Apply Newton's third law. F=ma. (Force of ship) = (Force of propellant) Think of this as recoil in a gun. Yes it works the same in space. Gravity doesn't affect recoil. So plugging in mass and acceleration in the above equation you get: (Mass of ship)*(Desired acceleration of ship) = (Mass of propellant)*(Acceleration of propellant). In space, going in a straight line, and not worrying about orbital mechanics, you would need either a ton of fuel to propel the ship or some propellant that moves crazy fast in the opposite direction of the ship. What you may be talking about "once in space" has to do with conservation of momentum, or Newton's first law. You won't be able to slow down that giant ship if you can get it going AND there's no opposing force.