Guy Norris/Los Angeles and Graham Warwick/Washington

The biggest barriers to development of economically viable and environmentally acceptable supersonic transports—sonic boom and airport noise—may be about to be breached. Through development of design tools allowing aircraft to be shaped to produce quieter booms, and noise-reducing nozzle concepts, NASA and industry are growing confident that routine supersonic overland flight is within reach.

Wind-tunnel tests in 2011 of Boeing and Lockheed Martin concepts for small supersonic airliners showed aircraft can now be designed that combine both low boom and low drag. Traditionally low boom and good performance have been mutually exclusive, but “we have broken that paradox,” says Peter Coen, NASA Supersonic Fixed Wing program manager. “We achieved low boom with good supersonic-cruise lift-to-drag ratio.”

Coen attributes the breakthrough to “better CFD [computational fluid dynamics] capabilities to more accurately model the flowfield, geometry manipulation tools to explore complex three-dimensional perturbations, and optimizers that can quickly identify realistic configurations with promise.”

While progress is being made, the rear of the aircraft—and particularly the propulsion system—remains the most challenging to shape for low boom. This is because of the highly three-dimensional flowfield and complex interaction between the fuselage, wing, empennage and nacelles. Shocks caused by the inlet flow and exhaust stream have to be controlled while meeting engine operability and accessibility requirements.

Gulfstream is coming to grips with the problem as it refines the design of a supersonic business jet and its potential precursor, the still-unfunded X-54 Low Boom Experimental Vehicle. The X-54 would prove that a complete aircraft can be shaped for low sonic boom and be flown to gather the data on public acceptance of shaped booms needed to convince regulators to change the rules barring supersonic flight over land—an essential step for market viability.

Designing a low-boom nacelle is difficult. Engine flow requirements set the inlet and nozzle size; the diameter of the engine and its external accessories determine the nacelle's girth. These set the intake cowl slope and nozzle boat-tail angle, which govern the strength of the forebody compression field and aft-body expansion zone in supersonic flight. These elements can be reduced by stretching the nacelle, but that increases weight and losses.

Working with the Rolls-Royce Tay turbofan for the X-54 demonstrator, Gulfstream hit a problem: The engine's externally mounted accessory gearbox caused a large nacelle bulge that adversely effected sonic boom. Its answer was the high-flow nacelle bypass concept. This encloses the powerplant in an aeroshell with a sharp intake lip and smooth internal flowpath that routes much of the captured air around the engine and gearbox with low losses. Curved channels overcome the problem of the gearbox blocking the flow.