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Outback Scramjet

A University of Queensland lab has supersonic success.

A small group of Australian scientists made aviation history July 30 with the successful atmospheric test of a supersonic air-breathing engine in flight. Working with a budget most big science programs would consider petty cash, the team had researchers around the world rooting for them. Their road to success can only be called unique.

Swooping across the south Australian outback in a rented Cessna 180 last November, Allan Paull learned the hard way that an aeronautics career doesn’t teach you how to keep your lunch down while airborne. Make that barely airborne: At times Paull could have leaned out and high-fived pedestrians, had there been any in this vast wasteland. But the nerve-racking maneuvers allowed him to better scan the desert for his missing scramjet. And he had a keen incentive to find it, for the remains of his papal-mitre-shaped contraption could hold information that would help him fine-tune his second scramjet, which sat in pieces some 1,500 miles away in his lab at the University of Queensland. 

The low-level aerial search failed, so he and his co-workers next took turns strapping themselves to the roof rack of a rented Toyota Land Cruiser. Balancing like skiers, they scanned the ground on either side, as the driver jostled along the route the scramjet should have overflown. They saw plenty of shrubs but no scramjet. Time to recruit reinforcements for a third outing in late February. Someone had a brainstorm: Why not enlist University of Queensland zoologists who had performed aerial surveys for kangaroos in these parts? The zoologists were accustomed to scrutinizing the monotonous landscape from airplanes without reaching for sick bags.

When working on a tight budget, it helps to be creative. It’s not for nothing that Paull’s scramjet has been termed a “scrounge jet.” With a budget of less than $2 million (“pin money,” Paull says, compared to the $185 million NASA has for its hypersonic program), Paull’s four-person team managed to be at the forefront of research that may help pull off an aviation dream: inexpensive vehicles that can fly at speeds measured in miles per second rather than miles per hour.

Many see the scramjet (short for “supersonic combustion ramjet”) as the key. Deceptively simple in principle, a ramjet is essentially a duct that funnels onrushing air into a combustion chamber, where it mixes with fuel. Its distinguishing feature is the way in which it raises the pressure of the incoming air in order to make the fuel self-ignite. Rather than use turbine-powered fans to compress the air, the ramjet forces the air to slow down and essentially compress itself as it passes through the engine’s narrowing intake duct. The end result is the same: As they escape through the rear nozzle, the burning gases produce forward thrust.

The ramjet’s simplicity offers practical advantages. It has no moving parts and therefore fewer chances for failure. It’s not limited by turbine blades’ inability to withstand engine temperatures associated with flying above Mach 3. In fact, the ramjet can’t fly below Mach 3 (it therefore requires a conventional engine to reach that speed). But it too has its limitations: Slowing down the air to subsonic speeds generates extremely high temperatures. A ramjet can therefore operate up to only about Mach 6; to operate beyond, the engine requires so much structure that it becomes impractically heavy.

In a scramjet, this problem is circumvented by slowing the air less dramatically, so that it passes through the combustion chamber at supersonic speed. A scramjet can therefore match rocket velocities, but unlike a rocket, it uses the air’s oxygen and so doesn’t have to carry tanks of oxidizer. The result, in theory, is a lighter (and therefore cheaper) craft capable of flying about three times faster than the long-standing speed record for rocket-powered aircraft, set by NASA’s X-15 in 1967: Mach 6.7. More tantalizing still, a scramjet’s upper speed limit is unknown.

On a typically hot and humid Brisbane summer day Paull, clad in short-sleeve shirt and shorts, receives me in his un-air-conditioned office at the University of Queensland. A cartoon-emblazoned punching bag sits wedged in the gap between the credenza and the window.

With wiry, sandy hair, and blue eyes, the six-foot-three Paull could pass as Christopher Lloyd’s chilled-out, antipodean cousin. Leaning back in his chair, the 42-year-old reflects on his groundbreaking work. Speaking in a broad Aussie accent, he liberally punctuates the tale with rightos  and underscores his drier observations with a slightly mischievous smile.

In 1985, after earning a graduate degree in applied mathematics, Paull netted himself a job crunching numbers for Ray Stalker, a University of Queensland space engineer of global renown who had designed and built one of the world’s most sophisticated wind tunnels at the university and used it for pioneering scramjet research. When Stalker suffered a stroke in the 1990s, Paull found himself in charge of the program. Progress remained hampered by a central limitation: Even the university’s most cutting-edge shock wave tunnel allowed a test window of only two milliseconds.

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