Aircraft designers believe they can take the loud boom out of supersonic travel, and there is demand for faster business jets. But can they convince regulators?
Two lines on a chart - one red, one blue - represent a breakthrough in supersonic aircraft design. The lines show the sonic-boom signatures laid down by two Northrop F-5Es as they flew at Mach 1.36 and 32,000ft, 45s apart, over California's Mojave Desert on 27 August 2003. The difference between the two lines vindicates a theory first proposed over 30 years ago, and paves the way for development of quiet supersonic aircraft.
The blue line is the sonic boom of an unmodified F-5E. It is a classic N-wave signature: the sharp rise to an overpressure peak as the shockwave from the nose of the aircraft passes, pressure dropping until it snaps back to ambient milliseconds later when the shockwave from the tail passes - the "double bang" of a classic sonic boom.
The red line is the signature of an F-5E modified to produce a shaped sonic boom. Instead of peaking at around 5.9kg/m2 (1.2lb/ft2), the initial overpressure flattens out at around 3.9kg/m2. The "flat-top" signature of the 27 August flight was historic: not only was it proof that an aircraft can produce a shaped sonic boom that persists through the atmosphere to the ground, but the results exactly matched predictions - designers now have the computational tools to shape an aircraft for low sonic boom.
SSBJ impetus
Flight tests of the F-5 Shaped Sonic Boom Demonstrator (SSBD) - an additional 21 flights were conducted in January this year - has added impetus to efforts under way at several aircraft manufacturers to design a supersonic business jet (SSBJ). But the tests, although ground-breaking, were not enough. The manufacturers believe a full-scale X-plane demonstrator is needed to convince regulators that sonic boom can be reduced enough to allow supersonic flight over land - the holy grail of SSBJ designers.
The theory that sonic boom could be reduced through aircraft shaping has existed since the 1960s, when it was first proposed by Albert George and Richard Seebass of Cornell University. But the opportunity to test the theory was provided by the US Defense Advanced Research Projects Agency's (DARPA) Quiet Supersonic Platform programme, which funded Northrop Grumman to convert an F-5E to the SSBD.
The modification dramatically altered the F-5's normally sleek appearance, adding an ungainly, pelican-like forebody fairing designed to generate a shockwave pattern that would produce a flat-top sonic boom. The baseline F-5 has a bow shock and strong inlet and wing shocks. These coalesce as they move away from the aircraft to produce the overpressure peak of the N-wave sonic boom. The longer, blunter nose and forebody shaping of the F-5SSBD strengthens the bow shock but weakens and stretches out the inlet and wing shocks, preventing them coalescing and reducing the overpressure peak.
The key unknown of the SSBD flight tests was whether the shaped sonic boom would persist all the way to the ground, or whether atmospheric conditions would cause the shockwaves to coalesce into a classic N-wave. The DARPA-funded Shaped Sonic Boom Demonstration - and the subsequent 21-flight, NASA-funded Shaped Sonic Boom Experiment (SSBE) - appear to have proved that shaped sonic booms will persist to the ground through various atmospheric conditions and aircraft manoeuvres.
The SSBD and SSBE programmes proved that sonic boom intensity can be reduced through aircraft shaping, and that the ground boom signature for a supersonic aircraft can be predicted. This gives manufacturers the tools, and confidence, to design aircraft with predictable boom signatures. The problem is, what signature will make sonic booms acceptable to the public and supersonic overland flight acceptable to the regulators?
Today, supersonic overland flight is banned. "Our real focus now is how to get regulatory action to change the prohibition to a rational rule," says Pres Henne, senior vice-president, programmes, at Gulfstream. Efforts are under way with the US Federal Aviation Administration to get sonic boom on to the agenda of the Committee for Aviation Environmental Protection at the International Civil Aviation Organisation.
Building belief
NASA, meanwhile, is proposing a five-year supersonic research programme that, if funded, would help pave the way for rational regulation. "All these things are building blocks. The real keystone would be a full-scale demonstrator vehicle," says Henne. "The regulators understand everything that is going on, but they need that final confirming tool - a vehicle that flies over people. They want to listen to it."
Meanwhile, Gulfstream is continuing design work. "Our focus is on sonic boom reduction and suppression," says Henne. "We are doing the research required to convince ourselves that we can develop an aircraft that is very quiet, and to convince the regulators that it is time to come up with a rational rule."
Gulfstream has developed a sonic boom simulation facility where it can create and listen to shaped signatures. "Our ability to create shaped signatures is getting pretty good," says Henne. "Instead of an abrupt pressure rise we get a series of small rises, stretched out." The goal is to reshape the initial and final pressure rises to turn the double-bang N-wave into an inaudible sine wave. "If we could set a perfect sine wave at 5-6Hz the human ear would not respond at all. It is not perfect, there is higher frequency content you can still hear, but we are still refining and getting closer."
Gulfstream has gone through several iterations of its conceptual Quiet Supersonic Jet (QSJ) design. In windtunnel tests at NASA Langley the company has reduced initial overpressure to around 2.4kg/m2. The QSJ Advanced+ configuration has a boom acoustic signature level more than 35dB lower than Concorde's. "That's below conversion-level noise," says Henne. The boom is minimised not only by shaping the aircraft asymmetrically to direct the shockwaves away from the ground, but by using a telescoping nose boom that stretches out the bow shocks.
