Fail 1: Stubborn DIY
Charles Zimmerman’s Flying Shoes (1947) was really two fans spinning in opposite directions, attached to the pilot’s feet. “What he had was, in essence, two upside-down lawn mowers,” Lehto says. “It doesn’t take a genius to look at that and say, ‘That could kill somebody.’ ”

Fail 2: No fail-safe
De Lackner Helicopters’ DH-5 Aerocycle (1955) featured large helicopter-like blades that a pilot stood atop, propelling him upward. It worked, but: “Imagine what happens if you lose your balance. You’d fall through those blades and disappear.”

Fail 3: Marketing overhype
Thiokol’s Jump Belt (late 1950s) featured a row of small propulsion canisters that encircled the pilot’s back. Its makers claimed that it could let you run 20 miles per hour or jump into a second-story window—“insane promises that could never be met by manufacturers.”

Fail 4: High risk, low reward
Bell AeroSystems’s Rocket Belt (1961) was a backpack made of two tanks of hydrogen peroxide and one tank of nitrogen, which together produce steam. It worked safely for 21 seconds. Doubling that would risk a chemical explosion.

Why We Still Don't Have Jet Packs

Sometimes, struggling innovators just need to take the hint. Steve Lehto, author of the new book The Great American Jet Pack, argues that was the case with this—"the most overpromised technology of all time." Its history is a lesson in unwise persistence.

[Illustrations by Jonathon Rosen]

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