Hypsersonic jet engines

After more than 30 years of research, a new sort of rocket engine starts to sizzle

SOME good ideas take years of dogged perseverance to come to fruition. That has certainly been true of a hypersonic engine which Alan Bond, a British engineer, began developing in 1982. Its first incarnation was as part of a government-backed spaceplane project called HOTOL (horizontal take-off and landing), run by Rolls-Royce and British Aerospace. When the money for this dried up in 1989, Mr Bond and two fellow Rolls-Royce engineers set up a company called Reaction Engines to keep the work going. This week BAE Systems (as British Aerospace is now called) bought 20% of Reaction Engines for £20m ($31m). That vote of confidence suggests Mr Bond’s novel propulsion system may be turning into reality.

His system is, as the firm’s name implies, a reaction engine. That means it relies on Newton’s third law of motion (to every action there is an equal and opposite reaction) to drive it forward. Broadly, reaction engines come in two varieties—rockets and jets—in which hot gases created by burning fuel are ejected out of the back, providing the action part of the law. The reaction part is forward movement. The distinction between rockets and jets is that rockets carry their own oxidant, as well as fuel, whereas jets use oxygen from the atmosphere for the purpose. However SABRE (synergetic air-breathing rocket engine) endeavours to have the best of both worlds. When the atmosphere is thick enough, it uses air and thus saves weight. But when it enters the vacuum of space, it switches to liquid oxygen. The result, if attached to a suitable airframe (see artist’s impression), which the firm dubs Skylon because it resembles a piece of sculpture of that name that was on display at the Festival of Britain in 1951, would create a fully reusable spacecraft. This could take off from a runway and fly into orbit carrying satellites, or might whisk passengers from London to Sydney in less than five hours.

Though the idea of such a hybrid between rocket and jet sounds reasonably straightforward, it is technically daunting. The biggest challenge is that at around the craft’s proposed cruising velocity, Mach 5 (five times the speed of sound, or 6,180kph) the oncoming air is heated by friction to 1,000°C. In air-breathing mode the engine needs to compress this onrush to 140 times atmospheric pressure before it is injected into the combustion chamber. But that would heat it further, and no materials exist that could be used to build such a compressor without melting. The consequence is that the incoming air has to be cooled to -150°C before it reaches the compressor—which happens less than a hundredth of a second after it enters the engine.

For this, SABRE’s engineers have devised what may be the world’s fastest refrigerator. It consists of thousands of pipes made from an alloy of nickel. Each pipe is a mere millimetre in diameter and has walls 27 microns thick. These tiny tubes greatly increase the surface area available for the coolant being pumped through the system to absorb heat. SABRE uses helium, which is particularly good for the purpose.

Nor is the absorbed heat wasted. First, the expansion of the helium that it causes is used to drive a fuel pump and a compressor. Then, the remaining heat is extracted by running the helium through a second set of pipes that are bathed by liquid-hydrogen fuel on its way to the combustion chamber. This evaporates the hydrogen, preparing it for combustion.

The switch to rocket mode, at an altitude of about 25km, where the atmosphere is too rarefied to support this process, involves closing the air intake at the front of the engine and then injecting liquid oxygen from a tank on board directly into the combustion chamber. This will take Skylon from Mach 5 to around Mach 25—and thus into orbit, whence it could deliver a satellite before gliding down to land.

Not surprisingly, SABRE has met with a fair amount of scepticism. But Reaction Engines has beavered away and, in recent years, has demonstrated in a series of trials that its heat exchanger works and is a robust piece of kit able to withstand high temperatures and pressures. Favourable assessments of the technology by the European Space Agency and America’s Air Force Research Laboratory have helped persuade the European Union to approve a £60m development grant from the British government, which the company should receive this year.

With BAE’s assistance the plan is, according to Mark Thomas, Reaction Engines’ new managing director, to have a ground-based version of SABRE running by 2020. Mr Thomas joined the firm in May, having previously worked for Rolls-Royce, where he oversaw a number of new jet-engine projects. Gradually, then, and with several changes of name, the spaceplane is returning to its roots. How long it will be before anything SABRE-powered actually takes off remains to be seen. But Mr Bond, who is now 71, lives in hope that he will yet see his baby take to the sky.


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