The air-turbo ramjet expander-cycle (ATREX) engine has been proposed for the main propulsion system in the Japan's TSTO space plane concept. The ground firing tests of its prototype core engine have been carried out successfully and its development study is now in the phase of planning flight demonstrations. This paper describes the concepts and the flight capability analysis of the flight demonstrator vehicles for the ATREX engine. Performance and weights of a subscale engine of a compressor fan diameter of 500mm are evaluated on the basis of the near-term technology. Vehicle configurations for carrying a single engine as well as twin engines are proposed. Their weights are evaluated by summing up their primary components designed preliminarily on the basis of the current technology. Their aerodynamic coefficients are predicted by a combination of the Newtonian approximation and Mach number dependence evaluated with DATCOM. Three-degree-of freedom flight trajectory calculation reveals that the single-engine configurations need a structural weight reduction for realization of a full demonstration up to Mach 6. On the other hand, the twin-engine configuration is found to be capable of carrying out a full demonstration, although a certain amount of its structural weight must be replaced by fuel for realization of its return flight.