Jetpack

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 apparent 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 . 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. The first pack design was developed in 1919 by the Russian inventor Alexander Andreev. This hot gas is used exclusively as the reaction mass and is fed directly to one or more jet nozzles. The great disadvantage is the limited operating time. The jet of steam and oxygen can provide significant thrust from fairly lightweight rockets, but the jet has a relatively low exhaust velocity and hence a poor specific impulse.