Abstract: The invention disclosed herein provides a subsonic wind tunnel capable for accurately maintaining a desired Mach number, pressure and temperature for use in aerothermal testing of materials. The invention allows the Mach number to be precisely controlled in the test section of the wind tunnel by employing a restricted outlet acting as a sonic throat for the wind tunnel. In the preferred embodiment, the restricted outlet is constructed to be variable in cross sectional area allowing a range of Mach numbers to be tested. The variable outlet is varied during the operation of the wind tunnel so that an actual trajectory with changing Mach numbers, pressure, and temperature is simulated.