NASA's Quiet Progress Towards a Low-Boom X-Plane

NASA continues to make steady progress towards the possible launch of a low-boom supersonic demonstrator program* later this decade. This would take boom-shaping techniques proved in the windtunnel and test them in the real world to gauge the public acceptance of shaped booms.

Photos: NASA Glenn

The goal is to reduce boom annoyance to a level where regulators can be persuaded to lift the ban on supersonic flight over land. Previous windtunnel work has shown that careful shaping of the aircraft can get the sonic boom for a small supersonic airliner down to what NASA believes is the threshold of acceptability.

Under Phase 2 of the so-called N+2 system-level experimental validation program, Boeing and Lockheed Martin’s Skunk Works are continuing work on small airliner concepts that are about the capacity and range of Concorde, but with dramatically quieter sonic booms – around 80 PLdB versus 105 PLdB.

And the shockwave signature observed on the ground would be more of a sine wave, resulting in a muted whoosh rather than the traditional double bang generated by the classic N-wave boom signature, with its sharp pressure rises caused by the powerful bow and tail shocks.

Producing these shaped signatures requires careful optimization of volume and lift distribution and results in an aircraft with extended nose and lifting tail - features that help prevent individual shockwaves coalescing into an N-wave as they propagate through the atmosphere to the ground.

Phase 2 of the experimental validation program is looking more closely at the impact of the propulsion system – inlet, nacelle, nozzle and plume – on sonic boom. Hence the top picture, which shows Boeing's 1.79%-scale model in the NASA Glenn tunnel, testing how changes in airflow at supersonic speed affect the performance of the top-mounted inlets. The earlier image below shows a model with simple flow-through nacelles.

After integration of the propulsion system has been optimized, the aircraft configuration will be updated and windtunnel tests that were conducted in Phase 1 to measure sonic boom and cruise efficiency will be repeated - another step in validating the tools needed to design a demonstrator.

* Thanks to Steve Trimble of Flightglobal for the update on NASA's demonstrator plans.


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