NASA is claiming a breakthrough in the design of supersonic aircraft, with wind-tunnel tests proving it is possible to design configurations that combine low sonic boom with low cruise drag, characteristics once thought to be mutually exclusive.

The tests involved scale models of small, supersonic airliners designed by Boeing and Lockheed Martin and aimed at entry into service about 2025. Although the measured shock wave signatures are at the high end of what would be publicly acceptable, they proved the design tools could produce a supersonic business jet capable of unrestricted overland flight, says Peter Coen, NASA’s Supersonic Fixed-Wing project manager.

NASA’s target for the under-track boom from a 2025-timeframe small airliner is a perceived noise level of 85 decibels (PNLdB). Boeing’s design achieved 81 PNLdB, and Lockheed’s 79 PNLdB. “That’s 25dB less than Concorde and 20dB less than the best we achieved under HSR [NASA’s High Speed Research supersonic-transport program, canceled in 1999],” he says.

“This is a breakthrough. It’s the first time we have taken a design representative of a small supersonic airliner and shown we can change the configuration in a way that is compatible with high efficiency and have a sonic signature than is not a boom,” Coen says.

NASA’s original goal was 65 PNLdB; 70 PNLdB is widely regarded as the threshold for public acceptance of routine overland supersonic flight. Boom is proportional to weight, and a small supersonic business jet is likely to meet that level, he says, while a larger airliner would need further technology development.

“We’ve learned that shaping technology will improve, and we will probably be able to reduce the boom further,” Coen says. With better materials and controls for a slender, flexible, 2035-timeframe aircraft, “we are much more confident we can get to 70 PNLdB.”

In addition to low boom, the designs showed good aerodynamic performance. “We’ve broken the low-boom/low-drag paradox, where you could get one, not both,” he says. “They achieved low boom with a good level of supersonic cruise lift-to-drag.”

Coen says the breakthrough results from development of better computational fluid dynamics tools to model the flow field more accurately; better geometry manipulators to explore complex, three-dimensional flow perturbations; and better optimizers to identify realistic configurations with promise.

Boeing and Lockheed are now working under Phase 2 contracts to refine the off-track shock wave signatures of their designs to reduce sonic boom over the full 60-mi.-wide ground “carpet.”

The companies will also increase the fidelity of the engine inlets, nacelles and nozzles on their models to measure the effect of the propulsion system on sonic boom. Real inlets and nozzles will have shock systems that could change the signature.

“The optimizer makes some pretty fine adjustments to the aft end of the aircraft,” Coen says. “Shock position is pretty important, and even small shocks from the nozzle flow could have an effect.”

Flight tests have been conduced at Edwards AFB, Calif., using an F-18 fighter in an effort to measure the public reaction to shaped sonic booms, but pressure is growing for a low-boom supersonic demonstrator.

The F-18 can fly a maneuver that produces a shaped boom at a certain position on the ground. “Ultimately, we would like to do a flight demonstration of low boom in steady level flight, as a way to look at community acceptance,” Coen says.

Industry regards a demonstrator as necessary to persuade regulators, such as FAA, to lift the ban on supersonic overland flight and enable the market for high-speed civil aircraft.