NBAA 2004 - Las Vegas witnesses birth of supersonic gamble

Two distinct business jet designs emerge at NBAA - now pioneers seek major backing if concepts are to pay off

Two distinctly different designs for supersonic business jets were unveiled at NBAA - both needing the backing of at least one major manufacturer before they can become reality.

Airframers from Boeing to Raytheon are being courted by Aerion, backed by billionaire US investor Robert Bass and led by former Learjet boss Brian Barents: and Supersonic Aerospace International (SAI), led by Michael Paulson, son of late Gulfstream founder Allen Paulson, who has paid Lockheed Martin's Skunk Works $25 million to design a supersonic business jet (SSBJ).

Both companies are trying to create industrial consortia to develop aircraft meeting similar requirements - eight to 12 passengers in a stand-up cabin, 7,400km (4,000nm) range and $80 million price tag - but via fundamentally different approaches.

Aerion's design combines existing technology with unique supersonic natural laminar-flow aerodynamics to produce a 40,800kg (90,000lb) gross-weight aircraft powered by existing engines and capable of cruising over water at Mach 1.6, then dropping to transonic speed over land to avoid sonic boom without sacrificing range. Development cost is estimated at $1.2-1.4 billion, and the aircraft could enter service as early as 2011, says Barents.

SAI's Quiet Small Supersonic Transport (QSST) is a low-boom aircraft designed to allow supersonic flight over land - which most manufacturers consider essential for a viable SSBJ. But airframe shaping to minimise sonic signature while cruising at M1.6-1.8 results in a 69,400kg gross-weight aircraft requiring new-technology engines. Development cost is estimated at $2.5-3 billion, says Paulson, with a first flight in 2010-11 leading to entry into service around 2012.

"Designs with acceptable boom over land may ultimately be successful, but it will add several years and substantial cost to development," says Barents. "Aerion's design reduces risk and speeds development and is a practical approach for day-to-day overland use." Paulson says SAI "will not accept the defeat" of being forced to fly subsonically over land.

Cautious reaction

Airframers reacted cautiously at NBAA, although both Aerion and SAI say they are in discussions with several manufacturers. Gulfstream, which is working on its own Quiet Supersonic Jet, is still looking for a solution, says Pres Henne, senior vice-president, programmes, engineering and test, adding: "I do not think it is there yet."

Pointing out that Bass "is a good customer of ours", Dassault Falcon president John Rosanvallon says the company is receiving information from Aerion and evaluating the aircraft in parallel with its own three-engined supersonic Falcon concept. "But we are not partners," he says.

Cessna "is not in contact at all" with Aerion, says chief executive Jack Pelton. SAI's design "looks very viable", he says, while expressing concern about the QSST's size, and cost. Cessna's own SSBJ design, with a smaller Citation X-size cabin, weighs in at 34,000kg "and it's still a big aircraft". Aerion's cabin is super mid-size class, but the design is still fluid, says Mike Henderson, chief operating officer. The final size will emerge from market research now under way. "We can trade cabin size for range," he says. "If we drop back to the next size cabin, we get a range benefit."

Patented by Aerion chief technology officer Richard Tracy, supersonic natural laminar flow reduces drag and allows use of existing Pratt & Whitney JT8D-219 engines. The unswept wing is also more efficient at subsonic speeds, allowing the aircraft to achieve its 7,400km design range even when flying at M0.95-1.1, says Barents.

Range of the delta-canard QSST drops to 7,000km at M0.92-0.98, says Paulson, because it is designed to fly supersonically over land. The shaped sonic boom generates an "imperceptible" overpressure of 0.3-0.5lb/ft2, says Tom Hartmann, Lockheed Martin programme manager. Aerion's design generates an overpressure of 1.2lb/ft2 at M1.6, almost half that of Concorde but still too high for overland flight.

