Materials Maketh the Jet Engine
The extreme conditions inside the jet engine presents a real challenge for engineers and designers, and they need to be able to rely on specially developed materials that can handle that environment whilst still being light enough to get aloft. Professor Dave Rugg, also at Rolls Royce walked me along the enormous Trent engine to explain the materials that go in to its manufacture.
Dave - Okay, at the front end, we sometimes have aluminium in small quantities and that can be for statics around the case, but largely it's titanium at front end. Titanium is king there. It's got very high strength : weight ratio, very, very good corrosion resistance as well so you don't get problems with salt water and all kinds of other aggressive environments that you might have. So over the years, titanium has actually worked its way back through the engine to progressively higher and higher temperatures. In the Trent series, it makes its way all the way through to the high pressure compressor assembly. And there, the temperatures can be in excessive of 600 centigrade.
Ben - And these are quite big components of the Trent, so presumably the pressure to use titanium is to cut down on weight?
Dave - Absolutely. Titanium has a got a density of about 4.5 grams per cc whereas nickels and steels will be up in the 7 and 8 so, big saving on weight there. And the benefit is both in terms of the actual mass of the entire engine, but also then the stresses that are induced on the rotating power it's because obviously, we've got self-induced loadings from the centrifugal force. We also should mention that containment case around the fan and again, that's titanium used for its high strength capability and it's also very good under high strain rate loading. So, quite a remarkable structure and very large as well as you can see.
Ben - Is this not the region of the engine where we're starting to look at using more and more composites as well?
Dave - That's absolutely right and the main reason for composite materials, they're both stiff and they're very light so we get improved weight saving over titanium although engineering and composites provides its own challenges as well. So, as we move back a little bit further, you see here, we've got a lot of accessories which people tend to try and ignore, but in reality, things like gearbox casings, a lot of cast light alloys, so there'd be aluminium. In some of our engines, we actually use magnesium alloys as well - very, very light, also, good resistance against vibrations so they naturally damp vibrations which is quite important.
Ben - I understand one of the problems with magnesium alloys is that they oxidise quite readily, so presumably, these are regions where either oxidation wouldn't be as big a problem or where they're a bit more shielded.
Dave - That's correct. They're in relatively cool environments, so you've not got you're high temperature oxidation concerns. The magnesium industry has been active in developing new alloys as well and the types of materials, we use a typically very high purity magnesium alloy so they minimise the use of introduction things like iron or copper which even at trace levels will badly affect corrosion resistance. And the other thing as you can see is we have very good surface treatment systems so that we don't get problems with corrosion, also galvanically insulating the magnesium components is beneficial so that you don't get battery on the side of your engine.
Ben - So, the front of the engine needs to be extremely strong while also being very light. So alloys of titanium and aluminium fit the bill. These components are responsible for the suck and the squeeze, taking the air in a compressing it. Once we get into the fuel burning region of the engine where the bang takes place, conditions are radically different.
Dave - Now, we're heading through into the back end of the compressor where we start to see nickel alloys coming in and they're pretty remarkable materials in terms of their strength retention at temperature which is why we're using them, so very good resistance against creep and the combustion chamber where again, it's nickel alloys that earn their money. The other thing you see in here is lots and lots of pipes. They can be in a variety of different materials but again, a lot of nickel used and some stainless steels as well.
Ben - So the real challenge in this bit of the engine is coping with that heat.
Dave - Absolutely. It's very high temperatures when you're burning kerosene basically under very high mass flows. So, the centre of the engine is white hot.
Ben - To cope with operating with huge stresses and very, very high temperatures, new alloys, single crystal components and high precision manufacturing technologies are employed. Meeting these demands drives the understanding of material science at universities worldwide. But even behind the white hot part of the engine, there's still work to be done.
Dave - The turbine has two main jobs to do. The first one is simply extracting the energy from the mass flow of the combustion products coming over it. So we use the turbine to both power the compressor assemblies to make the system work at all, but then we use the back end of the turbine to actually power the fan and it's the fan that provides around about 75% of the engine's thrust. An HP turbine blade which is about the size of a matchbox roughly, a bit thinner obviously, extracts getting on for 1000 horsepower's worth of energy from the gas stream which is quite incredible if you think - that's the entire energy output of the Merlin being taken by one small blade, the size of a matchbox.
Ben - But even the best high performance alloys such as those that make up these turbine blades couldn't be expected to work well in conditions above their melting point, so engineers have come up with incredibly complex and clever cooling vents and other systems that play a vital role.
Dave - Absolutely. If you try to just use monolithic nickel alloy in the turbine even on most advanced materials, they would just melt immediately. It would be fractions of a second and you have a molten puddle which wouldn't be very good. So, we have to use cooling for them and it's pretty amazing to think that the cooling gas that we use is actually several hundred degrees centigrade. The analogy that I've heard is that it's - with turbine blades, it's a bit like trying to use components in an oven that are made out of ice.
Ben - Reaching the very back of the engine, the vent itself is far more than just, act as an enormous exhaust pipe.
Dave - That's exactly right. As I said, the LP turbine provides a power to the fan, but some of the key jobs in turbine design for the low pressure turbine are to ensure you get very good mixing between the bypass air and the combustion products coming out of the back of the engine because if that isn't done correctly, the noise levels can be very substantive and clearly, there's a large number of reasons why we want to keep the engines quiet.
That was Dave Rugg, from Rolls Royce, for whom noise reduction is a priority. I met their Senior Project Engineer for Noise, Joe Walsh, and their Chief Noise Specialist, Andrew Kempton.