Jet pack, rocket belt, rocket pack, and similar names, are various types of device, usually worn on the back, that are propelled by jets of escaping gases (or in some cases liquid water) so as to allow a single user to fly.
The concept of these devices emerged from science fiction in the 1920s and popularised in the 1960s as the technology became a reality. Currently, the only practical use of the jet pack has been extra-vehicular activity for astronauts. Despite decades of advancement in the technology, the challenges of Earth's atmosphere, Earth's gravity, and the human body (which is not well suited for this type of flight)[citation needed] remain an obstacle to its potential use in the military and as a means of personal transport.
During World War II, Germany made late-war experiments of strapping two wearable shortened Schmidt pulse jet tubes of low thrust to the body of a pilot. The working principle was the same as the Argus As 014 pulse jet that powered the Fieseler Fi 103 flying bomb, though the size was much smaller.[citation needed]
The device was called "Himmelstürmer" (Heaven stormer) and operated as follows: when the flier ignited both engines simultaneously the tubes began to pulse modulate. The angled rear tube strapped to the flier's back provided both lift and forward thrust while the chest mounted deflector tube of lower thrust maintained a constant upward thrust. This lifted the flier up and forward. By opening the throttle to the rear tube, calculated "jumps" could be made of up to 60 meters (180 ft) at low altitude (under 50 ft, 15 m). The tubes consumed very little fuel but not much could be carried either.
The intended use for this device was for German engineer units to cross minefields, barbed wire obstacles, and bridgeless waters. The device was never intended for troop use, despite the imaginative depiction of it in that role in the comic book and film The Rocketeer (which was a prop bearing no resemblance to the real device).
At the end of the war this device was handed over to Bell Aerosystems which tested it on a tether out of fear of harm as no test flier was willing to risk his life with the German machine.[citation needed] What became of the device is not known.[citation needed]
The fictional device used by The Rocketeer was a rocket pack that was technically unique (at least in the film adaptation) because it was designed to remain cool. The Himmelstürmer, by comparison, never operated long enough to get extremely hot and both tubes were angled away from the body of the flier. In operation the thrust difference between pulse tubes acted as a push/pull/lift system. Flight time for jumps was in seconds with no lengthy descent time as altitude was minimal. As soon as the throttle was disengaged the device was shut off, a very simple operation and there was no report of any casualties.
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