Do ramjets use variable geometry supersonic inlets?
Why is that the ramjets do not require [variable geometry] to maximize pressure recovery?


Turbofans, in aircraft that have variable geometry intakes, such as in an F-111 or F15, operate from standstill, at the start of the runway, to about Mach 2.5. That means, they go from having no shock, to having quite a large (strong) shock. The intake needs to provide the about the same mass flow to the engine, regardless of the forward speed of the aircraft. And, at high speed, it doesn’t want to spill excess air, because that creates spillage drag, especially when the intake has sharp lips, need to reduce form drag at high speed. But, at low forward speed, it needs a bigger intake area, as the forward motion of the aircraft isn’t providing a lot of air. So, the intake has conflicting constraints. It needs a small area at high speed, but a large area at low speed. It also needs to form an oblique shock at high speed, as an oblique shock has less total pressure loss than a normal shock wave. The oblique shock wave needs to be kept out of the intake, hence typically a spike protrudes forward of the intake. The spike moves forward as the aircraft speed increases, and the geometry is made such that this spike reduces the intake area as it translates forward.

Now, consider a RAM jet. It typically operates from Mach 3 to 6, say. So, it’s going from a speed at which shocks exist, to a speed at which shocks still exist. It might still be an operating range of 6-3=3 Mach, compared with 2.5-0=2.5 Mach, so not much different. But, in terms of the change in fluid behaviour of the air, 0 to Mach 2.5 is a much much larger change in behaviour than Mach 3 to 6.

For example, the angle of an oblique shock at Mach 2.5 is 23.6 degrees. Mach 3 is 19.5 degrees and Mach 6 is 9.6. (See formula here at NASA). So, between zero speed an Mach 2.5, the shock changes from not being there at all, to starting as a normal shock of 90 degrees, to having an inside angle of 23.6 degrees (a change of 90-23.6=66.4 degrees). In comparison, for the RAM jet, the shock only changes angle by 19.5-9.6=9.9 degrees. It’s pretty easy to accommodate a change in shock angle of only 9.9 degrees with the same geometry.