Variable geometry
Gulfstream's baseline QSJ has a variable geometry wing to improve airport performance, reduce noise and increase subsonic range, at the expense of increased complexity and weight. The conceptual aircraft has a 45,500kg (100,000lb) gross weight, can take off in 1,830m (6,000ft) and carry eight passengers 8,900km (4,800nm) at M1.8. In comparison, Gulfstream's subsonic G550 weighs in at 41,300kg and with the same 1,830m take-off run can carry eight passengers 12,500km at M0.8.
The QSJ is a big aircraft - 42.7m long, with a span of 30.5m wings forward and 18.3m wings aft. The G550 is 29.4m long with a 28.5m span. The cabin is Gulfstream II size, and therefore substantially shorter than the G550's. The QSJ needs 38,000lb (170kN) of take-off thrust - compared with the G550's 30,800lb - 18,000lb of transonic acceleration thrust and 12,500lb of cruise thrust at M1.8. Gulfstream is aiming for airport noise performance 10dB below Stage 4, and an engine overhaul interval of better than 2,000h, compared with 7,000h for the G550's Rolls-Royce BR710s.
High sticker price
The QSJ price goal is $70-100 million, or roughly twice the cost of a G550. Despite this high sticker price, two Gulfstream market assessments have identified a "conservative" market for 180-350 aircraft. Independent market surveys have estimated the market at 300-400 aircraft. A big driver of SSBJ market optimism, given the likely pricetag, is the perceived demand for fractional ownership. Fractional operator NetJets makes no secret of its enthusiasm for an SSBJ, citing the popularity of its M0.92 Cessna Citation Xs.
NetJets feels a non-stop trans-Pacific range of at least 8,800km is a must for a supersonic business jet. The company believes a Citation X-size cabin, although smaller than the GII size assumed by Gulfstream, would be acceptable for flights of less than 5h and would make a trans-Pacific aircraft more feasible. NetJets says several long-time share owners have indicated "extreme interest" in paying $10 million for an eighth share in a M1.8 SSBJ with 9.250km range and Citation X-size cabin.
At the 200h/year an eighth share would buy, cost/hour would be under $25,000 for factional versus $55,000 for full ownership, NetJets estimates. This would make owning an eighth share in an SSBJ about as expensive as a quarter share in a Gulfstream G200 super mid-size business jet, NetJets points out. This helps explain the attraction of fractional ownership to SSBJ proponents, but again supersonic cruise over land is required for the market to materialise.
In addition to Gulfstream, Boeing, Cessna, Dassault, Raytheon and Sukhoi have expressed interest in developing a supersonic business jet - although most talk in terms of co-operating with other manufacturers as development costs are likely to exceed $1 billion.
Lockheed Martin's Skunk Works says it has already "closed the design" on a low-boom supersonic business jet, but the company shows little intention of proceeding with commercial development. Asked about its plans, Lockheed Martin says "the government is our customer" and points to the lack of a current military requirement for a low-boom supersonic aircraft. Despite this, the company has competed considerable design work, including windtunnel testing, on an SSBJ.
Lockheed Martin's SSBJ is a joined-wing aircraft, with a bracing tail allowing use of a long-chord, highly swept wing with underslung engines. The bracing tail, which connects the tip of the fin to the wing at mid-span and supports the engine nacelles, allows a configuration that distributes lift and area so as to minimise sonic boom, but which would otherwise be too flexible and prone to flutter. The design also features supersonic natural laminar flow, to reduce drag, and wing shaping to recover nacelle pressure drag.
The Skunk Works' SSBJ is 39.6m long, with a maximum take-off weight of around 65,000kg. Construction is conventional, and the engine inlets simple, for low cost. The boom signature is designed to resemble a sine-wave. Lockheed Martin believes it can reduce boom pressure at audible frequencies by 30 to 100 times, and that the perceived boom loudness will be well within acceptable limits at one SBJ flight a day and still within the region of acceptability at 10 flights a day.
Pros and cons
A 2003 study by Raytheon Aircraft for NASA Langley highlighted the advantages and disadvantages of low-boom design. Two aircraft were designed for the same mission, to carry six passengers 9,250km at M1.8 from a 1,830m take-off. One was constrained to have a boom initial overpressure of just 1.95kg/m2. Whereas the high-boom design came in at 46,000kg take-off weight, and was 40.7m long with a 19.2m span, the low-boom design weighed in at 54,000kg, with a 50.4m length and 21m wingspan.
As weight usually equals cost in aircraft, the high-boom design looks to have an advantage until the restriction on supersonic overland flight is factored in. Forced to fly subsonically over part of its route, the aircraft's range and average speed decrease while its mission duration and operating costs increase, limiting its usefulness and challenging the business case.
Raytheon concluded that a supersonic business jet is technically achievable given reasonable progress on key technologies over the next decade, and that the size and weight consequences of a low-boom design are small relative to the utility gained by being able to fly supersonically over land. But, the study warned, the definition of regulatory requirements allowing supersonic overland flight is "imperative".
GRAHAM WARWICK / WASHINGTON DC
Source: Flight International
