A simplified mathematical model is presented for the unsteady flow process of starting a transonic Ludwieg tube wind tunnel. The hardware modeled consists of a porous-walled test section surrounded by a plenum chamber with an exhaust system independent of the tunnel's main starting valves, which are located downstream of the diffuser-test section. In the present method, the hardware is modeled as three control volumes: the plenum, the test section, and the diffuser. The plenum is treated with the unsteady integral continuity equation with one-dimensional influx or outflux through the porous wall, through the plenum exhaust system, and through the flaps, which exhaust into the diffuser. The other two control volumes are treated with the steady integral continuity equation and a steady, adiabatic, one-dimensional energy equation whose stagnation conditions vary in time according to the classical solution for an unsteady expansion wave. Numerical solutions are compared with experimental pressure-time histories of a small, transonic, high Reynolds number tunnel referred to as HIRT. Agreement between the model and experiment is good. (Author).