The AIP Alternative
Air-Independent Propulsion: An Idea Whose Time Has Come?
By DON WALSH
Don Walsh served 24 years in the Navy, during which time he was involved in many aspects of Navy oceanographic activity. In 1975, he founded and chaired the Institute for Marine and Coastal Studies at the University of Southern California. He left that post in 1983 to devote full time to International Maritime Inc., which he founded in 1975 and still heads.
For nearly 100 years a primary goal for ship designers has been to increase the range and submerged-time capabilities of submarines. It was an elusive target until the introduction of nuclear propulsion in the mid-1950s. But nuclear propulsion was, and is, too costly for all but five of the world's major navies.
For the other 30 or so navies in the world diesel-electric boats would remain their only viable option. But even the most modern of today's diesel boats are only marginally better (in submerged-time and range) than the submarines of World Wars I and II. Development of the first practical air-independent propulsion (AIP) systems for diesel submarines, however, promises much greater improvements over the next 15*20 years.
The operational demands of World Wars I and II led to a major expansion of most of the submarine fleets of the warring powers. This led in turn to greatly increased investments in technological development. Even after World War II, designers developed a variety of enhancements for diesel-electric submarines--in streamlining and noise quieting, for example, in reducing manpower requirements in the design and production of more powerful batteries, and in snorkel improvements (for extended submerged range). Almost all of those, and other, capability improvements were incremental, though, and rather modest in scale.
Technical and Safety Problems
The development of air-independent propulsion systems actually began during World War II, when the Soviet Union and Germany developed AIP systems for their submarines. The Soviet-designed AIP system used liquid oxygen and diesel fuel to operate a closed-cycle diesel (CCD) engine that was installed in the submarine M-401 for an experiment that lasted from 1940 to 1945.
In Germany, Professor Hellmuth Walter, an engineer, developed an AIP system that used highly concentrated hydrogen peroxide to produce steam for a turbine-driven submarine. Towards the end of World War II the system was installed in the newly developed Type XXVI U-boat. As with the Soviet system, the Walter system was plagued by numerous technical and safety problems. Safe handling of the highly unstable peroxide in the closed space of a submarine proved to be simply too difficult and the Type XXVI U-boats never saw combat. Moreover, because it was so late in the war there was neither enough time nor enough resources to convert the Type XXVIs into effective combat units.
After World War II the Americans, British, and Soviets all obtained access to Walter's work and attempted to extend it to a safe conclusion. In the United States, the Navy's Engineering Experimental Station in Annapolis, Md., did extensive testing of a Walter Cycle AIP system. Eventually a reduced-size system was installed in the small experimental submarine X-1. However, by the mid-1950s the U.S. Navy had terminated this work. Nuclear-propulsion systems were being developed and the potential value of AIP-powered diesel submarines seemed to be no longer important.
In Britain the Royal Navy (RN) installed a Walter Cycle plant in HMS Excalibur to test the system under actual seagoing conditions. The results were not encouraging. In fact, the submarine was often referred to as "HMS Exploder." The experiments were stopped when the Royal Navy also shifted to nuclear submarines.
More Problems Than Progress
The Soviets continued AIP development for 15 years after World War II. Using data generated from their work on WWII closed-cycle diesel AIP systems, they built 30 Quebec-class submarines (from 1953 to 1957). They gained considerable operational experience with AIP, but the submarines--which ran on liquid oxygen and diesel fuel--were not satisfactory in fleet service. There were explosions, fires, and even the loss of some submarines. Russian submariners grimly called the Quebecs "cigarette lighters." AIP development was terminated in the mid-1970s, and the remaining Quebecs were scrapped. They had achieved much greater submerged endurance and range, but those gains were cancelled out by the unsafe nature of their AIP systems.
Meanwhile, the Soviets had also (in 1952) built an experimental Walter Cycle submarine designated Design Project 617, which entered service in 1958. An onboard explosion put an end to the program in 1959. From then on the Soviets also focused on nuclear propulsion--but they did carry out some further AIP research and development (R&D) for the diesel submarines they continued to build.
The CCD engines and the Walter steam turbines represented sound theoretical approaches to AIP. Increases up to 400 percent in submerged time and/or range were possible in the better systems; however, they still could not be made sufficiently safe for routine fleet operations. Nuclear power seemed not only the best but also the final answer to the submariners' dream of virtually unlimited submerged duration. Because it was such an expensive dream, though, nuclear propulsion was limited to only a handful of navies. Diesel boats were the only other choice available to less affluent navies with sizable submarine fleets. But many of those navies hoped for an affordable AIP system to be developed some day.
The problem was that only the major navies could afford the R&D needed in this area--and most of those navies had dropped AIP work in favor of nuclear propulsion. Eventually, though, submarine design groups in Germany, Sweden, and France resumed their work on AIP systems, following four different technical approaches: fuel cell, closed-cycle diesel, Stirling cycle engine, and steam turbo-electric.
European Advances in AIP
The Swedish Navy became the first to put AIP systems into its fleet operating units. The Kockums-built AIP system was first tested on the refurbished submarine Näcken in 1989. Today, three Gotland-class subs (Gotland, Uppland, and Halland) are fitted with Swedish Stirling cycle engines, which use liquid oxygen and diesel oil. The Gotlands are powered by hybrid diesel-electric propulsion units, with the Stirling engine supplementing the conventional diesel-electric system. The Stirling engine turns a generator that produces electricity for propulsion and/or to charge the vessel's batteries.
