You've all had a good stab at it but not one of you has explained the rocket, a thrown rock transfers kinnetic energy from itself to the thrower in the form of enertia, not thrust, the thrust is created by the thrower, not the rock. Someone important to this arguement once said, "For every action, there is an opposite and equal re-action", which when applied to your rocket in a vacuum theory doesn't work. The gases do indeed puch against the rocket engine propelling it forward, but, as we know, in a vacuum there in no resistance to thrust and therefore a rocket would not move based on this principal, it moves not because it created thrust but because the pressure behind the engine is greater than that in front of it, oh, by the way, if insect spray could propel a rocket with any deliberation NASA would own the Mortein company, instead, they prefer to spend billions engineering propellants. Slice of Heaven, don't believe the Universe has a boundary because some theorist wrote it in a white paper, believe it because you know it to be true or question it, it wasn't that long ago that someone wrote a white paper on how the earth was flat and sailing near it's boundary would result in doom. To believe the vacuum of space is zero pressure in relation to our atmosphere then you would have to have knowledge of other atmosphere's that must be more negative than zero? It is just as conceivable that space is a negative vacuum as it is that our atmosphere is a positive one, to argue otherwise would be egocentric, the earth and it's atmosphere are not unique as much as we would like to think it is, our atmosphere is not a yard stick by which we should measure the pressure of space, perhaps the pressure of space (space being a tad more substantial than our little rock) is a yard stick by which we measure the pressure of our atmosphere? I'll keep an eye on this thread.