Roller boaster

An amusement park may seem like a world away from the factory floor, but the development of the world’s tallest, fastest roller coaster presented its creators with a familiar set of design problems. Jon Excell reports.

The traditional roller coaster has a simple – yet appealing – formula. The interminable cranking of the chain as it drags the cars to the top of the hill. The simultaneous build-up of potential energy and rider excitement. The moment when time stands still and the train teeters on the brink. And finally – to an accompaniment of screams – the inevitable furious descent.

Courtesy of a groundbreaking hydraulic launch mechanism, Top Thrill Dragster, the world’s fastest and highest roller coaster bucks this trend with style.

At its dramatic setting on the shores of Lake Erie, in Ohio’s Cedar Point amusement park, this terrifying ride doesn’t give passengers the chance to worry, reaching 124 mph in just four seconds and immediately catapulting its cars 420ft into the air. Just to ensure that riders don’t get bored, the train also twists through 90 degrees during the climb and 270 degrees as it races back to the station.

Monty Jasper, vice-president of maintenance at the park, explained exactly how the launch mechanism works. The ride begins when an 18-passenger train arrives in the launch position and waits. Underneath the launch position a catch-car hooks to the train. This is essentially the leading edge of a slingshot that throws the train down the track. The slingshot is made of two forward cables that run around a big spool and another cable that pulls the catch-car back after the car has been launched.

To prime the ride for launch oil is pumped into a number of cylinders that each contain a piston. On the other side of the piston is nitrogen gas. As the cylinder fills with oil the piston compresses the nitrogen, forcing it into an accumulator. When the gas is fully pressurised the system is held by four servo valves.

As soon as these valves are opened the compressed nitrogen acts like a giant spring and forces the oil out of the cylinders and through 32 hydraulic motors. These motors – which Jasper likens to the motors found in a hydrogen electric dam – turn two big planetary gearboxes on either side of the spool and catapult the train out of the station. After launch the oil flows back to the tank, to be reused for the next shot. The system generates nearly 9,000hp.

But why a hydraulic system instead of, for instance, linear induction motors (LIM), which are already widely used in the ride industry?

The ride’s Swiss manufacturer, Intamin AG, spent a great deal of time evaluating different launch mechanisms. But Jasper explained that while it would be possible to generate as much speed with linear motors, they would take up far too much space.

‘The reason a hydraulic system was specified,’ he said, ‘is that it can get the train up to speed in a much shorter footprint/distance than would be possible with LIM. This keeps the ride as small as possible – around 1,100ft long. Had we used a LIM system we would have had to lengthen the overall ground footprint of the ride and that would have been disastrous for the park,’ he added.

Having accelerated the trains to such frightening speeds the ride’s designers also had to develop a method of bringing them to a complete halt in around 600ft, and the design of the braking system is just as ingenious as the launch mechanism.

Permanent magnets are mounted to the bottom of the trains. The magnets are mounted vertically underneath the train and put together in an attractive fashion so that there’s a field between them. Copper alloy fins are mounted on the track and to stop the train are popped into position and passed through the magnetic field generated by the train’s magnets. This creates a resistive force that slows the train down. ‘It’s uncanny,’ said Jasper. ‘There’s no metal-to-metal contact, no ropes and no pulleys – it’s like magic.’

From conception to completion the ride took three years to develop and cost around $25m (£15m). Rob Decker, corporate vice president of planning and design for Cedar Point, explained that, just like any other machine, roller coasters are subject to heavy software simulation and design before a single component is manufactured.

One area where simulation plays a particularly important role is in predicting G-forces. Nervous passengers could be forgiven for thinking that the speeds and angles encountered on the top-thrill dragster would subject riders to pretty extreme G-forces – but this, explained Decker, is something of a misconception. The G-forces experienced on a roller coaster are, he claimed, akin to those created by ‘flopping on to a couch’.

The important thing about G-force, he said, ‘is how long it is experienced, which for roller coaster riders is only a fraction of a second and for astronauts is more like 20 minutes.’ G-force is also generated during a change of motion, thus ride engineers concentrate on designing wide, sweeping transitions instead of quick conversions.

Just in case you’re still not convinced, the safety testing is said to be even more rigorous than that employed throughout the automotive industry. Tests include the use of water-filled torsos to simulate the average weight of riders, accelerometer tests conducted by biodynamic engineers and analysis with a crash test dummy hooked up to a laptop to give readings of what the human body will experience. ‘The same technology that allows for taller and faster roller coasters simultaneously helps engineers produce more advanced safety systems,’ said Decker.

The ride has not been without its teething problems. It was grounded for a couple of months shortly after it opened, although according to Jasper, it has now been running continuously since the beginning of August.

The problem revolved around the mechanism used to return the catch-car to its original position.

‘The system was designed well to launch the train but was having a hard time bringing the catch-car, which weighs a tonne, back into position at slow speed and positioning it within millimetres,’ explained Jasper.

This caused cavitation in the hydraulic motors – a common problem in hydraulic systems where low fluid levels draw in air. The cavitation led in turn to delamination within the motors and caused steel particles to get into the oil.

The solution is to return the catch-car at high speed, explained Jasper. ‘We then turn the system off and stop the catch-car with brakes in reverse. This spares the system moving the catch-car at slow speeds,’ he said.

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