Hypersonic aircraft?

Question

Is it true that when one of these aircraft enters the hypersonic envelope that the compressed air behind the aircraft actually forms a secondary combustion chamber thus increasing the thrust of the engine(s)?

Answer 1

Not really.

The point is that a hypersonic aircraft can have its countour designed in such a way that the rear performs as a expansion nozzle.

In the case of SCRAMJET (Supersonic RamJet), the combustion can be done externally, with the front of the airplane acting as a compression ramp, the underbelly as a combustion chamber, and the rear as a nozzle. It is not something that happens, it is something that is cleverly designed.

Note that, since a Scramjet cannot work at low speed, a hypersonic airplane would need additional systems to reach the speed where the fast engine can operate.