Lockheed Martin concept

How to... go around the world in six hours

Day trips to the other side of the Earth may soon be a reality thanks to Air travel at five times the speed of sound.

They say it's a small world, but anyone who has endured 24 hours of misery flying from the UK to Australia might be inclined to disagree. Imagine, then, zipping to Sydney for lunch, then popping over to LA to catch a movie premiere.

While hypersonic aircraft might not quite consign long-haul flights to the history books, travelling at five times the speed of sound would put almost anywhere in the world on the map for a day trip.

Over the past 50 years hypersonic passenger aircraft have been proposed many times, usually accompanied by illustrations of needle-thin spaceplanes soaring gracefully above the curve of the Earth. The US military, too, is keen on shipping troops (or merely warheads) around the world in a flash, and has funded several ambitious hypersonic research projects.

The reality today, however, is far more down-to-earth. No manned aircraft has yet flown under its own power at hypersonic speeds for more than a couple of minutes, and even that required a pilot who was literally out of this world. Neil Armstrong's sixth outing in an experimental X-15 rocket plane in April 1962 shot him to Mach 5.3 and straight into the record books, with a total flight time of just twelve and a half minutes.

It could even seem as if the dream of hypersonic flight is further away now than it ever was. The most promising hypersonic vehicle flying today is the X-51, another experimental aircraft, developed by the US Air Force, DARPA and Nasa, along with Boeing and Pratt & Whitney Rocketdyne. Despite over 40 years of technological advances, the X-51 is smaller and slower than the X-15, cannot carry a human pilot or be reused, and all three of its test flights to date have ended with premature splashdowns in the Pacific Ocean.

But those apparent failures mask many genuine achievements, as well as amazing progress from both the X-51 team and British and Japanese aerospace engineers. There has never been a better time, in fact, to ask exactly what is required to turn hypersonic passenger air travel from science fiction into reality, and to examine the technologies that might make it happen.

Hot stuff

The challenges to building a hypersonic aircraft are immense. At hypersonic speeds (generally regarded as Mach 5 and above), even thin, high-altitude air flowing around an aircraft can raise its surface temperature to 2,000°C - hot enough to melt steel and nearly vaporise aluminium. At these temperatures, the molecular bonds in air vibrate, changing the way it acts upon the aircraft. Go faster still and the molecules can break apart to produce an electrically-charged plasma that cloaks the vehicle.

Shock waves and expansions produce large variations in air density and pressure, creating aerodynamic conditions unlike those faced by subsonic or supersonic aircraft. And then there are control and navigation issues. At nearly 2km a second, even the tiniest impact or patch of turbulence could prove fatal.

With all these potential issues, perhaps the best way to learn how to build a hypersonic plane is to see how not to build one. Sadly, this is something at which the world has had plenty of practice, even if the US Air Force's X-15 initially made hypersonic travel look almost straightforward.

This single-seater aircraft, built from a heat-resistant nickel alloy, was designed to be carried to an altitude of 14km by a B-52 bomber. Upon release, its pilot would start a rocket engine to boost it quickly to supersonic and then hypersonic speeds, before gliding down to an unpowered landing on a traditional airstrip.

Between 1959 and 1968, 199 flights of the three X-15 prototypes demonstrated that hypersonic flight was feasible. Two X-15 flights in 1963 even exceeded the 100km altitude that officially represents the boundary of space, although with test pilot Joseph Walker rather than Neil Armstrong at the controls.

For all its record setting, though, the programme hinted at problems to come: during flight 191 in November 1967, an X-15 piloted by Major Michael Adams entered a hypersonic spin during its descent. Aerodynamic forces buffeted the plane, increasing accelerations to over 15g and eventually breaking it apart at an altitude of 18km, killing Major Adams.

Super cool craft

Atmospheric data and design lessons from the X-15 project, as well as one of its pilots, would later find a home in Nasa's Space Shuttle programme. The space shuttle holds speed and distance records for hypersonic flight, reaching Mach 25 on re-entry, albeit entirely unpowered. For hypersonic travel to be genuinely practical, it will have to combine the range of the space shuttle with the powered, controlled flight of the X-15.

The problem with a rocket-plane design, such as the X-15, is that it must carry both its fuel and its oxidiser, severely limiting both its range and payload capacity, while simultaneously increasing the risks of a catastrophic accident. Traditional jet engines, on the other hand, cannot handle air entering their turbines at more than about half the speed of sound. Supersonic aircraft have inlet systems to slow and compress the air, but these also heat it further. Flying at more than Mach 3 with existing technology risks melting the engine.

One obvious solution is to cool the air before it enters the engine - and ideally before it is even compressed. British Aerospace and Rolls-Royce proposed such a system for the HOTOL (Horizontal Take-Off and Landing) concept vehicle as early as 1982, using liquid hydrogen both to pre-cool incoming air and as fuel. This is easier said than done. A pre-cooler has to cool air extremely quickly to sub-zero temperatures, without suffering a build-up of ice from moisture in the air. HOTOL eventually lost its funding in 1988.

