The coaster with the moster

Forget the old clackety-clack climb traditional coasters take to the top of the first big hill. Don't expect a lot of time to enjoy the park's scenery on the way up. And don't even think about letting go of the safety bar to put your hands in the air. Top Thrill Dragster is not that kind of roller coaster. Indeed, catapulting up a 420-ft hill, going 0 to 120 mph in just 4 sec, will likely leave even the bravest coaster fanatics clinging for dear life.

Top Thrill Dragster, the newest among an army of giant coasters (including a 300-ft steel monster called Millennium Force) in Sandusky, Ohio's Cedar Point Amusement Park, is making a name for itself in more ways than one. The $25-million steel scream machine not only has the bragging rights to "world's tallest, fastest roller coaster," it lays claim to another special feature: a hydraulic launch system.

For years roller coasters have relied on chains to pull the cars to the top of a lift hill and build a reservoir of potential energy. But new propulsion systems have made it possible to launch coaster trains horizontally into the ride's elements rather than dragging them up a hill.

One catapulting system that's fast becoming a favorite uses linear-induction motors (LIM). LIM-powered coasters essentially ride a magnetic wave down the track as metal fins attached to the bottom of the trains pass stator coils anchored along the track. One such coaster at Cedar Point blasts riders out of the station five times, at five different speeds, topping out at 72 mph in 2.5 sec. Wicked Twister, as it's aptly named, uses LIMs to shoot riders up and down a U-shaped track with 450° corkscrews on top of each 215-ft vertical tower.

Another type of launching system uses electric motors to power high-speed drive tires, essentially dozens of rotating wheels arranged in rows along the track that grip the train and push it forward.

Hydraulics to the highest

The newest launch method relies on hydraulics. Though hydraulics has seen its share of amusement-ride applications, its use as a launching method is special, says Cedar Point's Rob Decker, vice president of planning and design. "We didn't set out to build a roller coaster with a hydraulic system," he says. "We left it up to the ride manufacturer, Switzerland-based Intamin AG, as to how we would top the top speed, which at the time was 93 mph produced by our own roller coaster, Millennium Force. Our goal was to set another world record, and our parameters were the tallest, fastest roller coaster. Intamin looked at all of the known roller-coaster propulsion systems and said 'We think we can get the speed you want with less length in less time out of a hydraulic system.'"

Intamin went with the hydraulic approach for basically the same reason the technology is used on stamping presses: The ability to precisely control applied force. In the case of Top Thrill Dragster, a hydraulic launch made it possible to hit top speed faster and required less upfront costs than a LIM coaster. And the hydraulic launch setup helped Cedar Point put the ride in a small footprint.

Though the technical details are sketchy on Intamin's proprietary hydraulic launch system, the operating principle is fairly simple. Hydraulic oil pumps from a reservoir into storage cylinders filled with nitrogen. "The nitrogen compresses and acts like a spring, and the hydraulic oil acts like potential energy," explains Cedar Point's Monty Jasper, vice president of maintenance and new construction. The ride operator gets a signal when pressure has built up and the train is ready to launch. Pushing the start button opens high-speed valves at the storage cylinders and sends the oil flowing to 32 hydraulic motors. Gears on each end of the motors turn a large planetary gearbox. Planetary gearboxes sit on either side of a cable-winding drum that turns at 500 rpm. "The drum works like a big fishing reel, taking up cable as it turns." Jasper explains. The launch cable attaches to a locking device called a catchcar that, in turn, attaches to the coaster train. The catchcar runs in the track and propels the train forward. Reaching the target speed of 120 mph disengages the cable from the train, closes the hydraulic valves, and restarts the process.

Hydraulics also keeps riders securely in their seats. A redundant safety system including interlocking seat belts and a hydraulic lap-bar assembly makes sure passengers stay in the train, which is decked out with dragsterlike trimmings, including tail fins and faux engines. Two hydraulic cylinders open and close the padded lap bar and lock it in place.

Once loaded and buckled in, passengers are in for a wild ride. The trains launch from a starting position and rocket straight up 420 ft, rotate 90°, crest the top, and free fall 400 ft while spiraling 270°. Under each car four road wheels ride on the track. Four side wheels position the train to the left and right, and four up-stop wheels keep the train from lifting off the track.

