Newton 2 is about momentum. Conservation of momentum.
Suppose you are on s frictionless surface - you are on blades on ice, say. Now you throw a heavy ball in any direction.
After you have recovered your balance you will me moving slowly backwards.
Why? You gave the ball some momentum in some direction. That momentum has to come from somewhere. It comes from you the thrower. That is why you are now moving slowly in the opposite direction of where you threw the ball.
That there is any air has nothing to do with it.
Quoting: MrZephyros Read carefully, I don't want you to miss this.
In the universe/omniverse there can never be a place where there is no atmosphere and no vacuum, it's has to be one of the two.
First to begin, let me correct your comment.
1) We are dealing in space not earth.
2) There is a vacuum not an atmosphere.
3) The rocket is not throwing a heavy ball anywhere, it is exhausting burnt fuel in the form of an expanding gas.
So we go on, the expanding gas is needed as a pressurised from of propellant in order to propel the rocket forward as it tries to relieve pressure by flowing through a designed nozzle (and the gas is not forced out in the same essences as anybody throwing a heavy ball). Because the section of the chamber facing the rocket is still subjected to pressure, it will force the rocket forward.
A vacuum however, will aid (I don't like to say suck) the gas in it's escape out of the chamber and increase expansion when gases exit the chamber. This will rob the engine of force needed to thrust the rocket forward. Hence this concept will not work in the vacuum of space.
