PVP 2, a very small normal shock with narrowly spaced slip lines formed close to the nozzle exit (see Fig 7. This small normal shock can be assumed as the first appearance of the Mach disk. As it can be seen in Fig. 8, during this time, the flow reached Ma 1 upstream the nozzle exit (within the nozzle); the flow accelerated Ma 1.02 at the nozzle exit. At t 23to (t 14.2 us) a sudden rise occurred the in-nozzle Mach number.