Recently, several laser propulsion vehicles have been launched successfully. But these vehicles remained in a very low subsonic flight. Laser‐driven In‐Tube Accelerator (LITA) is developed as unique laser propulsion system at Tohoku University. In this paper, flow characteristics and momentum coupling coefficients are studied numerically in the supersonic condition with the same configuration of LITA. Because of the aerodynamic drag, the coupling coefficient could not get correctly especially at the low energy input. In this study, the coupling coefficient was calculated using the concept of the effective impulse. © 2004 American Institute of Physics