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Top Thrill Dragster is one of the few roller coasters in the theme park world that requires no introduction.
Although this thrill machine was constructed more than 15 years ago, it has managed to
stand the test of time, and it remains as one of the top-ranked roller coasters in the
world today. When Dragster opened to the public in May
of 2003, it offered an experience that was unmatched by any other coaster, launching
guests from 0 to 193 km/hr in just 3.8 seconds, and catapulting them 128 m into the air at
a 90-degree angle over the massive top hat element.
At the time of opening, it was not only the tallest and fastest roller coaster on Earth,
but it was also the world’s very first strata-coaster; An exclusive title reserved only for closed-circuit
roller coasters exceeding 122 m in height. Even today, Top Thrill Dragster is one of
only two strata-coasters in existence. The second one being Kingda Ka at Six Flags
Great Adventure, which opened in May of 2005. Top Thrill Dragster and Kinga Ka were both
built by Intamin Amusement Rides in collaboration with Werner Stengel, and they are both variations
of Intamin’s hydraulically-launched roller coaster model known as the accelerator coaster.
The first accelerator coaster was aptly named Xcelerator, and it opened at Knott’s Berry
Farm in June of 2002, setting the groundwork for the taller and faster coasters that would
come later. Although this video will focus on the engineering
behind Top Thrill Dragster specifically, most of the information will also apply to the
other Intamin accelerator coasters like Kingda Ka and Xcelerator, however the exact figures
may vary from ride to ride depending on their size.
So now that the introduction is out of the way, let’s jump right into the mechanics
of Dragster. Once a train is loaded with passengers and
it is ready to be launched, it is propelled forward to the beginning of the launch track
by rubber drive tires that are powered by electric motors.
The catch car is then positioned underneath the train by a steel cable system that we
will discuss in more detail shortly. The catch car is a long piece of steel that
rides in a trough down the middle of the launch track, and it connects to the underside of
the third car of the train. There is a V-shaped groove in the top of the
catch car, which connects with a metal pin known as a launch dog that is dropped down
from the bottom of the car. The launch dog is normally held in place by
two permanent magnets, however when the train is positioned in the launch area, an electrical
contact is made between the train and the track which allows current to flow through
wire coils that are wrapped around the magnets. This generates a temporary magnetic field
that opposes the permanent magnetic field from the magnets, thus allowing the launch
dog to drop down onto the catch car. After the launch dog is dropped, the rubber
drive tires are retracted, and since the launch track is sloped upwards at a slight angle,
the train then rolls backwards and the launch dog slides into the groove in the catch car.
When the train is launched, the force between the train and the catch car holds the launch
dog in place, and when the catch car reaches the end of the launch track, the train simply
overruns the catch car, and the V-shaped groove pushes the launch dog back into place where
it is once again held by the permanent magnets. It is extremely important that the launch
dog retracts, because there’s always a possibility that the train will not clear the top hat
element, and it could fall back towards the launch track at nearly 200 km/hr in what’s
known as a roll-back. If the train were to reconnect with the catch
car at that speed, the result would likely be quite destructive.
After the train overruns the catch car and heads up the tower, the catch car is slowed
down using eddy current brakes, and it is returned to the beginning of the launch track
by the steel cable system for the next train. The steel cable system consists of three separate
cables, which connect the catch car to a large rotating drum that is powered by the hydraulic
launch system. Two launch cables are connected to the front
of the catch car, and they run in grooves down the length of the launch track.
At the end of the launch track, two sets of pulleys guide the cables into the hydraulics
building where each cable wraps around one end of the drum.
A retractor cable is connected to the back of the catch car, and it wraps around a pulley
at the beginning of the launch track before running along the bottom of the track to the
hydraulics building. The retractor cable wraps around the center
of the drum, and it is wound in the opposite direction to the launch cables.
When the drum spins clockwise in the orientation shown, the launch cables are wound onto the
drum which pulls the catch car to the right, while the retractor cable is unwound from
the bottom of the drum. Conversely, when the drum spins counterclockwise,
the retractor cable is wound back onto the drum which pulls the catch car back to the
left, while the launch cables are unwound from the top of the drum.
