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Enormous air pressures result in the thrust of a Scram jet or ramjet engine, more so the pressure required to drive the aircraft forward at supersonic and hypersonic speeds. Given that these engines are designed to function without a fan which sucks air and at those speeds would prevent air from coming in too fast or going back where at came from post being heated. What mechanism or design aspect prevents jet blast from escaping out the front of a Ramjet or scram jet, resulting in lost thrust and or perhaps negative thrust given that explosive pressure would drive air in any direction that would allow it to escape.

I'm also considering that the air entering the engine does not have that much pressure to prevent this cause if jet blast did exit out at the front, scram jets and ram jets would not be able to produce thrust

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Pressure is the greatest just before the combustor in both jet and ramjet engines - therefore air would have against the pressure differential for this to happen (turbine only has therefore an indirect role)

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  • $\begingroup$ The area of highest pressure before the combustion chamber is called the diffuser... $\endgroup$
    – jwzumwalt
    Jun 14, 2018 at 6:21
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Primarily the pressure differential between the stagnation pressure at the inlet and the ambient static pressure. Combustion takes place in an isobaric environment with the enthalpy of the exhaust gases being converted to kinetic energy by exiting the nozzle per Bernoulli’s Principle. Therefore the pressure differential between the combustion section and the intake vs exhaust nozzle only favors gas flow out the nozzle.

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Both ramjets and scramjets work on the principle of supersonic flow: a ramjet has supersonic flow before the intake, and a scramjet has supersonic flow throughout the engine.

When air encounters a disturbance, the pressure created by this travels at the speed of sound. This means that for supersonic flow, any information about a disturbance coming up does not move upstream in the form of pressure. All the incoming air particles are just going about their day, not moving out of the way (remember, by Newton's laws you need a force for something to start moving, and no pressure means no force), until suddenly, there's a disturbance! This is how you get shockwaves.

Now imaging an air particles behind the shockwave in the combustion chamber in a ramjet. It wants to escape, and for that it needs to move either upstream or downstream. Downstream is easy, just go out the back of the engine. Upstream however is impossible. It could go up to the shockwave, but there it will just get pummeled by unwitting air particles. No matter how much you increase the pressure, the incoming particles have no idea what's coming and will just hit our particle right back.

A scramjet is fully supersonic. There's not even a subsonic particle trying to escape to the front; it may try and form a shockwave but since this shockwave travels only at the speed of sound it will never try and catch up to the scramjet again.

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    $\begingroup$ “Both ramjets and scramjets work on supersonic flow.”? Negative. Ramjets use subsonic airflow through them. Ramjets are most efficient for use in supersonic ie Mach 2 to Mach 5 regeimes of flight. $\endgroup$
    – Romeo_4808N
    Jun 13, 2018 at 22:38
  • $\begingroup$ @CarloFelicione Better this way? $\endgroup$
    – Sanchises
    Jun 14, 2018 at 6:21

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