Henderson believes the aircraft will generate no boom at M1.1, with the right atmospheric conditions, but this will still require a waiver from the US Federal Aviation Administration's ban on supersonic flight over land. The company's plan is to cruise at speeds up to M0.99 over the USA, then seek permission to use existing supersonic corridors over sparsely populated areas such as northern Canada, Siberia and the Australian outback. "This avoids the need to change the rules on boom," says Tracy. A cockpit display of boom footprint would help the pilot stay inside the corridor.

SAI is already talking to the FAA about what will be needed to lift the prohibition on overland supersonic flight. Hartmann says there is data from NASA's cancelled High Speed Civil Transport programme that suggest the public would find a boom strength of 0.3ft2 acceptable. The company's plan is to use its three production-standard certification flight-test aircraft to validate its shaped sonic signature is acceptable. Paulson says SAI is considering building a 60%-scale proof-of-concept vehicle during the Phase 2 risk-mitigation effort to begin in January.

Hartmann says a demonstrator, which many manufacturers consider essential to validate low-boom technology and convince regulators to change the legislation, would add "tens of millions of dollars" to Phase 2, which is planned to cost $50 million and last 18-24 months. Barents says the need to persuade legislators to change the rules adds risk to the low-boom approach.

Aerion projects a market for 250-300 aircraft over 10 years - 40-50% of those for fractional ownership - not including the government market. SAI sees demand for 300-400 including government and special missions, reducing to 250-300 if supersonic overland flight is not allowed. "We would still proceed if we could not get the ban lifted, and work it out over time," says Paulson.

At his father's bequest, Paulson hired Lockheed Martin in 2000 to design an SSBJ. He has paid $25 million so far for work including 19 high-speed and low-speed windtunnel tests for performance and boom measurement. SAI plans to raise some of the Phase 2 funding from the risk-sharing partners Paulson hopes to sign up over the next 18 months. Lockheed Martin has signed a memorandum of understanding to continue its design involvement through to certification, but will not become a risk-sharing partner, says Hartmann.

Windtunnel models

Aerion completed Cycle 1 of its design in July. This converted the concept from a parametric study to an actual design, says Henderson. Cycle 2 is refining the design enough to allow windtunnel models to be built for testing in the second half of next year. Structural test samples will be built to validate that the thin carbonfibre multi-spar wing will resist flutter. Armed with data from these tests, Aerion plans to begin signing up risk-sharing partners. "At the end of 18 months we will be ready to go into detailed design," he says.

Aerion's design combines a conventional aluminium fuselage with a composite wing. The prominent strakes serve several purposes, housing the tandem-wheel main gear, systems and fuel while smoothing out area distribution. Strake-tank fuel is used for trim and fuel is also housed in the wing and centre and aft fuselage. When the aircraft goes supersonic its centre of lift moves aft, which must be balanced by moving the centre of gravity (CG) aft. "On a normal mission, use of fuel naturally moves the CG back," says Henderson. "If we have to go subsonic in an emergency, we rapidly pump fuel into the strake tanks to bring the CG forward."

The unswept wing is extremely thin, and is carbonfibre for stiffness - except the sharp and smooth leading-edge, which is solid titanium for heat resistance. Aerion is looking at a combination of electric and bleed-air de-icing. In supersonic cruise, a shockwave attaches to the leading edge, behind which a favourable pressure gradient keeps airflow over the wing laminar. Aerion expects to achieve laminar flow over all of the underside of the wing and over the upper surface to the spoiler hinge line at 70% chord.

There is supersonic laminar flow over the top of the horizontal stabiliser, which is hard-mounted to a fin that is hinged at its root. Subsonically, maintaining natural laminar flow requires careful management of airflow over the wing using the full-span flaps. Fly-by-wire is not required, but could buy its way on to the aircraft, says Henderson. The engines have conventional hydromechanical controls.

SAI's QSST is fly-by-wire because it is unstable in pitch in certain conditions, says Hartmann. The airframe is all-aluminium, with a so-called tail-braced wing - the inverted-V tail providing support for the aft-set engines and adding stiffness to the highly swept wing. The wing has both leading-edge and trailing-edge high-lift devices, but take-off distance at 2,400m (8,000ft) is substantially longer than Aerion's 1,800m.

Source: Flight International