Is the future of air travel hypersonic?

A Chinese company has successfully tested a prototype plane which could one day travel at hypersonic speeds, whisking passengers from London to Sydney in just four hours.

South China Morning Post reported that Space Transportation, headquartered in Beijing, has tested a commercial transport jet capable of flying at Mach 4 – or twice the speed of Concorde – with plans for a maiden voyage in 2027.

This would bring it within range of the hallowed Mach 5 speed, which is "the new Mach 1", said The Economist. "Just as aviators of old sought to break the sound barrier and travel supersonically, the search is now on for reliable and controllable ways to travel 'hypersonically', generally defined as more than five times the speed of sound."

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A massive technical challenge

While intercontinental ballistic missiles and spacecraft re-entering the Earth's atmosphere have been able to reach hypersonic "Mach 5" speeds for decades, the problem is "they are not very steerable", said The Economist. "Steering hypersonically using conventional control surfaces, like wings and ailerons, is out of the question. The forces and heat involved would destroy them."

Engineers are now exploring alternative means of propulsion. This includes magnetohydrodynamics – which uses plasmas controlled by electromagnets, instead of gases, to keep temperatures down.

Another "exotic design" reported by The Register is the SABRE (Synergetic Air Breathing Rocket Engine) which combines the fuel efficiency of a jet engine with the power of a rocket. Using innovative precooling technology and liquid oxygen it would allow an aircraft to take off from a runway as normal, then travel at velocities of about five times the speed of sound in the atmosphere.

The prospect of a flying time of under two hours from London to New York is "mind-blowing", agreed The Independent, but any talk of hypersonic travel in the near future comes with an "extremely important caveat".

It's not just that building a hypersonic passenger aircraft is a "massive technical challenge that has never even been attempted before – there are dozens of reasons that make it very nearly impossible".

Environmental and economic concerns

When Concorde ushered in the age of supersonic travel almost 60 years ago, "passengers cared about speed and governments wanted to wave the flag of national tech prowess", reported Politico.

Today there are "greener concerns", with Nasa calculating that supersonic jets burn around three to seven times more fuel per passenger mile than subsonic jets.

The economics also look "challenging" as hypersonic travel would require smaller fuselages which limit the passenger capacity of each flight. As was the case with Concorde, it may only ever be affordable for the super-rich.

Supersonic passenger aircraft also have a famously chequered safety record. A Soviet-made Tu-144 crashed just months after it was launched in 1973 while the Concorde crash in Paris in 2000 effectively signalled the end of commercial supersonic travel. Another issue that plagued Concorde was restrictions on overland supersonic flights, which were put in place to avoid noise pollution.

While the likes of Space Transportation and US-based Boom Supersonic, which has plans to launch its own supersonic Overture airline in the next five years, remain bullish, the British developers of SABRE offer a cautionary tale.

Just this week, Reaction Engines went into administration after failing to secure new funding, "ending ambitious hopes of revolutionising air travel by making hypersonic flight a reality", said the Financial Times.