The choices available for those interested in heavy-iron lift today are more numerous, capable and varied than ever — ranging from for-business Boeings and Airbuses to the fly-by-wire Falcon 7X, large-cabin Gulfstreams, Embraers and Globals. Extraordinary and unique as each may be, all the current aircraft have one thing in common: They are subsonic. And although various manufacturers have touted supersonic business jets (SSBJ) in recent years, not one has moved from paper to project launch.
Demand for SSBJs
"The demand for the [supersonic] aircraft will be substantial," says Teal Group Vice President of Analysis Richard Aboulafia. "We think there will be a requirement for 400 aircraft over the first 20 years based on growth in the high-end business jet market." Aboulafia is obviously one of the strongest proponents for the SSBJ.
He divides the market with half the aircraft based in the United States and the other half of the market in Asia, the Middle East, Russia and Europe; some orders will be placed by fractionals.
"Governments have historically absorbed 20 to 25 percent of the high-end business jet market, and for military applications it makes sense when you need to move a rapid reaction team. It can be emergency medical teams, special forces or VIPs involved in critical situations," Aboulafia said. "Can you move those people and their equipment on commercial airlines? I don´t think so."
But the big question is who can build, certify and support such an expensive project. Aboulafia suggests two things are required to bring the airplane to market: experience selling high-end business aircraft and knowledge of how to build a supersonic aircraft. Considering the expense of the undertaking and the potential market size, the best R&D and manufacturing option may be a consortium, but Aboulafia doubts one will ever take form since "history shows that successful aviation partnerships are very rare.
"Obviously, Boeing comes to mind," he said, "but it has a lot on its plate and would have to decide if it really want to get into such a market. Dassault has vast experience building supersonic fighters and also sells high-end business jets. For Dassault this is not a technical issue but a market decision. With Dassault bringing its new 7X to market, it may not want to confuse customers with another airplane."
Gulfstream is also absent from the consortium table. More than a decade ago, it worked with Sukhoi on the never-born S-21 SSBJ and recently it worked with NASA on its "Quiet Spike" project for minimizing sonic booms. Beyond acknowledging that activity involving a specially rigged F-15, the Savannah manufacturer has little to say about SSBJs. Aboulafia suggests that Gulfstream´s quiet stems from more immediate and practical reasons.
"One of the industry´s worst-kept secrets is the G600, which is supposed to be Gulfstream´s follow-on to the G500 and G550," he said. "Some suggest this secret airplane is a widebody and again, the last thing you want to do is spend money developing a new airplane and immediately introduce something else."
Two companies, Aerion and Supersonic Aerospace International (SAI), are actively pursuing the goal of developing SSBJs. Each is following a special course. Aerion, which is backed by financier Robert Bass, is promoting an aircraft, powered by commercial turbofans, that would fly subsonically over the United States and supersonically elsewhere. Meanwhile SAI, backed by Michael Paulson, is to flying supersonically everywhere, thanks in part to the fact that its boom would be hushed to a poof. Neither group intends to actually manufacturer anything, but rather advance their respective programs enough to attract a manufacturing partner.
Both approaches have challenges. "Aerion is using mostly proven technology and says it can work with current regulations and limitations," Aboulafia noted, while "SAI´s approach depends on newer technology and thus carries higher risks. The latter approach means a longer time to market, but possibly a better design. Either way, politics will play a role in all this."
On that last point, there´s no debate. Federal noise regulations prohibit supersonic flight over U.S. soil. That operational restriction helped slice orders and options for 78 Concordes to just 14 aircraft that subsequently flew for the flag carriers of Great Britain and France, the nations involved in building the thing.
While noise remains a big issue, any new supersonic transport will face increased challenges regarding other environmental concerns as public concern grows over global warming, water vapor in the upper atmosphere and nitrogen oxide emissions. And with various articles already vilifying SSBJs as mere "big boy supersonic limos" politicians may be in no rush to amend regulations regardless of recent studies showing the sonic boom can be modified for no-boom/low-boom.
"The SSBJ will happen," Aboulafia maintains. "The demand is there and this will eventually happen."
That prediction is seconded by Peter Edwards, CEO of Jet Aviation, and former head of Bombardier Business Aircraft. A member of Aerion´s board of directors, he says, "I think it´s a long-term project, but I do believe there will be a supersonic airplane in business operation."
At the moment, he said, the problem is political. "What´s really required is a regulatory road map from the FAA," Edwards said. "That´s the key blocking point." Once that is resolved, the engineering staffs will be able to focus on finding acceptable technical solutions and deliver what Edwards describes as a "very, very useful piece of hardware," one that serves an extraordinary next step for the globablization of business travel."
