The jet-pack design was patented in 1919, but practical models haven't been available until the past decade. Engineers spent the 20th century showing off versions able to fly for just 20 to 30 seconds. So, what took so long to develop jet packs that can fly for long amounts of time? Richard Browning, the founder of Gravity Industries, explains why efficient jet packs have taken so long to arrive and what his company's version can do.