A High-Speed Flight Demonstration (HSFD) program using scale vehicles is planned as part of NAL/NASDA joint research for the HOPE-X unmanned re-entry vehicle project. The program consists of two phases, and the objective of Phase II is to estimate the transonic aerodynamic characteristics of the HOPE-X configuration. The experimental method for Phase II is highly unique: the experiment vehicle is to be lifted to high altitude by a stratospheric balloon, from which it will be released and accelerate in free fall. During data acquisition, the vehicle will fly at a constant Mach number while changing its angle of attack quasi-statically. The vehicle's aerodynamic characteristics over a wide range of angles of attack will then be estimated from the flight data. After data acquisition, the vehicle will be recovered using parachute and airbag systems, and refurbished for the next flight. The flight of the vehicle is fully autonomous under the navigation, guidance and control of its on-board computer. The aerodynamic data obtained through the flight experiment will be used to reduce the uncertainties in the HOPE-X aerodynamic database, which are greater for the transonic region than for other speed regions, and to contribute reference data to improve wind tunnel testing and CFD (Computational Fluid Dynamics) technologies. The Phase II flight experiment will be conducted in collaboration with the Centre National d'Etudes Spatiales (CNES) of France, which will develop and operate the balloon system. Flight test data analysis will be conducted by