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Outback Scramjet

A University of Queensland lab has supersonic success.

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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.

Then opportunity came knocking, in the form of a Florida-based company called Astrotech Space Operations. The company had no interest in scramjets per se; it merely wanted to expand its sounding rocket business (selling cargo space for microgravity science experiments) into the Asia-Pacific region. What better way to make a public relations splash than by carrying “some sexy payload,” in Paull’s words, on the two demonstration flights the company had planned? An intermediary made the introductions, and in 1998 the two parties signed an agreement. Astrotech would provide the Terrier-Orion rockets for two launches; Paull would equip them with a cutting-edge scramjet experiment. The HyShot program was born.

Now the pressure was on: “We had to figure out how to make the engine fly and not fall apart” in a test time window hundreds of times longer than the one available in wind tunnels, Paull says.

Space programs from around the globe have tried to tackle the same perplexing dilemma for decades. The United States and Russia, in particular, have invested millions. What’s the carrot motivating their research? First and foremost, they hope one day to use scramjets as a cost-effective rocket replacement in space launch vehicles. Military planners want to add hypersonic missiles to their arsenals. On the commercial end of things, a scramjet-powered passenger airplane could, in theory, reduce travel time, allowing you to fly from, say, London to Sydney in two hours.

Indeed, eyeing such payoffs, the U.S. government began funding scramjet research in the 1960s; it now sponsors some half a dozen scramjet programs in the Department of Defense and NASA (see “A Matter of Seconds,” p. 76). Since 1994, NASA has worked with the much-lauded Russian program, which has launched some of the most successful tests to date. Today, half a dozen other countries have substantive programs as well.

Yet for half a century the scramjet has remained (excuse the pun) a pipe dream. Putting the simple theory into practice is fraught with engineering challenges. To begin with, there is the difficulty of igniting fuel with air that is traveling at supersonic speed. “It is like lighting a match in the middle of a blowing hurricane,” says Robert Mercier, head of HyTech, the U.S. Air Force’s scramjet program, located at Wright-Patterson Air Force Base in Ohio.

Then there is the heat issue: A vehicle traveling that fast can reach a temperature of 3,600 degrees Fahrenheit—similar to what the Apollo capsule experienced on reentry, and hot enough to warp, if not melt, most materials. (Some researchers are experimenting with heat-absorbing fuels that, prior to combustion, would circulate through channels in the engine walls to cool them down.) Also, to work effectively, scramjets must be integrated with the airframe; how best to do this remains a question.

The most advanced wind tunnels can accelerate a scramjet model to the required speeds and temperatures for only a few milliseconds. But actual flight tests are expensive, logistically challenging, and considerably vulnerable to things going wrong.

Just ask NASA. In June 2001, the agency hoped to record the first scramjet-powered hypersonic flight in a much-ballyhooed trial (the first of three in its Hyper-X program) off the California coast. But the rocket booster malfunctioned and, before the scramjet could be released to allow the real experiment to begin, propelled itself and its payload straight into the Pacific Ocean.

Because different scramjet teams have focused on different pieces of the research puzzle and pursued slightly different near-term goals, the question of who can rightfully claim some nominal “first” usually hinges on semantics and nuances; “It’s a bit fuzzy,” Paull says. What is beyond dispute is that until July 30 no one had demonstrated purely supersonic combustion in a scramjet hurtling unaided through the atmosphere.

The scramjet rode as the payload on a Terrier-Orion sounding rocket, which flew into space in a parabolic arc. The scramjet separated from the booster on the upward trajectory, rotated at its apogee toward Earth, and eventually operated for five seconds (at a speed of almost Mach 8) before it hit the ground. Attached instruments measured various parameters and transmitted a stream of data that researchers can now use to calibrate their design, analysis, and test tools to real flight conditions.

The promise of such data prompted researchers on the other side of the world to pay close attention to Paull’s HyShot program—so much so that NASA even became one of its sponsors. “We’re very hungry for flight data,” says Lawrence Huebner, manager of the Hyper-X scramjet propulsion program at NASA’s Langley Research Center in Virginia.

So what crucial decisions did Paull and company make to eventually achieve what Paull himself called their “beautiful” second launch? To start, Paull hired Hans Alesi, a German-Australian aerospace engineer who had read about HyShot and called to offer his services. Together the two men scrutinized potential engine designs, “trying to figure out how they could go wrong,” Paull recalls. “It’s like going out on a first date. There are a lot of ‘what ifs.’ ” They needed something that could withstand high temperatures, conduct heat well and did not bust their budget. They settled on an alloy of silver and copper, then commissioned university technicians to build it. The final result: a scramjet about half the size necessary to generate enough thrust to propel a craft, but large enough for their experiment.

