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Four independent paths may achieve greater “air independence” for diesel electric submarines. All four technological approaches provide secondary power sources for recharging the main ship batteries in order to
tery designs. Twenty-five years ago, nonnuclear submarines were typically required to “snorkel” on diesel generators one or more times each day. This was necessary to extract the air required to recharge the ship bat
Comparison of Air Independent Propulsion (AIP) Technologies
increase submerged endurance capabilities for diesel submarines, heretofore limited by evolutionary improvements in bat
teries and keep them in a “topped off” state for maximum flexibility.
Battery advancements now
allow modern diesel submarines to go as long as 4-10 days without snorkeling, depending on battery capacities, battery discharge rates, submarine patrol speeds, and just how low the submarine commander is willing to let the battery discharge. Air independent propulsion (AIP) technologies are intended to provide 100-400 kilowatts of power to allow slow-speed submerged operations (4-6 knots) for two weeks or more and still keep the batteries at full charge. The air independence is provided either by stored oxygen, by stored reactants, or by a low-power nuclear battery charger.
Closed-Cycle Diesel Engines. These attempt to recycle exhaust back to the engine intake, where it is mixed with oxygen, fuel, and possibly a working gas in just the right amounts to maintain a continuous, efficient combustion process. Two very different closed-cycle systems for submarines are currently being pursued by Western manufacturers. The first is the “toroidal” system that has been developed by the Italian firm Maritalia for
submarine that utilizes a closed-cycle diesel system to travel at least 1,600 nautical miles at eight knots or less speed. This translates to at least eight days of submerged endurance and is achieved by a clever “toroidal” pressure hull configuration that stores oxygen and exhaust gas. The mini-sub can be designed with either two heavyweight torpedo tubes, four lightweight torpedo tubes, or two swimmer/mine/combat delivery vehicles in a lock-in, lock-out chamber. In a confined sea region, this would represent a serious threat to warships, particularly if on patrol or at anchor.
The final advanced submarine propulsion concept that is coming to the Third World is high-power nuclear propulsion. The French Rubis submarine is the smallest nuclear attack submarine (SSN) in the world (weighing less than 3,000 tons) and is available for export. Its cost is more than twice that of a Type 209 diesel, but it provides unlimited submerged endurance. The core life of its reactor is reported to be approximately 25 years, which is an attractive feature for Third World navies. The Soviets also leased a Charlie nuclear-powered guided missile submarine to India in January 1989. Will they be selling, renting or leasing Victor Ills in a few years? In addition, several Third World countries are pursuing indigenous nuclear submarine production capability. India has had an indigenous SSN development program since 1974. Brazil has a similar program, and Argentina once had one, but it is on hold. By around 2010, a breakthrough in indigenous nuclear submarine capability is entirely possible.
Submarine Weapons Proliferation
All the diesel submarines previously identified have four to eight 53-centimeter (cm.) torpedo tubes and would typically carry 14-18 heavyweight (HW) torpedoes. Soviet exported submarines are equipped with 53 cm. torpe-
a 30-ton submersible used commercially by the oil industry.
This system is referred to as the '3GST9” and reflects the unique gas storage toroidal system contained in three-inch Pipes welded together to form a 'urge section of the pressure hull f°r this nine-meter-long vehicle. The three-inch pipes are used to store both liquid oxygen and engine exhaust gas, including carbon dioxide, which is used as a working gas for this closed cycle concept. Oxygen, carbon dioxide, and fuel are combined at the engine intake. Excess exhaust products are compressed and stored in the toroidal system. There is no need for any overboard discharge. Currently under development by Maritalia is a 150-ton mini-sub that is 27 meters long and has even larger diameter pipe sections for gas storage that will allow about a Week of submerged endurance. Whether this type of design will continue to scale up with larger submarines and produce even greater capacity for submerged endurance (two or three weeks) js uncertain, but this application ts being pursued by the Italian company Fincantieri, which owns Maritalia.
