Many process plant installations include cylindrical vessels which contain high temperature liquids with the remaining space above occupied by vapour or a vapour/gas mixture. If such a pressure vessel were to be ruptured, missiles (i.e. fragments) may be generated and equipment in the vicinity put at risk. There is a particular threat from large missiles. Theoretical models have been developed to describe the peak Velocity achieved by end-caps and ＇rocket＇ missiles generated by the circumferential failure of a vessel. The end-cap missile model assumes that the action of the escaping vapour/liquid on the end-cap is analogous to a missile driven by a gas jet from a constant pressure source. The ＇rocket＇ missile Velocities are derived via a simple approximation to the impulse applied to the internal face of the closed end of the ＇rocket＇. Experiments have confirmed the validity of these approaches and upper limit values to end-cap and ＇rocket＇ velocities have been defined.