To investigate effects of wall thermal conditions and small gaps between rockets and launch canisters on exhausted jets, a computational fluid dynamics (CFD) method was applied to simulate jet flows with different conditions. Results show that the small gaps between rockets and launch canisters influence the exhausted gas flows significantly with the rocket positioning in the canister due to the effect of ejected air from gaps. The maximum difference exists in the field near the nozzle outlet and the pressure error is about 10% between simulation models with or without gaps. The uniform temperature wall condition decreases the flow pressure and temperature about 5% referencing the models with adiabatic condition and coupled condition.