Do you love launching from 0 to 190 kph in less than 4 seconds? Then the Top Thrill Dragster is perfect for you. Motion Control’s editor, Kim Roberts, spoke to Monty Jasper, corporate vice president for safety and engineering at Cedar Fair in Sandusky, Ohio, about the engineering behind the Top Thrill Dragster.
Rated tops for aggressive thrill, the ride lasts less than 20 seconds but in that time it sends riders from 0 to 190 kph in less than four seconds, twisting them in spirals and hurling them up and over a 140-metre tower before plunging straight back down again, twisting through 270, to be brought to a halt within 50 metres by an eddy-current magnetic braking system where powerful rare earth magnets contactlessly oppose the motion of travel. The ride is specifically designed to simulate take-off from an aircraft carrier. Jasper says that Cedar Fair chose a hydraulic launch because of the power density and precision control it can achieve. The launching power is needed for only a few seconds, and hydraulics has the inherent ability to store energy and release it rapidly with controlled acceleration and velocity. “We didn’t have much space for this ride, but hydraulics did the job,” explains Jasper.
So how do you accelerate a 7-ton loaded coaster train from zero to 190 kph in only four seconds? You harness the power of 32 hydraulic engines.
The launch system uses two identical hydraulic systems which are highly automated. Each has 16 hydraulic motors that drive an internal ring gear. The pair of ring gears drives a dual-input planetary gearbox, which in turn drives a sheave that pulls a cable to launch a coaster. Fluid to the motors flows through a 5 cm 6-wire hose, and 15 cm hose is used for return flow to minimise pressure drop. Each motor is supplied by its own pump, so the launch system consists of 32 pumps and 32 motors. An auxiliary hydraulic drive provides power to pull the cable back to the loading area, where it engages the next coaster.
System operating pressure is 320 bar and peak power is 7,5 mW. Each pump-motor combination relies on a piston-type accumulator and gas pressure vessels to supply the majority of flow for a launch interval. A piston inside each accumulator separates its wet side from its dry side. The wet side is filled with hydraulic fluid. The dry side contains pressurised nitrogen and is plumbed to the gas pressure vessels, which are also filled with compressed nitrogen.
Before a launch occurs, each of the 32 pumps forces oil into the wet side of its respective accumulator. The additional volume of oil pushes on the piston and forces nitrogen out of the dry side and into the gas bottles. In less than a minute the gas bottles have achieved a full pressure charge, so the system can shift to a standby mode. A train enters the station, and attaches itself to a catch car that has cables pulling it. These cables are wound around a drum at one end like a fishing reel and a take-up reel on the back end.
When the launch sequence begins, an electrical signal opens a two-way valve, routing pressurised fluid from the wet side of the accumulator to the respective hydraulic motor. “The system acts like a spring,” he continues. This surge of fluid combines with flow from the respective pump to spin the motor up to speed, spin the drum, drag the cable and pull the train down the track and accelerate you from zero to about 190 kph. “It sling-shots you, just like an aircraft carrier when it launches a plane off the deck,” Jasper says.
The coaster also demonstrates numerous energy transformations through the short ride. There are five main areas where energy transformations take place. At launch the coaster has 100% mechanical kinetic energy because it is moving and has no height. When the train is about half way up the hill the cart has about 45% mechanical kinetic energy and about 45% gravitational potential energy. About 10% of the energy has left the system as sound kinetic energy and thermal kinetic energy. When the roller coaster is at the very top the passenger car stops for a split second. This is because all the energy left in the system is now potential gravitational energy, which means the coaster has no motion energy. Here there is about 75% potential gravitational energy, and about 25% of the total energy has now left the system. Halfway down the hill there is about 30% mechanical kinetic energy and 30% gravitational potential energy, and about 40% of the energy has left the system. At the point where the cart comes to a stop there is no energy in the system. All energy has left as sound, thermal and mechanical kinetic energy in the form of vibrations transferred to the track and breaks. All of these energy transformations happen in less than 20 seconds.
Two simple hydraulic valves ensure that riders are held safely and securely in place by a padded lap bar. The accumulator is physically attached to the cylinder, and each lap bar uses two of these circuits. Each cylinder has its own holding circuit, so one assembly is redundant. An operator pushing down on the lap bar causes the pair of double-acting cylinders attached to it to retract. This forces fluid from the cap-end of the cylinder into the rod-end. A check valve in the line connecting the rod-end port to the cap-end port prevents the cylinder from extending, so the lap bar cannot move upward. However, the volume of the cap-end of the cylinder is greater than that of the rod-end. So when the cylinder retracts, excess fluid from the cap-end flows into the accumulator. Once the coaster train reaches the exit area, an electrical signal energises the solenoid valve of all restraint circuits. When this occurs, the valve shifts, allowing fluid to flow freely from the rod-end of the cylinder to the cap-end. Pressurised fluid from the accumulator is also released and aids in lifting the lap bar.
Jasper says that a system with such high pressures and requiring such high precision is complicated and it took Cedar Point a few years to get the Top Thrill Dragster operating at its peak performance. “Maintenance and system evaluation are critical. It’s a cool system but it’s highly complicated,” he concludes.
© Technews Publishing (Pty) Ltd | All Rights Reserved
printer friendly version