How the XB-1 aircraft went supersonic without a sonic boom

When the experimental XB-1 aircraft broke the sound barrier three times during its first supersonic flight on 28 January, it did not produce a sonic boom audible from the ground, according to US company Boom Supersonic.

“This confirms what we’ve long believed: supersonic travel can be affordable, sustainable and friendly to those onboard and on the ground,” said Blake Scholl, Founder and CEO of Boom Supersonic, in a press release.

As an aircraft pushes through the atmosphere at high speeds, it changes the air pressure around it, creating sound waves. And when a supersonic flight surpasses the speed of sound – Mach 1, or about 1224 kilometres per hour – these sound waves combine to form a shock wave that spreads away from the flight path. This sonic boom can travel far enough to reach the ground, where it produces an extremely loud noise, rattles buildings and even breaks glass.

Sonic booms over land are so disruptive that they contributed to the retirement of fabled commercial airliner Concorde in 2003 and spurred many countries to prohibit commercial supersonic aircraft. Since then, aerospace engineers have been trying to develop aircraft designs that can go supersonic without the boom.

In this case, the XB-1 took advantage of a physics phenomenon called the Mach cutoff. Because sound moves more slowly at higher altitudes, an aircraft breaching the sound barrier at those heights will produce a boom that cannot reach the ground – if the boom moves downward, the increasing speed of sound will deflect it, pushing its shockwaves upward instead.

The trick is that temperature and wind also affect sound speeds, so the ideal altitude and speed for the supersonic aircraft will depend on atmospheric conditions. “The actual challenge is getting very accurate atmospheric forecasts on temperature and on wind – computing the practical Mach-cutoff flight speed is pretty straightforward from there,” says Bernd Liebhardt at the German Aerospace Center in Germany.

Boom Supersonic says that XB-1’s most recent and final test flight, on 10 February, also reached supersonic speeds without the boom. Now the company is using what it learned from the test flights to help its future commercial airliner, called Overture, achieve the same feat. Supersonic overland flights would be up to 50 per cent faster than today’s commercial airliners. That could make the travel time from New York to Los Angeles 90 minutes shorter.

Flying at supersonic speeds during shorter overland routes could also burn less fuel than flying at the “aerodynamically worst speed” – right below the sound barrier – says Liebhardt. But he cautioned that taking advantage of Mach cutoff is probably more of a niche use case for “supersonic business jet users”. It would deliver less of an economic payoff for a commercial airline service.

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