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Most roller coasters rely on fear-inducing heights and speed from gravity to excite riders. Top Thrill Dragster, at Cedar Point, Sandusky, Ohio, is no exception. However, instead of relying on a chain or cable to laboriously pull the coaster up its first hill -— in this case, a 420-ft hill — Top Thrill Dragster relies on kinetic energy to propel coasters up its hill in mere seconds.

Top Thrill Dragster accelerates coaster trains from zero to 120 mph in about 4 sec. At that speed, coasters have just enough kinetic energy to scale that monstrous hill. From there, gravity pulls the coaster down the other side of the hill, where the coaster once again reaches speeds of about 120 mph.

The biggest challenge to this application was finding a technology that could accelerate a 12,000-lb coaster train — plus the weight of 18 passengers — to 120 mph in 4 sec. Cedar Point's Monty Jasper, vice president, maintenance, reveals that ride designers initially considered the latest coaster technology — linear induction motors — to accelerate Top Thrill Dragster coasters.

"Linear motors would have been energy efficient and required little maintenance, but they wouldn't have been able to accelerate Top Thrill Dragster to 120 mph in four seconds," explains Jasper. "They would have required a longer approach to the hill, and had we gone that route, we would've had to remove or relocate two nearby attractions. Plus, riders would have been denied the excitement of such high acceleration."

Energy rollback

An important aspect of the launch system is that coaster trains accelerate to a speed that allows them to just barely scale the hill. Jasper continues, "When the coaster reaches the top of the hill, ideally, it will have just enough momentum to roll beyond the highest portion of the hill. From there, gravity takes over and pulls the coaster down the other side of the hill."

A ride computer regulates the amount of energy transmitted for each launch. In addition to operator inputs, the computer also receives signals from switches and sensors throughout the attraction. For example, a pair of inductive sensors mounted near the top of the hill detect when a coaster passes by. Because the sensors are spaced a known distance apart, the computer can calculate velocity by measuring the time interval between each sensor activation. If calculations indicate a coaster has gone too slow or too fast, the computer commands the launch system to increase or decrease speed of the next launch.

Jasper explains, "The computer actually evaluates speed of the last three coasters and calculates an average. This determines speed of the next coaster. So if three consecutive coasters are filled with lightweight passengers, then a bunch of football players fill the next one, that fourth coaster probably won't have enough momentum to make it up and over the hill."

When a coaster train doesn't have enough energy to conquer the hill, it rolls back toward the launch station. Jasper explains, "If a conventional coaster doesn't make it up the first hill, it's because of a malfunction. Plus, mechanical catches are always in place to keep conventional coasters from rolling back. This would allow evacuating the coaster — an extremely unlikely event.

"But Top Thrill Dragster is not a conventional coaster. It goes virtually straight up and straight down. So we don't want to stop the coaster somewhere up that 420-ft hill to evacuate passengers. The simple solution, then, is to let the coaster roll back. It is not a malfunction, and we let guests know ahead of time that a coaster could roll back."

When a coaster occasionally does roll back, it doesn't come charging into the loading area at 120 mph. Instead, a permanent magnet braking system automatically decelerates the train if it should roll back. Each car contains copper fins that fit between slots of a series of permanent magnets situated along the length of track between the launch station and hill. When a coaster is ready for launch, pneumatic cylinders move the permanent magnets out of the way so the coaster can pass. But once a coaster passes, the magnets move back into place so they can stop a coaster if it rolls back.

Because the braking system uses permanent magnets, coasters will be stopped even if a power failure occurs after a coaster is launched. Plus, they are actuated pneumatically in a fail-safe arrangement that requires an electrical signal to move the magnets out of the way for launching. So if power fails, the brakes will automatically be positioned to stop a coaster. A similar permanent magnet braking system is used to decelerate coasters after they have descended the far side of the hill. Position of these brakes, however, is fixed.