The Army Tactical Missile System (ATACMS) has been fielded with the US Army for almost 20 years as a deep strike precision weapon, capable of engaging time critical targets at high precision under all weather conditions. This paper will describe the use of the HWIL simulation in the design of the missile, validation of the simulation, the role of the simulation in the production and test process, how the simulation is used to support system shelf life and obsolescence issues and how the simulation is used to answer critical user questions on the performance of the system.