A propjet is a jet airplane that incorporates the use of a turbo-prop engine to propel itself through the air. Unlike a turbo-jet, the propjet uses the idea of a reciprocating engine prop airplane as its form of power but incorporated the concept of a jet engine to power the prop. Currently, turboprop engines are more so used on small subsonic aircraft, like charter jets with a common cruising speed of about 400 knots. However, there are some propjets that can have cruising speeds in excess of 500 knots.
The world's first turboprop engine was the Jendrassik Cs-1 and it was created by a Hungarian engineer called György Jendrassik. In 1945 the first ever-American turboprop engine was the General Electric XT31 and tool flight on the XP-81 in December. It was the first propjet to combine turboprop and turbojet technology.
Not realizing the potential of a propjet, America skipped over turboprop airliners in favor of the Boeing 707. This would later prove to be a mistake because the technology of the turbojet was fairly newer. However, the military saw the benefits of having a propjet capable yet not as efficient as a turbojet due to the relatively inexpensive price.
When people hear turbo-prop or turbo-jet they tend to associate them as the same thing the truth is they have two completely different modes of propulsion.
A propjets turboprop engine is an aircraft power plant that uses gas turbines to drive a propeller. The turbine is specifically for this so almost all of its output gets used to drive the propeller. The engine's exhaust gases contain almost no energy when compared to a jet engine and have almost no affect on the propulsion of the aircraft. In its simplest form, a turboprop consists of an intake, compressor, combustor, turbine and a propelling nozzle. Air is drawn into the intake and compressed by the compressor. Fuel is then added to the compressed air in the combustor, where the fuel-air mixture then combusts. The hot combustion gases expand through the turbine and drive the propeller.
Though today the propjet can be expensive due to its complex engines, they are greatly sort after by charter jet companies and smaller commuter business because they are more efficient at takeoffs and landings and allow for a less consumption of gas then their counterpart the turbojet. Since they have the ability to be more efficient when consuming gas they allow for a cheaper operating cost and thus increasing the charter companies profit gain.