NASA experimental X43 aircraft
Credit: NASA Dryden Flight Research Centre photo collection
SOMETIME SOON – sooner than you might think – it could be possible to climb aboard a craft that takes off from Sydney and touches down in London with barely enough time in between for a beverage and an afternoon nap.
To achieve this astounding feat, it will soar above the atmosphere at hypersonic speeds of Mach 8, or about 8,300 km/h, arcing across the globe in a parabolic flight path, then gliding gently down to the ground: the whole transglobal trip taking less than three hours.
Kevin Bowcutt, the chief scientist for hypersonics with Boeing Phantom Works, based in California, USA, says that flight could be a reality within 10 to 20 years. "There are no miracles, no major technological breakthroughs that we need to get from here to there," says Bowcutt. "But it won't be easy, quick, or cheap. It will take a lot of investment and years of hard engineering and development work."
He believes the project would cost about US$20 billion, and he's not sure where that amount of money can be found. But he's moving ahead, along with teams of scientists in Australia, to try to achieve that scenario, and they have a new batch of test flights scheduled for the Australian outback in May 2008 [Editor's note: this story was first published in Cosmos in April 2008]. Key to making it all work is a new engine: the scramjet.
Short for supersonic combustion ramjet, a scramjet is essential because it's fast, simple, powerful and economical. It has no moving parts, just an inlet that compresses the air and a combustion chamber where fuel (often hydrogen) is injected and ignited. The exhaust gases escape out the back, pushing the aircraft forward.
It differs from an ordinary ramjet engine, in that the air is shoved through at supersonic rather than subsonic speeds. And it's more efficient than a rocket, because it uses ambient air for combustion, while rockets must carry their own bulky oxygen supply. Scramjet-powered craft can be smaller, lighter and faster, providing much greater range and more payload for the same amount of fuel.
The scramjet also differs from ordinary turbojets that power today's airliners. Turbojets have a compressor fan that enables them to draw in air at any speed. A scramjet, on the other hand, needs an initial boost to get the air flowing at supersonic speeds before it can be ignited, so it would have to be part of a two or three-stage design. Aside from passenger aircraft, potential applications range from cruise missiles to orbital space access.
"Imagine you had an advanced turbine engine, like those on the Lockheed SR-71 Blackbird, that could take you from launch to Mach 3 or 4," says Bowcutt. "Then you could transition to scramjet power and accelerate to Mach 13 or 14, then switch to a rocket engine to reach orbital speed." The system would be simple, reliable and, adds Bowcutt, "You could bring down the cost of payloads from [US]$5,000 per pound today, to $100 or $200 per pound. Now it's a whole different story."
At that cost, space tourism becomes feasible. Satellites would be cheaper to deploy and repair. Orbital solar arrays could be economically viable. But first, Bowcutt needs to launch more experiments, and for that he needs lots of empty space: a commodity South Australia has in spades. At Woomera, the world's largest test range stretches across 127,000 km2 of unpopulated ochre soil and spinifex grass. Woomera has a long history of tests, from British nuclear detonations in the 1950s, to Australian and U.S. rocket and aircraft trials in more recent years.
In a new series of tests, scheduled to start around May 2008, a 13-metre-long rocket will shoot straight up out of the atmosphere into space, turn itself around, then return to the atmosphere and crash, reaching speeds of up to Mach 8. The rocket will carry a simple structure made up of a sharply pointed cone and a flared cylinder, plus an assortment of complex instrumentation.
The flight will last just 10 minutes, and will be used to measure how the air interacts with the outside surface of the mock-up, a region known as the boundary layer. "If the boundary layer flow is turbulent, that generates as much as five to six times as much heat and friction," says Bowcutt. The trick is to design a surface that will encourage a smooth airflow even at hypersonic speeds.
This test will be the first in a series of 10 experiments collectively called HIFiRE, or Hypersonic International Flight Research Experimentation. Flight tests will continue twice a year for about five years. "A couple of the tests will focus on various aspects of flight control," says Bowcutt. "Then we'll try out two different engine designs, one from the U.S. and one from Australia. Later tests will mate an engine with a controlled vehicle."
The A$74 million (US$68 million) HIFiRE project carries on a long tradition of scramjet research in Australia. "We got started with this research because we have the best shock tubes in the world," says Richard Morgan, director of the Centre for Hypersonics at the University of Queensland. The shock tubes are high-speed wind tunnels, and since flight testing is very expensive and not always successful, the tubes are vital to the advance of the technology.
Researchers at the University of Queensland developed a scramjet engine that reached an early milestone, creating more thrust than drag, in ground testing in 1993. The university also led the international HyShot program, achieving supersonic combustion in an atmospheric flight test for the first time ever, at Woomera in 2002.
HyShot tests continued through 2006. Then, in June 2007, a project called HyCAUSE launched a scramjet engine aboard a rocket from Woomera to an altitude of 530 km, reaching a speed of Mach 10 (more than 12,000 km/h) during re-entry.
Meanwhile, NASA is at work on its own scramjet-powered vehicle, the X-43A, which reached a speed of Mach 9.6 in 2004. And the U.S. Air Force is working on a design called the X-51 WaveRider, which could be deployed to power a cruise missile within about 10 years. The vehicle is designed to direct shock waves underneath the fuselage, so it can ride them like a surfer on a wave; this cuts down drag and boosts performance. Flight tests are scheduled to start next year.
Another U.S. project, FaCET (Falcon Combined- Cycle Engine Technology), aims to develop a fully functional hypersonic test vehicle by 2012. The craft will take off under turbojet power, accelerate to Mach 4, then switch to a liquid hydrogen-powered scramjet to reach Mach 10.
The basic idea of the scramjet may seem simple enough, but there's no guarantee researchers will be able to design a system that's safe, reliable and robust enough for civilian use. "We think we've got theoretical answers for everything," says Morgan. "But in practice, it's still hard to get more thrust than drag."
It seems those of us who are eager to arrive at the superfast hypersonic age may have to be patient with the slow crawl required to get there.
Mary Grady is a science and aviation writer based in the U.S. city of Providence, Rhode Island.
