Science Interviews

Interview

Sun, 3rd May 2009

Making Superalloy Jet Blades

Dr Howard Stone, Cambridge University

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Helen -   Well planes, trains and automobiles have been consistently evolving since they were first designed and itís certainly going strong still today. Scientists and engineers are finding new ways to make transport more efficient, more renewable and safer.  We are now joined by Dr. Howard Stone from Cambridge University Department of Material Sciences and Metallurgy and he is looking for ways to get the most out of metals as we use them to make jet engine blades so it can make them longer lasting, safer and more efficient.  Hello Howard!

Howard -   Hello!

Helen -   Thanks for coming on the show.  First off, what are the problems faced by jet engines that they have to overcome in order to work, to get these huge metal boxes up into the air? 

Howard -   Well, to actually get a gas turbine engine to work and to get the airplane in the air you actually have to suck the air in the front, compress it, mix it with fuel and then pass it out of the back over a series of turbine blades which if you like, extract the energy from the hot gases.  So there are great many engineering charges in that.

Boeing 737 passenger jetHelen -   I take it we are talking very, very hot temperatures here.

Howard -   We are talking very, very high temperatures and certainly one of the great challenges weíve got is just the great materials which are capable of surviving the environments in very core of the jet engines, and thatís really the focus on the work we do.

Helen -   Youíve got, I have to say this is very exciting, youíve got a bit of a jet engine with you in studio, it doesnít lookÖ well I donít even recognize as a piece from jet engine, what have you got there in front of you?

Howard -   I have one of the turbine blades so I'll describe it for your listeners as being the size of a couple of fingers and shaped roughly like a little wing and really this is the little component which extracts a lot of the energy from the hot gases that come out of the combustor in the engine, and therefore provide lot of the power for the engine to go forward.

Helen -   How is engineering that material itís made of helping to make it work and do what it needs to do?

Howard -   Well these things work in an extremely demanding environment.  I mean the gas stream temperatures are very, very hot and as a result we actually have to have them out of very special materials and the material I am holding here at the moment is known as a nickel based superalloy, a very apt name for it as it works on the conditions that most materials just really wouldnít survive.

Helen -   Whatís the super alloy as opposed to just an alloy?

Howard -   Itís one that is capable of working at conditions that are very demanding.

Helen -   Okay, and how hot are we actually talking about?

Howard -   Oh, actually the gas stream temperatures these things require to work in a pretty close to or exceed their melting temperatures.

Helen -   So they should actually melt if they wereÖ

Howard -   Well they are survived by some very complicated cooling passages and film coolings, ceramic coatings and so forth.  To give you an analogy its equivalent to having an ice cube in an oven and actually keeping the ice cube as a nice solid chunk of ice rather than a piece of liquid water in the bottom of your oven. 

Helen -   So by engineering that you could essentially keep an ice cube not to melting in the middle of an oven, just by making it cool, that cooling it down.

Howard -   Thatís a lovely analogy, yes.

Helen -   Okay, wonderful and so in that particular piece youíve got there, thatís involved in the very core of the jets where the fuel is being burnt and thatís where the thrust of the engine begins, is that right?

Howard -   Thatís right, you've put your finger on it perfectly there.  So this is the thing that extracts the energy from the hot gases that turns the big fan on the front, also it turns the various other parts of the engine which drives you through the sky, yes.

Helen -   And so that presumably has to move very fast as well.

Howard -   It does and itís spinning around at thousands of revolutions a minute, which is equivalent of hanging the weight for small car off each one of these little turbine blades.

Helen -   Off each one of those.

Howard -   Off each little blade which literally, as I said, they are more than probably the size of a couple of your fingers.

Helen -   And how many of those would you get inside one jet engine?

Howard -   Quite a few, quite a few.

Helen -   Itís certainly quite a small piece and last time I was on a plane and the jet engine looked like it was pretty huge.

Howard -   They are yes, indeed.

Helen -   So how do you want to make that material do what it has to do?

Howard -   Well, we are really trying to make sure that the composition of the alloy is optimised so that itís stable, it can survive under these very high temperatures and one of the most interesting developments is that these things are actually made of single crystals.  Itís one of the few examples you will find in an engineering application where an entire component of this sort of size is actually a single crystal.  Most of the things that we use to in the world around us, the steel in our cars, our cutlery and the like are made up of many little individual crystals.

Helen -   So you grow one crystal in the laboratory almost...

Howard -   Well itís very carefully grown to produce one beautiful single crystal.

Helen -   And what does that do?  How does that, what makes it work better?

Howard -   That helps make sure that it survives the very high temperatures and has the best creep resistance it possibly can.  So it doesnít elongate too much in the tremendous heats and tremendous stresses.

Helen -   I see and so I take it thatís the reason that weíve got to make all these improvements in efficiency of these materials is because we are worried about in air flights increasing in numbers and the amount of emissions and so on, and is this really going to help us make flying more efficient and use less fuel and all those sort of things that weíre concerned about at the moment?

Howard -   Thatís the idea, yes.  I certainly know that the increase in passenger numbers are, the governmentís forecast is something of the order of 495 million passengers by 2030 and that was a forecast in the 2007 report by the government and thatís quite an increase in the sheer number of people.  So we have to work out ways in which we can, if we fly a little more we have to work out ways in which we can make it as efficient as we possibly can.

Helen -   But material science and engineering, is at the core of that, itís really at the beginning of how we get those planes in the air.

Howard -   Itís one of the many solutions or many tools in our armoury that weíll have to pursue to try and get the most out of these things we going to carry on relying on for many years to come.

Helen -   Great.  Well thanks for coming into studio.  That was Dr. Howard Stone.  He is from Cambridge University and he was telling us all about how the metals that are right in the heart of a jet engine can be made to actually withstand of those incredible performances to get our planes into the air. 

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