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 ), forming in this case a steam-gas mixture at . 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. 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 very 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. In 1962 a backpack was created at Bell Laboratories, following Justin Capră's prototype. The backpack is now displayed in a museum where