The Reaction Engines Limited A2 (called the A2) is a design study for a hypersonic airliner. The airliner is intended to provide environmentally-friendly, long range and high capacity commercial transportation. It is being examined as part of the LAPCAT programme of the European Union. The plane has not been commercially launched yet, but Reaction Engines Limited, the British design firm, says it could probably be developed into a working aircraft within 25 years, if there is market demand for it.
The vehicle is intended to have about 20,000 km range and good subsonic and supersonic fuel efficiency, thus avoiding the problems inherent in earlier supersonic aircraft. The top speed is projected to be Mach 5+. It calls for the use of liquid hydrogen as a fuel, which has twice the specific energy of kerosene, and can be used to cool the vehicle and the air entering the engines via a precooler.
The A2 is designed to leave Brussels International Airport, fly quietly and subsonically out into the north Atlantic at Mach 0.9 before reaching Mach 5 across the North Pole and heading over the Pacific to Australia.
Another advantage of the design is that, while the 143 metre-long A2 is much bigger than conventional jets, it would be lighter than a Boeing 747 and could take off and land on current airport runways.
However, the A2 design does not have windows. The heat generated by traveling so quickly makes it difficult to install windows that are not too heavy. One solution Reaction Engines has proposed is to install flat screen panels, showing images of the scene outside.
Normally, as air enters a jet engine, it is compressed by the inlet, and thus heats up. This means that high speed engines need to be made of technologies and materials that can survive extremely high temperatures. In practice, this inevitably makes the engines heavier and also reduces the amount of fuel that can be burnt to avoid melting the gas turbine section of the engine, which in turn reduces thrust at high speed.
The key design feature for the Scimitar engines is the precooler, which is a heat exchanger which transfers the heat from the incoming air into the hydrogen fuel. This greatly cools the air, which allows the engines to burn more fuel even at very high speed, and allows the engines to be made of lighter, but more heat susceptible materials such as light alloys.
The rest of the engine is described as having high-bypass (4:1) turbofan engine features to give it good efficiency and subsonic (quiet) exhaust velocity at low speeds. Unlike SABRE the A2's engine would not have rocket engine features.