Anyone who’s ever winced and gripped their knees while squeezed into an economy class seat on a long-haul flight has always wished two things: that they were instead sitting in business class, or that the flight didn’t have to take so long.
The first of those is solvable with an extra couple of pounds; the second may have to wait until we’ve sorted out a little matter of physics.
If he has his way, Dr Ben Thornber will have you flipping from London to Manchester in a matter of minutes. Dr Thornber is a lecturer in the Department of Fluid Mechanics and Computational Science at Cranfield University, and he has a passion for hypersonic flight.
Speaking at the Dana Centre on 7th February as part of a presentation on the future of air travel, entitled Future Skies, Dr Thornber explained the benefits, and challenges, of travelling at over five times the speed of sound.
Airbus A-380
That, he explained, is the designated qualification for ‘hypersonic’ flight, a largely experimental area of aeronautics which makes supersonic flight look a little like a walk in the park. It may sound science fiction, but it’s very real, if a little unmanageable.
Picking up a little speed
To put things in perspective: at a brisk walk along the side of a road, you could clock up a speed of 5 mph (8 km/h). A car driving past at the maximum speed limit would register 70 mph (112 km/h) on a speed camera. An Airbus 380 (pictured right) passing overhead at cruise speed would be travelling at a little under 600 mph (965 km/h). A plane travelling at Mach 5 would overtake the Airbus in a way that would look similar to the car overtaking you on the road. It would be covering a mile every second.
North American Aviation X-15
But hypersonic flight is not a ‘future’ thing. It’s been done numerous times in the past. The X15 (pictured right) was the first to go hypersonic back in the 1960s, eventually reaching speeds of Mach 6.7. It still holds the record for the fastest speed in a manned rocket-powered aircraft.
However, for Dr Thornber, the fact that the X15 was rocket-powered means it was ‘cheating’. True hypersonic flight for the purposes of commercial air travel would require the craft to be air-breathing and therefore capable of sustainable flight.
He suggests that for the future of hypersonic commercial air travel, we’d need to look closer at planes like the SR71 Blackbird (pictured below right), an advanced medium-range reconnaissance aircraft, also operational in the 1960s, but unlike the X15, an ‘air breather’. It remained the world’s fastest and highest-flying operational manned aircraft throughout its tenure.
However, even though it broke a number of speed records in its time, it only managed speeds of around Mach 3.2, and so doesn’t qualify for acceptance to the exclusive ‘hypersonic club’.
X43 Nasa Scramjet
Too darn hot
Some craft that are flying today do make the cut, but they’re all small, experimental, unmanned, and have to be launched from larger aircraft. The X43 (pictured right) is an example. It manages a blistering Mach 9.8, and offers a glimpse of the capabilities of true hypersonic flight. But, like other hypersonic craft, it faces two major challenges.
The first is temperature. Air is compressed against the body of an aircraft as it moves – and as anyone who has held their thumb over the end of a bicycle pump and depressed the handle will tell you, when air is compressed its temperature rises.
For instance, the air compressed against the body of an Airbus A380 in flight can reach temperatures of around 50 degrees Celsius. At a speed of Mach 2.2, the air striking the surface of a Tornado fighter jet can climb to 300 degrees Celsius, and the SR71 Blackbird needed to be strong enough to withstand temperatures of around 700 degrees Celsius, hot enough to melt aluminium. And that’s why the Blackbird was built from titanium, which is light, has the highest strength-to-weight ratio of any metal in the world, and has a melting point temperature of around 1 650 degrees Celsius.
Lockheed SR-71 Blackbird
But even titanium – our most useful metal for operating under extreme conditions – would prove ineffective for hypersonic flight. At Mach 5 temperatures could reach 2,000 degrees Celsius, hot enough to melt titanium; and at Mach 10, the compressed air slamming against the body of an aircraft could reach 6,300 degrees Celsius – hotter than the surface of the sun.
I know what you’re thinking. “Didn’t he earlier report that there was a plane that had managed to reach Mach 9.8?” Yes, it was the X43, and it did, but only for 10 seconds – and now you know the reason.
Under pressure
Of course, the scientist in you has already figured out that if the temperature of the air compressed against the body of an aircraft travelling at hypersonic speeds reaches such extremes, the air pressure itself must be incredible.
In order to explain this challenge, Ben Thornber returns to his car analogy. At Mach 9.8, he explains, the air pressure outside is about 64,000 times greater than that you’d experience thrusting an open hand outside of the window of a car travelling at 70 mph (112 km/h). In other words, if you thrust your hand out of a plane travelling at Mach 9.8 it would be hit with the same force as an elephant jumping on your hand…and at a temperature of over 6,000 degrees Celsius. “It would not be very comfortable”, says Dr Thornber, forever the scientist.
A golden age
Artist's impression of The Skylon
Yet the concept of hypersonic air travel, he insists, is not doomed. In fact, it seems we’re in something of a “golden age” of unmanned hypersonic flight, with a number of experimental aircraft “knocking about”.
The reason for that bright outlook is not passenger flight, but space technology. Because satellites are becoming smaller, it’s no longer necessary to employ large, powerful, and expensive rockets to get them into orbit. Smaller, hypersonic craft could be the vehicles of choice to do this.
As an example, closer to home, an Oxfordshire company called Reaction Engines have a proof-of-concept unpiloted, reusable spaceplane intended to provide inexpensive and reliable access to space. It’s called the Skylon (pictured right)
Artist's impression of the LAPCAT A2
But the company’s sights are also set on designing something bigger, their pièce de résistance of hypersonic flight – the LAPCAT A2 (pictured right) - a proposed commercial jet airliner, that, in principle could make the journey from London to Sydney in a couple of hours. In reality, though, such a craft is still many years away.
There is, however, something right now that can reduce the suffering on your next trip down under; and what’s more, it can fit in your pocket.
It’s called a sleeping pill.
Images: Wikipedia Commons and Reactive Engines
Main image: Artist’s impression of the LAPCAT A2