Accelerator Coaster

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Storm Runner from Hersheypark.

The Accelerator Coaster is Intamin AG's term for their hydraulically-launched roller coaster model. These models are the tradename for some of the fastest rides on earth.

An accelerator coaster is named because of its long, straight launch track, its tower, known as a "top hat", and its magnetic brakes that smoothly stop the train without even touching it. After the top hat, the layout varies widely, ranging from a flat brake run to several inversions.

If this type of coaster is not launched fast enough to clear the top hat (which can happen for several reasons), it will roll backwards down the tower and along the launch track. For this reason, the launch track is fitted with retractable brakes that are retracted for the launch and extended at all other times. The main brake run uses the same type of brakes, which are fixed in place.

An Accelerator Coaster's hydraulic launch is much smoother than other launch technologies such as linear motors. While a linear motor-launched train's acceleration is greatest at the beginning of the launch and decreases throughout the launch, a hydraulic launch produces nearly constant acceleration throughout the launch.

Most accelerator coasters are launched from the station, but there are some that advance the train to a separate launch area, either for theming reasons (Superman Escape) or to allow multiple trains to be loaded simultaneously (Top Thrill Dragster, Kingda Ka).

[edit] Restraints

Over-The-Shoulder restraints found on most Accelerator Coasters. These are the restraints for Kingda Ka.

Accelerator coasters are noted for their unique over-the-shoulder restraints, which are used on all accelerator coasters except the first two, Xcelerator and Top Thrill Dragster, which use lapbars. These restraints feel like lapbar restraints while still providing the safety of traditional over-the-shoulder restraints. They consist of two major parts: a thick padded metal bar that swings down into one's lap, and two thin, flexible shoulder straps. For all but the largest riders, the shoulder straps won't even be noticeable.

These restraints are also notable for their locking system, which uses two hydraulic cylinders (for redundancy, in case one fails) rather than a ratchet. While a ratchet-based restraint only locks at each notch and will often be too loose or uncomfortably tight, the hydraulic system allows the restraints to be pulled down to any position, where they will stay. In the extremely unlikely case that both locking cylinders fail, the restraints are also held down by a seatbelt, as are most over-the-shoulder restraints.

These over-the-shoulder restraints also allow trains to be checked faster than with lapbars. The lapbars use a seat belt as a backup restraint. It must be checked by the ride attendants before the lapbar is lowered, as it cannot be checked properly with the lapbar down. This means that the guests must not pull down their own lapbars, but instead should buckle their own seatbelts, after which the attendants will check the seatbelts and lower the lapbars.

In contrast, the over-the-shoulder restraints are simply held down with a seatbelt. This means that the guests can pull down their own restraints and buckle their own belts, and all the ride attendants need to do is push down on the restraints and pull the seatbelts tight, saving significant time over the lapbar and seatbelt system.

[edit] How it works

The catch-car connects to the train to launch it.

An Accelerator Coaster's launch system operates on the same basic principle as a Super Soaker, but on a much larger scale. The coaster's power source is several hydraulic pumps, each capable of producing 500 horsepower (373 KW). These pumps push hydraulic fluid into several accumulators. These accumulators are divided into two compartments by a movable piston, one side filled with hydraulic fluid and the other with nitrogen gas. The nitrogen is held in large tanks directly beneath the actual accumulator. As the hydraulic fluid fills the accumulators, it pushes on the pistons, compressing the nitrogen. It takes approximately 45 seconds to pressurize the accumulators with all pumps operating. All of this pressure is released during each launch, which typically lasts between 2 and 4 seconds.

The heart of the launch system is a large winch, around which the launch cables are wound. This winch is driven by hydraulic turbines. The two launch cables are attached to the winch on its ends, and run through two grooves on top of the launch track. The cables are attached to the sides of the catch-car, which runs in a trough between the grooves. A third, single retractor cable is attached to the rear of the catch-car, it runs around a pulley wheel at the rear end of the launch track and returns to the hydraulic building along the bottom of the launch track, where it is wound in the opposing direction on the winch's drum.

The train connects to the catch-car with a solid piece of metal known as a "launch dog" that drops down from the center car. The launch dog is normally retracted and is held in place by a small magnet, but the launch area has electrical contacts that demagnetize the magnet and cause the launch dog to drop down. The launch dog drops down at an angle, similar to the chain dog that a lifted coaster uses to connect to the lift chain.

Once the train and catch-car are in position and all is clear, the operator presses the "Launch" button and the launch sequence begins:

1. The train's launch dog is released.
2. The drive tires that advance the train to the launch track retract. Because the launch track is sloped slightly upwards, the train rolls backwards a few inches, until it is stopped by the launch dog engaging the catch-car.
3. The anti-rollback magnetic brakes on the launch track retract.
4. Approximately five seconds later, the launch valves in the hydraulic room open. The compressed nitrogen in the accumulators forces the hydraulic fluid into the turbines that drive the winch. As the winch winds in the launch cables, the retractor cable is unwound from the winch. After the train moves off the electrical contacts in the launch area, its launch dog is held down only by the force of the accelerating catch-car.
5. Each section of brakes on the launch track pops back up immediately after the train passes a proximity switch.
6. When the train reaches full speed and all the pressure in the accumulators has been released, the catch-car, still connected to the train, enters its braking zone. The catch-car uses the same braking configuration as the train and is much lighter, so it slows down very quickly. As the catch-car begins to slow down, the train's launch dog retracts - the shape where it drops into is a "v" shape, so the dog is forced back into position as it runs over the catch-car and is held in place by the magnet, as the train continues on its way.
7. Once the catch-car has stopped, the launch system resets - the winch reverses direction, returning the catch-car to the launch area using the third retractor cable, and the pumps begin recharging the accumulators. This normally takes about 45 seconds, after which the next train can be launched.

If the train rolls back, it will be brought to a near stop (magnetic brakes cannot completely stop a train) well before the beginning of the launch track. Regardless of the position of the catch-car when the train passes it going backwards, there will be no interference as the train's launch dog will be retracted. After the train slows to a near stop, the brakes will be cycled up and down to control the train's speed until it is back in launch position. On the larger coasters, this "launch reset" process can take more than a minute as the train must be moved very slowly. Once the train is back in launch position, it can be launched again or can be returned to the station.

The basic launch sequence is often accompanied by various theme elements. The most common is "starting lights" that cycle down from yellow to green, the green light coming on just as the train begins to accelerate.

The number of pumps, accumulators, and turbines varies with the speed the coaster is designed to achieve. Kanonen has a design speed of 47 MPH, one pump, one accumulator, and eight turbines. Kingda Ka has a design speed of 128 MPH, seven pumps, four accumulators, and 32 turbines. The system as a whole is capable of producing up to 20,800 horsepower (15.5 MW) for each launch, although a typical launch uses less than 10,000 horsepower.

The catch-car is stopped by magnetic brakes identical to those used to stop the train. In order to give the catch-car room to slow down, only about three quarters of the launch track can actually be used to launch the train, the catch-car needs 64 feet(20m) on a 100km/h accelerator coaster (and significantly more on a faster coaster like Kingda Ka) to slow to a full stop.

One major advantage of this launch system compared to others is its low power consumption, the hydraulic pumps run constantly and actually use less energy than most chain lift drive motors^[1].

[edit] List of Accelerator Coasters