A computationally efficient model is developed for the coupled gasdynamics and kinetics in a pure source flow chemical laser. Average flow properties are calculated using the 1D equations for flow with area expansion and heat release. The flame sheet approximation is used to describe the mixing, and subsequent reaction, of the fuel and oxidizer streams. The gasdynamic properties of the mixed zone are assumed to be related in a simple way to the average flow properties. Application of the model to an HF laser indicates that the chemical efficiency of the laser is increased by increasing the nozzle exit Mach number, decreasing the initial source flow radius, decreasing the throughput, and increasing the mixing rate.