In 2008, the Japanese space agency JAXA announced that it had carried out successful ground tests of an air-breathing hypersonic turbojet engine pre-cooled with its own liquid hydrogen fuel. A flight test of the engine in 2010 reached Mach 2, substituting liquid nitrogen as coolant for safety reasons. JAXA is now planning to develop a complete experimental hypersonic aircraft called HYTEX, capable of speeds up to Mach 5.

Air cooling

While the HOTOL project expired, the expertise behind it did not. Its creator Alan Bond went on to form Reaction Engines, where he continued to work on hypersonic engines. In November 2012, Bond announced successful trials of a new pre-cooler heat exchanger using a nest of fine 1mm pipes running liquid helium.

"By adding a helium loop you introduce complexity and extra weight but the engine becomes significantly more efficient," explains Mark Hempsell, Reaction Engine's future-programmes director. "The helium gets energy from the hot air that you can use to power the turbine and liquid hydrogen pump. You can do much more sophisticated thermodynamic cycles with it."

Reaction Engine's heat exchangers can cool incoming air from over 1,000°C to -150°C in less than 0.01 seconds, without blocking with ice. They are intended for an innovative liquid hydrogen-fuelled engine called SABRE that runs as a pre-cooled jet turbine up to Mach 5 before transitioning to a liquid hydrogen/liquid oxygen rocket capable of reaching Mach 25. SABRE engines are hoped to eventually power a hypersonic spaceplane called Skylon capable of reaching orbit from a conventional runway take-off, although building that will take a decade or more.

If you want to make a hypersonic aircraft right now, your best bet is to use a supersonic combusting ramjet, or scramjet. Lacking the compressor found on normal jet engines, scramjets rely on high forward-velocity to compress and decelerate the incoming air, which nevertheless travels at supersonic speeds throughout the engine. Scramjets require no moving parts but do need to be moving very quickly (typically Mach 4 or faster) to function properly.

The University of Queensland in Australia flew the first scramjet in 2002, with a six-second test burn of a HyShot craft launched from a rocket. Nasa followed with its X-43 aircraft in 2004, clocking a record Mach 9.6 flight in a 10-second flight from a booster rocket. While an experimental success, the liquid hydrogen-fuelled X-43 was never intended to make a practical aircraft: its copper engine would have melted if the flight had lasted another 20 seconds.

France, Brazil, Germany, Australia, India and Russia are also working on their own air-breathing scramjets, while aerospace group EADS has promised a demonstrator of a Mach 4 hypersonic passenger aircraft by the end of the decade. Unsurprisingly, the most advanced hypersonic vehicle at the moment belongs to the US military, which wants a hypersonic strike weapon as soon as 2020, and a hypersonic surveillance aircraft (possibly piloted) ten years later.

X-51 trials

For the moment, the Air Force Research Laboratory is still working on the X-51, a fully autonomous, fuel-cooled hydrocarbon scramjet that uses the same kerosene-based JP-7 fuel as the Mach 3 SR-71 Blackbird spy plane. JP-7 is used because it is an endothermic fuel, absorbing heat as it combusts to regulate the engine's temperature. Although the X-51 itself is a single-use aircraft, its engine was designed to be reusable, and has been successfully restarted multiple times in ground tests.

Exotic carbon-carbon composites and a silica-based thermal protection system protect the X-51 from the fierce temperatures it experiences at full speed. Unfortunately, none of its three test flights so far have lasted even their intended length of just 4.5'minutes. The first and longest was 2.5 minutes, before hot gases escaped the engine compartment and interfered with control systems. The second flight failed after 10 seconds when unexpectedly high pressures moved the engine a scant 0.2mm and interrupted combustion. On the third flight, last August, an aerodynamic fin came loose and ended the test almost immediately. One X-51 remains, and it will probably be flown early this summer.

Joseph Vogel is director of Hypersonics at Boeing Phantom Works. He told E&T: "We have proved beyond any reasonable doubt the capabilities of these vehicles to travel at hypersonic speeds. What some might call failures we look on as opportunities for learning. But I'm ready to stop learning now, and show that the pieces we've put together can produce a vehicle that will travel the full duration planned for."

Looking beyond the X-51, Nasa and the US Air Force have set aside $30m ('18.75m) to fund three National Hypersonic Science Centres. These university/industry groups are dedicated to working on high-temperature materials and air-breathing propulsion, particularly on so-called combined cycle systems to integrate hypersonic scramjets and low-speed turbines into a single aircraft.

The first country to develop a practical and affordable hypersonic aircraft will enjoy tremendous military advantages, according to Charlie Brink, program manager for the X-51 at the US Air Force Research Laboratory. "War fighters are starting to get a feel for what these speeds could do for them in their job of protecting the nation," he says. "With a scramjet-powered system, we could take out all the bad guys in a building 500 miles away in less than 10 minutes. It's a paradigm shift."

And where soldiers fly, global day-trippers will soon follow. "There are synergies here with passenger aircraft," says Vogel. "As we move into higher speed systems for future commercial applications, the stuff we can build will benefit all humanity. Hypersonics is paramount to changing how we do business in the air."

So throw some shrimp on the barbie, cobber, I'll be over for lunch in about 20 years and three hours' time. *

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