Since the end of the Cold War,
navies have shifted their focus from preparing for openwater conflicts to
dealing with conventional as well as asymmetric threats from the littorals. At
the same time, navies have to fulfil new requirements such as supporting
intelligence gathering, counter-terrorism and special force operations.
Submarines offer a unique capability proposition as a stealthy platform, and
they also possess a robust capability
to meet both the conventional demands of
open water warfare and new demands of navies. However, acquiring and
maintaining a submarine fleet is costly. Therefore, the key challenge for
submarine designers is to strike a balance between the need for stealth, range
and adaptability, and the need to control the size and cost of the submarine.
In response to these challenges,
this article presents four critical solutions, namely:
• Modularity
• Design for Special Operations Forces
• Battery Technology
• Air Independent Propulsion (AIP)
MODULARITY
Cutbacks in fleet sizes and budgets have led
to the demand for more capabilities to be incorporated in submarines so that
they can perform multiple roles. As a result, submarine builders have
incorporated modular design in their latest generation of submarines. Modular
design allows the addition of capabilities and easier upgrades at the or a
shorter one for a dedicated role. Figure 2 shows a generic submarine type in
two variants with different engine modules.
The modularity concept is embraced
at all levels of submarine production and operation, namely
• Modularity at Design (Deconittignies, 2001)
• Modularity at Construction (Deconittignies, 2001)
• Deployment of
Modular Payloads
Modularity at Construction
Modularity at Construction’ refers
to the sharing of submarine manufacturing processes at different shipyards.
Each hull section can be fabricated and assembled in shipyards located in
different countries. The final assembly is then carried out at one of these shipyards.
Modularity at Construction avoids production bottlenecks and speeds up the
manufacturing process considerably. Such a concept, however, requires an
integrated information system, extensive quality control and tightly controlled
production tolerance. This concept has been used by the French submarine
builder, DCNS, in the manufacture of the Scorpene family of submarines for
Chile and Malaysia (Deconittignies, 2001). Both DCNS and Spainbased Navantia
share the construction and outfitting of the hull sections. The final assembly
is then carried out at either DCNS or Navantia.
Modular Payloads
The introduction of modular payloads is one of
the most significant and newest changes in submarine design. Instead of
carrying only a fixed payload of torpedoes, submarines with modular payloads
can be re-configured for a range of payload options. These include
intelligence, surveillance, reconnaissance and target acquisition systems; special
operations forces (SOF) support systems; strike weapons; mines and mine
countermeasures; undersea communications and sensor network systems; and
unmanned vehicles. Modularity of payload removes the need to incorporate a
single submarine with the complete set of payload options required for all its
capabilities. Instead, the submarine can be designed to allow the customisation
of payloads depending on specific mission needs. illustrates a modular sail section using the
Universal Modular Mast (UMM) concept, which consists of cartridges installed in
the sail of a submarine. These cartridges are used to house different
mission-specific masts as required. Besides masts, auxiliary systems can also
be fitted into the cartridge of the UMM. For example, a machine gun can be
stowed in a UMM cartridge, which can then be raised to deploy the gun. The UMM
concept has been adopted by new generation submarines such as the UK Royal
Navy’s latest Astute-class submarines.
DESIGN for SPECIAL OPERATION FORCES
The SOF form an important element
of modern warfare. Armed forces require the SOF to perform a wide variety of
missions such as Search and Rescue, Reconnaissance and Sabotage, and Forward
Observation. For these operations, the SOF may be sent to their mission areas
using submarines, aircraft, helicopters, parachutes or surface craft. As
submarines are the only platform that can guarantee covert delivery, they are a
vital element of the SOF.
Covert Delivery
Up to the 20th century, covert delivery of the
SOF could only be achieved by a large submarine with a dedicated external lock
that is carried as an attachment on the submarine hull. Figure 4 illustrates an
example of such an arrangement. This lock acts as an interface between the
submarine and the sea to transfer the SOF in and out of the submarine while
keeping the interior of the submarine dry. Following the flooding and
pressurisation of the lock, the SOF team is able to swim out to its mission
area. The SOF team also returns to the submarine via the lock after its
mission. These transfer locks are as heavy as 30 tonnes and very few navies can
afford submarines that are large enough to carry them. Designers of smaller
submarines which are incapable of carrying such heavy locks have explored innovative
means to achieve covert delivery. For example, the German builders of the U212A
class diesel-electric submarines overcame the lack of space in the submarines
by modifying part of the sail fin to become an internal lock (Wallner, 2006).
Modular Add-on for the SO F Operations
To overcome size constraints, submarine
designers have used modular design to create capacity to accommodate the
requirements of the SOF without compromising the submarine’s performance and
core mission capability. For example, the torpedo room can be designed such
that the designated torpedo racks can be removed to allow the fitting of
additional bunks for the SOF .This allows the submarine to be customised
without having to lengthen its hull to house the additional SOF crew.