Next they had to design and build the scramjet’s instrumentation module, which would control the flight and collect and transmit data. Several unexpected and time-consuming problems surfaced. Developing and testing a sophisticated attitude control device took a year and proved “a bigger challenge than the scramjet,” Alesi says in a lilting German accent. Paull and Alesi also had to design some of their payload test tools, such as a three-axis gimbal to simulate how the payload might rotate at the height of its trajectory.

The hitch-a-ride-on-a-sounding-rocket strategy was cheap, but was it cheap enough? As the HyShot timetable doubled and then tripled, Paull began to wonder if the team would run out of funds.

The University of Queensland, the Australian government, and QinetiQ (the privatized arm of Britain’s military research agency) had provided seed money for the project. Paull eventually secured additional funding from NASA,  the German and Japanese space agencies, Korea’s Seoul National University, and several Australian companies. Part of the appeal for the sponsors was Paull’s “clever and cheap way of getting this data,” says Terry Cain, a research fellow at QinetiQ, which is testing its own scramjet engine at one of the university shock tunnels.

Then, to stretch their modest budget, the team members got even more resourceful. Paull assigned students scramjet-related projects and in myriad other ways convinced people “to do things for free,” he laughs.

Some expenses were unavoidable: Wind tunnel tests (conducted an average of twice a day) cost about $500 per shot. To run them and also help out with other testing, Paull eventually hired one of his former graduate students, Myles Frost.

Lacking the resources to hire an expert for each task, he and his team members each wore many hats; Alesi sometimes found himself standing at a mill or a lathe, manufacturing some component for the payload. But they all say their big-picture perspectives encouraged them to come up with innovative solutions. Paull offers one such example: Forced to devise something that “weighed a kilo but could hold a ton” to keep the payload in place on the launch pad, Alesi designed a retractable lug that a private company has since expressed interest in buying.

Paull’s other big challenge was what he describes as an “amazing legal nightmare.” Like some B-grade horror movie plague, it ate up half his time, even after he hired another former graduate student, Susan Anderson, to help keep it at bay. The team had to secure authorizations from various state government agencies, coordinate with aviation bodies and insurance companies in both Australia and the United States (because of the involvement of U.S. funding), perform environmental assessments, and ensure their launch debris would steer clear of land claimed by Aboriginal tribes. They even had to visit area ranchers in person to allay their concerns. To complicate matters, the Australian government then grew jittery about anything taking place so close to a highly controversial refugee detention center. All told, the preparations took three and a half years. There were moments during that time when Paull wondered if transferring to a better funded program somewhere else might be the only way to fly a scramjet.

But surprisingly, at least to him, Paull had become something of a celebrity. In an isolated country accustomed to being a bit player on the world stage, his cutting-edge work was drawing considerable attention. Australian media coverage fired public enthusiasm to the point where new acquaintances congratulated Paull whenever he mentioned his work. HyShot was sometimes cast as a David versus Goliath affair, appealing to Australians’ affection for “battlers” persevering against all odds. “The strong effort we put in was appreciated,” Paull says. With that kind of moral support, “I couldn’t turn around and defect [to a program overseas] just to get the job done.”

Yet as the launch date drew closer, the prospects of meeting the deadline grew dimmer. Time to call in some family favors. During his student days, Paull had occasionally helped his father, Bert, who installed and maintained movie theater equipment, on emergency repair jobs. Now it was the 73-year-old retired father’s turn to step in and lend a hand. Scramjet electrical wiring may not have been part of Bert’s job description in the “picture business,” which he entered back when biplanes served the more isolated towns in his vast territory. But the fundamentals hadn’t changed: “All wires have got two ends,” he points out good-naturedly. Over the course of a month, Bert took time away from lawn-mowing and other pastimes (notably, monitoring airplane cockpit transmissions on his scanner radios) to install more than 40 yards of cables inside the payload and on the launch pad.

Also stepping up to the plate was Allan’s older brother Ross, an applied mathematician who had run his own machine tool business for more than 20 years. Ross helped develop the flight control software; he also lent a hand with the electrical work. Alesi says the family’s cutting-up helped take the edge off the many late nights and weekends on the job.

Finally, the day of reckoning drew near. In late October, Paull and his teammates drove halfway across the country with their precious cargo on the back of a utility truck. Paull’s family followed in their own cars. Their destination: Woomera Instrumented Range, a speck in the desert 300 miles north of Adelaide.

 “We knew the whole thing was fraught with problems,” Paull admits.

The launch itself went off without a hitch. But then the second-stage rocket veered off course and disappeared over the outback. Having experienced a similar misfortune, NASA’s Lawrence Huebner concludes with a laugh, “Rockets don’t want scramjets to take over their job.”

