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BALLAST
Surface ships must maintain a positive buoyancy at all
times or it is considered sunk. Submarines can
adjust buoyancy to be negative and they will be considered
submerged. Main ballast tanks accomplish this task
initially and are later supported by other systems.
MAIN BALLAST TANKS
There are two very large tanks outside of the pressure
hull (the "people space") on either end of the
submarine. When surfaced, these tanks are full of
air, combining the positive buoyancy of air in the tanks
with the air of the pressure hull to be greater than the
negative buoyancy from all the high density metal the ship
is constructed with. Despite always being exposed to
the ocean via holes in the bottom, water does not enter
due to the trapped air taking the volume. In order
to dive, vent valves at the top of the tanks are opened
allowing air to bleed out, making room for water to
enter. The tank space is now neutrally buoyant and
the internal air of the pressure hull is insufficient to
keep the submarine on the surface, resulting in an overall
negative buoyancy. Once these tanks are full, the
vent valves are promptly shut in preparation for the next
surface, especially since it might need to be an emergency
surface.
Both methods of surfacing involve putting pressurized air
into the tanks to overcome the water pressure and push it
back out of the bottom. The difference is that a
normal surfacing is an hour long operation starting from a
shallow depth using relatively low pressure air that is
generated as needed, whereas an emergency surface involves
the immediate expelling of ballast tank water at any depth
by releasing very high pressure air stored in dedicated
air flasks. Since it is stored air, the system is
designed to work even if the submarine lost all electrical
power. The more common name for this operation is
emergency blow and doing it will get the submarine to the
surface in under a minute from even the deepest of
depths. The activation valves for an emergency blow
are always within arms reach of the Chief of the Watch.
Fun Fact: The positive buoyancy of the air in the pressure
hull is so great that submerging with ballast tanks needs
some assistance from huge blocks of lead weight
permanently installed on either end of the ship.
TRIM SYSTEM
The wide variety of equipment and activities on a
submarine result in moving fluids into, out of, and around
the whole ship. Bilge water accumulation and
removal, toilet and shower usage, cooking, dish washing,
and many more are all constantly changing the center of
gravity of the submarine. This alters the ability to
control the trajectory of the submarine, so to compensate,
there exists the trim system. Unlike how the name
suggests, it can adjust much more than ship's trim.
While certain operations, such as jettisoning a missile,
requires odd angles put on the ship, steady state
operation generally maintains a level ship. There
are several tanks strategically placed in which a trim
pump moves seawater around. The pump and the valves
that control flow path is handled automatically by a
computer, but can be manually overridden.
In a very basic example, if freshwater is currently being
produced and sent to the potable water tanks, which are
located towards the rear of the ship, the trim system will
sense the ship developing a positive trim (pointing up)
due to extra weight in the back. It responds by
moving water from some of the aft trim tanks to the
forward trim tanks in order to compensate.
The trim system is one of the most versatile systems on
board and is capable of so many functions, the list is too
long to be worth displaying. However, there is one
very important function shared with it's sister system:
the drain system. This system is normally used for
routine pumping overboard, but is designed to accommodate
the potential of flooding. There are two pumps in
the drain system and the trim system is able to be
cross-connected to allow the trim pump to also contribute
to pumping capacity. Combined, they are capable of
pumping almost 1000 gallons per minute. Flooding
threatens the positive buoyancy of the pressure hull and
could therefore permanently sink the ship. Though
this is unlikely, since any one of the three watertight
compartments are capable of flooding to at least 70% full
and still allow the ship to surface. Watertight
doors are also located high up to allow sufficient time to
isolate the flooding compartment.
HOVERING AND MISSILE COMPENSATION
These are two ballast control systems unique to the Ohio
class due to the ship's main function as a missile
launcher. Buoyancy issues arise both during and
after a launch operation.
The principles of a missile launch are describe elsewhere in more detail, but
here, you only need to know that an upward force is
produced on the missile in order to get if off the
submarine. This force is tremendous as the missile
is very heavy and must be pushed through several meters of
water against gravity. Newton's Third Law holds that
a reaction force would be imparted on the ship.
While the ship is even heavier and would not move nearly
as much due to the conservation of momentum, it still gets
pushed down some. This becomes a problem if the ship
has been ordered to rapidly launch multiple or all
missiles in quick succession, which it can do. In
comes the hovering system, called as such because its
purpose is to make sure the ship hovers in place.
Unlike the trim system, hovering is designed with
immediate results in mind. It, too, operates
automatically when active. It has its own dedicated
tanks in which water can move into or out of so quickly
that the ship can, in fact, maintain a nearly constant
depth while launching a missile.
Missile compensation adjusts ballast for after the
missile launch. With yet another set of dedicated
tanks located near the missile tubes, this system fills
the tanks as needed to compensate for the weight
difference between the missiles that were on board and the
water that replaced them inside the tubes. A Trident
D5 missile has a mass of 59,000 kg. Water of the
same volume has much less mass. The difference is so
large that if multiple missiles have been fired then the
trim system would be incapable of handling it, thus the
existence of a separate system.
On the left side of the sticks, a Chief of
the Watch operates a Ballast Control Panel (BCP) at a
training facility. Almost all of the BCP's controls
is for ballast and there are many of them. The
emergency blow valves are circled above her head. IMAGE SOURCE: Navy
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