Realizing overland supersonic flight is likely to proceed
at a subsonic pace.
April 2007, page 24
Take the boom out of sonic booms,
and civilian supersonic flight could make a comeback. For that to happen, not only does the boom
have to be suppressed, but public acceptance of the quieter boom has to be won and regulations have
to be changed to permit supersonic flight overland.
The Concorde, a commercial
jet that flew from 1969 to 2003, had a devoted following of transatlantic commuters. But it was allowed
to fly supersonically only over water because its boom annoyed people on the ground.
Theoretical approaches
for suppressing the sonic booms that occur when planes fly faster than the speed of sound have been
around for three decades. But advances in computational fluid dynamics, plus a recent proof-of-principle
flight, have made the prospect of an acceptably quiet sonic boom seem within reach. Business jets
would be the first step.
In 2003 a Defense Advanced Research
Projects Agency-NASA-industry partnership flew a US Navy F-5E plane with a modified nose. The
Shaped Sonic Boom Experiment's boom was not quiet, but it did match predictions based on the modifications.
"They were able to produce the wavefront they wanted on the ground. It was the proof in the pudding,
and it got things stirred up," says Victor Sparrow, an acoustical physicist at the Pennsylvania
State University in University Park. The SSBE results "helped a lot of people get past a lot of unknowns
in the atmosphere," adds John Morgenstern, a Lockheed Martin engineer who works on shaping planes
to produce quiet booms and still meet takeoff, landing, and other aviation requirements. "On average,
turbulence reduces the strength of sonic booms. But many people were skeptical of shaped booms'
persistence through turbulence."
When a craft exceeds the
speed of sound, shock waves form at its surface and emanate outward. The shock waves coalesce into
a characteristic 𝖭 N
shape; the "boom-boom" heard on the ground comes from the abrupt pressure increases. Boom amplitude
scales with craft size, as does financial and technical risk, so, at least for now, the focus is on
small business jets. Beyond scaling down the size, says Morgenstern, "it's difficult to get rid
of the sonic boom, and it would be impractical from an energy standpoint. What we do is change the
shape of the waveform to make it far less audible."
"Viscous dissipation
and thermal conduction make a small contribution" to the shock wave's structure, says Sparrow.
"The largest factor is the molecular relaxation process. We know the quantum mechanical properties
of oxygen and nitrogen, but how will they affect the waveform? How do we shape the airplane to have
a number of smaller shocks instead of one larger one? The idea is to delay the coalescing of the little
shock waves into an 𝖭 N
wave."
That's where computational
fluid dynamics comes in. In the past, says Peter Coen, principal investigator for NASA's fundamental
aeronautics supersonics project, "we were using fairly simple linear-theory-based analyses
to start our sonic boom predictions. Now we are using CFD techniques to get flow fields all around
the craft. In order to create a low-boom signature on the ground, we use this analysis to control
the position and shape of the shock waves so that as the pressure signal propagates away and is affected
by the atmosphere, it forms the signal we want." The shapes of the wings and other lifting surfaces
play a role in sculpting the sonic-boom signature, he adds.
Cruising altitude is also
a factor. From higher up, a shock wave has more time to be attenuated by the atmosphere on its way to
the ground, but it can also coalesce more into an 𝖭 N
wave. And a higher-flying jet requires larger wings and a bigger engine. Another tradeoff is between
an airplane's speed and its engine size and other design parameters. "When you put everything in
the mix, the optimum Mach number is about 1.6 to 1.8"—or a speed of 1.6 to 1.8 times that of sound,
says Coen. At those Mach numbers, he adds, the optimum cruise altitude is about 50 000—55 000
feet.
Sonic puff
The market for business jets is growing,
according to a 13 February article in the Financial Times. Besides business executives,
says Supersonic Aerospace International (SAI) founder Michael Paulson, small supersonic jets
"will have utility for governments and for medical emergencies such as transporting organs for
transplants." The projected price tag is around $80 million apiece, or about double a subsonic
business jet.
