Reduced Sonic Boom the Key to a Successful SSBJ

The most interesting and most shadowy project on the corporate jet scene -- the effort launched two years ago by Lockheed Martin and Gulfstream to develop a supersonic business jet (SBJ) that does not trail a sonic boom behind it-continues to rumble on behind the scenes.

The Defense Advanced Research Projects Agency (DARPA) is getting ready to dispense some government money for the project, with a contribution from NASA. The U.S. Air Force is taking increasing interest in the project's military potential.

The SBJ project started when Lockheed Martin engineers made some still-classified advances in technology to reduce an airplane's sonic boom. Lockheed Martin contacted Gulfstream because a corporate jet would be easier and much less expensive to develop than a commercial jetliner. The project gained added political support from NetJets' owner Warren Buffett, and has also been pushed by political leaders who are concerned about the running down of NASA aeronautics research. (The agency's own supersonic transport program was scrapped in early 1999 after Boeing dropped support for the project.) Funds to start research on an SBJ-sized aircraft were added to the FY2000 defense budget.

DARPA has now launched a program called Quiet Supersonic Platform (QSP) and has issued requests for proposals (RFPs) covering a one-year first phase of work in three broad areas: basic technology (focusing on low-sonic-boom designs), the integration of the complete design, and propulsion.

At the same time, NASA intends to fund some specific engine-to-airframe integration work under its Revolutionary Concepts (Revcon) program. Work should start early next year. After the first year, DARPA will pick the most promising technologies for another year of more intensive 'technology maturation' work. Plans beyond that point are not as precisely defined, but the clear intention is to proceed with a flight-test demonstration.

DARPA has told prospective bidders to look at technology for a 100,000 pound aircraft that can carry a 20,000 pound payload for 6,000 nmi, at a cruising speed of Mach 2.4, and which meets current Stage 3 noise rules. The agency admits that these performance goals are "very aggressive" and says that they have deliberately been set to mandate the use of advanced technology. DARPA is also keen to use the QSP project as a test of new, more efficient and faster ways to develop new concepts in aeronautics.

Another important element of the QSP project philosophy is DARPA's belief that "no single technology will provide the 'silver bullet' required to solve the sonic boom problem." Instead, the project is aimed at the validation and integration of multiple breakthrough technologies.

For example, DARPA specifically suggests that designers should look not only at shaping the airplane to reduce its sonic boom signature-some ideas in this area were revealed in a Lockheed Martin patent filed late in 1999-but also at ways to reduce the boom by generating plasmas (ionized gas fields) around the airplane.

Supersonic laminar flow control-already evaluated under the earlier NASA program-would reduce the boom energy by reducing the airplane's size, as will more efficient engines. Size, cost, weight and noise could be reduced by using advanced materials such as high-temperature aluminum alloys and foamed metallic structures containing microspheres for reduced weight.

Developing an engine which can provide efficient Mach 2.4 thrust and meet noise rules is one of the most difficult challenges in an SBJ design. DARPA is looking at high-bypass supersonic engines and their associated inlet and exhaust systems, avoiding the need for a huge noise suppressor.

Reduced sonic boom, according to Gulfstream, is the key to a successful commercial project. It allows the SBJ to operate at supersonic speed between any two points and at least doubles the market for the airplane. At the same time, it has clear military value. The USAF is looking at QSP as the basis for a future reconnaissance and strike aircraft.

By Bill Sweetman