Unbelievably, six 16-passenger trains run on the track simultaneously. Operators control the traffic using PLCs and an advanced monitoring system. To keep trains from running into each other, the track is divided into different zones, each with a stopping mechanism and means of propulsion. Only one train at a time is allowed in a zone. Oncoming trains are held up until a block is clear. "It's a little like juggling," says Decker. "The safety systems are quite sophisticated and will not allow any human to override them," adds Decker.

Stop that train!

Once blasted from the starting position the last thing anybody wants is to see the train rolling backward after not making the 420-ft climb. That scenario is unlikely, says Decker, but Cedar Point officials are prepared nonetheless.

Traditional coasters that gradually ascend the lift hill have antirollback devices that keep trains from reversing. Those systems aren't appropriate for a coaster of Top Thrill Dragster's immensity because a train that didn't make the hill would come back down going more than 100 mph. Traditional braking mechanisms would give passengers too much of a jolt. And once stopped, there's no other propulsion method in place to get the train moving again. Intamin's solution: a magnetic-braking system.

Permanent magnets mount to the bottom of each train, and copper-alloy fins are affixed to the track. Pneumatic cylinders pull the fins down so the train can launch over top, and then let them pop back up after the train blasts off. When the fins are up, they pass between the magnets, trimming the speed and eventually stopping the cars. "Because we are using rare-earth magnets, there's no need for a power source, and they work all the time. The ride can come to a halt safely, even in the event of a total loss of power," explains Jasper.

The same system stops the trains at the end of the ride. "There are more than 600 sensors on the brakes alone," says Decker. "Two proximity sensors sit on each brake fin to make sure the fin is where it needs to be, and another sensor constantly monitors air pressure on the pneumatic line controlling the fins. The information all feeds back to a PLC and all systems must be go before any train can launch." The ride is dotted with proximity sensors, mechanical limit switches, pressure switches, and hydraulic-temperature sensors. Most are doubled up for redundancy so that no single failure would cause operators to lose sight of what's happening in a particular part of the ride.

Riders on the storm

Even with advanced safety systems in place, Top Thrill Dragster's menacing presence is bound to raise questions of just how high and fast coasters can go, and how much riders can physically take. "People assume at 420 ft and 120 mph, the forces applied to the passengers will be greater than on slower rides. That's not true," says Jasper. "Forces can be dissipated over wide curves. Even though Top Thrill Dragster is traveling 120 mph, the curve radii are long and smooth and the g forces are actually less than some of the smaller rides at the park."

The keys to g force are duration and direction: how long people feel the gs and the transitions over which they feel them. For example, on Top Thrill Dragster, riders might feel forces of 4 to 4.5 gs but only for a fraction of a second, says Jasper. "These are well within the tolerance levels set by the American Society for Testing and Materials and, in fact, are equivalent to such things as jumping on a pogo stick." Cedar Point also works with engineers to design wide, sweeping transitions instead of quick conversions, adds Decker.

Coaster designers work with what's called the heart line, keeping the CG around midchest so as not to send too much or too little blood to the brain, causing people to feel dizzy or faint. Case in point, engineers from Intamin designed Top Thrill Dragster with a 270° spiral during the 400-ft free fall. "We didn't tell them we needed to see that but Intamin thought it would be both a neat effect and a safe way to come out of the drop and transition from a vertical to a horizontal," Decker explains. "It helps push people into their seats. Though there isn't a problem of ejecting people, it's a good thing to keep passengers' CG around the middle of their body, both for comfort and to avoid lateral gs."

Before Top Thrill Dragster's maiden voyage, Cedar Point's maintenance, operations, and safety teams will have run extensive tests, says Decker. For example, water-filled torsos will simulate riders' average weight and accelerometers hooked up to crash-test dummies will read forces on the human body. Other methods will be used to study the safety envelope for riders. The ride must also be licensed by the Ohio Department of Agriculture's Amusement Ride Safety Div., and be in operation for a month prior to opening to the public.