This connection between acceleration direction and net force direction is often misunderstood by beginning students of Physics causing great trouble in their understanding of the force-motion connection. It states that the time rate of change of the momentum of a body is equal in both magnitude and direction to the force imposed on it. For a body with a constant mass m force is given by.
F ma where F Force m Mass and a Acceleration.
The rate of change is the speed of the body. According to Newtons second law of motion force acting on a body is equal to the rate of change of momentum. As the fuel is used up the mass m decreases but the force does not so we expect a to grow larger. Velocity is a vector quantity having both magnitude and direction while speed.