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.