An alternative closed cycle concept for submarines is being explored by the firm RDM of
Holland based on the “Argo” system designed by the British firm, Cosworth Engineering.
The Dutch “Spectre” system, as it is now termed, relies on a unique exhaust scrubber and water management/overboard discharge technique. This system dissolves carbon dioxide into sea water and maintains adequate pressure for overboard discharge down to depths of 300 meters without a significant increase in system power requirements. This represents a technical breakthrough, because previous closed-cycle systems attempting overboard discharge at depth have proved very inefficient because of increased power demands. Stored liquid oxygen is the key to achieving air independence for this system. Oxygen, an inert working gas (argon, to achieve proper specific heat), recycled exhaust, and fuel are mixed in correct proportions to maintain combustion. The submerged endurance capability achieved will be largely limited by stored liquid oxygen capacity and the overall efficiency of the closed cycle system.
Stirling Engines. The Stirling engine is a reciprocating, external combustion engine. The Swedish Navy is examining such a system for their next-generation diesel submarine. A 1,000- ton Nacken-class submarine is serving as an at-sea test vehicle. An eight-meter hull extension was required to accommodate two Stirling generators, additional fuel, liquid oxygen tanks, and a control system. The Stirling engine has thermodynamically connected pistons that transmit mechanical work to a drive shaft and also move a working gas (helium) through a regenerator/cooler (heat sink) between hot and cold sides of the engine. Expanding hot gases will force a particular piston down, with the hot surface of one piston being coordinated with the cold surface of another piston. Unlike internal combustion engines, Stirling engines feature continuous burning in an external combustion chamber, which is kept in overpressure to facilitate overboard discharge of exhaust gases down to 300- meter depths. Stirling engines are considered quiet in operation because of their lack of explosions and moving parts in the combustion chamber, low system vibration, and low engine revolutions per minute. Other external combustion engines include Brayton and Rankin cycle variants that rely on closed-cycle turbines and are smaller but less efficient than Stirling engines.
does with thermal propulsion that have relatively long endurance and high-speed features compared to torpedoes with electric propulsion. They also have large 400-kilogram (KG) warheads. Western export torpedoes such as West German (SUT/SST-4), United Kingdom (Tigerfish), French (F-17P) and Italian (A 184) designs have wire guidance and advanced acoustic homing capabilities. They generally have quiet electric propulsion and moderatesized 250 KG warheads.
Both Western and Soviet HW torpedoes are potentially lethal against surface combatants. The U.S. frigate Samuel B. Roberts (FFG-58) was nearly cut in half by a contact hit from a 100-125 KG World War I design mine in the Persian Gulf. During the Falklands Conflict the 13,000-14,000-ton Argentine cruiser Belgrano was sunk by two 340 KG warhead MK 8 torpedoes fired by the British SSN, HMS Conqueror. These straight-running torpedoes, based on designs more than 50 years old, killed 368 Argentine sailors. This was more casualties than the British suffered during the entire war, both on land and at sea. Neither the Roberts nor Belgrano was hit by a modern homing torpedo with influence fuzing designed to achieve an optimal underbottom hit.
In addition to torpedoes or mines, several antiship cruise missiles are capable of being launched from 53 cm. torpedo tubes. The United States has exported submerged- launch Harpoons to several Third World countries including Israel, Pakistan, and Egypt (due. to receive in 1993). France may follow suit with the SM39, the submarine- launched version of Exocet. The West Germans can adapt the 53 cm. tubes for their Type 209 variants to fire either Harpoon or Exocet.
Threat to Western Power Projection Forces
The German U-boat campaigns in the two world wars
Reducing the size/weight/ complexity/cost of Stirling engines are current goals as is assuring high system reliability in a marine application. Potentially, this system could provide for two weeks or more of submerged endurance at low submarine operating speeds.