Background on the Boom
Supersonic aircraft create a shock wave that radiates out in all directions. The "boom" is created by an initial overpressure. This is followed by a second wave that creates an underpressure before the final response, which returns the pressure to ambient. When plotted on a graph, the profile looks like the letter "N." Amazingly, the pressure change required to create an audible boom is quite slight, the equivalent to descending three floors in an elevator. So it is not so much the pressure differential, but the rate of change that creates the sharp loud boom or a barely noticeable dull thud.
Aircraft size, weight and shape as well as altitude and atmospheric conditions influence the boom. Speed, air temperature and wind can affect it significantly, but above 1.30 Mach, the effect becomes smaller. Larger aircraft have larger booms because they displace more air along their flight path and the heavier the aircraft, the stronger the boom as well. Sleek, thin aircraft produce less shock than large, blunt shapes.
NASA documents cite increased altitude as the most effective method to reduce the boom´s intensity. Also, because the boom spreads out in all directions, its intensity is not uniform but rather will be strongest directly beneath the aircraft and less so as one moves to the side of the flight path. For comparison, some well-known vehicles and their overpressures are listed here:
*SR-71: 0.9 pounds, speed of 3.o Mach, 80,000 feet
*Concorde SST: 1.94 pounds, speed of 2.0 Mach, 52,000 feet
*F-104: 0.8 pounds, speed of 1.93 Mach, 48,000 feet
*Space Shuttle: 1.25 pounds, speed of 1.5 Mach, 60,000 feet, landing approach
(Note: In 197,7 former Lockheed test pilot Daryl Greenamyer flew a rebuilt F-104 at 998 mph at 80 feet above the desert floor. A supersonic aircraft at 100 feet agl will produce an overpressure of 144 psf!)
NASA and Research
NASA has flown various vehicles to see if the sonic boom signature could be reduced or modified. In 2002, an extensively modified F-5E, dubbed by some the "Pelican," demonstrated a modified N-wave with a quieter boom. In 2006, the agency completed the aforementioned Quiet Spike project flying an F-15B with a 24-foot-long extendable spike. This past July it flew an F-18 in another series of tests. These tests are monitored in flight by a Blanik L-23 sailplane equipped with microphones to measure the boom before it enters the air a few thousand feet above the ground where turbulence can intensify it.
Ed Haering, NASA engineer, explained the "N-wave" of the sonic boom saying, "It is a plot of pressure versus time. Pressure rises for the first leg of the ´N´ and then there is a diagonal line where pressure decreases to less than ambient and the final leg is the upward spike returning to ambient pressure." If the "N" is tall and sharp, the boom will be very loud. The target is to make the "N" more of a sideways "S."
Haering has worked on all the recent projects studying the sonic boom. Asked about the Pelican, he said the airplane was quite ´draggy,´ but that was because it was not designed from scratch for low boom/no boom. "We only shaped the nose because that was the inexpensive way to do it. We could not interfere with the engine intakes and couldn´t change the canopy. A completely new aircraft would look different from the Pelican."
"The SSBJ is on the horizon and it will probably be a matter of years and not decades but again, the big question is what the FAA is going to do. How loud is too loud? If the laws are very restrictive, it may be technically unfeasible and if the noise is too loud, it may be publicly unacceptable," said Haering.
Aerion´s business model does not require changes on noise legislation and its SSBJ is intended to make extensive use of off-the-shelf hardware. For example, Aerion plans to power its SSBJ with the -219 variant of the ubiquitous and reliable Pratt & Whitney JT8D. For decades, more than 11,800 JT8Ds have accumulated over one-half billion flight hours powering 727s, 737s, DC-9s and MD-80s and the -219 variant has recently been selected as the powerplant for the U.S. Air Force´s 19 Joint Surveillance Target Attack Radar System (JSTARS) aircraft.
Pratt has placed a 3,000-hour TBO on the -219 when used in the typical Aerion SSBJ profile, which is not representative of airline operations. The engine is flat-rated at 19,600 pounds of thrust and will not require an afterburner to achieve the 1.6 Mach cruise. While some may consider the engine a bit long in the tooth, Aerion engineers say the -219 not only meets or exceeds current and pending environmental standards for noise and NOX emissions, it is an ideal SSBJ engine.
"It has an ideal bypass ratio of 1.7:1," according to Dr. Richard Tracy, Aerion´s chief technology officer. "An engine with a higher bypass ratio would mean a larger, heavier engine with a greater drag profile that would be overwhelming. A smaller bypass ratio would mean higher exhaust velocities and that means noise going up extremely quickly. More noise would mean sound-deadening systems that are heavy and cost thrust, not to mention they reduce engine efficiency."
Tracy said the engine would not use a FADEC, but would be equipped with a computer interface that mimics one. No decision has been made on moving vs. non-moving throttles.
