Abstract: During vehicle launch, the multiphase flow and load characteristics are significantly affected by the structure of the tube outlet and the flow field environment. This paper analyses the effect of initial and boundary conditions on the evolution of the air masses attached to the vehicle and the load characteristics of the flow field. The results indicate that during transverse traction velocity, the vehicle's head departs from the launching process with the air mass offset, broken, separated, and converged. Additionally, the high temperature and high-pressure air mass of the vehicle's tail fuses with the surrounding attached air mass upon exiting the tube, resulting in the phenomenon of re-entrant flow after expansion and necking. The formation of the tail cavity causes the air mass at the outlet cross-section of the second pull-off to experience the jet offset phenomenon. And the launch tube experiences a 'water hammer' effect in an inverted 'spiral' manner. Furthermore, A 50% reduction in the transverse traction velocity results in a 58.27% reduction in the peak load on the cylinder cover. The initial volume of the outlet cavity affects the size of the air masses attached to the vehicle. Increasing the height of the outlet cavity by 50% reduces the peak load on the cylinder cover by 17.64%. The loading characteristics of the flow field are influenced by the shape of the cover. Reducing the bending factor of the cover by a factor of one reduces the peak load on the cover by 31.48%. The primary cause of the second pull-off of the air mass and the deflection of the jet is the opening angle of the cylinder cover, and reducing the angle from 60° to 30° results in a 72.03% reduction in peak load.