Paulson's company is working
with Lockheed Martin Corp and is one of a handful in the worldwide aviation industry—another
is Gulfstream Aerospace Corp, which was founded by Paulson's father—pursuing quiet supersonic
flight. An inverted V-tail attached to the wings "allows us to place the engines very far aft on the
wings. It's what we need for shaping the sonic boom," says SAI's Paulson. The boom from SAI's design
would be hundreds of times quieter than that of the Concorde, he claims, adding that the company
aims to do its first test flight in 2013.
Last year, Gulfstream's
"quiet spike," a 24-foot telescopic nose, was test flown by NASA on an F-15B aircraft. The spike
dampens the boom by breaking the shock wave into many smaller ones. "Our computerized design shows
us that we can reduce the boom energy by 10 000 times to produce a sonic puff," says Gulfstream
spokesman Robert Baugniet. Acoustic laboratory tests, he adds, show a 40-decibel reduction in
boom sound compared to the Concorde. Neither Gulfstream nor SAI–Lockheed Martin has yet
tested a full-scale model of their supersonic design, and they present their boom suppression
claims in terms that are difficult to compare.
Many challenges
But even sonic puffs would require new
regulations. "Under our current regulation, there is an explicit prohibition on supersonic flight
over the continental United States," says Carl Burleson, director of the Federal Aviation Administration's
office of environment and energy. But thanks to inquiries from industry and the SSBE flight, the
FAA began funding some research on noise and human perception of sonic booms. With NASA and the Environmental
Protection Agency, the FAA also formed a team to explore high-altitude emissions and other issues
related to supersonic flight. And it started a task force at the International Civil Aviation Organization,
the United Nations agency that sets aircraft standards, to "see if supersonic operations and noise
certification should be revised," says Burleson. "The most important thing right now is developing
a metric to judge whether the booms are acceptable or not. We hope to have a metric in 2008."
Tests of human sound perception
include using sonic boom simulators, in which people compare recorded and simulated booms; having
people listen to sonic booms outside; and rigging a house with microphones and accelerometers
during sonic booms. "We don't know all the fundamental physics of boom interactions with structures,"
says Kevin Shepherd, head of structural acoustics at NASA's Langley Research Center in Hampton,
Virginia. "But we have reason to believe that how people react indoors and outdoors is quite different.
Inside you hear objects rattle and walls creak. This will influence people's perceptions." The
time of day, frequency of sonic booms, and ambient noise also play a role.
But computations, wind-tunnel
tests, and noise and rattle measurements only go so far. "If we are going to argue to change the rule,
someone is going to have to build an actual aircraft to demonstrate, as there [are] likely to be considerable
community concerns," Burleson says. That someone, he adds, will have to come from industry. No
one has stepped forward yet. "There is no way we as an industry are going to invest a whole pile of money
into developing airplanes until we know that the regulatory groups are going to move off of ground
zero. There has to be some kind of agreement," says an industry engineer who insisted on anonymity.
"There are so many challenges,"
says NASA's Shepherd, "and a lot of places where [a revival of supersonic flight] could fall down—the
sonic boom is not the only problem you can imagine. There is fuel efficiency, global warming, airport
noise. . . . If there was enormous pressure on oil consumption, then
producing a new supersonic aircraft would probably be poor timing. It would look crazy." Although
some industrial researchers claim they can make engines for supersonic jets that do not pollute
more per mile than subsonic planes, those data are not open to the public, and most researchers believe
the opposite is true.
A more detailed look at
the repercussions of flying at 50 000 feet is needed, Burleson says. But, he adds, "aviation
is a relatively small contributor to greenhouse gas emissions, 2 to 3%. If you have 12 000
to 14 000 aircraft flying around the world, adding a couple hundred more"—the projected
number of supersonic jets is 400 to 500—"is probably not going to add a huge [emissions] inventory
burden."
As for when supersonic
flight overland might become a reality, predictions start at about six years from now. Besides
the uncertainties of setting a metric and building and testing a prototype plane, says Burleson,
"once you get into the rule-making process, it's anyone's guess."
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