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 , founder and CEO of Boom Supersonic, in a .

As an aircraft pushes through the atmosphere at a high speed, it changes the air pressure around it, creating sound waves. And when a supersonic flight surpasses the speed of sound – Mach 1 – 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 can produce an extremely loud noise, rattle buildings and even break glass.

Sonic booms over land are so disruptive that they contributed to the retirement of the 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 accompanying 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 shock waves upward instead.

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The trick is that temperature and wind also affect sound speeds, so the ideal altitude and speed for a 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 at the German Aerospace Center.

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

But performing the Mach-cutoff flight “burns more fuel on the same distance than both subsonic and supersonic flight”, says Liebhardt. That makes it less economically viable than a regular supersonic flight and “the worst speed to fly at for fuel economy”. He sees Mach-cutoff flights as being more of a niche use case for “supersonic business jet users”, rather than for commercial airlines.

A pigeon-inspired robot has solved the mystery of how birds fly without the vertical tail fins that human-designed aircraft rely on. Its makers say the prototype could eventually lead to passenger aircraft with less drag, reducing fuel consumption.

Tail fins, also known as vertical stabilisers, allow aircraft to turn from side to side and help avoid changing direction unintentionally. Some military planes, such as the Northrop B-2 Spirit, are designed without a tail fin because it makes them less visible to radar. Instead, they use flaps that create extra drag on just one side when needed, but this is an inefficient solution.

Birds have no vertical fin and also don’t seem to deliberately create asymmetric drag. at the University of Groningen in the Netherlands and colleagues designed PigeonBot II (pictured below) to investigate how birds stay in control without such a stabiliser.

The team’s previous model, built in 2020, flew by flapping its wings and changing their shape like a bird, but it still had a traditional aircraft tail. The latest design, which includes 52 real pigeon feathers, has been updated to include a bird-like tail – and test flights have been successful.

Lentink says the secret to PigeonBot II’s success is in the reflexive tail movements programmed into it, designed to mimic those known to exist in birds. If you hold a pigeon and tilt it from side to side or back and forward, its tail automatically reacts and moves in complex ways, as if to stabilise the animal in flight. This has long been thought to be the key to birds’ stability, but now it has been proven by the robotic replica.

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The researchers programmed a computer to control the nine servomotors in Pigeonbot II to steer the craft using propellers on each wing, but also to automatically twist and fan the tail in response, to create the stability that would normally come from a vertical fin. Lentink says these reflexive movements are so complex that no human could directly fly Pigeonbot II. Instead, the operator issues high level commands to an autopilot, telling it to turn left or right, and a computer on board determines the appropriate control signals.

After many unsuccessful tests during which the control systems were refined, it was finally able to take off, cruise and land safely.

“Now we know the recipe of how to fly without a vertical tail. Vertical tails, even for a passenger aircraft, are just a nuisance. It costs weight, which means fuel consumption, but also drag – it’s just unnecessary drag,” says Lentink. “If you just copy our solution [for a large scale aircraft] it will work, for sure. [But] if you want to translate this into something that’s a little bit easier to manufacture, then there needs to be an additional layer of research.”