Propulsion and The X-43A
What is propulsion? The word is derived from two Latin words: pro meaning before or forwards and pellere meaning to drive. Propulsion means to push forward or drive an object forward. A propulsion system is a machine that produces thrust to push an object forward. On airplanes, thrust is usually generated through some application of Newton's third law of action and reaction. A gas, or working fluid , is accelerated by the engine, and the reaction to this acceleration produces a force on the engine.
The ramjet is the most basic type of jet engine. They find use only in guided air launched missiles. The aeroplane firing them must be flying at supersonic speeds.
Below is a picture of a liquid propelled ramjet engine:
Usually a rocket is used to bring the ramjet up to speed before it produces thrust. Because the ramjet uses external air for combustion, it is a more efficient propulsion system for flight within the atmosphere than a rocket, which must carry all of its oxygen. Ramjets are ideally suited for very high speed flight within the atmosphere.
Below is a picture of the X-15 which uses a ramjet engine.
Scramjet is an acronym for Supersonic Combustion Ramjet. The scramjet differs from the ramjet in that combustion takes place at supersonic air velocities through the engine. It is mechanically simple, but vastly more complex aerodynamically than a jet engine. Hydrogen is normally the fuel used.
One program currently under development that will test “air-breathing" engine technologies is NASA's Hyper-X. If scramjet technologies work then an aircraft will not require to carry an oxygen supply for it's engine and this will increase payload size, reduce the size of the aircraft and also reduce flying costs. This would be an ideal technology for reusable space launch vehicles.
One test of this technology will involve the X-43A as shown below:
The experiments will take place as follows:
NASA Dryden's B-52 aircraft will climb to about 20,000 feet for the first flight and release the launch vehicle. For each flight the booster will accelerate the X-43A research vehicle to the test conditions (Mach 7 or 10) at approximately 100,000 feet, where it will separate from the booster and fly under its own power and pre-programmed control.
Currently there is ongoing research and development around the world regarding hydrogen and hydrocarbon fuelled scramjet engines. The hydrocarbon fuelled scramjet, though less energetic than the hydrogen fuelled engine has proven to have more potential for safety and achievability.
Ramjets are operational from about Mach 3 to Mach 6; theoretically, scramjets would work from about Mach 6-7 to Mach 10.
Below is a comparison of a standard jet engine (turbojet and turbofan) vs a ramjet engine.
And below is a more recent test of an X-43 class prototype....(Click on each image to view a large high quality picture including description)