ROCKETS are a spectacular way to get into space. But they are also inefficient. Unlike jet engines, which combine atmospheric oxygen with stored fuel to obtain thrust, rockets must work in a vacuum, and so must carry their oxygen with them. That increases their weight, with the result that a large fraction of a rocket’s fuel is expended just lifting the rest of the fuel off the launch-pad.
Now Reaction Engines, a small British firm, reckons it has a way around the problem. On November 28th it said it had completed a series of tests of its SABRE engine, a high-tech hybrid of a rocket and a jet. At low altitudes and speeds (up to about five times the speed of sound), SABRE would operate as a powerful, lightweight form of jet engine. At higher speeds and altitudes, it would switch to internal oxygen and act as a rocket.
By cutting the amount of oxygen a spacecraft needs to carry, this engine may at last allow aerospace firms to build a truly reusable winged space-plane that can fly into orbit and then glide back to land on an ordinary runway. (The Space Shuttle’s giant, disposable external fuel tank meant it was not truly a reusable craft.) That would be a big advantage: all modern rockets are single-use machines. Reaction Engines reckon that its Skylon concept vehicle could, by contrast, undertake a flight every couple of days, slashing the cost of space flight.
The key to the engine’s performance is a small, light and powerful heat exchanger, designed to cool incoming air, which, at high speeds, will be heated by friction to around 1,000°C. The heat exchanger will bring the air down to around -150°C. Among other benefits, that removes the need for the sort of heavy-duty construction that would otherwise be required to stop the engine from melting, helping to save precious weight.
Space cadets may object that they have heard this all before. Spaceplanes are a long-held dream of aeronautical engineers, and the SABRE engine is a development of an abortive British spaceplane project called HOTOL, undertaken in the 1980s by Rolls-Royce and British Aerospace (now BAE Systems). But SABRE has survived close scrutiny by the European Space Agency, which could find no show-stopping problems, and has performed well in extensive ground tests. Nor is space travel the only possible application: the firm talks of the military and civilian uses of a Mach five air-breathing aircraft, as well as the potential usefulness of its compact, ultra-light heat exchangers for earthbound applications.