We investigated the influence of femtosecond short pre-pulses on the expansion of thin foils for laser-ion acceleration. By analysing the hydrodynamic simulation results, we develop a simple numerical model to describe the evolution of foil as a function of intensity I0 and time t at which pre-pulse impacts the foil prior to the main pulse. We find this investigation particularly helpful for the new generation of laser ion accelerators and the respective quantification of temporal contrast requirements in the era of Petawatt laser driver.