To maintain their trajectories, reentry vehicles use reaction control systems (RCS), which consist of jets of air, shot out from the craft as it descends through the atmosphere. The jets of air twist and turn the craft, changing its orientation and flight characteristics. Errors in reentry trajectory can over heat the craft, send it off course, or subject the passengers to excessive accelerations. To predict the behavior of RCS, the interaction between the RCS jets and the airflow around the vehicle needs to be better understood. The RCS jet is a high density, sonic airflow, which is modeled using continuum dynamics. The airflow around the reentry vehicle is a low density, hypersonic flow, which is modeled using ratified gas dynamics. Neither model can represent the interaction between the continuum and rarified flows. Computer models, which take into account the airtlow in the interaction region, are in development at NASA Langely. However, there is not sufficient experimental data to verify the accuracy of computer models. To obtain experimental data and a better understanding of the interaction region, a wind tunnel that mimics reentry conditions and simulates RCS jets has been designed and built at the University of Virginia. The wind tunnel has been equipped for use with flow visualization and optical measurement techniques. In the future the technique of planar laser induced iodine fluorescence will be used to obtain quantitative data on the temperature, pressure, and velocity in the flow field. This paper covers the first phase of this research; the design, building and calibration of the tunnel. In addition to constructing the tunnel facility; the data and calculations obtained from the tunnel calibration will be useful to future researchers who wish to operate this facility. Researchers wanting to run the tunnel will have a graphical representation of the calculations to allow a fast and efficient means of performing hypersonic research. Additionally the flow visualization techniques have provided qualitative pictures of the flow field. © 1999 by the American Institute of Aeronautics and Astronautics, Inc. All Rights Reserved.