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Subsonic ramjets have high specific fuel consumption. This indicates to me they must have a low pressure ratio.

Is there a limit to how high pressure ratio one can achieve with subsonic air? Why can't you just increase the area ratio between inlet and the diffuser exit?

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Yes, they have a limit at around 2. And this can only be achieved at Mach 1, as Peter Kampf points out. Increasing the inlet to diffuser area ratio to extreme values will just lead to your engine being filled with slow moving air. The mass and volume of air in your engine will increase, but neither the pressure nor the rate of airflow will increase appreciably

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    $\begingroup$ And that limit is only possible at Mach 1. At a slower speed the pressure ratio will become even lower. $\endgroup$ – Peter Kämpf Feb 11 at 17:48
  • $\begingroup$ which equation is this deducible from? $\endgroup$ – Francis L. Feb 12 at 1:13
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    $\begingroup$ @FrancisL. there you go $\endgroup$ – Peter Kämpf Feb 12 at 4:57

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