Supersonic business jets will use aerodynamic shaping to minimize sonic booms. Don't be alarmed by the lack of windows: Cameras will send exterior images to the cockpit and cabin.
Aerospace engineer David Graham and his three colleagues had a deadline, and a little brown tortoise was putting it in jeopardy. In a few hours, as the sun rose over the Mojave Desert on an August morning last year, two Northrop Grumman F-5E fighter jets would come racing over the horizon. Flying 30,000 feet above Harper Dry Lake and traveling at 920 mph, the airplanes would be trailing long sonic booms–the distinctive aural signatures of supersonic flight that ordinarily make high-speed passages over land impossible.
The engineers, all members of a Northrop Grumman?led research team working to make those signatures significantly less distinctive, expected the two booms would be different from one another–a difference too slight to hear, even with your ear cocked to find out whether a 30-year-old theory aimed at mitigating supersonic shock waves worked in the real, turbulent and bubbly atmosphere, but one big enough to be detected by the instruments in the back of their SUV.
But this SUV, crammed with gear that had to be set out across the lake bed, wasn’t going anywhere until the desert tortoise moved its reptile rear out of the way. The Bureau of Land Management’s instructions were strict: Startling the endangered animal could threaten its life. The predawn hours are the male desert tortoise’s time to roam in search of water, food and female company. That is arduous work, as every tortoise knows, and sometimes a guy just needs a rest. It was 15 long minutes before the beast waddled on its way.
Finally on the lake bed, NASA investigator Ed Haering supervised the placement of the portable instrument packages he’d designed, each containing an ultrasensitive Brel & Kjaer 4193 microphone, in an array about 2.5 miles wide. Away to the north, Northrop test pilot Roy Martin lined up his F-5E, which Graham had disfigured until Welko Gasich’s elegant 1956 design was barely recognizable. Martin pushed the stick forward and the pelican-nosed F-5E began to pick up speed in a shallow dive, accelerating through the sound barrier.
Pointing the aircraft accurately wasn’t easy. Graham, Haering and Wyle Laboratories boom expert Ken Plotkin had chosen dawn for the test because, later on an August day, thermals rise from the desert floor and the atmosphere gets turbulent. The chosen course put the rising sun smack in Martin’s face. Squinting at the instruments, Martin lined up and locked in the test speed–Mach 1.36, 36 percent above the speed of sound.
The cone of the sonic boom trailed miles behind Martin’s jet, and by the time the pressure wave swept across Haering’s array, he was slowing down, turning for a possible second run. Down the course after him, trailing by 45 seconds, came a standard F-5E from Fallon Naval Air Station in Nevada. Plotkin had reckoned that the comparison between the two booms would be fair if the two fighters were more than 30 seconds and less than two minutes apart.
The second boom arrived on the lake bed with its usual authority–a thunderous double bang (one for the front of the aircraft, another for the aft) audible for miles–and the data was complete. “I definitely heard a difference,” Graham recalls. That might be debatable. The two shock waves of each boom were less than 1/10 of a second apart, and the team had not even tried to alter the second half of the boom from the modified airplane. But within moments, the engineers were viewing the two booms on a laptop computer perched on a car trunk. A blue line showed the pressure wave of the modified F-5E; a red line represented the Navy fighter.
It was a dead-on match for the predictions. Ken Plotkin, who’d been in the boom business longer than anyone else present, danced a little jig. Graham says he saw tears in his eyes. Plotkin placed a call to Cornell University in Ithaca, New York. “It worked!” he said. The reply was calm: “I knew it would.”
The August test flights over the Mojave Desert have answered a critical question about low-sonic-boom design: Engineers now know that they can predict how the sonic boom develops as it travels from the airplane to your ear. That means there’s much less speculative risk involved in designing and building a low-boom airplane. Within a few years, a low-boom X-plane could be paving the way for a supersonic business jet–surveys have consistently shown strong demand for such an airplane, even at a $100 million price tag, and Boeing and Gulfstream are known to have SBJ efforts under way–or a quiet supersonic bomber, capable of sneaking into enemy territory at high speed without having its presence betrayed by piercing sonic booms. There are even signs that Lockheed Martin, with its long history of flying airplanes with capabilities that most observers deemed unattainable, could already be capable of building an operational supersonic business jet, under development in a code-locked vault and bankrolled by an unidentified sponsor.
