The vehicle is intended to have about range and good subsonic and supersonic speedfuel 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 impulse of kerosene, and can be used to cool the vehicle and the air entering the engines via a precooler. The developers say it would be able to fly from Brussels to Sydney in about 4.6 hours, compared to around a complete day of travel with normal aircraft. The cost of a ticket is intended to be roughly business class level.
Design
Capabilities
According to Alan Bond, the A2 design could fly subsonically from Brussels International Airport into the North Atlantic then reaching Mach 5 across the North Pole and over the Pacific to Australia. The great circle route is not used in this example because the route travels mostly over land. The sonic boom generated by travelling at supersonic speed can cause great discomfort for people on the ground, which was why Concorde was prohibited from flying supersonically over land. Another advantage of the design is that, while the 143 metre-long A2 is much longer 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 the hypersonic airflow over the body puts constraints on window design which would make them too heavy. One solution Reaction Engines has proposed is to install flat panel displays, showing images of the scene outside.
Engines
The Scimitar engines use related technology to the company's earlier SABRE engine, which is intended for space launch, but here adapted for very long distance, very high speed travel. Normally, as air enters a jet engine, it is compressed by the inlet, and thus heats up. It needs much more power to compress that heated air further by the engine's compressor section, which reduces the compressor's efficiency dramatically. Furthermore, this means that high-speed engines need to be made of 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 burned, to avoid melting the gas turbine section of the engine. This in turn reduces thrust at high speed. The key design feature for the Scimitar engines is the precooler, which is a heat exchanger that 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 engine inlet diffuser also has to slow the incoming air to subsonic speeds because if the air moved through the precooler and compressor at supersonic speeds, it would cause damage to them. The rest of the engine is described as having high-bypass turbofan engine features to give it good efficiency and subsonic exhaust velocity at low speeds. Unlike SABRE the A2's Scimitar engine would not have rocket engine features.
