The submarine of conventional propulsion, far from being nullified by the nuclear alternative, has been largely perfected to increase her speed, operational range and stealth.

During the Second World War submarines were technically submersibles, rather than true submarines. The German inventive created new types of submarines (the Type XXI and Type XXIII, among others) and also developed the snorkel, a tube fitted with valves that allowed to use the Diesel engines and recharge the batteries while navigating submerged. The Germans also created anaerobe propulsion systems, such as the closed-circuit Diesel engine and the Walter turbine, as well as diverse torpedoes of acoustic or cable guidance.

The pictures below represent the submarines Type XXI and Type XXVI. This latter was a design that never entered service, made for the new turbine Walter moved by hydrogen-peroxide. It has been said that this fuel would make submarines to be one thousand times more expensive to operate. In the Type XXI we can see a large amount of batteries stored in two decks beneath the crew accommodation, to ensure an autonomy as long as possible for the electric motors. Even if using the snorkel, recharging the batteries always posed a risk for the submarine, for aircraft could spot submarines standing at snorkel/periscope depth with relative easiness.

Type XXI attack submarine
Type XXVI attack submarine
Inspection Window
After the war the Allied countries started the design of submarines based in the German types XXI and XXIII. A good number of already existing vessels was modified, increasing the capacity of their batteries, fitting them with snorkel, making their hulls and conning towers more hydrodynamical, changing their propellers and anything possible to increase their speed and operational range, while reducing their acoustic signature. These submarines, the most direct descendants from the aforementioned German submarines, are those of the first generation.

For example, the four submarines of the Dutch class Dolfijn, as the one shown just below, belonged to the first generation. These submarines were a rather particular design with three individual pressure hulls, whereas a normal submarine has only one. The upper hull, the largest one, housed the crew and the largest part of the equipment, whereas beneath that one ran longitudinally two smaller hulls, each of them containing the machinery and the storages. This layout granted great resistance to the submarine but the space in the lower hulls was very tight, causing great difficulties to access the machinery for maintenance.

Dolfijn class attack submarine
We can consider that the largest part of submarines of conventional type (propelled by Diesel engines while in surface and by electric motors while in immersion) in service in the last years of the 20th century belong to the third generation. It is not so easy to classify the vessels according to a particular generation, for they may have characteristics from different generations. A number of new technologies, whose application will be progressive from the beginning of the 21st century, constitute the fourth generation.

Type 205 attack submarine
Note: Diesel-electric patrol/attack submarines are often referred as "SSK".

The conventional submarine of third generation

This one can be described as a vessel designed to navigate permanently in immersion, by charging her batteries at snorkel depth by means of generators attached to her Diesel engines, and not emerging unless strictly necessary. Her hull is highly hydrodynamical and has a single propeller, usually of five or seven blades, so the crosswise-arranged tailplanes can "shadow" only one blade at a time. The hull is topped by a conning tower, where the sentinels remain while the vessel is surfaced, and which is in turn topped by periscopes, antennas, the snorkel and other elements.

In some vessels the access hatch is double, with an airlock that allows to operate with a group of commandos; in others the airlock can be found in one of the access hatches on the deck, or not existing at all. However it is mandatory to have a specific hatch for loading torpedoes, sometimes used also for crew access or as airlock. In some cases it can be found a sealed opening to pass large pieces - such as engines -, which is opened only when the vessel is subject to extensive maintenance operations in which every service in inspected.

The maneuverability of single-propeller submarines is rather poor in restricted areas (docks and bases), requiring the help of auxiliary berthing means. But their performance when submerged is better, showing better military prestations. The steering rudders are usually two, one on the lower section of the hull and the other in the upper section, as well as the aft hydroplanes, with which they form a cross. In the fore section there is a second pair of hydroplanes, which in some cases are retractable. Hydroplanes mounted in the conning tower have some advantages: good control while in immersion, less exposition to damages, better stability at periscope depth and avoidance of flow noise near the bow sonar. However they make much more difficult to emerge through ice caps.

Propulsion is effectuated thanks to several Diesel engines which actuate on electric generators that provide energy for the main motor, which can also be fed by the circa 400 elements that form the battery. This one is of the classic type acid-lead, whose electrolyte is agitated by means of bubbling to obtain better performance, while the connections between the plates and the batteries are cooled by a fresh water closed circuit. The power of the battery is enough to navigate for over a week at silent speed. The main motor is cooled by air which in turn is cooled by a water-cooled heat exchanger circuit. The silent motor, which allows to navigate only at low speed, can be an independent one or being part of the main one, using in such case only a part of the windings.

The weapons of a third-generation submarine are torpedoes, encapsulated missiles that are fired from the very torpedo tubes and mines. Depending on type, and as an approximate rule, they can carry up to two dozens of torpedoes/missiles and twice of mines. Sensors comprise a radar and very sensitive sonars of passive and active type (for detection and attack, respectively), of which a wide diversity exists, including towed and side swept models.

A third-generation submarine can descend to considerable depths, being usual 300 meters, which means a pressure on the hull of about 30 kilograms per square centimeter, something which is possible only with special designs, ribs with adequate profiles and sections, and the adoption of very resistent and flexible steels; because of this, even if the hull gets flexed and deformed, it does not crack.

The so called "indiscretion rate" is the ratio between the time that a submarine navigates by means of batteries and the time that she uses the snorkel, with the Diesel engines working, moment on which she can be detected by infrared sensors that detect the heat from the exhausts, or the radar, whose waves are reflected by the snorkel head.