Major obstacles remain, however, including manufacturing engines that are up to the task and can lower their noise output to tolerable levels during takeoff and landing, and refining the quiet supersonic design to be more visually acceptable than the admittedly ghastly-looking F-5E mod. But the biggest challenge when it comes to clearing supersonic flight for overland travel: Nobody knows how low is low enough for people on the ground, whether the boom comes from an Air Force jet or a billionaire’s express ride. And that is ultimately a question of politics, not engineering.
Still, the final political hurdle could be easy work, compared with everything it took to get to it. It has been a 30-year race that began with the voice that Plotkin spoke to from the Mojave lake bed, which belonged to Cornell’s Albert George. George was Plotkin’s thesis adviser at Cornell in the late 1960s, when sonic booms were a hot issue. The Concorde supersonic airliner and its Russian counterpart were flying, and Boeing was designing a 300-foot-long, 1,800 mph monster. But these aircraft were hamstrung by their shattering booms. Working from pure acoustic theory, George had found a way to reshape the boom from the sharp-edged double bang into a soft, harmless pressure wave. His colleague Richard Seebass–”Seebass would have come up with the theory about 30 seconds later,” Plotkin says–built a mathematical structure behind it. The result became the Seebass-George theory.
The researchers published their theory in January 1971. Congress scrapped Boeing’s supersonic transport two months later, and for more than 30 years the theory would remain exactly that. The math was complicated: It worked for simple shapes but nobody knew how to use it to design a practical airplane. If you tried, the only way to know if you’d got it right, even in part, was to test a model in a wind tunnel, observe mistakes, modify the model and test again–a long, expensive effort with no real assurance that the next test would produce better results instead of different problems. Seebass-George “is an ideal,” says Plotkin. “When you get into a real aircraft, the method is not as precise as you need.”
Early in the 1990s, a solution to this problem came into sight, thanks to more powerful computers and computational fluid dynamics, or CFD–the use of computers to model the airflow around an airplane. With CFD, a design could be evaluated much more quickly than in a wind tunnel, the results could be analyzed in fine detail and modifications could be made almost instantly.
But even if you could design an airplane according to Seebass-George, there’s still a showstopper. The boom on the ground is not the same as the boom next to the airplane. As the pressure wave expands through the atmosphere, it changes shape, with individual pulses flowing and blending together into the double bang that hits the ground. Seebass-George says that proper design would prevent that from happening–but only in an idealized, stable atmosphere that progressed smoothly from near vacuum at supersonic cruise heights to the thicker air at ground level. The real atmosphere is not like that–the air density is variable and the atmosphere full of turbulence and wind shears and the like–and it is too big for any CFD to encompass. Many skeptics thought the imperfections might be enough to invalidate the theory.
To prove Seebass-George, it was clear you would have to build the airplane and fly it, with a strong chance that it would be a total flop, due to the vast quantity of unpredictable factors. The researcher who most people credit with devising a solution to this dilemma is Domenic Maglieri. Long retired from NASA, Maglieri’s experience with sonic booms is unmatched. “I consider myself the world expert,” he says, “because I’ve outlived everyone else.”
In the early 1990s, as NASA started to take another look at supersonic transport airplanes, Maglieri realized that the only way to answer the critical question–”Will the boom persist with some stability, or will it go into an N-wave as it moves away from the airplane?”–was a flight demonstration. In 1993, Maglieri put forward a plan to make boom-reducing modifications to a Firebee 2, a supersonic pilotless target drone. When money got tight, the program was canceled, as was a NASA plan to modify a Lockheed SR-71 Blackbird spy plane for similar tests. But the people who had built the SR-71 at Lockheed Martin’s famed Skunk Works were secretly taking a new look at low boom. In the mid-1990s, the Skunks hired low-boom expert John Morgenstern from McDonnell Douglas and engaged Richard Seebass, then at the University of Colorado at Boulder, as a consultant. By 1998, the Skunk Works team believed they had cracked the low-boom problems and they teamed with Gulfstream on a supersonic business jet. The two companies talked to potential customers, including
NetJets. berinvestor Warren Buffett’s company had pioneered the business of selling shares in business airplanes and already had hundreds of jets on order. If low boom worked, NetJets was ready to buy.
