Design considerations
The various components named above have constraints on how they are put together to generate the most efficiency or performance. However the performance and efficiency of an engine can never be taken in isolation; for example fuel/distance efficiency of a supersonic jet engine maximises at about mach 2, whereas the drag for the vehicle carrying it is increasing as a square law and has much extra drag in the transonic region. The highest fuel efficiency for the overall vehicle is thus typically at Mach ~0.85.
For the engine optimisation for its intended use, 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 us consider design of the air intake.
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Air intakes
See also: Inlet cone
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Subsonic inlets
Pitot intake operating modesPitot intakes are the dominant type for subsonic applications. A subsonic pitot inlet is little more than a tube with an aerodynamic fairing around it.
At zero airspeed (i.e., rest), air approaches the intake from a multitude of directions: from directly ahead, radially, or even from behind the plane of the intake lip.
At low airspeeds, the streamtube approaching the lip is larger in cross-section than the lip flow area, whereas at the intake design flight Mach number the two flow areas are equal. At high flight speeds the streamtube is smaller, with excess air spilling over the lip.
Beginning around 0.85 Mach, shock waves can occur as the air accelerates through the intake throat.
Careful radiusing of the lip region is required to optimize intake pressure recovery (and distortion) throughout the flight envelope.
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Supersonic inlets
Supersonic intakes exploit shock waves to decelerate the airflow to a subsonic condition at compressor entry.
There are basically two forms of shock waves:
1) Normal shock waves, which are perpendicular to the direction of the flow.
2) Conical, or oblique, shock waves, which are angled rearwards, like the bow wave on a ship or boat.
Note: Comments made regarding 3 dimensional conical shock waves, generally apply to 2D oblique shock waves
Normal shock waves tend to cause a larger drop in stagnation pressure, than the weaker conical shock waves. Basically, the higher the supersonic entry Mach number to a normal shock wave, the lower the subsonic exit Mach number and the stronger the shock. Although conical shock waves also reduce Mach number, the outlet flow remains supersonic.
A sharp-lipped version of the pitot intake described above for subsonic applications performs quite well at moderate supersonic flight speeds. A detached normal shock wave forms just ahead of the intake lip and 'shocks' the flow down to a subsonic velocity. However, as flight speed increases, the shock wave becomes stronger, causing a larger percentage decrease in stagnation pressure (i.e. poorer pressure recovery). An early US supersonic fighter, the F-100 Super Sabre, used such an intake.
pressure recovery improvements resulting from the use of complex shock wave systemsMore advanced supersonic intakes exploit a combination of conical shock wave/s and a normal shock wave to improve pressure recovery at high supersonic flight speeds. Conical shock wave/s are used to reduce the supersonic Mach number at entry to the normal shock wave, thereby reducing the resultant shock losses. An example of this was found on the SR-71's Pratt & Whitney J58s that could move a conical spike fore and aft within the engine nacelle, preventing the shockwave formed on the spike from entering the engine and stalling the engine, whilst keeping it close enough to give good compression.
Many second generation supersonic fighter aircraft featured an inlet cone, which was used to form the conical shock wave. This type of inlet 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 ramp. An oblique shockwave will form at start of this ramp. The Century series of US jets featured a number of variations on this approach, usually with the ramp 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 ramp 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.
One of the problems with supersonic intakes is that they can deliver more corrected (or non-dimensional) flow than the engine itself can handle, particularly if the engine is throttled back. Some of the difference can be absorbed by the normal shock wave moving forward to a smaller flow area/lower entry Mach number, which weakens the shock, thereby reducing the outlet corrected flow. However, steps must be taken to prevent the normal shock from going forward of the intake lip, as this will disrupt the flow entering the intake. More extreme excesses in corrected flow can be accommodated by spilling air overboard through a trapdoor or supplementing the secondary flow of an ejector type final nozzle.
Concorde intake operating modes[edit]
Compressors
Compressor stage GE J79Axial compressors rely on spinning blades that have aerofoil sections, similar to aeroplane wings. As with aeroplane wings in some conditions the blades can stall. If this happens, the airflow around the stalled compressor can reverse direction violently. Each design of a compressor has an associated operating map of airflow versus rotational speed for characteristics peculiar to that type (see compressor map). Many compressors are fitted with anti-stall systems in the form of bleed bands or variable geometry stators to decrease the likelihood of surge. Another method is to split the compressor into two or more units, operating on separate concentric shafts.