Aerion does not plan to use variable ramps and moving exhaust nozzles, but Tracy said engineers have spent considerable time designing the -219´s unusual intakes, nacelle and exhaust nozzles. The intake, which is sharply raked away from the fuselage, employs a "cheese cutter"-like splitter plate to control airflow spillage and shockwave. Supersonic, the shockwave falls just on the outside lip of the intake. The exhaust nozzles, raked almost parallel to the intake, are also innovative.
According to Tracy, "Nice things happen from the exhaust nozzle design. The aerodynamic coupling of the nozzle and aft fuselage shaping near the exhaust eliminate a lot of boat-tail drag and create some suction on the forward facing surface of the aft fuselage, which also helps with drag. Having the longer ramp on the outside provides some noise shielding as well as changing the thrust line slightly, which improves engine-out handling."
While a biconvex wing is not new, Aerion has created a wing that will have no leading edge devices, but will use a sort of split flap to improve takeoff and landing performance so the aircraft can routinely operate in and out of 6,000-foot runways. A section of the wing design has been flown beneath a NASA F-15 to verify projections. Also, with its patented natural laminar flow over about 70 percent of the upper wing and 100 percent of the lower wing, tests have shown the airfoil produces considerably less drag than a delta or highly swept wing. MMO is 1.6 but best supersonic range should be achieved at between 1.4 and 1.5 Mach. Subsonic and at 0.95 Mach, range is expected to be 4,700 nm. Maximum ceiling target is 51,000 feet and a typical end of mission approach speed is put at 120 knots.
Currently, the fuel load is set for 45,400 pounds of Jet-A stored only in fuselage tanks. Fuel burn and transfer will be done automatically during both subsonic and supersonic flight to maintain optimum c.g. Of course, manual control is provided as a redundancy.
Brian Barents, Aerion vice president, says its market survey projects 250 to 300 aircraft in the first 10 years with 20 percent of the orders coming from fractionals. The survey did not include purchases by governments. Barents emphasizes Aerion´s approach differs from others in that its SSBJ makes sense even with current regulatory limitations. "We will operate in the current mix of aircraft. Unlike others, our business plan assumes current rules for noise and emissions remain unchanged," he said.
"We will be flying at 45,000 to 51,000 feet with the other business jets, but we don´t pay the drag penalty for subsonic flight incurred with a delta wing. And supersonic flight over land is permitted by ICAO as long as there is no sonic boom," he said. "This means we can fly up to 1.1 or 1.2 Mach depending on atmospheric conditions. Only the United States prohibits any Mach 1 flight over land. In many areas such as Northern Canada, Siberia and Australia, unlimited speed is permitted regardless of the boom strength."
Asked if he thought the U.S. legislators were stuck on the number, Mach 1, or the boom itself, Barents suggested that ICAO may adopt rules that allow supersonic flight as long as there is ". . . no noise nuisance or boom. Then the United States may quietly adopt those regulations."
Aerion´s timeline consists of one year after signing with an OEM and another five years after that to build and certify the airplane. Deliveries are to begin almost immediately after certification. Cost is still pegged at around $ 2 billion to bring the airplane to market and price is put at between $ 80 and $ 100 million per copy.
Supersonic Aerospace International (SAI)
"We will be the best in the market because our design comes from Lockheed and no one can doubt their designing ability. They have proven this over 50 years with revolutionary designs and so we are not just talking paper airplanes." So spoke Michael Paulson, whose SSBJ effort is a part of the legacy of Paulson´s late father, Allen, the former head of Gulfstream who wanted to build an SSBJ in partnership with Sukhoi.
Over the last six years, SAI has completed extensive tests for its Quiet SuperSonicTransport (QSST). SAI has filed for 27 patents based on its QSST research, with 13 patents having been awarded. The patents range from synthetic vision to some involving ". . . secret sauce from Lockheed´s Skunk Works."
SAI´s design includes an inverted V-tail, which is said to provide multiple benefits.
"The inverted V-tail is an essential part of the QSST´s revolutionary design," Paulson said. "It shapes and mitigates the QSST´s sonic signature and it acts as a truss, allowing us to attach the engines as far aft as possible on the wing. It also creates an additional lifting surface."
"Our design is the only one out there that shapes the sonic signature," Paulson continued. "We will work for new regulatory approval showing we can fly over populated areas with an overpressure of 0.3 to 0.5 psf, which is a very insignificant level. In fact, if we were having a conversation on a city street, you would probably not hear it. It is comparable to the rumble of a truck in the distance. For comparison, our airplane will have about one-one-hundredth of the sound signature of Concorde."
Paulson said his group has had preliminary discussions with several airframe builders and believes "it´s just a matter of time" before they land one to build and support the aircraft. Failing that, however, he said, "Our current business plan is structured to stand up SAI as a full-fledged OEM in the event that a suitable existing OEM is not forthcoming, as there are really only a handful of companies capable of building this revolutionary aircraft."
