Note: last post for a little while thanks to real life concerns making a nuisance of themselves. Will post again this time next week. Probably.
Sultana Josh asks
Jetpacs. How would they work?
This should be phrased “Jetpacks. How do they work?” Because jetpacks aren’t sci-fi. They’re a real, working thing. You know that bit in Bond flick Thunderball where Sean Connery straps on a jetpack to escape some angry pursuers? That sequence is real. The flight is real. What the stuntman is using there is something called the Bell Rocket Belt developed for the US Army in the early 1960s, and it works using the fundamental principle of any rocket or jet engine: shoot reaction mass out the back to provide thrust. The Bell version used a mix of nitrogen and hydrogen peroxide which decomposed to superheated steam1 and oxygen when exposed to a catalyst which was then blasted out of a pair of curved nozzles. On the front of the belt was a pair of joysticks which the wearer could use to change the facing of the nozzles, the direction of the thrust, and the direction of his travel.
So we’ve had functioning rocket packs since the 1960s. Why haven’t they caught on in a big way? The Wikipedia article mentions two factors, one of which was obvious to me and the other which you don’t often think about but is actually kind of important: not only do jetpacks only have room for extremely limited quantities of fuel (the Bell could fly for 20 seconds, meaning that short hop in Thunderball was actually all it was capable of) but they operate at altitudes too low for parachutes to function. This means that if something goes wrong the person wearing it is super-dead. While they are spectacular they are also impractical, expensive and dangerous compared to other forms of transport; the US Army concluded that helicopters would be far more useful and discontinued the Bell contract in the mid 60s, and that’s pretty much been the end of serious jetpack research. Enthusiasts and speciality equipment manufacturers still make and sell their own models but they are not significantly better than the fifty year-old Bell design, and they are unlikely to ever become so.
Jetpacks – or more accurately, rocket packs – in space are more effective. Since they are operating in a more or less Newtonian environment and do not have to constantly act against the Earth’s gravity the fuel expenditure involved is much less gratuitous. You may be familiar with this picture of astronaut Bruce McCandless in freefall around the Earth. Strapped to his back is something called the Manned Manuevering Unit, which actually functions more like the reaction control systems found on most spacecraft than it does a classical jetpack: it used small squirts of reaction mass to set the astronaut in motion and gave him a great deal of fine control over where he was going. The MMU was used on three missions in 1984 reasonably successful, but after the Challenger disaster in 1986 it was viewed as too much of an unnecessary risk to the astronaut when the tasks it was used to carry out could be accomplished more safely by robotic arms and tethered spacewalks. So even in space they’re viewed as too impractical compared to more boring forms of doing the job, which is pretty much the concept of jetpacks in a nutshell.
- And by “superheated” I mean 740oC, so I hope Bond was wearing some fire-retardant underwear. ↩