What Metals Make A Jet Engine?
Material Scientist, Caroline Goddard, explained to Chris Smith and an audience at the Cambridge Science Centre, how new alloys are made and how you select metals for different tasks...
Caroline - I work on developing alloys which is when you mix a couple of metals together, a bit like baking, you mix things together, heat it up, see what happens. So, the reason we do this is because essentially, when it's in its pure state, metal isn't that good. You need to add something else to it that's got some good properties that you also like to it to make it much better. For example, steel is made up of iron and carbon. The carbon is what makes it strong and more brittle.
Chris - So, what is the difference between iron and steel? If I looked down a microscope, they're both iron, right? But it's just the trace element carbon that's there. Why does it make a difference?
Caroline - So, the carbon sits in the spaces between the iron atoms which stops the atoms from moving around which makes it much, much stronger.
Chris - Why is it then that a piece of iron, if I just put it out on the street will go rusty, but stainless steel doesn't?
Caroline - Stainless steel includes another element which is really, really environmentally resistant, which is chromium because chromium reacts with the oxygen and creates this layer of oxide on the surface which prevents it from corroding. So, if you put some chromium into your steel then you'll get this layer on the surface which prevents it from rusting.
Chris - Tell us about the jet engines because I'm dying to know what you do with those. How does one work? Just a simple starter for 10 points.
Caroline - So, a jet engine works by, air goes in through the front and is compressed as it moves through the engine. It's compressed really, really small and then at that point, it gets really hot. You inject some fuel into it and ignite it, and the air expands very quickly, and goes out back of the jet engine which creates some thrust. This expulsion of air creates enough power to turn the turbines in the engine which turns a huge fan at the front which you've probably all seen when you fly off in your holidays which sucks in a huge more amount of air which flings it back, which makes the plane go forwards.
Chris - Brilliant! Suck, squeeze, bang, blow! I think that's what Rolls Royce say how an engine works. So, what are you doing to make these things better? They work pretty well, I've just taken a very long flight, and I think there were Rolls Royce engines on the plane because you can tell because they paint those little spirals on the nose cones and they also go clockwise. Interesting fact I learned about Rolls Royce aero - you can tell a Rolls Royce engine because they go clockwise. All the other engines go the other way. But why are these not good enough? What can we do to make them better?
Caroline - To make an engine go fast, it has to be very efficient at burning fuel and to make it more efficient, you need to make it go at higher temperatures. So, in order to do that, you need to develop a material that can reach higher temperatures.
Chris - So, how hot is it inside the jet engine then?
Caroline - It depends on where in the jet engine. At the front, it's relatively low, perhaps 300 degrees, something like that. When you get to the combustor and the turbine at the back of the engine, you're reaching 800, or even higher degrees. So, it's very, very hot. So, what happens is, the engineers design an engine that can tolerate all of these temperatures and they come to the material scientists and said, "We need a material that can reach this temperature."
Chris - "So, this is what we want to do. Go and make the material that can do it." And no material like that exists yet.
Caroline - Exactly, so then they get lots of PhD students like myself to go around and make lots of materials that will reach this temperature and then the engineers will come back to us and say that they want it to go even higher. So, we have to go back to the drawing board again.
Chris - So, there's the challenge - how do we actually make materials better? Over to you. What would you like to know? Yes, sir. What's your name?
Errol - Hi, it's Errol again. It can't be trial and error. You must start with some base materials with some obviously, standard sort of functionality. How do you decide which ones to put together to come up with the problem you're trying to solve?
Caroline - So, choosing your base metal gives most of the properties. So, if you use iron then most of your properties are very steel-like. If you use titanium, you'll have a very lightweight material, that's strong for its weight. So, you choose a base metal based on that. High temperatures, in the turbines, we use nickel as our base. And then you put chromium in to make it environmentally resistant and then put some other elements in that will make it stronger or make it better at resisting vibrations. But the proportions, it's not guess work.