Cost to market will be about $ 2.5 billion to $ 3.0 billion and Paulson expects the money to come from overseas investors as well as from within the United States. If the entire package cannot be raised from investors, he may try to take the company public and make SAI an OEM.
Paulson is funding the initial portions of the project and said SAI will require another $ 250 million before they can begin cutting metal.
Gulfstream´s 1990 concept — the 114,000-pound, 1.4 Mach S-2,1 — faded into aviation history without any metal being cut and the OEM has been rather quiet about its subsequent SSBJ activities. Although it has invested in sonic boom research and noise suppression, Gulfstream maintains there´s no business case for such an aircraft until the ban on over-land supersonic flight in the United States is lifted.
Its partnership with NASA on the 470-pound composite Quiet Spike was deemed successful in that the technology modified the normal supersonic shock waves to produce considerably less noise. The Quiet Spike also showed that one could design an aircraft with a larger fuselage without compromising the desired low sonic boom.
Gulfstream´s Quiet Supersonic Jet (QSJ) presentations (available online at www.aiaa.org/events/aners/Presentations/ANERS-Henne.pdf ) argue the market requirements call for an airplane with a 1.6 to 2.0 Mach cruise and a 4,000 nm range. If the over-land ban is rescinded, it sees a potential for 200 to 400 aircraft, priced at $ 70 million to $ 100 million each, with fractionals offering a large potential market.
"From the outset it has been understood that the sonic boom must be reduced to acceptable levels before consideration could be given to developing a prototype quiet supersonic jet," said Gulfstream´s Pres Henne, senior vice president, programs, engineering and test. "These tests are just a few in a series of activities that must be undertaken to prove to numerous regulatory agencies and environmental groups, both at home and abroad, that supersonic flight over land is achievable in a way that will significantly reduce the impact of the sonic boom on people and on the environment. It is only one step, but a very important step, for all of us."
Gulfstream says the Quiet Spike technology "transforms the sharp crack of sonic boom into a quieter sound." Company literature shows the modified boom noise level at or slightly below routine talking dB levels and much lower than the noise generated by city car traffic. A patent was issued for the technology in March 2004.
Gulfstream´s design work assumes a 100,000-pound airplane flying at 1.8 Mach and carrying eight to 14 passengers at least 4,000 nm. Like Aerion and SAI, Gulfstream assumes a 6,000-foot runway for takeoff and landing.
Summary notes for the Quiet Spike jet says there is a "significant market" for small supersonic civil aircraft and that progress is being made to address environmental concerns. But in the end supersonic flight over land must be permitted for any such project to move forward.
Also in the 1990s, Dassault proposed a supersonic Falcon, but the lack of a reliable engine capable of sustained 2.0 Mach cruise clipped its would-be wings. Still, the French manufacturer continues to study the concept.
Having produced Mirage and Rafale fighters for decades, Dassault is intimately familiar with the demands of supersonic flight, and now it is working with 37 organizations on the high speed aircraft through the HISAC project whose main objective is to establish the technical feasibility of an environmentally compliant supersonic small size transport aircraft (S4TA), through a Multi-Disiplinary Optimisation (MDO) approach and focused technological improvements. Dassault is in charge of program coordination. For more information on the HISAC project, visit their Web site at www.hisacproject.com.
Meanwhile, Bombardier says it is "keenly aware of developments across corporate aviation," including work on SSBJs. However, it too sees numerous roadblocks to the concept including the absence of a new-technology powerplant designed for long-range, supersonic flight. And it believes the performance of any such aircraft would demand runway lengths exceeding those of most general aviation airfields, thus forcing the use of major, airline-centric airports.
Growing concern over emissions — both noise and toxic — are significant challenges, the Canadian manufacturer maintains, and it also questions whether there´s enough demand for an $ 80 million aircraft to support the investment required to develop, certify and deliver it.
The capper, according to Leo Knaapen, Bombardier´s director of public affairs, are the regs already on the books among regulators in the United States and in Europe. Changing those, he said, "will likely prove to be a stiff sociopolitical challenge given the increasing prominence of the environment as a dominant issue, and the emergence of the pro-Green movement among governments, corporations and the general public."
So, six decades after Chuck Yeager first went supersonic in the X-1, with the pace and volume of world of commerce growing dramatically, the opportunity for extremely long-range, high-speed aircraft has never been stronger. And yet, the old aviation mantra of "Higher, Farther, Faster" seems to have been suspended.
Many believe that suspension is temporary, that technology and commercial demand will ultimately force regulators to allow the industry to realize its full performance potential, delivering speed while respecting the environment in which its aircraft operate. Their faith may been well founded — after all, the advance of technology seems inevitable — but their timetable seems to be printed with invisible ink — non-toxic, of course.