supersonic nozzle

Betty

For a nozzle with design lip Mach number of 2.2, if we are given the pressure ratio = pstatic/pambient = 1.2 , then if the isentropic relations were used to calculate the exit Mach number(using po/pstatic = 1.2) it gives a really small value Mexit=0.103.

I know something is wrong here. How do I get the stagantion pressure just from the pressure ratio? Or what would be the right way to compute the exit mach number given the above pressure ratio??

I would appreciate any help.

Thanks