Modern aircraft carriers are impressive warships that play a key role in global military operations.
Although they are not particularly well-suited for combat on their own, they are invaluable
assets because they serve as mobile airbases that can be deployed just about anywhere around
the globe.
There are approximately 50 aircraft carriers currently in service around the world, and
they are owned by 14 individual countries.
Incredibly, the United States Navy owns 24 of them, which is nearly half of the global
total.
11 of the American aircraft carriers also belong to a special class of supercarriers,
each with a price tag of more than 10 billion US dollars accounting for inflation.
These nuclear-powered supercarriers exceed 330 m in length and 75 m in width, and they
tower more than 20 storeys above the ocean surface.
Each ship can carry more than 75 aircraft between the flight deck and interior hangar,
and the total weight of each ship exceeds 90,000 metric tons when fully loaded.
Despite their astonishing size, even the of largest supercarriers do not have flight decks
that are long enough for planes to take off and land under their own power alone.
This has been a fundamental problem since the very first aircraft carriers were commissioned
in the early 1900’s, and it led to the development of assisted takeoff and arrested recovery
systems.
Prior to World War II, the United States and other countries experimented with a number
of systems that utilized technologies such as gunpowder, flywheels, gravity, and hydraulics.
Up until the end of World War II, the hydraulic systems were most common on aircraft carriers,
but this changed in the 1950’s when the Royal Navy introduced the first steam-powered
aircraft catapults.
Steam catapults quickly became the method of choice for assisted takeoffs, while hydraulic
systems remained in use for arrested landings, and these systems are still widely used today.
The 10 Nimitz-class supercarriers of the United States Navy are all equipped with these systems,
although the Navy does have plans to replace this fleet with a new class of carriers that
will use electromagnetic catapults.
The first ship of this class is the Gerald R. Ford, which was commissioned in 2017 following
heavy criticism of the new electromagnetic system, which has been reported to have a
probability of critical failure that is 9 times greater than the traditional steam-powered
system.
The United States and other countries have also adopted alternatives to assisted takeoff
and arrested recovery such as ski jumps and short takeoff and vertical landing aircraft
like the Harrier jet.
Nonetheless, this particular video is going to focus on the engineering behind the steam
catapults and hydraulic arresting systems that are used on the American Nimitz-class
aircraft carriers.
The flight deck on each of these ships is equipped with four 100 m long catapults.
2 are located at the front, or bow, of the ship, and 2 are located on the left, or port,
side of the ship.
The 2 catapults on the port side cross an angled landing runway, which spans from the
back, or stern, of the ship all the way to the front of the port side.
The runway is angled like this so that pilots can abort a landing and take off again without
running the risk of crashing into planes and personnel at the front of the ship.
Each catapult is powered by steam generated by the two nuclear reactors that power the
carrier.
Prior to a launch, the steam is collected under high pressure in a large accumulator
tank that is located underneath the catapult.
The pressure inside the accumulator is monitored by the catapult officer as it is filled, and
a flow control valve is closed once the desired pressure is reached.
The catapult officer is commonly referred to as a shooter, and they operate the catapults
from a small control pod that protrudes above the flight deck.
The steam pressure that is required for each launch depends on the weight of the aircraft,
and the maximum operating pressure is around 3.2 MPa or 465 psi.
As the accumulator is being pressurized, an aircraft is positioned at the beginning of
the catapult and a jet blast deflector is raised behind the aircraft by hydraulic actuators
to protect equipment and personnel from the jet blast.
A tow bar on the nose gear of the aircraft is connected to a shuttle that protrudes through
a slot in the flight deck, and a metal bar called a holdback is secured to the back of
the nose gear to hold the aircraft in place until the catapult is fired.
The shuttle is attached to 2 spear-shaped pistons that run inside parallel steel cylinders
with open slots in the top.
These cylinders are positioned just below the flight deck in a long trench, and they
run the entire length of the catapult.
Flexible strips are used to seal the open slots in the cylinders to prevent steam from
escaping, and the shuttle assembly simply bends the strips out of the way as it travels
down the length of the catapult.
