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I've noticed that the bypass valves in J-58 are opened after M=1.6. I don't understand why and what are the differences between the J-58 and a normal turbojet that reeaches more than M=1.6.

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At that point, the rearmost fan discs in the compressor stage were near their temperature limits (remember, the first stage/inlet compressor fan disc was managing air at 800 F). The bypass ducts limited the temperature rise in the final compressor stages. By that point the compression work performed by the inlet duct system and the remaining stages of the compressor were sufficient to run the afterburner.

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  • $\begingroup$ Not only that, but compressing the air further would had heated it so much that combustion would had become inefficient.At these temperatures, chemical energy would go into molecular dissociation instead of further heating. $\endgroup$ Oct 8, 2023 at 12:07

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