The High Speed Flight Demonstration Phase II (HSFD Phase II) is a flight test program, which is planned to acquire transonic aerodynamic characteristics and accumulate autonomous control technique of HOPE-X, a Japanese re-entry vehicle which has a large base.1 The aerodynamic analysis in the HSFD Phase II program consists of flight tests, wind tunnel tests and CFD analyses. The objective of this program from the aerodynamic point of view is clarifying the difference between the results of those three kinds of analysis and evaluate validity (or invalidity) of conventional ground testing methods in estimating the aerodynamic characteristics of the vehicle with a large base. In this program, a highly unique method is hired to accelerate the experiment vehicle. It will be pulled up to high altitude by a stratospheric balloon. Then vehicle released from there will accelerate due to the gravity force to transonic speed under the control of a onboard flight control computer. Before the flight tests, the pre-flight wind tunnel test was performed. The main objectives of it are mainly two. The first one is validation of the aerodynamic model built with the wind tunnel test of HOPEX because external deviations from original HOPE-X configuration, such as a long ADS probe and GPS antennas are added to the HSFD vehicle. The other is eavluation of various model support systems such as a straight sting support, a dorsal support with blade strut above the model and a ventral support with blade strut underneath the model, to find the good support system to reproduce base flow in the flight condition and to acquire the reference data of the flight test. When the blade strut was used, a dummy sting was able to be added to reproduce the base flow in the straight sting support configuration. The wind tunnel test was performed in the 2m × 2m transonic wind tunnel of the National Aerosapce Laboratory of Japan (NAL-TWT1) and the test Mach number was M∞ = 0.8, 1.05 and 1.2. It is validated that the external deviations have little effect on the aerodynamic model and acquired a good knowledge about the support systems. The results of the preflight wind tunnel test are examined in comparison with the CFD computation as well. This HSFD program is promoted in corporation among NAL (National Aerospace Laboratory of Japan), NASDA (National Space Development Agency of Japan) and CNES (Centre National d'Etudes Spatiales) of France.