Up to Ten Times the Speed of Sound

Scramjets Could Rocket Australia Into 21st Century
by Pat Sheil

Source: Space Daily

January 6, 2001

Dr. Allan Paull and his crew at the University of Queensland’s Centre for Hypersonics are about to make the first test flight of their brand new toy - the world’s first operational scramjet.

Sydney - Dr. Allan Paull and his crew at the University of Queensland’s Centre for Hypersonics are about to make the first test flight of their brand new toy - the world’s first operational scramjet. If the thing works, the UQ scramjet will be the fastest air-breathing engine ever built, capable of pushing aircraft along at up to ten times the speed of sound. This sounds fast, and it is fast - faster by a factor of three than any existing military plane. The current speed record is held by the Lockheed SR-71, an American high-altitude surveillance machine with a top speed of Mach 3.6, or around 3,500 kilometres per hour.

This is about as fast as it’s been possible to go for quite some time - the SR-71 first flew as long ago as 1965. An F-18 Hornet fighter/bomber, the frontline aircraft of the RAAF, has a top speed of Mach 2.5.

Allan Paull has spent the last fifteen years trying to break this speed limit. To smash it to pieces, in fact. And now, after countless tests in the University’s remarkable T4 shock tunnel (the windiest wind tunnel on earth), the Hypersonics crew will finally haul their gadget to Woomera next March and let her rip.

"We were the first group in the world to show that you could make these things work - that we could actually get net thrust out of a scramjet engine," says Paull.

"We developed techniques which allowed us to measure the thrust performance and be able to see what was going on inside the engine. I joined the team as an applied mathematician way back in 1985, and now we’re finally going to flight test the thing."

So what is a scramjet, and how is it that they come with so much grunt? In essence, a scramjet is the simplest jet there is, but that hasn’t meant that building one is a simple job.

A scramjet is a supersonic ramjet, and a ramjet is basically a turbojet stripped down to its bare essentials. Turbojets are the engines that power commercial airliners. They use a compressor fan - those big windmill blades at the front - to shove air into a combustion chamber, where it’s combined with fuel and ignited. The exhaust gases are forced out the back, and on their way they power the compressor, which forces more air into the front of the engine, and so on.

The beauty of this arrangement is that it allows the engine to be fired up while it’s stationary - essential for any commercial application.

A ramjet is much simpler, in that it dispenses with the compressor. The downside is that the engine must be moving through the air at high speed to generate the pressure required to compress the air and fuel in the combustion chamber.

What this means is that a ramjet must be brought up to a certain critical speed before it can be turned on, typically by piggybacking the ramjet to a turbojet or a rocket engine until ignition speed is reached.

A scramjet works on the same principle, except that unlike a ramjet, the combustion takes place while the gases are moving at above the speed of sound.

And at the Centre for Hypersonics, they’ve been forcing air at enormous speeds and pressures though countless configurations of scramjets, dreaming that one day they could unbolt their monster from the bench, strap it to a rocket and see what the hell would happen in a real live atmosphere.

Unfortunately, there was a small matter of money to be sorted out. Wind tunnel tests are a lot cheaper than flight tests, and when you want to test an engine at Mach 8, they’re much, much cheaper.

"The problem was that a test launch would have cost us a minimum of about $10 million, which was out of the question. Then we got very lucky."

Paull heard that British aerospace company Astrotech had been given approval to test-launch their new breed of sounding rockets from Woomera, and after some complex negotiations his group was able to hitch not one, but two free rides for their scramjet. At last the first flight tests, dubbed HyShot, were on.

After lift-off in late March or early April, HyShot will reach an altitude of over 300 kilometres before plunging back into the desert. On the way down, around 25 kilometres above the middle of nowhere, the scramjet will kick in, hopefully sending back a torrent of data before it is incinerated or pulverised.

The main purpose of the tests is to establish that sustainable combustion actually takes place in the howling maelstrom inside the engine.

Paull admits that the forthcoming launch has put him "on the steepest of learning curves". When it became clear that the launch was actually going to happen, he soon realised how little he knew about some of the finer points of sub-orbital space flight. He has had to re-invent himself, transforming from applied mathematician into payload specialist in short order.

"We’ve had to learn about things like turning rockets around in space, we’ve had to develop new sensing equipment, thrusters, the lot. These are tough problems."

The window of opportunity to glean information from the flight is terrifyingly narrow. The correct test conditions for combustion occur for only five seconds as the engine screams through the atmosphere on re-entry.