Every time a train is launched, the two launch cables are put under a great amount of tension,
which causes the cables to stretch under the load.
In order to keep the cable system taut, the pulley at the beginning of the launch track
is equipped with a tension regulator to pick up the added slack in the retractor cable.
The pulley is housed inside the launch track and it is positioned on a horizontal track
of its own, which allows it to move back and forth as the cables change length.
The tension regulator connects the pulley to the launch track, and I’m fairly certain
that it simply uses a pre-loaded spring to apply force onto the pulley in order to maintain
a minimum tension in the retractor cable. The force on the pulley does not need to be
that high since the retractor cable is not directly loaded during a launch, but the cable
does need to remain taut so that it does not slip off of the pulley.
On the other hand, the amount of force that is needed to launch one of Dragster’s trains
is huge, and we can estimate this force using Newton’s second law by multiplying the average
mass of a train with its acceleration. According to the park, an empty Top Thrill
Dragster train has a mass of about 15 tons, or 13,600 kg.
Each train can carry 18 passengers, and the average mass of an American adult is 82 kg,
so we can approximate the total mass of the passengers as 1,500 kg.
This gives an average total mass of about 15,100 kg for a fully loaded train.
The ride accelerates the trains from 0 to 53.6 m/s in 3.8 seconds, which corresponds
to an acceleration of 14.1 m/s2 or about 1.4 g’s.
Using these figures, the average force required to launch a train can be calculated as approximately
213 kN, or 48,000 lbs. Since there are two launch cables pulling
the train, the force in each cable would be half of this value.
In reality, the actual force would also be slightly higher because the hydraulic launch
system needs to overcome friction and drag. In addition to force, we can also estimate
the average amount of power that the hydraulic system needs to provide in order to launch
a train. At top speed, the kinetic energy possessed
by a train can be calculated as ½ of its mass multiplied with its velocity squared.
Using the velocity and average mass of a fully loaded train from before, this gives about
21.7 MJ of energy. I also did some back-of-the-envelope calculations
to estimate that an additional 3.1 MJ of energy needs to be provided to overcome friction
and drag during a launch. This means that on average, the hydraulic
system needs to provide about 24.8 MJ of energy in just 3.8 seconds.
These figures correspond to a power output of about 6.7 MW, or just under 9,000 HP, which
is actually comparable to the power output of a real top fuel Dragster.
The technicians who work on the ride estimate that the hydraulic system is capable of generating
between 10,000 and 15,000 HP at its peak output, however I was not able to confirm an exact
value. The exact amount of power provided by the
hydraulic system varies between each launch, and it depends on the mass of the passengers,
as well environmental conditions such as humidity, atmospheric pressure, and temperature, which
affect friction and drag. Before launching a train, the ride computer
uses data from the previous 3 launches to estimate how much power it needs to provide.
Sensors along the track are used to measure the speed of each train, and if the trains
are running slower than expected, then the hydraulic system will provide additional power
for the next launch. Conversely, if the trains are running faster
than expected, then the hydraulic system will provide less power.
This is the reason why a sudden change in the weather can cause a train to stall on
the top hat element and roll back down the tower.
The massive hydraulic launch system itself is housed inside a masonry building that is
located at the end of the launch track. The large rotating drum that we saw earlier
is located at the center, and 16 red hydraulic gear motors are positioned around the circumference
on each side, for a total of 32 motors. On each side, the 16 motors connect to a massive
steel gearbox, which transfers power from the individual motors to a single output shaft.
I have seen a few sources claiming that this as a planetary gearbox, however I was able
to reach out to the company that manufactured it, and they confirmed that this is not actually
the case. Inside the gearbox, each hydraulic motor connects
to one of 16 helical pinion gears that are arranged in a circular fashion around a central
bull gear. These pinions work together to spin the bull
gear, which is connected to a large hollow output shaft.
