European SSBJ study holds promise but powerplant remains undefined

 - October 2, 2009, 9:23 AM

High-speed aircraft (Hisac), a European research project to study the feasibility of a supersonic business jet (SSBJ), is coming to a close at the end of this year, having shown better understanding of the achievable performance but without an answer to the big question mark on engines. Dassault has led the program, which counts Italy’s Alenia Aeronautica and Russia’s Sukhoi among its other major stakeholders. A total of 37 organizations, including engine makers Rolls-Royce and Snecma, participated in the five-year effort.

The program studied various concepts for supersonic flight. Some were optimized for overwater supersonic flights, with laminar, variable-geometry or delta wings. Other concepts targeted overland supersonic flights, thus focusing on reducing the sonic boom. Also envisioned are transonic concepts in the Mach 0.95 to 1.2 range.

The sonic boom can be reduced, and designers should focus on lightness, suitable shapes and an optimized speed/ altitude combination, researchers determined. The now-retired Concorde created a boom (measured in air pressure difference) of 100 Pa. By virtue of size alone, an SSBJ would have a 50-Pa boom. With an optimized shape–such as mounting the engines on top of the aft fuselage section–this could be cut to 20 Pa.

Transonic cruise could be a solution to the sonic boom because below a speed of about Mach 1.15 the shockwave cannot reach the ground. However, it might be difficult to make a business case for an aircraft that is difficult to design and reduces transatlantic flight time by only a couple of hours.

Beyond the sonic boom, noise in the airport environment presents a major challenge for a supersonic aircraft. Designers have reduced the environmental impact of conventional jets by using high-bypass (and thus large-diameter) turbofans, but their drag renders them unsuitable for an SSBJ. Hisac research engineers believe a bypass ratio of 3:1 is most suited to an SSBJ.

Variable-cycle engines could be a solution by adapting the powerplant in flight to best match the demands of speed and altitude in all phases of flight. However, such concepts–the holy grail of engine makers–are too expensive to have found their way to civil aircraft. To limit development costs, engine makers could use an existing core or share costs between civil and military applications.

The research group also studied pollutant emissions, particularly those that have a greenhouse effect, such as CO2 and water vapor. Simulations were performed with various aircraft configurations and various engine combustors at different speed/
altitude combinations. Nevertheless, for a given distance, an SSBJ would burn 50 to 100 percent more fuel than a conventional bizjet to complete the same trip.

As with Concorde, the longer nose section of an SSBJ limits the crew’s forward visibility. Rather than using a drooping nose, researchers are studying synthetic vision and periscopes.

Hisac conducted wind-tunnel tests of scale models at Ruag Aerospace’s facilities. However, Bruno Stoufflet, Dassault v-p for scientific strategy, R&D and advanced business, acknowledged Hisac is still behind U.S. research, which has advanced to the point of flight tests by NASA and Gulfstream. In Europe, a demonstration program–if Hisac does give birth to a follow-on project–could cover two aspects: the engines and their nozzles, and the sonic boom.

SSBJ concepts remain viable even in today’s unfavorable economic climate and against the current environmental backdrop, according to Dassault. First, “the market is there,” the company asserts. Second, the U.S., Japan and Russia are also involved in “intense” research activity. Because of the small anticipated market (250 to 500 aircraft) for the $80 to $100 million SSBJ, Dassault is hoping such an airplane would be the result of transatlantic cooperation.￶