Success in the wind tunnel is increasing pressure on NASA to find funds for a supersonic flight demonstrator to take low-boom testing out of the confines of Edwards AFB, Calif., and into communities not inured to sonic booms.

While confidence is growing that aircraft can be designed that have both low sonic boom and good cruise efficiency, exactly what level of boom the public will judge acceptable for routine supersonic flight over land is not so clear.

NASA is conducting flight research to validate its ability to reduce sonic booms, and to measure public response, but has to use Boeing F/A-18 supersonic fighters that fly a special maneuver to produce a shaped boom at a specific location. Meanwhile, the X-54 Low Boom Experimental Vehicle remains unfunded.

The F/A-18 was used in 2011 for the Superboom Caustic Analysis and Measurement Program (Scamp) to assess the focusing effect that occurs when a supersonic aircraft accelerates to cruise speed. As it goes from subsonic to supersonic speed, acoustic rays from the aircraft converge on a surface called a caustic, and the lens-like effect can produce a boom 5-10 times louder than in cruise.

Scamp is aimed at improving prediction codes and developing mitigation maneuvers. The 13 flights along the remote Black Mountain supersonic corridor north of Boron, Calif., generated 70 sonic booms, measured by a 2-mi. microphone array on the desert floor, a blimp tethered at 3,500 ft. and a quiet motor glider flying between the F/A-18 and the ground at 4,000-10,000 ft.

NASA also plans flights to validate its ability to predict over-the-top or secondary booms, caused by shockwaves that go up and away from the aircraft and travel long distances before reaching the ground. Compared with the traditional N-wave “double-bang,” such booms are less predictable and have a more random, low-frequency rumbling signature.

F/A-18s were used for NASA's Waveforms and Sonic-boom Perception and Response (WSPR) project, completed in November. “WSPR was a pilot study for future sonic boom community studies,” says Peter Coen, supersonics project manager. “More than 100 volunteers provided subjective live responses to booms over a two-week period.” From Nov. 4-18, F/A-18s conducted 22 flights, generating 82 quiet and five normal booms. Precise times and boom intensities were recorded and participants used a phone app, web page or paper questionnaire to provide information every time they heard a boom while at home.

The volunteers were Edwards residents used to hearing loud booms. “Their reactions to low-noise booms will be a valuable guide for future work in sonic boom perception and response,” says NASA, but WSPR's purpose was to develop data-collection methods and test protocols for future public perception studies in communities that do not usually experience sonic booms. “We have to get away from people who experience booms into more normal communities,” says Coen.

The F/A-18 is being considered, but has limitations. “We can use the F/A-18 in a maneuver that creates a low boom in a certain position on the ground, and can fairly accurately control the loudness at that location,” says Coen. “But ultimately we would like to do a flight demo of low boom in steady level flight as a way to look at community acceptance.”