The baseline design for Gulfstream's Quiet Supersonic Jet (QSJ) is a slender aircraft with highly swept wing, T-tail and two engines mounted on the fuselage above the wing trailing edge. The company has also studied underwing engines, but the fuselage- mounted installation promises to reduce sonic boom, although it does pose aerodynamic challenges. "We are focused on somewhere between M1.6 and M2, probably 1.8; a range of more than 4,000nm [7,400km]; and eight to 14 passengers," says Henne. Take-off field length needs to be below 2,000m (6,500ft) to access the 3,500-plus smaller US airports currently used by Gulfstreams.
The cabin is similar in size to that of the Gulfstream II. "We don't need a GV-size cabin because the passengers are not going to spend the same amount of time en route," says Henne. In an executive layout, the QSJ would seat eight passengers with the lavatory aft and the galley and baggage forward. In a high-density layout, the aircraft would accommodate 14 passengers in two single rows of seats. Cabin height, aisle width, seat pitch and seat width are all greater than in Concorde.
Gulfstream believes the QSJ's speed capability "redefines a 12h work day". A M1.8, 8,500km-range QSJ could fly from New York to Moscow within 5h. "You could fly from New York to Moscow for a two hour meeting and be back in New York within the same 12h period. For a business person who has to travel and who wants to preserve some sort of family lifestyle, that has to be a tremendous draw," says Henne. On a US transcontinental mission, the QSJ could save 4-6h over a subsonic aircraft, Gulfstream calculates.
Dassault's initial specification for a supersonic Falcon looks similar to Gulfstream's: a range of 9,400km at M1.8 and a Falcon 50-size cabin. The baseline configuration is a highly swept cranked delta with foreplanes and the manufacturer is studying both twin-engine and trijet versions. The companies are also in accord in identifying the issues that must be overcome before an SSBJ can become reality. These include sonic boom, engine life and emissions.
Principal among the challenges is reducing the sonic boom. "The aircraft must be capable of overland supersonic flight, and that is the biggest technical hitch," says Henne. "The boom overpressure must be reduced to whatever it takes for acceptable overland supersonic flight." Defining what constitutes an acceptable sonic boom is just one of the problems designers face, but there is optimism that it is possible with an SSBJ.
Several techniques will have to be applied to minimise the sonic boom, which is caused by the shockwaves the aircraft generates as it flies supersonically. Shockwaves originate at different points on the aircraft, such as the nose, inlets, wing and tail. As they travel through the air, the waves coalesce, forming a front shock and a rear shock. The sound heard on the ground as a sonic boom is the sudden onset and release of pressure as these shocks pass. Although the pressure changes involved are not great, they occur within milliseconds, resulting in the characteristic "double bang" of a sonic boom.
According to the US Air Force, the strongest sonic boom ever recorded resulted in a peak overpressure of 706kg/m2 (144lb/ft2), produced by a McDonnell Douglas F-4 flying at just over M1 at an altitude of 100ft. More typical overpressures produced by supersonic fighters in normal operations are under 50kg/m2, the USAF says. Concorde generates a boom overpressure of 10kg/m2. The goal of the QSP programme is an initial shock strength of less than 1.5kg/m2.
A sonic boom has an N-shaped signature, pressure rising rapidly at the front shock to an overpressure peak, then decreasing to an underpressure, before returning rapidly to ambient pressure at the rear shock. Efforts to minimise the sonic boom mainly involve modifying the shape of the N-wave by reducing the pressure rise through the front and rear shocks, reducing the maximum and minimum pressures and increasing the duration of the pressure wave.
Reducing aircraft weight reduces the overpressure while increasing aircraft length increases the N-wave's duration. This favours a smaller aircraft like a business jet and explains why slender designs are preferred. Careful airframe shaping promises to reduce the pressure increase - key to making any boom acceptable. Sonic boom experts have calculated that a 30.5m-long, 27.2t SSBJ cruising at M1.6 and 40,000ft would have front and rear shock pressure rises of under 1.25kg/m2.
While this appears acceptable, manufacturers caution that what constitutes an acceptability is as much political as technical, since US laws will have to change to allow supersonic overland flight. "It is important not to lead the public to expect too much," says Olivier Villa, Dassault vice-president of Falcon programmes. "If you are going to do it, you'll do it with a small aircraft. The question is how small can we make it so that it has acceptable boom, yet be large enough to meet market need?" says Henne.
Boom minimisation techniques have yet to be flight-tested. Next year, under QSP, Northrop Grumman will fly an F-5E with a modified, lengthened forward fuselage designed to produce a differently shaped, quieter sonic boom. The aircraft will be flown against an unmodified F-5E to measure the boom reduction.
Next on the list of challenges is finding a suitable SSBJ powerplant. Engine life is a major issue for an aircraft that will spend most of its time cruising supersonically. While Gulfstream's goal is a 2,000h time between overhauls, this is likely to result in an expensive engine. Dassault is looking at the possibility that a cheaper, shorter-lived engine might be a better solution. The company's concept is that hot-section modules could be changed "on-wing" every 500h. This would be once a year for corporate operators, but could be several times a year for fractional operators. "We would have to get our logistics system right," says Villa.
Equally challenging will be meeting emissions requirements. Gulfstream's airport noise goal is Stage 4 -10dB. This will probably require development of a variable-cycle engine with ejector nozzle to increase bypass ratio and reduce noise on take-off. Dassault says a low-emissions combustor is needed to meet stringent requirements for operations at 50,000-60,000ft, particularly for NOx, but also for reduced water emissions to prevent contrail formation above the tropopause. "We think cruise emissions will be our second biggest hurdle," says Henne.
Inlet design also poses difficulties as, in a supersonic aircraft, shockwaves slow and compress the air before it enters the engine. A "mixed compression" inlet emerged from US research into supersonic transports as the favoured design because of its performance. In a mixed compression inlet, the terminal shock is located inside the inlet. But if the airflow is disturbed, the inlet can "unstart" - the shock popping out to stabilise in front of the inlet, causing thrust to drop, and drag to rise, resulting in large transient forces on the aircraft.
Gulfstream's focus is on simpler external compression inlets. "We would like to keep it simple. If we could have a pitot-style inlet we'd probably give up performance to do it," says Henne. "If you have a mixed compression inlet you'd get unstarts. With this kind of market, if you had an unstart you'd have an aircraft for sale."
The series of hurdles facing the SSBJ is long. Henne has a chart showing the "barriers to QSJ". Of the eight items, only one is green for "go", and that is market demand. Five are red, and they include boom mitigation, engine noise and emissions, and certification. He says: "There's an awful lot of red on this chart. The question is are we smart enough to build this aircraft?"
Source: Flight International