A jet pack, rocket belt, or rocket pack is a device worn on the back which uses jets of gas or liquid to propel the wearer through the air. The concept has been present in science fiction for almost a century and became widespread in the 1960s. Real jet packs have been developed using a variety of mechanisms, but their uses are much more limited than their fictional counterparts because of the challenges of the Earth's atmosphere, gravity, the low energy density of utilisable fuels, and the human body not being suited to flight, and they are principally used for stunts. A practical use for the jet pack has been in extra-vehicular activities for astronauts because of the weightlessness and lack of friction-creating atmosphere in orbit. The term jet suit is used for a system incorporating a jet pack and associated jets attached to the arms to increase manoeuvrability (e.g. the Daedalus Flight Pack). Overview In the most general terms, a jet pack is a wearable device which allows the user to fly by providing thrust. With the exception of use in a microgravity environment, this thrust must be upwards so as to overcome the force of gravity, and must be enough to overcome the weight of the user, the jet pack itself and its fuel. This necessarily requires the jet pack to continually push mass in a downwards direction. While some designs have power and/or mass supplied from an external, ground-based source, untethered flight requires all of a flight's fuel to be carried within the pack. This results in problems relating to the overall mass ratio, which limits the maximum flight time to a few minutes, rather than the sustained flight envisaged in science fiction. Liquid-fuelled rocket pack Andreyev: oxygen-and-methane, with wings The first pack design was developed in 1919 by the Russian inventor Alexander Andreev. The project was well regarded by Nikolai Rynin and technology historians Yu. V. Biryukov and S. V. Golotyuk. Later it was issued a patent but apparently was not built or tested. It was oxygen-and-methane-powered (likeliest a rocket) with wings each roughly long. Hydrogen peroxide-powered rocket packs A hydrogen peroxide-powered engine is based on the decomposition reaction of hydrogen peroxide. Nearly pure (90% in the Bell Rocket Belt) hydrogen peroxide is used. Pure hydrogen peroxide is relatively stable, but in contact with a catalyst (for example, silver) it decomposes into a mixture of superheated