In emergency situations, modern submarines are built to
allow for a rapid surfacing, a technique called an
emergency main ballast blow. This maneuver is quite
breathtaking when done correctly. The submarine flies out of
the water with tremendous force.
The USS Bremerton performing an emergency
main ballast blow Photo
courtesy U.S. Navy
When mistakes are made, an emergency main ballast blow can
be extremely dangerous. The U.S. Navy's USS Greeneville was
performing this maneuver when it collided with a Japanese
fishing ship in February 2001.
Under normal operation, a submarine surfaces by pumping
compressed air into the ballast tanks. The water that is in
the ballast tanks is then forced out to make room for the air
until the submarine's density is less than the water outside
the submarine. In an emergency situation, a switch is thrown
that blasts large volumes of compressed air into the ballast
tanks, which rapidly brings the submarine to the surface.
In this edition of HowStuffWorks,
you will see how a submarine dives and surfaces in the water,
how life support is maintained, how the submarine gets its
power, how a submarine finds its way in the deep ocean and how
submarines might be rescued.
Diving and Surfacing A submarine or a ship
can float because the weight of water that it displaces is
equal to the weight of the ship. This displacement of water
creates an upward force called the buoyant force and
acts opposite to gravity, which would pull the ship down.
Unlike a ship, a submarine can control its buoyancy, thus
allowing it to sink and surface at will.
To control its buoyancy, the submarine has ballast
tanks and auxiliary, or trim tanks, that can be
alternately filled with water or air (see animation below).
When the submarine is on the surface, the ballast tanks are
filled with air and the submarine's overall density is less
than that of the surrounding water. As the submarine dives,
the ballast tanks are flooded with water and the air in the
ballast tanks is vented from the submarine until its overall
density is greater than the surrounding water and the
submarine begins to sink (negative buoyancy). A supply
of compressed air is maintained aboard the submarine in air
flasks for life support and for use with the ballast tanks. In
addition, the submarine has movable sets of short "wings"
called hydroplanes on the stern (back) that help to
control the angle of the dive. The hydroplanes are angled so
that water moves over the stern, which forces the stern
upward; therefore, the submarine is angled downward.
Buoyancy in a submarine Click on the Surface
and Submerge buttons to watch buoyancy in action.
To keep the submarine level at any set depth, the submarine
maintains a balance of air and water in the trim tanks so that
its overall density is equal to the surrounding water
(neutral buoyancy). When the submarine reaches its
cruising depth, the hydroplanes are leveled so that the
submarine travels level through the water. Water is also
forced between the bow and stern trim tanks to keep the sub
level. The submarine can steer in the water by using the tail
rudder to turn starboard (right) or port (left) and the
hydroplanes to control the fore-aft angle of the submarine. In
addition, some submarines are equipped with a retractable
secondary propulsion motor that can swivel 360 degrees.
When the submarine surfaces, compressed air flows from the
air flasks into the ballast tanks and the water is forced out
of the submarine until its overall density is less than the
surrounding water (positive buoyancy) and the submarine
rises. The hydroplanes are angled so that water moves up over
the stern, which forces the stern downward; therefore, the
submarine is angled upward. In an emergency, the ballast tanks
can be filled quickly with high-pressure air to take the
submarine to the surface very rapidly.
Life Support There are three main problems
of life support in the closed environment of submarine:
Maintaining the air quality
Maintaining a fresh water supply
Maintaining the Air
Quality The air we breathe is made up of significant
quantities of four gases:
Nitrogen (78 percent)
Oxygen (21 percent)
Argon (0.94 percent)
Carbon dioxide (0.04 percent)
When we breathe in
air, our bodies consume its oxygen and convert it to carbon
dioxide. Exhaled air contains about 4.5 percent carbon
dioxide. Our bodies do not do anything with nitrogen or argon.
A submarine is a sealed container that contains people and a
limited supply of air. There are three things that must happen
in order to keep air in a submarine breathable:
Oxygen has to be replenished as it is consumed. If the
percentage of oxygen in the air falls too low, a person
Carbon dioxide must be removed from the air. As the
concentration of carbon dioxide rises, it becomes a toxin.
The moisture that we exhale in our breath must be
Oxygen is supplied either from pressurized tanks, an oxygen
generator (which can form oxygen from the electrolysis
of water) or some sort of "oxygen canister" that releases
oxygen by a very hot chemical reaction. (You may remember
these canisters because of their problems on the MIR space
station -- see this
page for details). Oxygen is either released continuously
by a computerized system that senses the percentage of oxygen
in the air, or it is released in batches periodically through
Carbon dioxide can be removed from the air chemically using
soda lime (sodium hydroxide and calcium hydroxide) in devices
called scrubbers. The carbon dioxide is trapped in the
soda lime by a chemical reaction and removed from the air.
Other similar reactions can accomplish the same goal.
The moisture can be removed by a dehumidifier
or by chemicals. This prevents it from condensing on the walls
and equipment inside the ship.
In addition, other gases such as carbon monoxide or
hydrogen, which are generated by equipment and cigarette
smoke, can be removed by burners. Finally, filters are used to
remove particulates, dirt and dust from the air.
Maintaining a Fresh Water
Supply Most submarines have a distillation apparatus
that can take in seawater and produce fresh water. The
distillation plant heats the seawater to water vapor, which
removes the salts, and then cools the water vapor into a
collecting tank of fresh water. The distillation plant on some
submarines can produce 10,000 to 40,000 gallons (38,000 -
150,000 liters) of fresh water per day. This water is used
mainly for cooling electronic equipment (such as computers and
navigation equipment) and for supporting the crew (for
example, drinking, cooking and personal hygiene).
