is this supersonic shock wave that is important to ramjet air intake functioning, and it was the cause of major headaches in the design of ramjets that would work reliably. It is important to remember that the air molecules in this wave are moving at the speed of sound, and no faster.
The simplest air intake design is just a hollow tube with a circular opening - a pipe. Imagine a pipe fastened to a supersonic airplane or rocket. When the pipe is propelled through the air at supersonic speeds the edge of the opening pushes air molecules out of the way, forming a shock wave. On the outside edge of the pipe the shock wave moves outwardly just like it does around the nose of a supersonic bullet. However, on the inside of the tube the shock waves from all around the opening converge, as shown by the dashed lines. The air molecules moving away from the inner edge of the opening run into other molecules moving inward and the pressure wave can go no farther. Pressure builds up behind the shock wave, compressing the air in the tube and slowing the rate at which it flows through the tube.
Normal shock front
The air passes through the tube slower than the speed the tube passes through the surrounding air. As a consequence, air
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The inside of the tube is smooth, and there are no surfaces for pressure to push on in a forward (or aft) direction. Because air flowing into the tube is slowed and compressed, it acts as drag working against motion through the air. This added drag consumes some of the power from the engine propelling the tube. The thrust from the exhaust gasses pushes forward against the compressed air at the front of the tube. This reduces the energy that the engine moving the tube has to expend, so the tube can move faster. The faster the tube moves, the more air enters the front, causing temperature and pressure to rise inside the tube, causing the gasses to escape at higher speeds at the rear, resulting in greater