Ups and Downs of Jetpacks: This is a really fantastic article about jetpacks and rocketbelts, that mythical pair of technologies that would finally deliver us into the world of the Jetsons we were promised when we were kids.
The article explains a lot of things about the technology that I’d always wondered about, primarily why the only working version can only fly for 25 seconds.
Not only do rocketbelts have limited fuel capacity, but they use this fuel inefficiently compared with a propeller engine, says Wells. A propeller works by accelerating a large amount of air to relatively low speed — a few hundred kilometers per hour — whereas a rocket accelerates a smaller amount of gas to several thousand kilometers per hour. A rocket engine is most efficient at converting its fuel into thrust when moving close to the speed of its exhaust velocity — when the relative velocity of the exhaust and the atmosphere is a minimum. But a rocketbelt is used to simply hover or move relatively slowly, so this is a very inefficient way to fly.
Worse, hydrogen peroxide is a “monopropellant” — it isn’t burned or combined with anything else — so pilots must carry all the fuel they need on their back. A jet engine, on the other hand, combines a combustible fuel with oxygen from the atmosphere, which does not have to be carried on board.
And just in case you get bored, there’s a murder in there. And a kidnapping.