The Gotland was delivered in 1996. Submerged endurance (without snorkeling) for the 1,500-ton submarine is 14 days at five knots. A crew of five officers and 28 enlisted personnel is required to operate the submarine. Kockums now offers the similar T-96 submarine for export. The "unit cost" of the T-96 is about $100 million.
Some of today's most advanced AIP developmental work is being carried out by the German Submarine Consortium (GSC). This group consists of two shipyards--the Howaltswerke-Deutsche Werft (HDW, in Kiel) and the Thyssen Nordsee Werke (TNSW, in Emden)--plus the IKL design bureau and the Ferrostaal trading company. Over the past 30 years the two shipyards have delivered 122 submarines to 16 navies either as new construction or as "kits" for local production.
For the past 15 years both shipyards have been working on parallel development of two different AIP systems. HDW offers a fuel cell (developed with Siemens Electric), while TNSW is marketing a closed-cycle diesel engine. After extensive prototype testing ashore, both systems were sea-tested in 1988*1990 on the U-1, a former German Navy Type 205 diesel-electric submarine.
The HDW fuel cell is scheduled to enter fleet service in 2003 on GSC's new 1,800-ton 212-class submarines. This AIP system also will be a "hybrid," with the submarine retaining a basic diesel-electric propulsion system. A fuel cell cannot deliver sufficient electrical output for high-speed operations, but the conventional storage battery can (for a short period of time, after which the fuel cell can recharge the battery as well as provide energy for low-speed operations).
Artificial Air But Tangible Improvements
HDW estimates that the 212, with its crew of 27, will be able to remain submerged for more than a month and to cruise (at four knots) for over 3,000 miles. Four of the $250-million submarines will be delivered to the German Navy--two built by HDW and two built by TNSW. Two also are being built for the Italian Navy under license at Italy's Fincantieri Shipyard.
GSC recently announced the availability of the 214 class, an improved version of the 212 with greater diving depth (more than 1,400 feet), a newer dual-fuel-cell design, and a slightly larger crew of 30 officers and men. It has been reported that Greece intends to order three of the 214s.
Thyssen Nordseewerke's closed-cycle diesel system uses liquid oxygen, diesel fuel, and argon gas to fuel its AIP system. The oxygen and argon gases are combined to make "artificial air" for the diesel. Argon, an inert gas, is recovered and continuously reused. The same diesel is used as a conventional air-breathing engine for main propulsion on the surface or when snorkeling. TNSW's CCD AIP system is considered to be particularly cost-effective for the retrofit of existing diesel-electric submarines, but it also can be installed in a new-construction boat.
Both HDW and TNSW estimate that the AIP option will add only about 15 percent to the overall cost of a newbuild submarine. To get that much added performance for such a small addition in cost is considered quite a bargain. It also appears that most AIP systems will require, on average, the addition of a hull section approximately 30 feet long.
In France the DCN International naval shipbuilding company has developed the "MESMA" (Module d'Energie Sous-Marine Autonome) AIP steam-turbine system, which basically burns ethanol and liquid oxygen to make the steam needed to drive a turbo-electric generator. DCNI offers the MESMA option for its Agosta 90B and Scorpene classes of submarines. The company claims that its AIP option increases submarine underwater en-durance "by a factor of 3 to 5." The design of the MESMA system permits it to be retrofitted into many existing submarines simply by adding an extra hull section.
Pakistan has bought three Agosta-class submarines, the first of which was commissioned earlier this year. The third one, expected to be built in Pakistan, will be fitted with the MESMA AIP system and thus in all likelihood become the world's first MESMA-powered submarine.
Outlook for the Future
In addition to the builders of the four Swedish submarines and the GSC and DCNI boats, there are other "players" who have done considerable R&D work on AIP systems. Russia is offering a fuel-cell option for its "improved" Kilo- and Amur-class attack submarines. None have yet been built with an AIP system, but reports suggest that China may add an AIP unit to one of its Project 636 Kilos.
The Netherlands' RDM submarine shipyard offers its "Spectre" CCD option for the yard's 1,800-ton Moray 1800 H submarine; none have been built yet, but RDM estimates that a hybrid-powered Moray could remain submerged for 20 days while cruising at two knots. Negotiations started earlier this year to build an AIP Moray for Egypt, but as of early November there had been no firm commitment. The average cost of a Moray is estimated to be about $250 million.
The Japanese Maritime Self-Defense Agency has undertaken studies to add AIP systems to its latest models of diesel-electric submarines. The leading candidate systems are the Swedish Stirling engine and the German HDW fuel cell.
It is estimated that 100*150 diesel submarines will be purchased in the next 10 years. Naval experts--and shipbuilders--throughout the world are closely monitoring the operations of the Swedish Navy's four AIP submarines and eagerly await the first GSC-Type 212 submarine. By 2005 there should be sufficient fleet operating experience to determine what are the most likely operational and cost benefits that can be derived from shifting to AIP systems. By then the unit cost for a modern diesel-electric submarine should be between $200 million and $300 million. Paying only 15 percent more to add or retrofit an AIP unit--a relatively small cost for greatly improved submerged performance--should be a very attractive option, therefore.
AIP submarines could be a particularly formidable threat when operating in coastal waters, marginal ice zones, or maritime straits and other global "choke points." Add to that the virtual certainty that new underwater weapons will help equalize the performance disparity between AIP boats and nuclear-powered submarines and it may well happen that the U.S. Navy will want to reassess the desirability of developing an AIP submarine of its own, if only to learn how to counter this new and potentially revolutionary undersea challenge.
