CONCORDE may be languishing in various museums, but the dream of supersonic air travel lives on.
Innovative, reusable rocket engines are being developed that are so powerful, they could one day fly a suborbital spaceplane from London to Sydney in just 4 hours. While Concorde cruised at Mach 2 – twice the speed of sound – with regular jet engines, these new engines are shooting for Mach 5 and beyond – a speed known as “hypersonic”.
Rocket-powered aircraft are capable of hitting far greater speeds than those with simple jet engines. But they are extremely expensive because they must be powerful enough to carry the mass of the liquid hydrogen and liquid oxygen fuel they need. Making them lighter would make them cheaper to fly, but how do you ditch weight? The answer is obvious: the air the spaceplane passes through has plenty of oxygen. If you can suck up the air and burn it with hydrogen you can instantly slash the amount of on-board oxidant you would need to just that required to propel the spaceplane when it leaves the atmosphere.
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That’s the idea behind Sabre, a hydrogen-and-air-burning engine that is being developed and tested by Reaction Engines of Culham in Oxfordshire, UK. It is designed to reach orbit in a future satellite-launching spaceplane called Skylon (illustrated above), and may be used suborbitally in a point-to-point Mach 5 passenger aircraft.
Sabre’s main advantage is that it could power a reusable spaceplane able to take off from a runway and climb to space with just one type of engine. That puts it well ahead of scramjets – engines with no moving parts that use the superfast motion of the plane to compress air into fuel. Scramjets only begin working at about Mach 4 and need boosting to that speed by a rocket. “If you need different engines for different parts of the trajectory, you’re carrying expensive dead weight,” says Ben Gallagher, business development manager at Reaction Engines.
Sabre’s key technology is a “precooler”, a superfast, lightweight chiller that processes air to burn with hydrogen.
“Air entering the Sabre engine at Mach 5 heats up to over 1000 °C. That would melt a normal rocket engine,” says Gallagher. “So our precooler chills that air to -150 °C in one hundredth of a second.” This frigidity means that when the air is compressed, in preparation for combustion with hydrogen, it does not get too hot.
“Air entering the Sabre engine at Mach 5 heats up to 1000 °C. That would melt a normal rocket engine”
The precise details of how the precooler works are a secret – so secret that Reaction Engines has not even filed any patents on the technology. All we know is that it passes air over a great many thin, helium-chilled tubes that present a large surface area for cooling.
At the Farnborough International Airshow in the UK last month, company chief Alan Bond said that in recent tests the Sabre precooler has run steadily, free from vibration, for over 6 minutes – the time it would need for a launch to orbit as part of a spaceplane.
We don’t have to take their word for it, either. Mark Ford, a propulsion engineer at ESTEC, the European Space Agency’s research centre in Noordwijk, the Netherlands, acted as an independent witness at the Sabre tests in Culham. “The initial results are very promising and we’re very impressed,” he says. “The precooler provided stable subzero-temperature airflow through to the engine.” More challenging tests will be carried out later this month to see if the engine produces its target of supplying super-dense air at -150 °C to the compressors – or freezes solid.
Meanwhile, a second attempt at a Mach 5 spaceplane engine emerged during the Farnborough airshow, courtesy of a joint venture between MBDA Missile Systems, the aerospace firm EADS, which owns Airbus, and the Lavrentyev Institute of Hydrodynamics in Novosibirsk, Russia. The collaborators revealed that they are developing an as-yet-untried variant of the pulse jet engine technology that powered the German V-1 flying bomb in the second world war.
The pulse jet engine – the distinctive buzzing sound of which struck terror into Allied civilians, particularly when it cut out above their heads – worked by injecting small pulses of fuel and air into a channel and igniting the flammable mixture. At the same time, shutters were closed at the front of the engine to ensure the combustion products were fired backwards, propelling the vehicle. A pulse detonation engine takes this further by exploding the fuel and air while travelling at supersonic speeds.
The collaborators are attempting to advance that still further into the hypersonic realm with what they are calling a engine. In this, they modify the fuel injection pressure several thousand times per second, rather than in slower pulses, to create ignition in the form of a continuous series of shock waves which combust the fuel more fully and can generate thrust levels up to Mach 5 speeds.
Such blisteringly fast suborbital flights for ordinary passengers are still some way off, of course, but these recent engine-test successes have prompted the UK’s science minister David Willetts to urge the European Aviation Safety Agency to look into certifying European Union airports for use by spaceplanes. “It’s not that EASA has been hostile to spaceplanes,” Willetts told New Scientist. “It’s just that it has no regulatory regime.”
Meanwhile, Virgin Galactic, which already has plans for spaceplane-based suborbital tourism, also has ambitions for high speed point-to-point travel that actually involves entering orbit. “Point-to-point travel would be the most amazing thing,” says Julia Tizard, deputy vice president of operations at Virgin Galactic in Mojave, California. “An orbit takes 90 minutes, so you could leave the UK and have lunch in Australia in 45 minutes.”
