Diving and Surfacing

Photo courtesy U.S. Navy

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.

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

• Maintaining the air quality
• Maintaining a fresh water supply
• Maintaining temperature

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)

• Oxygen has to be replenished as it is consumed. If the percentage of oxygen in the air falls too low, a person suffocates.
• 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 removed.

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 the day.

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).

Maintaining 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 reactor, diesel engine, or batteries (emergency).

Power Supply

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 smaller reactor.

Navigation

Photo courtesy U.S. Department of Defense Sonar station onboard the USS La Jolla nuclear-powered attack submarine

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

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 minimized.
• Oxygen use must be minimized so that the available oxygen supply can hold out long enough for possible rescue attempts.
• 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.

Photo courtesy U.S. Department of Defense DSRV secured to the deck of a submarine

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 occurs.