BATTERIES FOR SUBMARINE APPLICATIONS
Batteries are standard features in all
submarines to provide standby and propulsion power (Szymborski, 2008). Prior to
the advent of AIP and nuclear technology, a submarine’s submerged endurance
depended entirely on its battery life. Thus, the time required to charge its
batteries remains as one of the submarine’s key performance indicators – this
determines how long a submarine has to snorkel and risk detection by
adversaries. While lead-acid batteries have been the standard used in
submarines, their dominance is increasingly challenged by a new generation of
batteries that offer better power and energy density.
Lead-Acid Battery
The lead-acid battery has been the
battery of choice for submarine applications. Despite having a low
energy-to-weight ratio and a correspondingly low energy-to-volume ratio,
lead-acid batteries are able to supply high surge currents and maintain a large
powerto-weight ratio.
This explains why they are chosen for submarine
applications. The leadacid battery stores electrical energy in the form of
chemical energy and releases this stored energy into an electrical circuit as
the battery discharges. The operation of a leadacid cell is described in the
reversible chemical reaction.
Lithium-Ion Battery
Lithium-ion (Li-ion) batteries are
currently one of the most popular types of battery for portable electronics.
They have a superior energy-to-weight ratio and a slow loss of charge when not
in use. Lithium is one of the lightest metals and has great electrochemical
potential.
In addition to the wide-ranging applications of Li-ion batteries in
the consumer electronics domain, there is also a growing demand for it in the
defence, automotive, and aerospace industries. This is due to the high energy
density and technological maturity of Li-ion batteries. Figure 7 shows a
simplified diagram of the charging and discharging sequence of a Li-ion
battery.
AIR INDEPENDENT PROPULSION
The period of time that a conventional
submarine can remain submerged continuously is limited by its battery capacity.
Despite emerging battery technologies, it is difficult to achieve a quantum
leap in the submerged endurance due to volumetric and cost constraints.
Submarines with AIP capabilities can extend their submerged endurance from days
to weeks.
Conventional submarines run on diesel engines which require oxygen
for combustion. AIP enables the submarine to operate without the need to
surface or use its snort mast to access atmospheric oxygen. The oxygen required
for combustion is stored on board as liquid oxygen. AIP technologies include
Stirling engines, fuel cell (FC) systems, steam turbine systems, and closed
cycle diesel systems. Thus, nuclear propulsion can also be considered as AIP.
However, due to the size of nuclear submarines and their seemingly limitless
submerged endurance, nuclear propulsion is seldom mentioned under the same
category as non-nuclear AIP submarines.
Stirling Engine
The Stirling engine
solution developed by Kockums is regarded as a well-established AIP technology.
All Swedish submarines employ the Stirling AIP system. The Japan Maritime Self
Defence Force also deploys Stirling engines in its Soryu-class submarines. The
Stirling engine produces heat by burning low sulphur diesel fuel and oxygen
(stored in cryogenic tanks) in a pressurised combustion chamber. The heat is then
transferred to the engine’s working gas (usually helium) operating in a
completely closed system. The working gas forces the pistons in the engine to
move, thus producing mechanical energy to drive the alternator as illustrated
in Figure 8. The combustion pressure is higher than the surrounding seawater
pressure, which allows exhaust products dissolved in seawater to be discharged
overboard without the use of a compressor. This results in low infrared
signature and noise emission levels. The submarine’s submerged endurance is
determined by its storage capacity for the liquid oxygen.
Fuel Cell System
The FC system is developed by HDW
in Germany. The system uses Polymer Electrolyte Membrane (PEM) fuel cells. PEM
fuel cells are electrochemical energy converters in which hydrogen ions and
oxygen ions are combined to produce electrical charge as illustrated Similar to the Stirling engine, the FC system generates electricity at a
slow and steady rate, suitable for low-speed submarine operations. For
high-surge operations, the submarine relies on its regular battery system,
which in turn is recharged by the FC system. The FC system is operational on
board the German U212A class submarines and the Type 214 class submarines.
It
has also been selected for the S80 class submarines built for the Spanish Navy.
The FC system is advantageous because the only by-product is pure water and it
does not generate any exhaust gas. Furthermore, the FC system is much quieter
than other AIP systems, has the lowest oxygen consumption rate and potentially
offers the highest underwater endurance. However, the hydrogen required in FC
systems is stored in the form of liquid hydrogen in metal hydride which
requires costly maintenance and support facilities. The S80 class submarines
will feature reformer technology that generates the required hydrogen from
ethanol through a reformer.
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