"It’s a big step, a lot different to doing tests in the wind tunnel. This thing’s got to work; you’ve got to get it right. It’s a very big change moving from academic life to something that’s very, very real."

And he’s had to make an almighty leap from lab to launch on a budget that would be considered laughable in the United States or Europe. Paull is eloquent on the subject of research funding in Australia, and has a dim view of accountants who insist on short-term results from long-term projects. It is a familiar gripe, one that can be heard in just about every lab in the land, but if nothing else, financial adversity has necessitated inspired improvisation.

"We’re doing this for a tenth of the price that anyone else could do it for. At the core of the project, it’s just two or three dedicated people, working non-stop on dozens of different problems.

"We’ve had to build all sorts of ancillary equipment, and do it on the cheap. For instance, one of things we had to do is ensure that the payload’s spin was correct. To get the spin balance right we bought a second-hand car tyre balance from Bob Jane. It works beautifully!"

Paull knows that the flight is experimental by its very nature - no one really knows what’s going to happen - and he’ll be happy just to get the scramjet into the air. Data is data, and in that sense, any results will be good results.

"We’ve obviously got to crawl before we walk, and this test will be our first chance to get supersonic combustion results in the real world.

"But you have to remember that everything we do on this project - the equipment, the people, the physics - is pushed right to the limit. It’s the nature of the game. But if you take risks and ask new questions you get new information - there’s no point in asking the same old questions and getting the same old answers.

"The way I see it, even if we get the payload to the platform we’ve already succeeded. The chances are about 1 in 5 that the thing’s going to work. and we’ve got two goes at it. I’ll let you work out the odds!"

For the sheer hell of it, maybe. Knocking together a Mach 10 jet engine in the backyard has a Boys’ Own, Airfix appeal that cannot be denied. But if these things prove to be practical, they will indeed have their practical side.

They’re bloody fast, that’s for sure. In terms of getting from A-to-B in next to no time, they’ll be hard to go past. They also burn hydrogen, which is renewable and non-polluting (the only exhaust gas is steam), and with no moving parts, scramjets are simple and efficient.

They may well power the next generation of supersonic airliners and low earth orbit boosters, and it’s in this role that Paull sees a big future for scramjet technology.

Unlike the rocket engine, which caries both fuel and oxygen, the scramjet only has to carry fuel, as it takes its oxygen from the atmosphere. This cuts the launch weight by almost half, and while there’s no oxygen in space, a scramjet-powered plane would be capable of taking a satellite at very high speeds to an altitude of over 15 kilometres. After that a small rocket engine could take the payload the rest of the way.

"The possibility of cheaper satellite launches is the main commercial aspect of the research, and I think that’s where it will be used first."

But there is a darker side to the scramjet. There had to be a dark side to something this powerful.

"Naturally there are serious military implications as well. You wouldn’t want an unfriendly overseas country having one of these things. The Defence Department people seem to think that this technology is too far in the future to worry about, but I think they should be taking it very seriously indeed.

"If we as a university can undertake a flight test, imagine what a country determined to develop these engines could do if they put their minds to it."

It’s true that a scramjet fighter would make the F-18 look extremely pedestrian, and scramjets could power an armada of low-cost, long-range missiles.

But the potential uses of scramjet technology extend well beyond simply pushing payloads around at mind-boggling speeds. When they were invented back in the 60’s, no one expected that the biggest use of laser beams would be in domestic stereos, and Paull expects that his group’s work with high temperature supersonic gases will have ramifications that he hasn’t even thought of.

"This technology already has a lot of really diverse applications. One application we’re looking at is working with the Japanese on the chemical analysis of constituents of volcanic plumes, which could lead to much better early warnings of major eruptions."

So what happens next if the Woomera tests are successful? Planning is already well advanced for HyShot 2, which Paull hopes will conclusively show not only that combustion takes place, but that the engine generates more thrust than drag in the real world. Once that’s established, he’s got a fair-dinkum scramjet on his hands. Demonstrating that the engine works outside the wind tunnel might be the easy part. Convincing the aerospace industry to invest billions in developing commercial scramjet aircraft could prove to be the biggest hurdle of all.

But convincing Boeing or Airbus to build a Mach 10 airliner is the last thing on Allan Paul’s mind right now, and anyway, he doesn’t really see that as being his department.

"It will happen one day. It could take as long as sixty years. When you think about it, this is how long it took to get from the Wright brothers to the 747. But I suspect it will happen a lot sooner than that."

But for now Allan Paull just wants to get to Woomera and watch his baby fall out of the sky at 8,600 kilometres an hour.

After that, who knows?

Center for Hypersonics:

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