NASA experimental X43 aircraft
Credit: NASA Dryden Flight Research Centre photo collection
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.