The shuttle assembly is also fitted with wheels that run in a track between the cylinders
and the underside of the flight deck.
Just behind the shuttle, there is a mechanical device called a grab that travels along the
same track.
This grab can be moved along the track by a hydraulic cylinder and a system of cables,
and it is used to retrieve the shuttle from the end of the catapult following a successful
launch.
After the aircraft is connected to the shuttle, a hydraulic tensioner is used to push the
grab forward slightly, which in turn pushes the shuttle and moves the aircraft forward
in order to eliminate any slack in the system.
When the pilot is ready for takeoff, they throttle their engines to full power, however
the holdback initially prevents the aircraft from moving forward.
As soon as the catapult is fired by the shooter, a launch valve in the accumulator is opened,
and the steam surges into the two cylinders.
The force from the steam pushes the pistons forward with enough force to break the holdback,
and the shuttle accelerates the aircraft forward towards the bow of the ship.
The catapult is capable of launching a 20 metric ton aircraft from 0 to 265 km/hr in
just 2 seconds, while the pilot experiences nearly 4 g’s.
For reference, this is more than twice the acceleration that you would experience on
the world’s fastest roller coaster, Formula Rossa at Ferrari World.
To accomplish this, the catapult needs to exert around 750 kN, or just under 170,000
lbs of force.
At the end of the catapult, the spear-shaped pistons plunge into a water brake that consists
of water-filled cylinders, which brings the pistons and shuttle assembly to a stop.
This also triggers a sensor which closes the accumulator launch valve and opens an exhaust
valve to release the spent steam.
The grab is then moved to the end of the catapult by the hydraulic system, where it latches
to the shuttle and pulls it back to the starting position.
This marks the completion of a single launch cycle, and the next aircraft can be moved
into position to start the whole process over again.
At peak operating efficiency, the flight deck crew can launch two aircraft every 40 seconds
using the 4 catapults onboard.
Although the mechanical systems of the aircraft catapults are quite complex, the takeoff procedure
is relatively simple from the perspective of the pilot.
Landing a plane back on the flight deck, on the other hand, is far more difficult and
requires a great amount of skill from the pilot.
There are 3 to 4 steel cables called arresting wires that span the width of the landing runway
at the stern of the ship, and the pilot must catch one of these cables using a tail hook
that is deployed from the rear of the aircraft.
It is necessary for the aircraft to approach the runway at a precise angle in order to
accomplish this, and so an optical guidance system is used to help guide the pilots in
for landing.
The guidance system is comprised of many lights and lenses that are mounted adjacent to the
runway, and the pilot will see different colors and patterns depending on the angle of their
trajectory as they approach the ship.
As soon as the aircraft touches down, the pilot throttles the engines to full power
as a precaution in case they were not able to catch one of the arresting wires.
If they miss the cables, then the aircraft needs to gain enough speed to takeoff again
since the landing strip is too short for aircraft to stop under their own power.
If the pilot is successful in catching one of the cables, then the kinetic energy of
the aircraft is absorbed by a hydraulic system that is located beneath the runway, which
brings the plane to a stop.
The end of each arresting wire is attached to a steel cable and pulley system that wraps
around a hydraulic cylinder.
As the cables are drawn out by the landing aircraft, the hydraulic cylinders are compressed,
which forces hydraulic fluid out of the cylinders and into two piston accumulators.
The pressure inside the accumulators increases as they are filled, reaching a peak pressure
of about 4.5 MPa, or 650 psi, just before the aircraft is brought to a complete stop.
Once the aircraft is detached from the arresting wire, the hydraulic fluid is released back
into the cylinders, which causes them to extend and retract the wire.
This system is capable of stopping an aircraft travelling at 240 km/h in just 2 seconds over
a distance of about 100 m.
The aircraft catapults and arresting systems on modern aircraft carriers employ some impressive
engineering to make these giant floating airbases possible.
Without the ability to launch and land planes on a confined flight deck, warships like the
Nimitz-class supercarriers simply wouldn’t be feasible.
These incredible ships are among the largest and most complex vehicles ever built, and
they are sure to remain as some of the greatest military assets that United States has at
their disposal for many years to come.
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