Fuel Cells. The West German Navy is attempting to achieve air independence for its future submarines by using fuel cell technology. A 450-ton Type 205 submarine with a several-meter hull extension has been used to demonstrate this technology.
Fuel cells have the highest potential efficiency (50-70%) of any of the nonnuclear AIP concepts being explored. This overall efficiency is possible, because there is no heat transfer or combustion taking place. Fuel cells are designed to convert chemical reactions directly into electrical energy. In the German case, the chemical reactants are liquid oxygen and hydrogen stored as a metal hydride. In other words, the hydrogen bonds to the metal and is driven off by waste heat produced from the reaction, which is a safer method than storing the hydrogen as a gas under pressure or in liquid form. Separate electrodes bring each of the reactants into contact with an electrolyte solution of potassium hydroxide that is porous to the reactants, thus allowing electron transfer. Fuel cells are purported to have five times the net energy density of a lead-acid battery. In the future, if technical difficulties can be overcome, up to about one month of submerged operation may be achievable for fuel cell-equipped diesel submarines. This could be possible with relatively “silent operation,” because fuel cells rely on no rotating machinery to produce their electricity.
Low-Power Nuclear Reactors. A “nuclear battery charger” could allow unlimited submerged endurance at low submarine speeds consistent with the low power output of the reactor. Canada’s ECS group is currently attempting to develop just such an autonomous marine power source (AMPS). They are adapting a 1960s “Slow Poke” university reactor design that has previously been licensed for unattended operation because of its “fail safe” automatic shutdown features. This small reactor is non-pressurized, non-boiling (95°C), light-water cooled, and employs low-enriched uranium fuel (<20%). The heat produced by the reactor is transferred to a secondary loop in which liquid freon is vaporized to spin a turbine generator. To accommodate this type of marine reactor and associated equipment could require up to a 10-meter plug on some submarine designs. The first application of AMPS technology is scheduled for the French SAGA-I 545-ton commercial submersible, perhaps as early as 1995, when it will be renamed SAGA-N. Currently, the AMPS technology has uncertain government backing and has generated no military sales.
AIP technologies are being developed by the West and possibly by the Soviets as well. As AIP military hardware is developed for Third World diesel submarines in the future, it could dramatically change the ASW equation for dealing with them. The main uncertainties are which AIP technologies will ultimately prevail in the near term, based on various considerations including ship impact, safety, reliability, performance, and affordability. In the very distant future, as these power sources continue to improve, they may replace the diesel generators on nonnuclear submarines.
—Fitzgerald and Benedict
demonstrated that submarine operations force navies to maintain a large and costly ASW capability. In many Third World regions, although only a few submarines can be brought to bear, it will again require a relatively large ASW force capability to counter them. This is particularly evident against modem submarines and in view of other important factors that may offset smaller numbers of submarines. First, risk to naval forces must be consistent with the benefits of the operation. For example, if the objective is a naval “show of force,” then the price of losing a ship and large number of sailors may be too high. Second, neutralizing even a single diesel submarine could prove difficult in view of its limited acoustic signature and the adverse acoustic environments often associated with Third World regions.
Although Third World submarines often have limited seaward reach against modem naval forces, certain other factors may offset that weakness. First, naval forces may be required to operate in close proximity to hostile shores in order to perform various missions such as naval gunfire support, evacuation of civilians, landing of forces ashore, and asserting right of free passage. Second, these naval forces often occupy restricted operating areas for protracted periods, a fact that greatly simplifies the encounter problem for nonnuclear submarines.
Several characteristics of Third World contingencies may offset the limited training and operational expertise of their submarine crews. First, the Third World adversary has a mobile “home field” advantage in waters with which the naval forces may be unfamiliar. Second, less focused intelligence on Third World navies could make them more unpredictable in terms of tactics and intentions. Third, difficult mles of engagement may prohibit precursor operations and restrict attacks on subsurface contacts to avoid collateral damage.
The Falklands Conflict displayed many of the character-