The maximum speed of these vessels is about 15-20 knots while in immersion and 10-12 while in surface. But in this case their operational range with Diesel engines reaches 4000-6000 nautical miles at 6-8 knots, whereas only 2000-3000 while in immersion. Silent speed is only about 4-6 knots, but in this case the submarine is harder to detect. If necessary, these vessels can stay uninterruptedly between 20 and 30 days in the sea. The new Air Independent Propulsion (AIP) allows to considerably increase the prestations of these vessels, but in this case underwater autonomy is increased at expenses of a more complex and costly design.

Type 206-A attack submarine

The fourth generation

Albeit several are the differences between the third and fourth generations, two features affecting the propulsion system are the most notable ones: the aforementioned AIP and the new electric motors of permanent magnet. Regarding sensors there is a standard utilization of towed sonars, to prevent the own noise emissions, and lateral sonars. The high degree of computerization and automatization will allow to reduce the complement and hence to improve the habitability in the restricted space of a submarine. However, the possible incorporation of women in the complement of a modern submarine imposes a redundant distribution of the accommodation.

Returning to the key factor of propulsion, military submarines always had an AIP propulsion, which was the batteries. But these have a very limited endurance, and nowadays we consider as such a propulsion system that allows to increase considerably the immersion time, without compromising the stealth of the submarine. This propulsion system is not intended to increase the maximum speed while in immersion or the operational range in general.

The purpose is to increase the operational range when navigating at silent speeds, so it is not necessary to provide a high amount of power in a given moment, but a relatively low amount in a continuous way, allowing to navigate in stealth mode during more days. This anticipates the future antisubmarine war, which will demand more patience than ever to the crew members, forced to remain submerged as far as their submarine allows. Who makes the first mistake or is forced by adverse circumstances to show signs of life, loses the battle.

The following picture shows an approximate internal view of an attack submarine of the Scorpčne class, jointly designed and built by the Spanish company Empresa Nacional Bazán and the French company Direction des Constructions Navales. These submarines can be delivered to the buyers with or without AIP module, which can be later added if required.

The specifications are: length, 60 meters; beam, 6 meters; maximum displacement, 1565 tonnes; propulsion, Diesel-electric plus AIP; main machinery, permanent-magnet electric motor; power, 4000 horsepower; speed, 20 knots while submerged; operational range, 550 nautical miles at 5 knots while submerged; armament, six 533-millimeter torpedo tubes for torpedoes Black Shark, antiship missiles Exocet or mines; complement, 31.

Cutaway of a Scorpene-class attack submarine
Submarine or spaceship? We can truly look towards the future in the command room of a submarine of the Scorpčne class. The minimalist appearance of this prototype is simply astonishing.

Control console on a Scorpene-class attack submarine

Air Independent Propulsion

Several are the AIP systems in study, each one having specific advantages and inconveniences. The aforementioned Empresa Nacional Bazán and Direction des Constructions Navales focused in the MESMA (Module d'Energie Sous Marin Autonome), which uses a turbine moved by steam obtained from a primary circuit or combustion chamber fed by a mixture of ethanol and oxygen. The turbine can actuate on a propeller or move a generator to feed an electric motor. This is a very silent system that triples operational range while in immersion.

Germany has focused in the energy cell, which basically is an inverse electrolitic reaction on which electricity, heat and water can be obtained from hydrogen and oxygen. The electricity can be applied to the electric motor or the batteries, while the heat and the chemically pure water can be either used for any pertinent purpose or discarded.

Sweden has used the Stirling engine in its most recent models of submarines. Such engine, firstly devised in 1816, remained largely unnoticed during almost two centuries. It is an internal combustion engine of regenerative closed cycle which works with air or other gas. The advantages of this engine are much lesser vibrations, noise and contamination, therefore being an interesting application for the submarine weapon.

Another system is the Diesel engine of closed circuit, basically a conventional Diesel engine which has filters applied to its exhaust as well as oxygen intakes on the combustion chambers. So, the gases are permanently recycled and the engine becomes autonomous from external air. The origins of this system can be traced back to German projects from the Second World War. The drawbacks are the notable vibrations and noise produced by these engines.

Finally, another method is the application of new types of batteries. The most promising type is that called LAIS (Lithium Aluminium/Iron Sulphide), which can produce about triple the energy of the best conventional lead/acid batteries. The drawback is the high amount of heat generated, which is probably not welcome inside a submarine.

The Type 212, depicted below, is the "last cry" in German attack submarines. This model is propelled by an electric motor Siemens Permasyn of 1700 KiloWatt (olive color) and one Diesel engine MTU 16V 396. Electric energy is generated by a number of Siemens PEM fuel cells which consume hydrogen and oxygen. These cells combine superb energy efficiency with minimal noise generation and a high degree of reliability, while extending immersion time by several weeks. The fuel and oxidizer tanks (cyan color) are located outside the pressure hull for safety reasons.

Cutaway of a Type 212-A attack submarine

The electric motor of permanent magnet

This type of electric motor can work at lower revolutions per minute than the conventional type. Besides it lacks brushes, for the permanent magnets are in the rotor and the windings are in the stator, changing the magnetic fields by means of thyristor converters. It is much more compact and allows better cooling for it can use a closed and direct water circuit. Its greater torque allows to use propellers of larger diameter/pitch, reducing noise and cavitation. This is possible thanks to permanent magnets based in rare earths such as samarium and neodymium. Besides, the weight of these motors is about two thirds of that of a conventional type.

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