The Scramjet

A scramjet is a type of engine designed to operate at the high speeds usually associated with rocket propulsion. Different from a classic rocket, a scramjet uses air collected from the atmosphere to combust its fuel, as opposed to an oxidizer carried with the vehicle. Normal jet engines and ramjet engines use air collected from the atmosphere in this way. The problem is that collecting air from the atmosphere causes drag, which increases quickly as the speed increases. Also, at high speed, the air collected becomes so hot that the fuel no longer burns efficiently thus producing lesser energy

The scramjet is a proposed solution to both of these problems, by modifying the ramjet design. The main change is that the blockage inside the engine is reduced, so that the air is not slowed down as much. This means that the air is cooler (due to absence of lesser friction), so that the fuel can combust more efficiently. Unfortunately the higher speed of the air means that the fuel has to mix and combust in a very short time, which is difficult to achieve.

To keep up the rate of fuel combustion, the pressure and temperature in the engine need to be kept steady. However, the blockages which were removed were useful to control the air in the engine, and so the scramjet is forced to assume a particular speed for each altitude. This is called a "constant dynamic pressure path" because the wind that the scramjet feels in its face is constant, making the scramjet fly faster at higher altitude and slower at lower altitude.

http://en.wikipedia.org/wiki/Image:Scramjet_operation.png

The inside of a very simple scramjet would look like two kitchen funnels attached by their small ends. The first funnel is the intake, and the air is pushed through, becoming compressed and hot. In the small section, where the two funnels join, fuel is introduced, and the combustion makes the gas become even hotter and more compressed. Finally, the second funnel is a nozzle, like the nozzle of a rocket, and a propulsion is produced.
Note that most artists' impressions of scramjet-powered vehicle designs depict waveriders where the underside of the vehicle forms the intake and nozzle of the engine. This means that the intake and nozzle of the engine are asymmetric and contribute directly to the lift of the aircraft. A waverider is the required form for a hypersonic lifting body.

Sources:
http://en.wikipedia.org/wiki/Scramjet