In addition to friction (boundary layer) losses, the supersonic diffuser must be designed to minimize losses in stagnation pressure across shocks. The ideal supersonic diffuser is one which decelerates the flow until it reaches Mach 1.0 at the throat, with no shocks, in an isentropic process. This requires a continuously-varying shape, which is complex to manufacture. Even if this were possible, such a design would be isentropic for only one upstream Mach number and angle of attack: at other conditions, strong shocks may form.
Since shocks will occur in any practical supersonic diffuser, we recall that the stagnation pressure loss across a shock increases as the Mach number of the velocity component normal to the shock increases. Thus, the loss can be reduced by making the first shock oblique. Also, the flow must go through a normal shock of some finite strength before it becomes subsonic. Thus, the design problem becomes that of finding the optimum combination of oblique shocks and normal shock which will provide the lowest loss in stagnation pressure.
Given N-1 variable ramps, to produce N-1 oblique shocks of controllable strength, and one normal shock, it can be shown that the minimum loss in stagnation pressure occurs at approximately the condition where the normal Mach number in front of every shock is the same.
For aircraft which fly at low supersonic Mach numbers, only a normal shock inlet is used. For higher Mach numbers, as many as three variable ramps have been used.