Paull admits that his own team lost momentum after that first failed test. Alesi, seeking more financial security for his young family, left to work for Boeing in the United States. Ross Paull took his place, and the team took up the challenge of finding their missing scramjet. Both the Royal Australian Air Force (which had sent a reconnaissance helicopter soon after the launch) and Paull’s own search-and-recovery efforts had been more search than recovery, until the University of Queensland kangaroo spotters signed on for the job. By a bizarre coincidence, the likely points of impact lay within their annual survey area.

The new recruits went airborne but saw little of note the first two days, aside from the extraordinary sight of Paull teetering on a stool set atop an upright oil drum, the entire ensemble strapped to the roof of the four-wheel-drive vehicle he and his brother were using for their concurrent ground search. Eventually, someone in the airplane spied a grounded rocket. But hopes were dashed as quickly as they’d been raised when, upon closer inspection, it turned out to belong to some mystery third party.

Finally, on the third day, their optimism now flagging, one of the zoologists spotted something resembling a rubbish dump. The absence of wheel tracks suggested it might have fallen from the sky. Zoologist Gordon Grigg radioed Paull and company, who raced to the sight. “We could tell from their body language it was the right one,” Grigg says. “Myles, the first to arrive on the scene, began pointing very excitedly and jumping up and down.”

An analysis of the wreckage helped the crew prepare their second prototype. They worked furiously to meet their self-imposed deadline of July 30, 2002, for the second launch. This time the Terrier-Orion Mk70 rocket did its job and took the scramjet into the upper atmosphere, where it kicked in 22 miles above Earth, reaching speeds of more than 5,000 mph before ramming into the ground.

No flawed leftovers for the kangeroo spotters to find this time.

 


A Matter of Seconds

Before HyShot’s July 30 flight added another five seconds to the tally, the few minutes of scramjet flight data that had been gathered had all come from Moscow’s Central Institute of Aviation Motors. The Russian bureau conducted experiments in the 1990s, giving three scramjets rides on the noses of rocket-powered SA-5 anti-aircraft missiles. On the last launch, supported by NASA in 1998, the scramjet operated for 77 seconds with internal flows that were subsonic during parts of the flight.

Aerodynamicists have used supercomputers to simulate the airflow within scramjets, but combustion chemistry in turbulent flow has proved too complex, even for the most powerful computers. Flight tests produce far more reliable data—when they work. Had NASA’s Hyper-X program succeeded last June with the launch of the X-43A, it would have added 12 seconds to the total. (The X-43A also hitched a ride on a rocket, an Orbital Sciences Pegasus, launched from the wing of a B-52 from NASA’s Dryden Research Center in California.) Unlike the Australian and Russian engines, the X-43’s scramjet is integrated with the vehicle’s airframe. The 12-foot-long, 2,200-pound aircraft will separate from its rocket booster and fly alone through the atmosphere. The scramjet will ignite for the 12-second experment after the booster has accelerated the vehicle to Mach 7. The next launch attempt, according to program manager Lawrence Huebner, will take place in summer 2003.

In an independent effort, the U.S. Air Force is broadening the range of scramjet fuels, with an eye toward using the engines in operational missiles. Scramjets built to date, including those for the X-43A, have used hydrogen. It mixes and burns readily in an engine, but it is difficult to store and handle. The Air Force program, called HyTech, is relying on JP-7, a hydrocarbon fuel in use at most military bases.

Scramjet fuels must mix and burn far more rapidly than JP-7 can in ordinary  circumstances. But the HyTech effort “cracks” the fuel’s molecules, breaking them into fragments that mix and burn more easily. In the laboratory, the cracking is accomplished by heating the fuel in a separate installation, but engineers believe that in flight the heat from the engine can break down the JP-7 molecules. The fuel, circulating as a coolant through the walls of an operating scramjet, will absorb heat and crack before it is injected into the combustor.

In the spring of 2000, and again in November of that year, a scramjet that had been built by Pratt & Whitney and that used cracked JP-7 as fuel produced a significant, though classified, amount of thrust in a wind tunnel. That version, built of heavy copper, weighed about 2,000 pounds and had no cooling. But a follow-on test engine using flight-weight components and weighing less than 200 pounds is now being tested with JP-7 as both the fuel and the coolant.

Program manager Robert Mercier expects to begin testing a third engine, this one a complete flight-ready system incorporating fuel pumps and valves, in 2003. In the meantime, NASA, a partner in the HyTech project, plans to develop an advanced X-43 to test the HyTech engine in flight in 2006 or 2007. The mission goal will be to accelerate under scramjet thrust from Mach 5 to Mach 7.

But first the next X-43A, which already is being prepared at the Dryden center, must collect its 12 seconds of scramjet flight data. Lawrence Huebner points out that although 12 seconds seems very short, it was the duration of the Wright brothers’ first powered flight in 1903.

—T.A. Heppenheimer

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