First of all, the rocket is moving in a straight line at a uniform speed, so the resultant force acting on the rocket must be zero. In the horizontal direction, because the gas friction resistance on the surface of the moon is almost zero, the rocket will not have power in the horizontal direction, otherwise it will accelerate in the horizontal direction. So the rocket is only subjected to the same recoil force as gravity in the vertical direction. Because the recoil direction is opposite to the air injection direction, the air injection direction must be vertically downward. ...