An Accelerator Coaster is a hydraulically-launched roller coaster model from Intamin. The model usually consists of a long, straight launch track, a top hat tower element, and magnetic brakes that smoothly stop the train without making contact. The technology was developed by Intamin engineers as an alternative to electromagnetic launch systems, such as the Linear Induction Motor (LIM) and Linear Synchronous Motor (LSM), that are found on earlier launched roller coasters like the Flight of Fear and The Joker's Jinx. Unlike the earlier linear induction motors, the Accelerator Coaster's launch system exhibits constant acceleration and is capable of reaching greater speeds.
Accelerator Coasters with the top hat element are known to perform an occasional rollback – when the train is unable to complete the top hat element and rolls back to the starting point – which can occur due to a number of different factors. Despite this, Accelerator Coasters have a great track record for safety and are extremely power efficient, especially compared to older technologies such as the classic chain lift found on most roller coasters. Formula Rossa, the world's fastest Accelerator Coaster, and Kingda Ka, the world's tallest, are some of the most well-known installations around the world.
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 (370 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:
- The train's launch dog is released.
- 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.
- The anti-rollback magnetic brakes on the launch track retract.
- 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.
- Each section of brakes on the launch track pops back up immediately after the train passes a proximity switch.
- 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, but 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.
- 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 (the world's slowest Accelerator Coaster) has a design speed of 47 mph (76 km/h), one pump, one accumulator, and eight turbines. Kingda Ka (the world's second fastest roller coaster) has a design speed of 128 mph (206 km/h), 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 (7,500 kW).
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 100 km/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. An Accelerator Coaster's hydraulic launch system also provides constant acceleration, unlike the acceleration from electromagnetic linear induction motors which begins to decrease or trail off after the initial push.
Most Accelerator Coasters use restraints that consist of a U-shaped lap bar that locks into place. The lap bar is also secured by seat belts that run between the legs of riders. In addition, the restraint system features a unique over-the-shoulder restraint (OTSR) harness designed for comfort and fast loading times. Only three Accelerator Coasters have a lapbar restraint without the over-the-shoulder harness - Xcelerator, Top Thrill Dragster and Formula Rossa.
Another notable feature is the locking system, which uses two hydraulic cylinders for redundancy (in case one fails) rather than relying on the older ratchet design. While a ratchet-based restraint locks at one of several positions that can be too loose or uncomfortably tight, the hydraulic system allows the restraints to be pulled down and locked in any position to better match a rider's body dimensions. In the extremely unlikely case where both locking cylinders fail, the restraints are still held down by a seat belt.
The newer over-the-shoulder design allows for faster load times, as opposed to lap bars. With lap-bar designs, riders must rely on a seat belt around their waist as a backup restraint. It must be checked by ride attendants before the lapbar can be lowered, significantly slowing load times. In contrast, the over-the-shoulder restraints are simply held down with a seatbelt. This means that guests can pull down their own restraints and buckle their own belts saving ride attendants time.
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 and Kingda Ka). For layouts that have the top hat element, a mechanism is in place to deal with the occasional occurrence of rollbacks where a train fails to pass the peak height of the element and rolls backwards returning to the point of launch. Another set of magnetic brakes found at the final brake run also exist on the launch track to stop the train during a rollback.