Engineering Expedition Everest, complete with a yeti

Aug. 10, 2006
Walt Disney Imagineers faced a monster of a challenge when they planned Walt Disney World's latest park attraction, Expedition Everest.

The yeti is supported by a structural beam connected to its back. The beam attaches to a vertically mounted actuator that gives the creature 2 ft of vertical motion. The vertical actuator, in turn, connects to a sled that moves 5 ft horizontally on metal ways to give the yeti back and forth motion.

At just under 20-stories high, the man-made mountain in Disney's Expedition Everest is the tallest peak in Florida. It took three years and more than 38 miles of rebar, 5,000 tons of structural steel, and 10,000 tons of concrete to build the mountain.

They chose to build the largest mountain in Florida, lay track through that mountain, and populate it with one of the largest, fastest-moving robotic creatures in history — a 25-ft tall, 20,000-lb yeti, the Abominable Snowman. Riders see the simian yeti only as a quickly darting silhouette or hear it as a lonesome howl before they get up close and personal with it.

"As the ride goes along and guests get closer to the yeti, the more real he becomes, building excitement and bringing the ride to a climax when they meet faceto-face," says John Van Oort, principal mechanical engineer for the Show Mechanical Group, Disney Imagineering, Glendale, Calif.

The yeti relies on Disney's Audio-Animatronics technology, which was first developed in the 1960s to animate 3D robots of characters from Disney movies. And although Disney won't say what that technology consists of, or even whether it's analog or digital, the results are "awesome," according to those who have ridden the Expedition Everest. We also know that hydraulics power much of the yeti's movements.

"It has 19 axes of motion, but it's a bit different than past Audio-Animatronic creatures, " says Van Oort. "Because he is so large and heavy, and moves so fast, he could not be balanced and supported on his legs." Instead, a structural boom connected to his back keeps the entire mass airborne. It also keeps much of the workings out of sight.

The boom attaches to a vertically mounted linear actuator that gives the yeti about 2 ft of vertical motion. The vertical actuator connects to a sled that moves about 5 ft fore and aft, letting the yeti approach or retreat. The sled supports such a large cantilevered load that bearings could not handle it. Instead, the slide rides on metal ways. "We had to revert to sliding ways, which is World War Two-era technology," explains Van Oort.

With the boom supplying motion, the legs, along with the left arm, are unpowered. "They all just go along for the ride," says Van Oort. But Imagineers do get lifelike motion out of the legs as they are "puppeteered" by the rest of the moving yeti structure.

Moving parts of the creature include the pelvis (side bend, fore bend, and twist), neck ( forward or hunch, turn, nod, and tilt), and the right shoulder, elbow, wrist, and fingers.

At the heart of the hydraulic system is a 60-gallon pressurecompensated pump capable of putting out 400 gpm. A duplicate pump acts as an online spare. The pump feeds a rack of eight 15-gallon accumulators (again, one is an online spare), which act as storage. The accumulators feed hydraulic lines going out the boom and into hydraulic cylinders in the yeti. Cylinders moving larger masses, such as the pelvis actuator, have a 3.25-in. bore, while those for lighter movements, such as the neck, use 1.5-in cylinders.

"The show, or the time the yeti is onstage is so short, that the net volume needed to perform a show is only 25 gallons," notes Van Oort. He also likes to point out that if all the actuators in yeti were turned on at the same time, it would generate more thrust than a 747 jetliner.

For some yeti movements, such as those for his eyes, the Disney team chose servocontrolled pneumatics. "The 6-inchdiameter eyes are small compared to the other yeti components, so we use a compact pneumatic in the head, with the valve close to the actuators so there's little noise or lag."

The yeti performs the same movements for each show and systems engineers had to ensure yeti movements mimic those of a living creature and look real from a wide range of angles. To get the profiles right for all 19 actuators, engineers use a control box built in the 1960s by Disney specifically for "electronic puppeteering."

The finishing touch on the yeti is his furry skin. It covers a layer of Spandex that protects the skin from the moving parts beneath. The 1,000-ft2 skin is held in place by 1,000 snaps and 250 zippers.

But even without the yeti, Disney's Expedition Everest is a firstclass roller coaster with plenty of yanking and banking. And Disney Imagineers used the yeti and Himalayan setting to transform the coaster into a mini movie. Park guests are not so much riders but rather participants traveling through the scenario.

Guests first climb aboard a "steam-powered" train for a trip through a Himalayan mountain pass. But that illusion would be shattered if they heard the familiar sound of a pawl clicketyclacking over antirollback teeth as the coaster ascends the first hill. To get rid of that telltale sound, Disney has a special wheel on the underside of the coaster that only touches and turns on a rail on the first uphill section. The wheel generates magnetic eddy currents that lift the pawl and keep it from knocking against the antirollback teeth.

To complete the illusion of a steam locomotive, Disney Imagineers placed steam vents under the platform. When the train comes into the boarding area, steam pours up between the train and loading platform. Steam also winds through special venting to emerge from the train's smokestack.

One of the big challenges in building the track was a section in which the train stops on a steep hill for about 6 sec, long enough for riders to see the railroad tracks torn up in front of them and hear a yeti wailing in the distance. Then they zoom backward down the track and race away from the yeti. In those 6 sec, however, Disney engineers had to switch the track to let the coaster continue backward. They devised a plan in which the track needed for the backward run is underneath the regular section. When the coaster stops, four pneumatic actuators —two pulling and two pushing — spin the 200,000-lb length of switch track 180°, slowing down prior to the final position engagement to minimize wear.

The entire length of track, 4,424 ft, moves and vibrates as the train carrying 34 passengers barrels down hills at 50 mph and careens up and around a 60° banked spiral. The track sits on supports designed to be dynamic, so they can move a bit without yielding. Designers also had to snake the track and its supports through a rigid and static structure that carries the skin or outer surface of the 20-story mountain. The outer structure cannot move, not even a little. Its outer rockwork is plaster, so if it moves, it cracks. And Disney artists spent months carving and shaping that plaster to make the edifice look like a real mountain. One of their newfound techniques for getting crisp, sharp edges on the rock facade was to press aluminum foil onto roughly shaped outcroppings and faces.

An engineering challenge in detailing the outside of the mountain was that regular vertical scaffolding wouldn't provide a practical work platform. It would be too far from the face of the rock, especially on higher slopes. Traditional scaffolding would also block the view of the mountain. And much like sculptors who must be able to see what they are doing, Disney's crew of mountain artists had to see their work to get it right.

The solution was to let 2,000 interior support beams or tabs extend about 5 ft through the skin of the mountain to serve as attachments for planks. Workers stood on the planks to complete the rockwork, and the resulting structure didn't obscure the view of the mountain as it was being finished. After the work was done, all the supports were cut away, a clean-up task that took three months.

The outside skin of the mountain also relied on these tabs. After generating 24 clay models of the rocky crag, engineers scanned and digitized the final version, turning it into a CAD model. They added coaster tracks, support structures, and all ancillary wiring, ducting and engineering work to the model. The final detailed CAD model took Imagineers 18 months to create, instead of the three to four years normally required. The outside of the mountain was then divided into 3,000 rebar "chips," each with a unique and predetermined position on the mountain. These chips are attached to tab arms and give the mountain its rough shape.

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