Conservation of momentum, the end.
If you say conservation of momentum doesn't apply to gasses, don't planes break conservation of momentum, the plane gains momentum yet nothing can gain momentum to cancel it out.
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Conservation of momentum is a fundamental law of physics which states that
the momentum of a system is constant if there are no external forces acting on the system. It is embodied in Newton's first law (the law of inertia).
http://scienceworld.wolfram.com/physics/ConservationofMomentum.html
So, what is the external force on a rocket (system)? Or is the cited definition incorrect?
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Here is the Wikipedia definition:
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In a closed system (one that does not exchange any matter with its surroundings and is not acted on by external forces) the total momentum is constant. This fact, known as the law of conservation of momentum, is implied by Newton's laws of motion.
A rocket is clearly not a closed system according to this definition. So, the conservation of momentum does not apply.
Even if it did, you would still get no motion without an external force acting on the system.
Unless both definitions are incorrect?