This good news arrived just as the NASA supersonic transport was shot down by pessimistic forecasts from Boeing.
But Lockheed Martin, Gulfstream and their friends can pull strings on Capitol Hill, and the first slice of money for new low-boom research appeared in February 2000 in the Pentagon’s budget. The Defense Advanced Research Projects Agency’s Quiet Supersonic Platform (QSP) project began the following year. DARPA is not in the business of building corporate jets, so QSP’s goal was to develop so-called dual-use technology for either a business jet or a long-range supersonic bomber. That brought Northrop Grumman into the picture.
It has been said that there is a way to identify an extroverted engineer–he looks at your shoes when he talks to you. But that most certainly does not apply to the enthusiastic, voluble and nattily dressed Charles Boccadoro, who took over Northrop Grumman’s QSP program in 2000. Previously, Boccadoro had taken part in a study of future strike aircraft, and concluded that the QSP’s Mach 2 speed–about 1,320 mph–was the “sweet spot” for a next-generation airplane.
The appeal of quiet supersonic aircraft is evident now more than ever, Boccadoro says. “Anti-access” threats–from terrorism to missiles–will mean that U.S. forces will be based farther from war zones. “You’re not going to bring hundreds of short-range fighters into the theater,” Boccadoro notes. But subsonic bombers like Northrop’s own B-2 take so long to fly from distant bases that they can’t respond quickly, while ultrafast hypersonic vehicles would be difficult to maintain even if they can be built. With stealth, altitude and speed, a Mach 2 bomber “can fly day in and day out.”
Boccadoro wasted no time in signing on Eagle Aeronautics and Wyle Laboratories to his QSP team–and thereby gained exclusive access to Maglieri, Plotkin and Juliet Page, who had been a key member of the High Speed Civil Transport team at McDonnell Douglas. Their influence was crucial, says designer Jim Kerswell. “DARPA had some extremely aggressive goals,” he says, “and everyone was looking for some magic mechanism to suppress the boom.” Researchers were proposing everything from plasmas to supersonic biplanes. “Ken, Domenic and our in-house people all agreed that some of the things that were proposed either didn’t match the laws of physics or weren’t robust,” says Northrop’s Steve Komadina, current QSP program manager. “They might work directly under the airplane but not off to the side.” The solution was clearly shaping, using computer-enhanced versions of the Seebass-George theory.
At that point, Maglieri revived his idea for a Firebee-based demonstrator–but everyone knew there were disadvantages, the foremost being that unmanned test vehicles tend to crash a lot. Aerodynamicist David Graham had another idea: Northrop Grumman’s own F-5E fighter. As Northrop test pilot Roy Martin was passing Graham’s cubicle, the engineer buttonholed him and asked what he thought of flying an F-5E with a heavily modified nose. Martin had only two conditions: Don’t make the front fuselage wider (it could affect airflow into the inlets) and leave the main landing-gear doors alone (the procedure for belly-landing an F-5E is simple: Don’t).
Graham’s team spent 16 months designing a new nose for the F-5E, struggling with everything from inlet spillage–a side effect of supersonic flight that pushes air that can’t be swallowed by the inlet forward, creating shock waves near the nose–to gaping differences between the Lockheed Martin, Northrop and Boeing code used to formulate the boom-shaping modifications. The team slogged away through most of 2002 without getting much closer to a flight test. “The final design was identified as 24B4, if that gives you any idea of how many we went through,” says Graham.
In December 2002, DARPA hosted a meeting in Huntington Beach to review the proposed F-5E modification. Plotkin, Maglieri and the Northrop Grumman team got together in a room before they met with NASA and DARPA, and “we asked ourselves, ‘Are we there?'” Graham recalls. The final design was a gross modification, a giant pelican beak, but the team decided that it would work. Now Northrop Grumman could start modifying the test airplane–a Navy F-5E. It had flown 7,200 hours and the Navy was ready to ship it to the boneyard. Northrop Grumman secured a 50-hour life extension.
