Flying Planes at Mach 6

25 March 2007

Interview with

Jenny Goodman from the University of Oxford

Chris - Now we're very pleased this week to welcome Jenny Goodman who's a researcher at Oxford University and works on getting us all to the other side of the planet much more quickly. So tell us about your research. A plane wingJenny - What I work on is something called the Sustained Hypersonic Flight Experiment. It has the rather unfortunate acronym of SHiFE.Chris - Which could be mistaken!Jenny - It's being done by a company called QinetiQ and the aim is to build something called a ramjet engine, which is a special kind of jet engine. This is going to fly on a rocket up to four times the speed of sound. It's then going to launch itself and accelerate up to six times the speed of sound, which is two to three times the speed of Concorde.Chris - So that's two kilometres a second isn't it?Jenny - Yes that's right. Then it's going to fly along at Mach 6, six times the speed of sound, for 100 seconds before running out of fuel and crashing to the ground. But 100 seconds, Mach 6, ramjet, is all very exciting and never been done before.Chris - How's it actually work and why does it need to be launched off a rocket? Why doesn't it just take off?Jenny - A ramjet engine works just the same as a jet engine that you get on your standard jumbo jet. What happens is that it needs to suck in air and it needs to compress it so it increases the pressure before it gets to the combustion chamber. You then squirt in some fuel, you combust that fuel and just exhaust it out of a nozzle. You get really high-speed air coming out the back, which creates the forward motion of the engine. Now a ramjet engine isn't like a jet engine because what happens is instead of having that big spinning set of blades you have in front of a jet engine, in a ramjet engine you can carefully shape the intake so that it does all the compression through something called the ram effect. That's why it's called a ramjet. You compress your air without this enormous lump of metal spinning at the front, so it's much lighter. Fantastic!Chris - It must be quieter I imagine.Jenny - Not necessarily quieter because you've got all that air coming out the back so it still makes a bit of a noise. Bu the trouble with it is that in order to get the ram effect to work, you need to be flying fast and creating something called shock waves in the intake. This means that you need to be flying at more than Mach 2 really for them to work effectively. In our case, our ramjet is only going to work at Mach 4 to 6, so in that case you have to get it there first of all with a rocket. Chris - So how is this different from what rockets already do?Jenny - The thing with a rocket is that it has to carry its fuel and its oxygen on board. With a ramjet engine it sucks in the oxygen in the form of air from the atmosphere, which reduces a lot of weight because you don't have to carry all that oxygen around with you. Chris - And once it actually gets going, how far are we away from seeing this being able to be plugged on to an aeroplane? One could see how you could apply this: you would need something that would go pretty fast to get it up to that threshold speed and then these things could kick in and then take you much much faster.Jenny - Well we've actually already got it on an aeroplane. The Blackbird SR71 actually has a turbo ramjet engine. So it uses a normal jet engine to get it up to around Mach 1 or Mach 1.5 and then it changes into a ramjet engine, but it only flies at around Mach 2 to 2.5, so our one is going to go faster. Chris - Does it use the same fuel as a normal aircraft engine? Jenny - Our one is actually using diesel, but it can also use the same fuel, kerosine, that we use in a normal engine. Chris - Because that's important because you don't want to be carrying two loads of fuel aloft, do you?Jenny - No, you wouldn't. If you then go faster still into something that's called a scramjet, which a supersonic combustion ramjet engine, they tend to use hydrogen because it's got different properties to diesel.Chris - So what are the major constraints with getting this going? Jenny - One of the major constraints is that once you go that fast, so above Mach 5 which we call hypersonic, things get very hot. If you can imagine the friction forces just on the outside, inside our combustion chamber we've got 2400 Kelvin, so that's about 2200 degrees Centigrade, so you're talking 1000 times hotter than your normal room.Chris - But things run hot anyway, I mean your average jet engine must be pumping out gases that are 1000 degrees or hotter at least.Jenny - They are hot but they're not as hot as that for sustained periods. The other thing is that you want to keep the whole thing light. So your standard jet engine or gas turbine engine is cooled by leading some of the air that goes into it through complicated passages, and that keeps the turbine blades at the back cool enough by film cooling and things. We don't want to anything as complicated as that when we're that hot and trying to keep the weight down, so that's complicated. Chris - And is that surmountable? I mean if we can put a probe so that it can slam into the atmosphere of Titan, Saturn's largest moon, at 13 000 miles an hour, get to 2000 degrees, withstand that and then plummet into a bath of liquid methane, surely it's not beyond the realms of possibility that we can design something that's light enough for what you need it to do.Jenny - Of course not, that's what we're doing. We're going to fly this thing in two years or so. We can do it! We're using something called carbon silicon carbide, which is a really fancy sort of composite material that's worth its weight in gold. That can withstand these kind of temperatures pretty much indefinitely. It's great stuff but you can see how it gets pretty expensive. You fly a probe to Titan and you can do it once. It costs millions and billions of pounds. You want to fly continually from London to Sydney, do you really want your air fare to be that high?Chris - No, of course not. Now what sort of altitude will these things run at? Presumably they would be able to go much much higher. If they're using that kind of approach they would probably being going twice as high as we fly at at the moment wouldn't they?Jenny - Our one's going to fly at 30 000 metres, so 30 kilometres, which is a lot higher than normal. We're usually at about 32 000 feet, I think.Chris - Yes because aeroplanes go along at about the height of Everest don't they. This would be three times that wouldn't it?Jenny - Yes it would be. That adds additional challenges for when you're designing your plane and things. Chris - But I would think that at those kinds of altitudes, because the air is so much thinner there will be much less resistance and it should be quite economical as far as flying goes.Jenny - I guess it is. Would you get less drag? I'm not sure. Drag goes against an aeroplane to stop it going along. I think there are an awful lot of challenges with going high because the pressure outside is lower, which means you have to pressurise your aeroplane a lot more, which means it needs to be stronger and therefore heavier. So there are a whole load of pros and cons to flying high. Chris - How long have people been trying to develop this? When I was little I swear I heard people talking ten or fifteen years ago about how notionally this technology could be used, and they were saying that in a couple of years they'd be able to build it. Here were are fifteen or perhaps even twenty years later and although it's taking to the air in a prototype form, it's still pretty prototype.Jenny - Well ramjets have been around for a very long time. I'm struggling with my history here but I'm tempted to say around the 1930s or 40s we actually had ramjet planes flying. The thing is that to be able to fly this fast, were talking hypersonic speeds, and to make it reliable and safe, now that is something that we're really grappling with. We've only just got there with subsonic flight, so moving up to supersonic and then hypersonic flight - we've had supersonic flight and let's face it, people weren't prepared to pay. Putting this extra amount of money in the get to these higher flight speeds is a challenge in itself.Chris - And my last question for you Jenny, what would be the sonic boom you would get from one of these things? Because Concorde upset quite a few people in Surrey if it went a bit too fast and they all got these sonic boom effects. Will this do the same thing and will we have a noisy future if we plug this into aeroplanes?Jenny - You will still get a sonic boom as far as I'm aware because at some point you will have to create that shockwave as you go over people. I believe these days if they created a new supersonic, a Concorde 2, they can make the sonic boom a lot less by the way they shape the actual aeroplane. So probably by the time you're taking off over people you're probably going at Mach 5 anyway, so we can actually do that better now, so it's probably going to better for people than Concorde.

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