A silent jetpack would be like swimming in air, but it is likely beyond the physics of thrust
In "Too Hard for Science?" I interview scientists about ideas they would love to explore that they don't think could be investigated. For instance, they might involve machines beyond the realm of possibility, such as particle accelerators as big as the sun, or they might be completely unethical, such as lethal experiments involving people. This feature aims to look at the impossible dreams, the seemingly intractable problems in science. However, the question mark at the end of "Too Hard for Science?" suggests that nothing might be impossible.
The idea: "I've always loved flight," says Kamen, who pilots and owns helicopters and jets. "I would love a jetpack that doesn't make noise, a device that, without commotion, would allow me to defy gravity. You would just liberate yourself from the ground, go swimming in the air."
The problem: "The basic physics of thrust mean that if you want to fight against the gravitational field of Earth and weigh, say, 100 kilos, you need to accelerate that amount away from you at 9.8 meters per second squared," Kamen says. "Air is ethereal, it has low density, about 1 kilo per cubic meter, so you have to continuously accelerate 100 cubic meters of air at 9.8 meters per second squared or the equivalent to compensate for the gravitational attraction of Earth."
"You can try to do this by moving a lot of air relatively slowly with rotors, or a small amount of gas really fast, like with a rocket, to get enough thrust," Kamen says. "If you're using a rocket, typically the gas goes hypersonic, which means it's very loud and you need a lot of energy to do it. If you're using large rotors, those are very noisy too."
The solution? On Kamen's show Dean of Invention, he discussed NASA's concept for a super-quiet, hover-capable one-person electric aircraft named the Puffin that looks much like a jetpack with a cockpit.
"That might be close," Kamen says. "Still, I wish there was something you could strap on your belt and allow you to leap tall buildings with a single bound. I don't need Superman's cape, but I'd love that experience in every other aspect."
Image of Dean Kamen from his Web page.
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About the Author: Charles Q. Choi is a frequent contributor to Scientific American. His work has also appeared in The New York Times, Science, Nature, Wired, and LiveScience, among others. In his spare time he has traveled to all seven continents. Follow him on Twitter @cqchoi.
The views expressed are those of the author and are not necessarily those of Scientific American.