A simple model has been developed for predicting the velocity of "rocket" missiles generated by failure at the lower end of a vertical cylindrical pressure vessel containing gas and a cold liquid. This model accurately predicts the velocities observed in a parallel experimental investigation. When modified to take account of liquid flashing to vapour during depressurization, the model has also been successfully applied to the case of "rockets" generated by failure of a vessel containing a hot liquid and a cover gas at a pressure sufficient to suppress boiling in normal operation. However, where "rockets" are generated by failure of a vessel containing a boiling liquid, the model significantly under predicts the "rocket" velocity in many cases. Here the enhanced experimental velocities are attributed to the increase in the apparent liquid volume, caused by the presence of the vapour bubbles, prolonging the liquid expulsion period. Simple correlations of the experimental data for all three cases have been developed using a parameter derived from the theoretical predictions. These define upper limit velocities for use in hazard assessment.