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File:Marquardt RJ43-MA-9 Ramjet Engine - sectioned.jpg|[[Marquardt RJ43]] ramjet cutaway museum exhibit. A ramjet is a propulsive duct in which high velocity air is converted into pressure in a diffuser, heat is added and the air leaves with a higher velocity. For this particular supersonic ramjet compression takes place starting at the tip of the inlet spike and ending at the red-coloured high-blockage grid, this length constitutes the diffuser. Combustion occurs from the beginning of the cylindrical section to the nozzle and expansion takes place in the convergent-divergent nozzle.
File:Pratt & Whitney JT3.jpg|[[Pratt & Whitney J57]] turbojet (1/4 scale model). A turbojet uses its thermodynamic cycle gas as its propelling jet. The jet velocity exceeds the speed of a subsonic aircraft by too great an amount to be an economical method of subsonic aircraft propulsion. The purpose behind the jet engine is to convert fuel energy into kinetic energy of the cycle air but after the thrust-producing momentum has appeared the unwanted byproduct is the wake velocity which results in kinetic energy loss, known as residual velocity loss (RVL). The wake velocity behind a turbojet-powered aircraft at subsonic speed is about 600 mph. At maximum propeller-driven speeds, the wake velocity behind the propeller it replaced as a thrust producer is about 10 mph with an insignificant RVL.<ref>{{Cite book |last=Smith G. Geoffrey |url=http://archive.org/details/in.ernet.dli.2015.19428 |title=Gas Turbines And Jet Propulsion For Aircraft |date=1946}}</ref> It is impossible to transform completely the kinetic energy acquired inside the engine into thrust work. The whole increase in kinetic energy obtained inside the engine is expended in thrust work and losses of kinetic energy outside the engine. There is thus kinetic energy inside the engine which will remain unused. In the case of the stationary engine before take-off the whole kinetic energy turns into losses since the thrust force does no work.<ref>{{Cite report |url=https://apps.dtic.mil/sti/citations/AD0722283 |title=Theory of Jet Engines |language=en}}</ref>
File:Klimov VK-1F (1948) used in MiG 17 at Flugausstellung Hermeskeil, pic2.jpg|[[Klimov VK-1]]F turbojet with afterburner. An afterburner is a propulsive duct in which high velocity exhaust from an engine turbine is converted into pressure in a diffuser. Afterburner fuel is burned with the oxygen in the dilution air which was not involved in the engine combustion process. The gas expands in a nozzle with an increase in velocity. The turbojet afterburner has the same three requirements as a ramjet, both being propulsive ducts. These are conversion of high velocity gas into pressure in a diffuser, combustion and expansion to a higher velocity in a nozzle.
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Since the introduction into service of the bypass principle in xx a progressively greater proportion of bypass air compared to that passing through the power-producing core has been enabled by increases in core power per pound a second of core airflow (specific core power).
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