Jet Engine > Compressor

In the compressor, air coming from the Fan Section is sent through a series of blades increasing the pressure and making it more dense.  This compression usually happens in two sections with each section increasing the temperature and stress on the components in the compressor.  The compressor is made up of rotors which spin and stators (or nozzle guide vanes) which remain stationary but can vary their angle for better engine performance.  Each set of compressor blades increases the pressure by only 30-40% in order to prevent losses from flow separation and the air stalling over the blades.  Given a Trent 900 has a total of 14 compressor stages, this means the compressor takes the incoming air and increases its pressure by almost at 40 times.

 

The early stage compressor sits directly behind the fan section and creates a step increase in the density of air travelling through the engine.  An increase in air pressure is comes with an increase in temperature thanks to the Ideal Gas Law.  This means air coming through the fan section at 50°C is compressed and is now at approximately 300°C.  Although the early stage compressor experiences increased temperature and pressure compared to the fan section, the most common material used to make this component is also Ti 64 or Ti-6Al-4V.  This material is capable of handling the stresses and temperature during the initial stages of compressing the air, whilst being lightweight as these are still very large weighty components.

In the later stage compressor air is made more dense as it enters a far smaller part of the engine. This means the air temperature rises above 300°C and Ti 64 can no longer handle both the increased temperature and stress.  So far Ti 64 has been able to handle everything we have thrown at including a few birds, but these stresses and temperatures go beyond what it can handle.  At this point we turn to Ti 6246 or Ti-6Al-2Sn-4Zr-6Mo, which is stronger and capable of handling temperatures up to 450°C.  This late stage compressor has a second function in that it is also how a jet engine starts up.  To turn on a jet engine, a small motor is connected to the late stage compressor, this turns the compressor fast enough for air with a bit of fuel to ignite in the combustor.  This starts the process of turning the turbines which through the shaft can now turn the compressor.  Once the turbines spin the motor is retracted and the jet engine is now operational.

In new generation jet engines that have a HP compressor (HP=high pressure), the stress and temperature goes beyond the capability of titanium based materials.  A HP compressor ups the pressure and temperature of the air again above 450°C.  Titanium based alloys can no longer handle these conditions so nickel based alloys are used for the compressor. When we talk about nickel based alloys in aerospace we mean superalloys because we need a material with super powers to deal with the stress and temperature.  Older generation superalloys such as Waspaloy have been replaced in modern times with higher performance materials such as Inconnel 718, Udimet 720 and RR1000.

To make a compressor blade, we start with a lump of metal that is squeezed out much like toothpaste into a specific size a.k.a. an extruded billet.  Some 21st century blacksmithing is then done where the metal is heated and beaten into shape i.e. hot forged.  The details of the compressor blade are made by machines cutting away material, then the blade is then polished for aerodynamics.

The design of the blades has become more and more complex thanks to better knowledge of aerodynamics and greater computing power, meaning the precision required for each blade and leading edge profile is beyond what can be done manually, and therefore future compressor blades will be made robotically.