Chris - So, it's guess work, yeah.
Caroline - We do have informed guess work.
Chris - It's called trial and error. Thank you, great question. We've got another question here. What's your name?
Olivia - My name is Olivia. I'm just wondering, can you separate an alloy?
Caroline - Can you separate an alloy?
Chris - So, you mean Olivia, can you get the other things that have been added, mixed in? Can you get them out again?
Olivia - Yes.
Caroline - I don't know. I don't think you can.
Chris - Because when I was going to South Africa and I met this bloke, this is the kind of thing you meet in an airplane. I met this guy as we flew into the airport in South Africa and he said, just looking out the window, "Ah! They've got another airplane for me." I said, "What do you mean?" And, he said, "Well, they line up all the airplanes over there because I recycle airplanes." And they line it all up on the runway and then he'd sort of literally takes them to pieces. He said, "We can't re-use the aluminium in the shell of the airplane because it's got stuff added to it. You can't turn that into another airplane because no one will guarantee the composition of the alloy and if you mix that up with other metals, you don't know that it's got a purity that you can rely on. And so, it may have characteristics that you can't predict. With something like an airplane, it's a bit too dangerous. So, I think it's really tricky, but Dave, what do you think?
Dave - Basically, to separate something you need to have properties which are different. So, if you have an alloy with something with quite a low melting point or a very high melting point, or very high boiling point, you can heat it up until the low boiling point one boils off and you'd get slowly a more and more concentrated solution of one of them and other one kind of boils off. But if you've got two metals which are very, very similar, it gets very, very nasty. I don't know if you've heard of the rare Earth metals, things like neodymium. They're actually not very rare. The problem is, that when you find them, they're mixed all up together and splitting them apart is incredibly difficult. You can do it, but it's very, very difficult and messy chemistry.
Ginny - Could you do it if you actually went down to the molecular level because I remember there was that video that was made recently where people were actually moving individual molecules around. They made a little video of a guy playing with a ball. So, if you went down to the molecular level, could you just tweak out the carbons?
Chris - So Caroline, how many atoms are there?
Caroline - I guess in theory, you could do that, but it would take a very long time.
Chris - There's a question at the back.
Jamie - Hi. I'm Jamie from Cambridge. I was wondering if you could explain how turbine blades operate at temperatures above their melting point. Is that correct?
Chris - Maybe we should just explain the context to the question, how do these turbine blades operate above their melting point because the reality is in these engines, the stream of gas going by is hotter than the metal melts at, isn't it?
Caroline - Yes.
Chris - So, you think I've got an engine running here which has got stuff going on inside it, hotter than the engine components actually melt at. Sounds a bit worrying, but the engines stay up there. How do they deal with that?
Caroline - So, it's a combination of two things. The first is, we coat the blades in a ceramic which has a much higher melting temperature and it conducts heat through it very slowly, so it doesn't reach the metal. And the second reason which sort of - they work together is that we blow cooling air through the blades themselves which works with the coatings to stop that.
Chris - There's one more question here. What's your name?
Freya - Hi. My name is Freya. I have two questions. I was wondering how many parts would be in a common jet engine and my second question is, how many metals would be in a common jet engine?
Chris - Pretty simple there Caroline. So, how many bits to an engine? How many different metals?
Caroline - Bit's - a lot.
Chris - Is that an SI unit then?
Caroline - Yes. So, with the jet engine, you've got the disks and the blades and the shaft which do the main amount of work. But then you've also got the casings and they all have to have sensors on them. So, there's lots of electronics all over them, so there's lots and lots of parts. For different metals, well, in a jet engine, you'll have steels, titanium and nickel based alloys. They are the main three, but within those alloys, you'll have a lot of other elements. In some cases, up to 13 different elements that you've thrown in there.
Chris - Thirteen in one alloy? And do you know exactly what composition of all those different elements there is?
Caroline - Yeah.
Chris - It's amazing. No wonder it's so expensive.