Before tracking the species evolution behind the shock, the Shock Reactor Model first determines the initial conditions immediately behind the shock, using gas-dynamic relations. There are different relations employed for incident and for reflected shocks. To facilitate the discussion, we first familiarize the reader with the two time-frames that are relevant to this system: the shock tube "laboratory" time-frame and the gas "particle" time-frame. Understanding the relationships between these two time-frames is important to the interpretation of experimental data and application of the gas-dynamic equations.