An unsteady quasi- ID numerical simulation model has been developed in order to predict the behavior of large solid motors during the ignition transient. In particular, this model is finalized to be used as a numerical tool during the preliminary design phase, when no information about the future behavior of the motor is available. An Euler flow model has been adopted coupled with suitable semi-empirical models that take into account the main phenomena affecting the ignition transient. Special attention has been devoted to simulate the effects of the impingement of the igniter jets on the grain propellant surface (heating, ignition and combustion of the impingement region). A radiation model is also proposed. The simulation model has been extensively tested and the numerical results have been compared with the experimental results obtained for largely different motor concepts and configurations. Significant information about the role of some phenomena affecting the ignition transient has been also deduced from the critical analysis of these results.