Harrison Martin takes a jet pack for a test flight at the annual Experimental Aircraft Association Airventure Fly-in in Oshkosh, Wis. Martin hovered for 45 seconds in the contraption on Tuesday. At left is Martin's father, Glenn Martin, who designed the jet pack.

OSHKOSH, WIS. ? This isn’t how a jet pack is supposed to look, is it?

Hollywood has envisioned jet packs as upside-down fire extinguishers strapped to people’s backs. But Glenn Martin’s invention is far more unwieldy: a 250-pound piano-sized contraption that people settle into rather than strap on.

As thousands looked on Tuesday, the inventor’s 16-year-old son donned a helmet, fastened himself to a prototype Martin jet pack and revved the engine, which sounded like a motorcycle. Harrison Martin eased about three feet off the ground, the engine roaring with a whine so loud that some kids covered their ears.

With two spotters preventing the jet pack from drifting in a mild wind, the pilot hovered for 45 seconds and then set the device down as the audience applauded.

The Martin jet pack can – in theory – fly an average-sized pilot about 30 miles in 30 minutes on a full 5-gallon tank of gas. The apparatus was unveiled Tuesday at AirVenture Oshkosh 2008, the annual aviation convention of the Experimental Aircraft Association.

“Wow, that went better than expected,” Glenn Martin said afterward. “People will look back on this as a moment in history.”

That remains to be seen. Federal regulations limit the use of such devices, and it’s unclear whether people will shell out $100,000 for a jet pack whose capabilities have been demonstrated on paper but not in the air.

The Martin jet pack is designed to conform to the Federal Aviation Administration’s definition of an ultralight vehicle, which weighs less than 254 pounds and carries only one passenger.

Although the FAA could always change its mind, the ultralight designation means riders won’t need a pilot’s license.

But don’t expect to see commuters rushing to work by air instead of land. Ultralights can’t be operated over congested areas, according to FAA regulations, and are to be used “exclusively for sport or recreational purpose.”

That’s fine, Martin said. He predicts the jet packs will start out as toys for the wealthy. Then, as law enforcement officials become more familiar with them, Martin envisions jet packs used by the military, border-patrol officials and search-and-rescue teams.