Two projects are making serious progress towards realising supersonic business travel – and the hunt is on in earnest for industrial partners
One year after unveiling their respective supersonic business jet (SSBJ) designs at the 2004 National Business Aviation Association (NBAA) convention, Aerion and Supersonic Aerospace International (SAI) are alive and well and starting a determined search for long-term industrial risk-sharing partners.
Those prospective partners face a difficult choice between two teams pursuing fundamentally different approaches to achieving affordable supersonic travel. Aerion, a three-year-old Nevada-based company supported by US billionaire Robert Bass, is developing a Mach 1.6-capable aircraft that will avoid sonic booms by dropping to transonic speed over land. SAI’s Quiet Supersonic Transport (QSST), on the other hand, is a technologically more ambitious low-boom aircraft that could cruise supersonically overland.
New partners are expected to be named by both companies over the next year, and a few likely suspects may even begin to emerge at this year’s NBAA. Aerion, which is already teamed with Pratt & Whitney on the powerplant, has a wealth of industry experience in its ranks, ranging from vice-president Brian Barents, former president of Galaxy Aerospace and Learjet, to chief operating officer Michael Henderson, the former Boeing High-Speed Civil Transport programme manager. “It is extremely important to get the right players on board,” says Henderson, adding: “It is ever the case, particularly where there is significant perceived technical risk – even if, as we believe, the actual risks are a lot lower than most people think.”
SAI was established by Michael Paulson in 2001 funded by money bequeathed by his father, the late Gulfstream founder Allen Paulson, and is using the design know-how of Lockheed Martin’s Skunk Works to perfect the QSST configuration. “We’re really working heavily on establishing partnerships,” says Paulson, who adds: “We have signed a letter of intent with one aerospace company already, and we’re looking at signing another soon.”
The success or failure of both projects almost certainly rests on the strengths of the industrial teams formed over the next year or so. The solidity of these formations, in turn, will be based on the technical foundations now being built by the two projects as they progress through the early design-definition phases.
World tour
Aerion completed Phase 1 studies in September and is “literally going around the world” to talk to potential partners in the run-up to the forthcoming launch of Phase 2, which will see formation of the industrial team, freezing of the lines of the SSBJ and the start of preliminary design. “We’re going out to find OEMs [original equipment manufacturers], or even strong contractors, to build it and we’re going out to see how much they believe our story when we shown them our data,” says Henderson.
SAI began its Phase 2 effort in January, expecting it to run for around two years. Phase 3 will cover detailed design and first flight in 2010-11, with certification and entry into service, planned for 2012-13. The 40m (130ft)-long QSST will have a span of 19.8-20.7m, seat between eight and 12 people in an executive configuration, or up to 18 in a high-density arrangement, and be capable of ranges of up to 7,950km (4,300nm) with instrument flight rules reserves using a typical NBAA flight profile, says Paulson. “The data shows we have an abundance of performance, and the power to go right to altitude and supersonic cruise.”
SAI, which has doubled its funding allocation for Phase 2 to $50 million, is also using the second phase to “engage with regulators in the USA and overseas, and we are really working on the aircraft as far as the propulsion system is concerned”, says Paulson. General Electric, P&W and Rolls-Royce have all been asked to submit proposals and “we’re getting good news from the manufacturers. All three have new engine concepts that are fuel efficient and we expect to downselect to one towards the end of Phase 2 – so that will probably be around late 2006 or early 2007 – and possibly even sooner the way the data is going,” Paulson says. The decision will be based on picking the best specific fuel consumption, lowest weight and best efficiency as SAI works on “final tweaks and drag-reduction efforts to get the maximum range”, which Paulson says is slightly greater than originally envisaged. “On the boom side we’re very pleased as we literally have no boom. We are looking at an over-pressure of 0.3–0.5lb/ft2 [1.46-2.44kg/m2], and a very soft-shaped sonic signature,” he says. Boom reduction is achieved through airframe shaping and a “tail-braced wing”, with an inverted-V tail supporting the underslung, aft-positioned engines and stiffening the swept, gulled wing.
Aerion’s dramatically different design, which relies on the concept of supersonic natural laminar flow pioneered by Aerion chief technology officer Richard Tracy, is based around an unswept, tapered wing with a sharp leading edge and a thin, bi-convex aerofoil section. At supersonic speed, the design is expected to achieve laminar flow over 70% of the upper surface of the composite wing, and up to 100% of the lower surface.
Countering pitch-up
A series of windtunnel tests conducted in May at the University of Washington, with the aid of overnight model changes made by Aeronautical Testing Services, helped confirm the benefits of several key design modifications. These were made to counteract “severe pitch-up” problems with the T-tail configuration discovered after earlier windtunnel tests confirmed the results of analysis using computational fluid dynamics.
The revised configuration now incorporates several external and internal design changes. From nose to tail, these include a slightly downward-kinked nose, elevated cockpit floor, and a revised windshield angle to ensure adequate forward visibility. “The cockpit isn’t quite as commodious as before, but there is much better visibility over the nose,” says Tracy. The nosegear leg has been moved aft to a spot below the cockpit floor, reducing the turning radius to less than 23m. The fuselage cross-section has also been refined to improve cabin volume as well as high-speed aerodynamic performance, and now transitions from a vertical ovoid in the forward cabin to a constant circular cross-section. In addition, a flat floor runs the full length of the 9.15m-long cabin section, allowing for up to three seating areas.
