A jet pack, rocket belt, rocket pack or flight pack is a device worn as a backpack which uses jets to propel the wearer through the air. The concept has been present in science fiction for almost a century and the first working experimental devices were demonstrated in the 1960s. Jet packs have been developed using a variety of mechanisms, but their uses are limited because of factors including the Earth's atmosphere, gravity, the low energy density of extreme fuels, and the human body not being suited to flight, so 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-generating 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.
Rocket pack
Gunpowder-fueled rocket pack
In 1632, Evliya Çelebi recorded a story about an Ottoman artist and engineer Lagâri Hasan Çelebi flew a platform with seven rockets strapped to it. When the fuel ran out, Lagari jumped out with a winged glider device and landed in the Sea of Marmara. The sultan awarded him a bag of 70 silver coins and registered him as a sipahi, a heavy cavalry class soldier in the Ottoman Army. This figure existed but the event is physically impossible.
Liquid-fueled rocket pack
Andreyev: oxygen-and-methane, with wings The first pack design was developed in 1919 by the Russian inventor Alexander Fedorovich 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 1 m (3 feet) 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 steam and oxygen in less than 1/10 millisecond, increasing in volume 5,000 times: 2 H2O2 → 2 H2O + O2. The reaction is exothermic, i.e., accompanied by the liberation of much heat (about 2,500 kJ/kg [1,100 BTU/lb]), forming in this case a steam-gas mixture at 740 °C [1,360 °F]. 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 advanced rockets, but the jet has a relatively low exhaust velocity and hence a poor specific impulse. Currently, such rocket belts can only fly for about 30 seconds (because of the limited amount of fuel the user can carry unassisted). A more conventional bipropellant could more than double the specific impulse. However, although the exhaust gases from the peroxide-based engine are very hot, they are still significantly cooler than those generated by alternative propellants. Using a peroxide-based propellant greatly reduces the risk of a fire/explosion which would cause severe injury to the operator. In contrast to, for example, turbojet engines, which mainly expel atmospheric air to produce thrust, rocket packs are far simpler to build than devices using turbojets. The classical rocket pack construction of Wendell Moore can be made under workshop conditions, given good engineering training and a high level of tool-making craftsmanship. The main disadvantages of this type of rocket pack are:
Short duration of flight (a maximum of around 30 seconds). The high expense of the peroxide propellant. The inherent dangers of flying below minimum parachute altitude, and hence without any safety equipment to protect the operator if there is an accident or malfunction. Safely learning how to fly it, given that there are no dual-control training versions. The sheer difficulty of manually flying such a device. These circumstances limit the sphere of the application of rocket packs to spectacular public demonstration flights, i.e., stunts; for example, a flight was arranged in the course of the opening ceremony of the 1984 Summer Olympic Games in Los Angeles, USA.
Justin Capra's flying backpack Justin Capră claimed that he invented a "flying rucksack" (Romanian: rucsac zburator) in 1956 in Romania, and, without arousing any apparent interest, informed the American Embassy of his idea. However it was tested by Henri Coandă, a paratrooper who crashed it the first time but managed the second after he advised to change the fuel and improve on the design. In 1962 a backpack was created at Bell Laboratories, following Justin Capră's prototype. The backpack is now displayed in a museum.
Jump Belt In 1958, Garry Burdett and Alexander Bohr, Thiokol Corporation engineers, created a Jump Belt which they named Project Grasshopper. Thrust was created by high-pressure compressed nitrogen. Two small nozzles were affixed to the belt and directed vertically downward. The wearer of the belt could open a valve, letting out nitrogen from the gas cylinder through the nozzles, which tossed him upward to a height of 7 m (23 ft). While leaning forward, it was possible with the aid of the jump belt's thrust to run at 45 to 50 km/h (28 to 31 mph). Later, Burdett and Bohr tested a hydrogen peroxide–powered version. The jump belt was demonstrated by a serviceman in action, but as no financing was forthcoming, there was no further testing.
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