A motorjet, also called a thermojet or hubrid jet, is an early type of propulsion system in which a separate power source, usually a piston engine, drives a compressor. This compressor forces cold-intake air into a combustion chamber, where the air is ignited and accelerated through the exhaust, providing thrust. Motorjet technology is similar to that of modern ducted engine fans. Many aviation researchers experimented with this form of propulsion system in the early 20th century, but it was overtaken by turbojet technology shortly after World War II. Although motorjets are considered all but obsolete at the commercial level, devoted hobbyists regularly employ motorjet technology when building models and RC aircraft.
Rene Lorin first experimented with motorjets in 1908. The Frenchman intended motorjets to be placed on the wings of aircraft to provide thrust, which would have been feasible because motorjets were independent from the crankshaft. Others picked up on the concept and used the motorjet in their own designs. A motorjet built by Henri Coanda powered the first jet-propelled flight in 1910, although historians dispute his claims that the aircraft ever flew. The term was patented in 1917.
The two most commonly known aircraft to employ the motorjet are the Caproni Campini N.1 and the A.I. Mikoyan-M.I. Gurevich I-250(N). The Caproni Campini was an Italian research experimental aircraft developed in 1939 and studied into the early 1940s. The term thermojet was coined by Campini at this time. The Mikoyan-Gurevich (MiG) was a Soviet jet aircraft that was the only motorjet ever used by a military. It flew from 1946 to 1950 and was considered a hybrid, as it also had a nose propeller.
Late in World War II, the Japanese also experimented with the concept. They developed the Tsu-11 motorjet that was used in the Japanese Ohka kamikaze plane. It was reasoned that use of the Tsu-11 would overcome a major liability of the plane. It had to get very close to enemy aircraft in order to strike and was frequently shot down before it could carry out its mission, a problem the Japanese hoped the Tsu-11 would correct. An example of this is in the Smithsonian National Air and Space Museum in Washington, DC.
In 1942, Americans investigated motorjets in the Naca Jeep project, but never completed research because of interest in turbojets. The German company BMW experimented with motorjets and turbojets in the early 1940s. In the late 1940s, interest in motorjets had declined because turbojets tend to be more fuel-efficient than motorjets.
As the need for more powerful jet aircraft grew, turbojet technology became the norm. Some argue that motorjets are actually more fuel-efficient than turbojets at the lower speeds, lower altitudes, and shorter flights which characterize most air travel. Although motorjets are generally considered a primitive type of jet power for commercial use, many airplane hobbyists enjoy them since they are easy to build.