Changes to wing geometry have resulted in area reducing from 130m2 (1,400ft2) to 110m2, while the addition of raked tips to keep the vortex flow outboard has increased both tip chord and aspect ratio, which grows from 3.0 to 3.3. The prominent fuselage-mounted wing strakes, identified as the root cause of the post-stall pitch-up problem, have been reduced in size by around half. The strakes, first introduced to improve take-off and approach performance while helping reduce high-speed drag, still help keep balanced field length below 1,830m at maximum take-off weights up to 42,220kg (93,000lb), even with reduced size.
The intersection of the strake and wing leading-edge also generates shock waves that disrupt the all-important laminar flow over the lower surface. To mitigate this, the revised wing leading-edge sweep at the root “goes through zero and is partially negative”, says Henderson. However, as with all such modifications, there is a trade-off and the zero-sweep leading edge becomes susceptible to stall. To counter this, the smaller strake incorporates a Krueger flap, which deploys switchblade-like from the fairing for take-off and landing.
The strake, originally a significantly larger structure, was intended to house fuel and, as a result of its smaller size, Aerion has been forced to use the wing as a fuel tank. “As the aircraft was originally conceived it was to have a dry wing, and fortunately that fuel is right on the centre of gravity,” says Henderson, who adds that the whole wing is now “wet”.
The wing structure is now simpler following the decision to remove spoilers from the design. “It looks like we don’t need those,” says Tracy, who adds that the fuel volume has grown as a result. The trailing-edge flaps, which serve as flaperons for roll control, have also been improved with the addition of partial split flaps to increase lift and reduce deck angle by up to 4º on approach.
Significant effort has also gone into refining the design of the engine inlet, nacelle and exhaust nozzle, which have evolved from two-dimensional, rectangular openings to virtually circular inlet and exhausts. “They are scarfed from inboard aft to the outboard, with localised spillage that provides better aerodynamic and structural properties,” says Tracy.
Revised tail
Although appearing to be wing-mounted, the engines are structurally connected to the aft fuselage, but with accessories housed within the wing. Access panels for engine system maintenance are below the wing.
The revised tail shape is a Dassault Falcon-style lower cruciform with zero sweep and raked tips on the horizontal stabiliser. The changes also allow Aerion to achieve laminar flow on the fin and stabiliser, helping provide the aircraft with the critical ability to cruise at boom-free transonic speed over land without loss of range. Stall characteristics are improved and the tail has increased control power for stall prevention and recovery. To further improve handling qualities, the SSBJ will also be equipped with a fly-by-wire (FBW) flight-control system.
“We’re using a very simple stability augmentation system,” says Henderson, who adds that the feature will enhance stability and handling qualities, particularly in extreme situations such as a rapid descent following an engine failure. This procedure would normally involve the need to rapidly transfer fuel to shift the centre of gravity, “but even if you couldn’t transfer the fuel as fast as you’d want, the stability augmentation system would make the aircraft entirely controllable”.
Henderson adds that “we were also told it is very difficult to certify a mechanical flight-control system that cannot be grandfathered”. Tracy says the FAA requirements for redundancy in a mechanical system are “almost punitive, so once you go to FBW you might as well do what you like with the non-linear and feel systems to get better handling qualities”.
Aerion is expected to brief NBAA attendees on the latest design changes, as well as court potential partners and customers. Earlier this year, the results of a survey conducted for the company by SurveyOne Consulting indicated a potential market for 250-300 Aerion SSBJs over a 10-year period, with around 20% going to the fractionals. The company adds that production over a 20-year life could exceed 500 aircraft.
That production run would be a welcome fillip to P&W, which is working with Aerion on studies of a slightly modified JT8D-219 to power the SBJ. “The cycle of the JT8D is compatible with what they’re looking for in the overall mission,” says P&W advanced programmes vice-president Simeon Austin. The extended time at high power and high speed of the SSBJ mission do, however, require some modifications to cope with “oxidation, creep and things like that. Things that were not design drivers originally.”
Noise reduction
Specific changes being considered for the 1.7:1 bypass-ratio engine include using upgraded materials in the high-pressure compressor (changing disc material from steel to nickel alloy), and a new mixer nozzle for reduced take-off noise. Aviation Fleet Solutions is working with P&W on a Stage 4-compliant noise reduction kit for the Boeing MD-80 and is expected to be in line to work with Aerion and P&W on a similar solution for the SSBJ. However, due to tailpipe differences between the two applications, the final system adopted for the SSBJ will not be a copy of the MD-80 solution. Despite this, Peter Robertson, P&W advanced commercial engines programme manager, says: “Some of the noise reduction features for this product will become standard for the JT8D.”
In the meantime, while Aerion is busy at NBAA hawking its SSBJ and displaying the most recent windtunnel model, SAI will be distinct by its absence. According to Paulson “we have so many major developments going, we think our efforts are better focused on getting these tasks done”.
GUY NORRIS/LOS ANGELES
Source: Flight International
