The ease with which a manta ray can zoom through the water has offered scientists and engineers an incredibly challenging task: They want to see if they can build a mechanical substitute for one of nature's best performers.
An underwater robot that could travel great distances at high speed while consuming very little energy could be useful for a wide range of projects, from carrying scientific instruments into the ocean deep to measuring pollutants in bays and waterways to, of course, unobtrusively delivering explosives to a distant target.
That's a long range goal, but first they have to figure out how the ray works its magic.
"We want to learn from the biology," Hilary Bart-Smith, associate professor of mechanical and aerospace engineering at the University of Virginia, said in a telephone interview. "We want to see how rays have found the optimal solution for underwater propulsion, or if there is another solution out there that could be even better."
Bart-Smith leads an interdisciplinary project involving scientists and engineers at four universities whose collective effort has resulted in a mechanical version of a cownose ray that looks remarkably lifelike as it swims through the water in the University of Virginia's swimming pool. At this stage, it's no match for the real thing, but it will serve as an instrument in an ongoing effort to learn from the Batoidea ray, a collection of 500 species in 13 families including stingrays and mantas.
This project is still in its early stages. The mechanical ray is controlled, for now, by a joystick linked to the robot through a tether, so it can only reach a maximum depth of 10 feet. The ultimate goal, of course, is to build a robot that performs at least as well as a real ray and can be controlled remotely, even at 10,000 foot depths, where some rays live.
But studying rays in their natural environment can be a bit tricky. They can deliver a real punch, as Captain John Smith learned in 1608 when he was stung near Chesapeake Bay so severely that his crew thought he was going to die.
Marine biologist Frank Fish (that's really his name) of West Chester University in Pennsylvania, a member of the team, traveled to the remote Micronesian island of Yap to videotape rays in their natural habitat.
He has also studied rays in his lab as they swam down a channel through water containing particles highlighted by a laser, thus showing some of the fluid dynamics at work.
The rays move forward through the water by flexing their pectoral fins, commonly referred to as "wings" because that's what they look like. But how they do that is still something of a mystery. An aircraft uses propulsion to drive itself forward, creating lift as air passes over its wings. A ray flaps its wings up and down -- but has no visible means of forward thrust.
Tetsuya Iwasaki, an expert on animal locomotion at the University of California, Los Angeles, has been addressing that question. Animals, including humans, are propelled by a linear series of electrical pulses from the brain that cause us to put one foot in front of the other, a pretty simple solution to a complex problem. But rays flap their wings up and down to produce forward thrust and maintain proper depth instead of sinking to the bottom of the ocean.
The robot now being tested in Virginia is the first major step in learning exactly how the ray does that. Work is already underway on second and third generation mechanical rays, and that will give the researchers greater flexibility. They should be able to determine how the rays react to a change in current, for example, and how it gets that forward thrust by flapping its wings.
But going from a creature made of cartilage and muscles to a device made of rods and cables and actuators is a challenge. It would be easy to add a forward thrust mechanism to the robot, but that would defeat the purpose of the research. In the end, the robot has to perform exactly like a live ray, somehow moving forward as it flaps its wings up and down in an amazingly efficient manner.
The process is called bio-mimicry: creating machines that copy nature.
"It's an amazing species to watch swimming," said Bart-Smith, who has also swum with wild rays. "It just looks effortless. It's a fast, efficient swimmer that can operate in many ocean environments from open ocean to close to shore, from near the surface to on the bottom."
The researchers didn't know if their first attempt to build a mechanical ray would be successful.
"We were excited when we took it to the pool to test it out," she said. "It was our first attempt to see what was going on. Have we got something that could actually swim? We were amazed at how life-like it looked."
She said she took the video home and showed it to her husband and her daughters.
"They said, 'That looks like a real ray,'" she recalled.
The next step will be to find a pool that's more than 14 feet deep -- the maximum depth of the university's swimming pool -- and free the robot from its tether. They will also need to develop a communication system so they can control the device remotely, even at great distances.
Then it will really look like a ray. And that could pose a new problem. Build a mechanical ray, paint it realistically, let it loose in the ocean, and it may look so realistic that sharks will try to prey on it .
"If it looks like a ray and swims like a ray there's a potential for a predator thinking it would be a tasty morsel," she said.