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Personal Jet PackFew ideas for getting from A to B have gripped popular imaginations in the modern era as strongly as the personal jet pack. The ultimate gravity- and traffic-defying machine has appeared in comic books, science fiction novels, Hollywood films, and circus side shows for decades.Although people aren't flying around with rocket-propelled backpacks today as Sean Connery's James Bond once did (Thunderball, 1965), inventions of fiction, sketch pad and exhibition have surprising links to real-world transportation systems.

The rotors of Leonardo da Vinci, the lightweight materials of Buckminster Fuller, the robust designs of military strategists: Ideas like these helped to inspire the lineage of today's air, land, and sea vehicles. Even a flight of fancy like Harry Potter's "Knight Bus" has an analogue in the efforts under way to better match mass transit to customer demand. National Geographic's Great Energy Challenge pulled together a look at some amazing inventions that could point the way to a smarter transport future, and some that are just plain fun.

Just over 50 years ago, a pioneering jet pack pilot named Harold "Hal" Graham maneuvered a 140-pound "Rocket Belt" by Bell Aerosystems for 112 feet (34 meters). Graham flew "at an altitude best measured in inches," Bell rocket engineer Robert D. Roach later wrote. But on that chilly April morning at Niagara Falls Airport, Graham completed "man's first controlled, individual free-flight with a rocket-belt."

The dream is alive and well today. This photo shows a flier strapped into a jet pack from Jetpack International over Denver, Colorado. The company makes three jet pack models, each weighing 180 pounds. One model, sold only to specially trained pilots, can fly for an estimated nine minutes or 11 miles (18 kilometers), up to 250 feet (76 meters) in the air.

Two other models, built for demo use only, run on hydrogen peroxide. The mild antiseptic and hair bleach can be used as a rocket propellant in concentrated form. (It doesn't burn but releases a great deal of energy when it decomposes in the presence of a catalyst.) But as an energy alternative, hydrogen peroxide has disadvantages, including the amount of energy required to produce it. And don't expect to go far in one of these demo jet packs; maximum flight time is less than a minute.(Related: "As Jet Prices Soar, A Green Option Nears the Runway")--Josie GarthwaiteThis story is part of a special series that explores energy issues. For more, visit The Great Energy Challenge.

Photograph courtesy Jetpack International