the world economic situation, and the knock-on effect on military budgets, may affect the number of states that will actually buy into the technology.
Nonetheless, despite conceding defence cuts will inevitably impede the growth of the world submarine market, May's ICD Research report currently values it at $16.4 billion annually, and predicts a near 11% rise - to $18.2 billion - by 2021. Significantly, in this forecast, Asia will account for a cumulative value of $44 billion over the period, which represents 23.6% of the total market worth.
Advances in AIP notwithstanding, nuclear-powered submarines are expected to account for the largest share of the overall market - some $87.4 billion over the next ten years - according ICD Research, presumably at least in part because a nuclear sub is second only to an aircraft carrier as a statement of national prestige.
Submarine reactors need to be robust and resilient enough to withstand the rigours of decades at sea. The physical stresses of onboard operations, coupled with the potential for rapidly changing power demands, and the innately harsh conditions within the reactor plant itself, combine to produce a uniquely challenging set of circumstances for nuclear propulsion.
It requires what the Federation of American Scientists' Military Analysis Network describes as "an active, thorough and far-sighted technology effort to verify reactor operations and enhance the reliability of operating plants, as well as to ensure naval nuclear propulsion technology provides the best options for future needs."
Currently, achieving that often involves some level of technology transfer. Brazil's first nuclear-powered submarine, for instance, will use French technology, and the Royal Navy's newly announced, safer PWR3 reactor combines US design with British reactor expertise. However, other nations - most recently Argentina - have expressed ambitions to equip vessels independently with home-grown nuclear propulsion. How practical this is remains to be seen, but against a growing global backdrop of regional rivalries and territorial security concerns, the logic is clear.
Although the absolute number of active submarines has fallen since the end of the Cold War, there are more navies operating them than ever before, and for different reasons. With propulsion a key driver on submarine innovation, the growing interest in AIP, collaborative agreements and, ultimately, the rise of domestic nuclear-power programmes, are trends that seem set to continue.