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Main article: Turbofan
If the propeller is better at low speeds, and the turbojet is better at high speeds, it might be imagined that at some speed range in the middle a mixture of the two is best. Such an engine is the turbofan (originally termed bypass turbojet by the inventors at Rolls Royce). Turbofans essentially increase the size of the first-stage compressor to the point where they act as a ducted propeller (or fan) blowing air past the "core" of the engine.
This type of engine runs best from about 250 to 650 mph (400 to 1,000 km/h), which is why the turbofan is by far the most common type of engine in aviation.
The bypass ratio (the ratio of bypassed air mass to combustor air mass) is an important parameter for turbofans. Early turbofans (and most modern jet fighter engines) are low-bypass turbofans with bypass ratios less than 1. However, the "large mouthed" engines on almost all modern civilian jet aircraft are high-bypass turbofans which generally have bypass ratios of 3 or more.
Turbofans (especially high bypass engines) are fairly quiet. The noise of a jet engine is strongly related to the temperature of the air coming out the back. In the turbofan this hot air is mixed with the cold air bypassing the engine, so the result is a much lower temperature. Jet aircraft are often considered loud, but a conventional piston engine delivering the same power would be much louder.
The components of a jet engine are standard across the different types of engines (noted above). The parts include:
The various components named above have constraints on how they are put together to generate the most efficiency or performance. Important here is air intake design, overall size, number of compressor stages (sets of blades), fuel type, number of exhaust stages, metallurgy of components, amount of bypass air used, where the bypass air is introduced, and many other factors. For instance, let's consider design of the air intake.
For aircraft travelling at supersonic speeds, a design complexity arises, since the air ingested by the engine must be below supersonic speed, otherwise the engine will "choke" and cease working. This subsonic air speed is achieved by passing the approaching air through a deliberately-generated shock wave (since one characteristic of a shock wave is that the air flowing through it is slowed). Therefore some means is needed to create a shockwave ahead of the intake.
The earliest types of supersonic aircraft featured a central shock cone used to form the shock wave. This type of shock cone is clearly seen on the English Electric Lightning and MiG-21 aircraft, for example. The same approach can be used for air intakes mounted at the side of the fuselage, where a half cone serves the same purpose with a semicircular air intake, as seen on the F-104 Starfighter and BAC TSR-2. A more sophisticated approach is to angle the intake so that one of its edges forms a leading blade. A shockwave will form at this blade, and the air ingested by the engine will be behind the shockwave and hence subsonic. The Century series of US jets featured a number of variations on this approach, usually with the leading blade at the outer vertical edge of the intake which was then angled back inwards towards the fuselage. Typical examples include the Republic F-105 Thunderchief and F-4 Phantom. Later this evolved so that the leading edge was at the top horizontal edge rather than the outer vertical edge, with a pronounced angle downwards and rearwards. This approach simplified the construction of the intakes and permitted the use of variable ramps to control the airflow into the engine. Most designs since the early 1960s now feature this style of intake, for example the F-14 Tomcat, Panavia Tornado and Concorde.
In one unusual instance (the SR-71), a variable air intake design was used to convert the engine from a turbojet to a ramjet, in flight. To get good efficiency over a wide range of speeds the Pratt & Whitney J58 could move a conical spike fore and aft within the engine nacelle, to keep the supersonic shock wave just in front of the inlet. In this manner, the airflow behind the shock wave, and more importantly, through the engine, was kept subsonic at all times. Additionally, and unusually for this engine, at high mach, the compressor for the J58 was unable to carry the high air flow entering the inlet without stalling its blades, and so the engine directed the excess air through 6 bypass pipes straight to the afterburner. At high speeds the engine actually obtained 80% of its thrust, versus 20% through the turbines itself, in this way. Essentially this allowed the engine to operate as a ramjet, and actually improving specific impulse (fuel efficiency) by 10-15%.
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