The failure to develop a useful jet pack is primarily due to the physics of making a human being fly. We're just not aerodynamic creatures. Nothing in the shape of a human creates lift when we're moved through the air. That means that a jet pack has to create all the lift with pure thrust. Generating that much thrust uses up a lot of fuel -- quickly, too. The true barrier to useful jet pack development is the weight of fuel. We have jet packs, but the reality is that they only work for about 30 seconds. Adding more fuel to prolong flight time would make the jet pack heavier, requiring even more fuel. See the dilemma?
The fuel weight issue has severely limited the usefulness of jet packs. Thirty seconds of flight time isn't enough to do anything other than look for a good place to land. This brings up the second major problem with jet packs: safety. Strapping a rocket or jet engine to your back is inherently dangerous. Shooting yourself up into the air, knowing you only have 30 seconds to get back down, makes it even more dangerous. Since every extra pound that you carry reduces flight time even more, there's not much room for back-up safety systems.
The final flaw with jet engines is one most people don't consider when they fantasize about soaring smoothly through the sky: noise. If you've ever been near a jet engine or a large rocket when it was running, you know they are incredibly loud. One of the early proposed military uses for jet packs was for reconnaissance; however, as soon as the U.S. Army realized that any soldier scouting with jet pack would be heard by the enemy literally miles away, they knew it would never work. The noise would be a problem even in applications where you're not in danger of being shot at. Imagine even one jet pack equipped construction worker in a crowded city. The deafening noise would cause a lot of problems for other workers and anyone unfortunate enough to live or work nearby.
Of course, in the more than 50 years since the first jet packs were developed, we've made some amazing advancements in technology. Surely, if we applied our best scientific minds to the problem, we could overcome these flaws and create functional, useful jet packs, right? It's entirely possible that we might, but there's no demand. It turns out there's not much use for jet packs beyond the, "Hey, cool, jet pack!" factor. Just about any use you can think of for a jet pack can be accomplished with a far cheaper and more reliable technology. It's also not very efficient to transport a single person by air. If we need to get someone somewhere through the air, we can use an airplane or a helicopter and take several people or even some extra cargo.
What was the driving force behind the creation of the first jet pack? Who wanted it built and why? Take a look at the next page to find out.