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A variable-geometry supersonic inlet is used to

  • A.Eliminate the need for a diffuser
  • B.Increase the mass flow at low speed only
  • C.Match the shocks to Mach number
  • D.Reduce the inlet's weight

Answer: C

Correct answer: Match the shocks to Mach number

A supersonic inlet decelerates the flow through a system of oblique shocks, and the angles at which those shocks stand depend on the flight Mach number. A fixed geometry can therefore be correct at only one speed.

Variable ramps or a translating centrebody move the shock system so that it remains properly positioned across the flight envelope, keeping recovery high and avoiding the unstable shock oscillation known as buzz. The mechanism adds weight and complexity rather than saving it, and a diffuser is still required.

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