Jet Engine > Shaft & Structural
To make an engine work there are a number of other components that are not directly involved in the air flowing through the engine. The most obvious of these components is the shaft which transfers the rotation of the turbine through to the compressor and fan section. Other important components include the struts which give the engine structural integrity in normal and extreme conditions. To get an engine approved for use, a fan-blade-off test is done where a small explosive is used to break off a fan blade whilst the engine is operating. The structural components of the engine have to be able to handle this simulated disaster and the resulting imbalances in the engine. In addition when designing components able to handle such challenges, we still need to make them lightweight as jet engines like the Trent 900 are not small (over 6000kg each!).
The shaft of a jet engine connects the fan section, compressor and turbines together, transferring energy from the back of the engine to drive the front. Acting as the backbone of the engine, a shaft experiences different forces and temperatures along its length as it runs from the cool lower pressure fan section, driving the high pressure warm compressor, passing through the hot combustor, then is turned by the high temperature fast rotating turbines. This means the shaft needs to be strong to transfer the force from the turbines all the way down its length to power the fan and compressor. It also needs to be able to cope with warm conditions as even with a cooling system, it is hot inside a jet engine.
The shaft of a jet engine is a complex design problem. At the front of the engine the fan section takes in outside air and mildly compresses meaning the stress on the shaft at this point to turn the fan isn’t too high, and the temperature is fairly cool. In the compressor, the shaft needs to turn a series of blades to make the air a lot more dense and so it needs to have a lot of force to squeeze the air down, which in turn increases the air temperature and thus shaft temperature. Passing through the combustor, the shaft needs to still be structurally sound as it transfers the force from the back of the engine to the front, so it experiences stress and temperature even if no load is applied from the combustor. The back of the engine is where the shaft gets its spin from. This part of the engine is hot and being turned very fast by the air coming through the turbine blades so the shaft needs to be able handle these conditions. Adding to this, each of these sections spin at a different rate and to get the best performance out of an engine we need to have the rights bit spinning at the right speed at the right time.
To deal with this almost impossible problem Rolls Royce don’t just use one shaft, but instead use three. Using three shafts we can connect one of the turbines to the fan section, then the other two shafts to connect different turbines to different compressor sections. This means we can have the best combination of turbines driving the fan and compressor sections, spinning independently.
Having 3 interconnected shafts mean that you can use different materials depending on what stresses and temperatures each shaft will experience. Steel is one of the most prolific materials used to build things because it has good strength, can be heat treated for specific characteristics, and can be easily made into different parts. Weight is also an issue when choosing a shaft material as a full shaft can weigh literally tonnes. The shafts of current jet engines are made out of specially designed and heat treated steel like Super CMV. The next generation of engines is looking to replace the first shaft with F1E which arguably is not simply a steel, but an iron based superalloy.
Struts & Structural
In a jet engine there are a number of other structural components that keep the engine rigid. There are four main rotor support structures for the spinning parts of the engine and keep them centered. The first is behind the fan section where the air is either sent through the engine for combustion or along the engine for cooling. The second and third supports are located in the middle of the compressor. The final support structure is at the rear of the engine behind the turbines.
These struts are static parts that experience different stresses during take-off, climbing, cruising, descent, landing and even sitting on the tarmac idle. These components are usually far enough away from the centre of the engine that they don’t experience the extremes of temperature, but still have a demanding job in keeping the engine structurally sound. These often large components are generally made from either high performance steels or more recently composites.
Blisks & Bling
Used in the compressor and turbines, blisks and blings are a method of creating structural integrity whilst reducing the weight of these components. A blisk where blades and disc are made out of a single structure. By using blisks, this can reduce the weight of the compressor or turbine by 30%. Small blisks are usually machined from a solid block whilst larger blisks are made by friction welding forged aerofoils onto the disc. This joining technology produces a very high quality weld and maintains the material strength across the join.
Further weight loss is possible by removing the inner part of the disc with the remaining ring carrying all the centripetal loads. Using this weight loss strategy, a bling is a blade-ring which is more extreme than a blisk and can save even more weight. Blings are being developed utilising a titanium metal matrix composites.