In what might seem counter-intuitive at first, the U.S. Army supported the development of a helicopter that had no engine. One can even visit the Army's Aviation Museum at Fort Rucker in Alabama to catch a glimpse of this design by the American Helicopter Company that is fondly called Jet Jeep.
The Jet Jeep was thought of many decades ago as the solution for a light observation needed by the Army. The U.S. Army was looking for a flight-capable option for light surveillance and by that, it meant enough to carry one or two people at the most. This is quite like the problem jet pack makers are trying to solve these days. But this was way back in the 1950s and helicopters and aircraft were largely the way flying worked.
So, the U.S Air Force took upon this task and made a lighter version of the helicopter, XH-26, by skipping the bigger engine. Instead, it put two AJ7.5-1 pulse jets at the end of each of its rotors and was also successful in avoiding the transmission system, which reduced its weight further, the U.S. Army's website said.
The prototype that resulted from this experiment weighed under 300 pounds (136 kg), was collapsible, and could be put into a storage container that could then be towed by a Jeep. It just needed two men to put it back together and that could be accomplished in under 20 minutes, according to the National Museum website of the U.S. Air Force.
The jet pulses also offered an additional advantage. One could skip the tail rotor system that is used in helicopters to deliver the anti-torque capability. However, as the Army began testing the prototype, the flaws of the design came to the fore. The jet pulses were extremely loud and gave the position of the aircraft's location at night.
If one of the pulse jets failed, the aircraft would simply crash and that's where another major problem was uncovered. Due to its small footprint, the XH-26's fuel tank was located just under the pilot's seat and did not safeguard the pilot from sustaining a serious injury in any way.
Even though the likelihood of carrying out surveillance using the aircraft was out of the question, the Army reportedly attempted a two-person aircraft a few years down the line. It built a total of five prototypes to test the concept time and again but finally shelved the project and the Jet Jeeps have become a part of the Army and Air Force's museums.