Turbofan
Sir Isaac Newton's three laws of motion should explain this. Also it's patently bleeding obvious :-}
The momentum of an object is constant unless an outside force acts on that obejct.
If the spacecraft is in the atmosphere, it requires thrust to maintain it's velocity or to accelerate. Just like an aircraft, in steady level flight, thrust = drag
In space....no one can hear you scream. And of course a body will continue in a straight line and at a constant velocity unless outside forces, (gravity, asteroids, green people) influence it.