An ideal ramjet has no rotating machinery at all, so every bit of pressure rise has to come from decelerating the incoming stream through the shape of the duct. Checking each listed process against that fact settles the question.
- Oblique shock: at supersonic flight Mach numbers, the intake is designed with ramps or a cone that generate oblique shocks. These slow the flow in stages with a much smaller stagnation pressure loss than one strong shock would cause. This is a genuine part of ramjet compression.
- Mechanical compression: this needs a compressor rotor driven by a shaft, which is exactly what a ramjet does not have. A turbojet uses mechanical compression; a ramjet never does. This process is absent.
- Normal shock: after the oblique shock system has brought the Mach number down close to 1, a normal shock finishes the transition from supersonic to subsonic flow, usually right around the inlet throat. This is a standard part of the supersonic diffuser.
- Subsonic diffusion: once the flow is subsonic, the duct keeps widening so the flow keeps slowing and its pressure keeps rising, right up to the face of the combustor. This subsonic diffusion is the last stage of compression.
Three of the four listed processes, oblique shock, normal shock and subsonic diffusion, are exactly how an ideal ramjet raises the pressure of the incoming air. Mechanical compression is the one process that a ramjet cannot use, since it has no compressor.
Let's summarize:
- A ramjet compresses air only through the shape of its inlet duct: oblique shocks, then a normal shock, then subsonic diffusion.
- Mechanical compression requires a compressor rotor, which a ramjet does not carry.
So the correct processes are oblique shock, normal shock, and subsonic diffusion.
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