An explicit equation for critical quantity of heat in condensing flow in a shock tube was derived.The equation is of general validity and applies tO vapor flow with or without cagier gas.It shows that critical quantity of heat in internal heat flow for phase change is greater than external heat flow.As flow is choked thermally,a shock wave is generated and propagates upstream.A location where local static pressure begins to rises,which has long been regarded as onset of unsteady condensation in a steady supersonic nozzle,is not acceptable in shock tube flow.It is before the zone of condensation.Furthermore,a second-order AUSM scheme is applied tO two-dimensional compressible flow with condensation.Computation results reproduce integral predictions well.