Ramjets and Scramjets

The ramjet is the most basic type of jet engine.

In comparison to turbojets, they have no moving parts.
They find use only in guided air launched missiles. The aeroplane firing them must be flying at supersonic speeds.
Ramjets operate by subsonic combustion of fuel in a stream of air compressed by the forward speed of the aircraft itself, as opposed to conventional turbojet engines, in which the compressor section (the fan blades) compresses the air.

Interesting Related link: NASA Ramjet simulation, JAVA Applet

Scramjets (supersonic-combustion ramjets) are those in which the airflow through the whole engine remains supersonic.

It is mechanically simple, but vastly more complex aerodynamically than a jet engine.
In a scramjet powered aircraft, there must be tight integration between the airframe and the engine.
Scramjet technology is challenging because only limited testing can be performed in ground facilities.
Long duration, full-scale testing requires flight test speeds above Mach 8.
X-43 Hyper-X, NASA's testbed for the scramjet, serves this purpose. To get the engine to that speed, some other power has to be used.
In the NASA Hyper-X, this will be provided by OSC's pegasus booster. It must be noted here that scramjets are good only for sustaining hypersonic speeds, not for achieving them from zero.

Scientists realized during early experiments that evolving the well-known ramjet to a supersonic combustion engine was a challenging and complex task. In the ensuing years, scientists conducted several programs in the US with the objective of proving the scramjet as a viable propulsion system, however, none advanced to the flight test stage.
Today, scientists are working two programs to demonstrate the nearly half-century-old vision. Under the National Aeronautics and Space Administration's Hyper-X program and the Propulsion Directorate's Hypersonic Technology (HyTech) program, scientists plan to demonstrate hydrogen- and hydrocarbon-fueled scramjet engines respectively. The hydrocarbon-fueled scramjet, though less energetic than the hydrogen-fueled engine, is much more logistically supportable.
The objective of the directorate's HyTech program is to demonstrate the operability, performance, and structural durability of a Mach 4-8 hydrocarbon-fueled scramjet propulsion system.
HyTech scientists employ a building block approach to solve progressively more complex challenges using the knowledge garnered from preceding developments. This approach begins with discrete component build and test, and culminates with a flight test in an appropriate mission-sized vehicle. The long-term goal is to develop scramjet engine technologies for routine, affordable, on-demand access to space.

In july 2002, the Australina HyShot program succesfully launched a hypersonic, sramjet propulsed rocket, to a speed of Mach 7.6, or 7.6 times the speed of sound. BBC news article Anoter article on the test flight