Some very embarrassing statements from somebody here.
There is much information on the internet about this subject, if you would only search and find it.
First, best not to believe what you read on the internet.
All I can do is hopefully lead you to water.
Bob Abernethy's Untold tales is very good. It is too short unfortunately to explain much but we are in luck because we have the essential complement to his 'tales' in the form of
the 'bible' he wrote on how he fixed the J58 to work at Mach 3.2. It is called US Patent 3,344,606.
From Aerostories
Mach numbers the new engine would be unable to cope with the volume of air coming through the air-intakes
Read the patent and then tell us what you have learned.
installing a series of fixed flow-vanes downstream
Read the patent and then tell us what you have learned.
From Aircraft Engine Design:
Turbojet/Ramjet Combined Cycle Engine.When these tubes are in use, the compressor, burner, and turbine of the turbojet are essentially bypassed
Bear in mind 'Bob' designed and built real engines that made money. He didn't write textbooks.Read the patent and then tell us what you have learned.
If you are still having trouble understanding,
Embarrassing statement
The air in the compressor could stagnate causing compressor blade flutter or even compressor stall where the turbojet engine would cease to function at all
Embarrassing statement. Read the patent. Tell us what you have learned.