  The problem of engine unstart of scramjets has not been resolved. In this
paper, the mechanism of engine unstart is discussed from the point of view of
shock/shock interaction and deflagration-to-detonation transition. The
shock/shock interaction leads to the nonlinear, transient and discontinuous
process of the supersonic combustion flow field. This process is similar to the
deflagration-to-detonation transition process. If the velocity of
pre-combustion shock wave is faster than the velocity in the isolator, it will
propagate upstream and cause the engine unstart. The C-J detonation velocity is
defined as the stable operation boundary of scramjets, which is the maximum
shock wave produced by combustion theoretically. The scramjets will work stable
if the velocity in the isolator is faster than the corresponding C-J detonation
velocity. The combustion characteristics and propulsive performance of
scramjets is theoretically analyzed by using C-J detonation theory. For high
Mach number scramjets, the velocity in the isolator is much faster than the C-J
detonation velocity. Therefore, extra fuel and oxygen can be injected into the
combustor to increase the thrust as long as the shock wave velocity driven by
the combustion products is slower than the air velocity in the isolator. The
theoretical results agree well with the existing experimental results, which
can be used as a baseline for the development of scramjets.
