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I discovered a new theoretical engine called the shock-ramjet (schramjet) and I was wondering, how would it work exactly? I've read that its a high-speed engine that could be used around Mach 10, but I can't figure out how it actually works. My idea is that the engine uses the shockwave to compress the air for combustion later, but by reading some more I found out this wasn't correct at all. Can anyone explain it more fully?

Source: https://en.wikipedia.org/wiki/Shcramjet

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  • $\begingroup$ The link explains it, you need to be more specific with your question. $\endgroup$ Aug 4, 2023 at 18:20
  • $\begingroup$ My idea of how it works is that a ram at the beginning uses the shockwave to compress the air for combustion $\endgroup$
    – Mateo
    Aug 5, 2023 at 4:19

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Scramjet is the portmanteau of supersonic ram jet. A normal ramjet tries to decelerate the air flowing through it to almost a standstill in order to allow combustion within a limited length. However, deceleration causes heating, and at flight speeds above Mach 5 the deceleration of air to subsonic speed would heat the air so much that combustion would become inefficient, and heat loads would exceed any materials known to man.

In order to reduce heating, the air is delelerated less so combustion has to happen at still supersonic flow speed. In order to happen inside the engine, this needs a fuel with very high flame speed, and the best fuel for this is hydrogen.

A variation is the shcramjet (now the sh part stands for shock and the following c for combustion) where the heat of an oblique shock is used to ignite the fuel-air mixture. Here, fuel is injected into the intake stream so it can use the length of the intake for mixing with the air and burns after the last compression shock. The longer mixing length allows for a wider variety of fuels such as methane besides only hydrogen in a scramjet. Since the flow speed past an oblique shock is still supersonic, this is still in principle a scramjet, but it differs from the classical scramjet in the design of the combustion area.

Now much depends how close to the shock the combustion happens since it can influence the shock. In pulse detonation engines the deflagration of regular combustors is replaced by a detonation of the fuel-air mixture which results in a much faster reaction. This releases the chemical energy along a much smaller flow path, making the reaction more efficient for propulsion.

A deflagration is a subsonic explosion where the shock wave travels ahead of the flame front, with significant separation between the two. A detonation is a supersonic form of combustion where the shockwave and flame propagate as a coupled pair, augmenting the pressure significantly in comparison to deflagrations.

The detonation is a discontinuous process because it changes the flow pattern momentarily. In consequence, a pulse detonation engine uses a cyclic process while a scramjet or a shcramjet with combustion downstream of the shock employ a continuous process.

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  • $\begingroup$ @ZeissIkon I should have finished the answer before posting but was too tired yesterday. $\endgroup$ Aug 5, 2023 at 6:08
  • $\begingroup$ Much better when it's all there downvote replaced with upvote. Carry on. ;) $\endgroup$
    – Zeiss Ikon
    Aug 6, 2023 at 16:23

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