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This is not 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 113-kilogram 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 a metre 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 50 kilometres in 30 minutes on a full 19-litre tank of gas. The apparatus was unveiled Tuesday at AirVenture Oshkosh 2008, the annual aviation convention of the Experimental Aircraft Association in east-central Wisconsin.

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

That remains to be seen. U.S. federal regulations limit the use of such devices, and it is 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 115 kilograms and carries only one passenger.

Although the FAA could always change its mind, the ultralight designation means riders will not need a pilot's licence.

But commuters are not likely to rush to work by air instead of land – ultralights cannot be operated over congested areas and are to be used “exclusively for sport or recreational purpose.”

That's fine, Mr. 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, Mr. Martin envisions jet packs used by the military, border-patrol officials and search-and-rescue teams.

His white jet pack with black trim stands on a brick-sized base with two legs sprawled behind it. The pilot steps backward into the straps of a shoulder harness, his shoulder blades resting against two wide upward-facing fans that provide the thrust.

An emergency parachute is effective above about 120 metres, and an impact-absorbing undercarriage that can soften a rough landing or shortfall, Mr. Martin said.

He is still refining the safety features for those heights in between.

“A lot of it comes down to how do you fly, at what speed, at what angle,” he said.

Like Kent Couch, the Oregon man who flew almost 380 kilometres earlier this month with 150 helium balloons attached to his lawn chair, Mr. Martin always wanted to soar through the air. He quit his job as a pharmaceutical sales rep to launch his jet-pack company.

Mr. Martin says venture capitalists are backing him, although he did not reveal nay names.

Reaction to the test flight was mixed. Attendees with aviation backgrounds raved, calling it an engineering marvel and saying the 45-second flight was fantastic proof that the idea works. Others who hoped to see the machine go higher and move in different directions seemed generally disappointed.

Mr. Martin began taking orders Tuesday for jet packs to be delivered at next year's AirVenture, although he is keeping his sales expectations in check. After all, other entrepreneurs who chased the idea for about 50 years were unable to get off the ground.

German scientists experimented with jet-pack technology during the Second World War as a way to help soldiers avoid mines.

Then scientists at Bell Labs produced a version that ran on hydrogen peroxide and provided a few seconds of lift.

Later a California company spent six years and millions of the military's dollars on the 2.4-metre-tall SoloTrek Exo-Skeletor Flying Vehicle. During a disappointing 2002 test flight the machine hovered about a metre off the ground for 19 seconds.

Two other companies are trying to sell jet packs now. Tecnologia Aeroespacial Mexicana in Cuernavaca, Mexico produces a custom-made rocket belt that costs $125,000. It uses hydrogen peroxide to power 20-second flights, according to the company's Web site. The rocket belts are mostly sold for use in advertising and promotions, such as halftime appearances at football games.

Jet Pack International, based in Denver, produces two hydrogen-peroxide models and one $200,000 jet pack that runs on jet fuel. An average-sized pilot could travel about nine minutes and 17 kilometres on the 19-litre tank, the company said.

Jet Pack has “hundreds” of people on a waiting list for its jet fuel pack, spokeswoman Kelly McLear said. She would not say when it would be available.

“Our No. 1 priority is safety,” Ms. McLear said. “We're not going to put a product on the market unless we've checked it a million times over and worked all the bugs out.”

No other major companies have revealed plans to produce jet packs.