One worrisome problem: The time required to modify the
F-5E pushed the first flight tests into the summer, when high temperatures would make the air more turbulent and increase the local speed of sound, putting the Mach 1.4 target out of reach. “The good news was that our drag prediction was right on,” Martin says. “That would get us to Mach 1.3.” General Electric gave its blessing to a “throttle push” on the tired J85 engines, on condition that nobody would use them again. The modified airplane proved reliable and straightforward to fly. Plotkin says now that no one who has worked with the Seebass-George theory ever doubted it–and the straight-out-of-the-box success of the August 27 tests and the almost uncanny match between prediction and theory was worth performing a dance over. “There were still people who said you’d never get those results in the real world,” he says. Adds Maglieri: “Shaping wasn’t really what the test was all about. It was a matter of proving that the effects of the shape would persist in a real atmosphere, at a real distance.”
With a few precious hours left on their aging test airplane, the team secured funding for a series of follow-up tests to see whether the results could be reproduced under different conditions. The flights were made in January 2004, when cooler temperatures opened the speed envelope to Mach 1.45, and made it possible to fly as late as 1 p.m. and still get good data. The weather was perfect. “We got very lucky, nine days in a row,” says Haering. The F-5E ran like a new Honda, and the team logged 21 more flights.
Haering launched a sensor-equipped F-15 to probe the shape of the boom and borrowed a Blanik sailplane from the USAF test pilots’ school to pick up measurements 6,000 to 8,000 feet above the ground, clear of eddies and turbulence. Plotkin designed a “push-over” maneuver, which would focus the boom on a selected area of the ground, as might happen when an airplane turned: Once again, the result matched the theory. “We got to see the Plotkin dance of joy again,” says Graham.
Today the Northrop Grumman team is looking for a museum home for the research airplane and, like other companies, is looking to the next stage. Boccadoro detects increasing Air Force interest in a supersonic bomber, and half a dozen companies are looking at supersonic commercial airplanes. They include Gulfstream–which has developed a swing-wing design and a telescopic swordfish-like nose spike that breaks up and weakens the forward shock wave–Lockheed Martin and Boeing. The potential market is enormous, says Harvey Maclin, a manager of advanced technology marketing and government programs for GE Aircraft Engines. “NetJets will tell you that they’d order 100 supersonic business jets right away,” he notes, quoting a number of forecasts that put the market for supersonic business jets at close to 500 aircraft, even at a price approaching $100 million apiece.
Most people agree that before either the Pentagon or NetJets kicks off a full-scale development program, there will have to be an X-plane, an all-new airplane designed to achieve a low-boom signature. That will take money. “The technology is in great shape,” says Maclin, “but we don’t have the resources today to accomplish it.” NASA is formulating plans for an X-plane that would be modified over its lifetime to demonstrate a series of technologies, according to Peter Coen, who heads a new supersonic vehicle team at NASA’s Langley Research Center in Virginia. Its first mission would be to prove to the Federal Aviation Administration and the public that today’s outright ban on supersonic commercial flight could be replaced by a limit on boom intensity.
The engines, in particular, will require special attention. The main engine makers–General Electric, Rolls-Royce and Pratt & Whitney–are studying supersonic cruise engines for military and civil aircraft. Key challenges: Civil engines have to be quiet on takeoff and landing (but military engines don’t) and supersonic engines run at full blast all the time, making it hard to run several thousand hours between overhauls.
Perhaps the furthest along is Lockheed Martin. Funded by an unidentified sponsor, the company’s supersonic business jet project has been under way since 2001. Program manager Tom Hartmann won’t discuss the effort, except to give credit to Seebass, who worked with Lockheed Martin before his death in 2000. “When the history is written,” he says, “Seebass will be the Orville and Wilbur Wright of low boom.”
A Lockheed Martin press release in November stated matter-of-factly that their airplane’s low-boom technology had been demonstrated and that the aircraft design was “closed,” meaning that the company had a combination of shape, size, performance and weight that would work. No demonstrator was needed, the company suggested, just a new FAA rule allowing limited supersonic overflight at a peak overpressure of half a pound per square foot. The company–and its unidentified sponsor–believes it can scoop a 500-airplane market if the airplane gets airborne before its competitors.
By 2010, an X-plane or even a business jet prototype could do something that 15 years ago most people would have considered contrary to the laws of physics: Fly at supersonic speed without trailing a perceptible boom across the ground. If that happens, a very ugly modification to an elegant little fighter will have been a crucial step along the way.
It Ain’t Pretty, But it Worked
From “Boom” to “Pop”