Temperature The temperature of the ocean surrounding
the submarine is typically 39 degrees Fahrenheit (4 degrees
Celsius). The metal of the submarine conducts internal heat to
the surrounding water. So, submarines must be electrically
heated to maintain a comfortable temperature for the crew. The
electrical power for the heaters comes from the nuclear
engine, or batteries
Power Supply Nuclear submarines use
nuclear reactors, steam turbines and
reduction gearing to drive the main propeller shaft,
which provides the forward and reverse thrust in the water (an
motor drives the same shaft when docking or in an
Submarines also need electric power to operate the
equipment on board. To supply this power, submarines are
equipped with diesel
engines that burn fuel and/or nuclear
reactors that use nuclear fission. Submarines also have batteries
to supply electrical power. Electrical equipment is often run
off the batteries and power from the diesel engine or nuclear
reactor is used to charge the batteries. In cases of
emergency, the batteries may be the only source of electrical
power to run the submarine.
A diesel submarine is a very good example of a hybrid
vehicle. Most diesel subs have two or more diesel engines.
The diesel engines can run propellers or they can run
generators that recharge a very large battery bank. Or they
can work in combination, one engine driving a propeller and
the other driving a generator. The sub must surface (or cruise
just below the surface using a snorkel) to run the diesel
engines. Once the batteries are fully charged, the sub can
head underwater. The batteries power electric motors driving
the propellers. Battery operation is the only way a diesel sub
can actually submerge. The limits of battery
technology severely constrain the amount of time a diesel sub
can stay underwater.
Because of these limitations of batteries, it was
recognized that nuclear
power in a submarine provided a huge benefit. Nuclear
generators need no oxygen, so a nuclear sub can stay
underwater for weeks at a time. Also, because nuclear fuel
lasts much longer than diesel fuel (years), a nuclear
submarine does not have to come to the surface or to a port to
refuel and can stay at sea longer.
Nuclear subs and aircraft carriers are powered by nuclear
reactors that are nearly identical to the reactors used in commercial
power plants. The reactor produces heat to generate steam
to drive a steam turbine. The turbine in a ship directly
drives the propellers, as well as electrical generators. The
two major differences between commercial reactors and reactors
in nuclear ships are:
The reactor in a nuclear ship is smaller.
The reactor in a nuclear ship uses highly enriched fuel
to allow it to deliver a large amount of energy from a
Navigation Light does
not penetrate very far into the ocean, so submarines must
navigate through the water virtually blind. However,
submarines are equipped with navigational charts and
sophisticated navigational equipment. When on the surface, a
sophisticated global positioning system (GPS)
accurately determines latitude and longitude, but this system
cannot work when the submarine is submerged. Underwater, the
submarine uses inertial guidance systems (electric,
mechanical) that keep track of the ship's motion from a fixed
starting point by using gyroscopes.
The inertial guidance systems are accurate to 150 hours of
operation and must be realigned by other surface-dependent
navigational systems (GPS, radio, radar, satellite).
With these systems onboard, a submarine can be accurately
navigated and be within a hundred feet of its intended course.
To locate a target, a submarine uses active and passive
SONAR (sound navigation and
ranging). Active sonar emits pulses of sound
waves that travel through the water, reflect off the target
and return to the ship. By knowing the speed of sound in water
and the time for the sound wave to travel to the target and
back, the computers can quickly calculate distance between the
submarine and the target. Whales, dolphins and bats use the
same technique for locating prey (echolocation). Passive
sonar involves listening to sounds generated by the
target. Sonar systems can also be used to realign inertial
navigation systems by identifying known ocean floor features .
Rescue When a submarine goes down because of
a collision with something (such as another vessel, canyon
wall or mine) or an onboard explosion, the crew will radio a
distress call or launch a buoy that will transmit a distress
call and the submarine's location. Depending upon the
circumstances of the disaster, the nuclear reactors will shut
down and the submarine may be on battery power alone.
If this is the case, then the crew of the submarine have
four primary dangers facing them:
Flooding of the submarine must be contained and
Oxygen use must be minimized so that the available
oxygen supply can hold out long enough for possible rescue
Carbon dioxide levels will rise and could produce
dangerous, toxic effects.
If the batteries run out, then the heating systems will
fail and the temperature of the submarine will fall.
Rescue attempts from the surface must occur quickly,
usually within 48 hours of the accident. Attempts will
typically involve trying to get some type of rescue vehicle
down to remove the crew, or to attach some type of device to
raise the submarine from the sea floor. Rescue vehicles
include mini-submarines called Deep-Submergence Rescue
Vehicles (DSRV) and diving bells. The DSRV can
travel independently to the downed submarine, latch onto the
submarine over a hatch (escape trunk), create an
airtight seal so that the hatch can be opened, and load up to
24 crew members. A diving bell is typically lowered from a
support ship down to the submarine, where a similar operation
the submarine, typically after the crew has been extracted,
pontoons may be placed around the submarine and inflated to
float it to the surface. Important factors in the success of a
rescue operation include the depth of the downed submarine,
the terrain of the sea floor, the currents in the vicinity of
the downed submarine, the angle of the submarine, and the sea
and weather conditions at the surface.
For more information, check out the links on the next page.