Although this arrangement looks similar to a planetary gearset, it is fundamentally different
because there is no ring gear around the outside. The output shaft from each of the two gearboxes
is connected to the 2 m diameter cable drum, and at top speed, this drum spins at approximately
540 RPM. That’s equivalent to 9 full rotations per
second. To accomplish this, each of the red hydraulic
gear motor uses two internal gears to convert the flow of high-pressure hydraulic fluid
to rotational motion. As fluid passes through one of the motors,
it is forced to flow around the outside of the two gears, which causes them to rotate.
One of the gears drives the output shaft of the motor, which is connected to one of Dragster’s
two gearboxes. The hydraulic system that supplies the high-pressure
hydraulic fluid to power the motors is comprised of 4 identical subsystems, each powering 8
of the 32 motors. To make things a little easier to follow here,
let’s isolate one of the subsystems to see how it works on its own.
At the core of the system is a hydraulic piston accumulator, which is used to store hydraulic
fluid under high pressure. The accumulator is divided into two chambers
that are separated by a floating piston, and one of the chambers is filled with nitrogen
gas. This nitrogen chamber is connected to additional
back-up bottles that allow the system to use a greater volume of gas.
Hydraulic fluid is pumped into the second chamber by a 500 HP hydraulic pump, which
compresses the piston and pressurizes the gas.
The hydraulic fluid is practically incompressible, and the pressure inside the accumulator is
dependent on the volume of gas displaced by fluid.
Prior to a launch, the accumulator is pressurized to about 32 MPa, or 4,600 psi, and this process
takes about 45 seconds. When the ride operator hits the launch button,
the cartridge valves at the end of the accumulator are opened, and the pressurized hydraulic
fluid is delivered to the motors which spin the drum, reeling in about 100 m of cable
in just 3.8 seconds. As the hydraulic fluid passes through the
motors, it is collected in a large reservoir to be used again for the next launch, and
the whole process starts again. The exact same process also takes place in
the other 3 hydraulic subsystems simultaneously, and they have to work together in perfect
unison to power the ride. While the accumulators are recharging in between
launches, an auxiliary motor is used to spin the drum in the opposite direction, which
unwinds the launch cables and returns the catch car to the beginning of the launch track.
In total, the entire system contains approximately 15,000 L, or 4,000 gal, of hydraulic fluid,
and it is designed to launch a train about every 60 seconds.
Now ofcourse if a train is going to be launched 128 m into the air at 193 km/hr, then there
also needs to be a way to slow the train down, and this is accomplished with eddy current
brakes. The brake run at the end of the ride has fixed
metal fins that protrude up from the track, and these align with permanent magnets that
are mounted on the bottom of the trains. The fins are conductive but non-magnetic,
and as a train passes over them, the magnetic field from the permeant magnets induces circular
electric currents, called eddy currents. These electric currents create a magnetic
field of their own, which opposes the magnetic field from the permanent magnets.
This generates a drag force that acts on the moving train opposite to its direction of
travel, and the magnitude of this force is proportional to the train’s velocity.
Once the train is slowed down, rubber drive tires are used to bring it to a stop before
moving it into the station. In order to handle roll-backs when a train
does not clear the tower, Dragster’s launch track is equipped with a braking system of
its own that functions in the same way as the final brake run.
However, the metal fins on the launch track can be moved up and down using spring-loaded
pneumatic cylinders so that they do not generate a braking force while a train is being launched.
The springs inside the cylinders hold the brake fins above the track, and just before
a train is launched, the cylinders are pressurized, which compresses the springs and lowers the
fins. As the train speeds down the track, proximity
sensors are triggered, which tell the pneumatic cylinders to depressurize so that the fins
are raised back to their default position. The spring-loaded mechanism inside the cylinders
ensures that the braking system is fail-safe because the brake fins will rise automatically
in the event of a power failure. The ride also uses more brake fins than are
necessary to stop the trains, and the computer will not allow a train to be launched if any
of the sensors are not working properly. In total, there are about 400 moving brake
fins along the launch track, and the ride uses more than 800 sensors.
The experience on Top Thrill Dragster may only last 17 seconds, but it takes many complex
systems all working together to make it happen safely and reliably.
From the towering steel structure all the way to the powerful launch system, you really
have to admire just how much engineering went into this ride.
Of course, whether you choose to do that from the ground, or from 128 m above it, is entirely up to you.
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