And gasses follow the path of least resistance from higher to lower pressure.
Why do gasses move from higher to lower pressure.
Hint: It has something to do with forces acting on the gas.
Extra Hint: If a force acts on the gas, what does newtons third law tell us?
Wtf?
Extra extra hint: For anything to move away from something containing it, it has to accelerate away.
Wtf?
If something is sitting stationery, then it moves away from something, clearly it has gained velocity.
I'm practically telling you the answer, come on.
Wtf?
http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node33.html
Are you totally ignorant of all the "Laws of Motion"?
Just look at the illustration from your own reference (you did read it?).
Between the first and third diagram, the gas clearly moved, as shown by the arrows in the middle diagram.
Now, as you have asserted,
gas has mass, so the
centre-of-mass of the gas moved to the right, from the centre of the left box to the centre of the pair of boxes.
Hence, to keep the
centre-of-mass of the whole system unchanged, the
centre-of-mass of the pair of boxes has to move left.
Therefore Joule-Expansion or free-expansion can cause motion. Now here, the whole system was just the isolated boxes but for a rocket the system can include as much of your
infinite-vacuum-of-space as you like. As in this previous post:
Joule-Thomson Expansion - Free Expansion of a Gas
Imagine a gas confined within an insulated container as shown in fig 1. The gas is initially confined to a volume V1 at pressure P1 and temperature T1. The gas then is allowed to expand into another insulated chamber with volume V2 that is initially evacuated. What happens? Let’s apply the first law.
We know from the first law for a closed system that the change in internal energy of the gas will be equal to the heat transferred plus the amount of work the gas does, or ∆U = Q + W. Since the gas expands freely (the volume change of the system is zero), we know that no work will be done, so W=0. Since both chambers are insulated, we also know that Q=0. Thus, the internal energy of the gas does not change during this process. | | 
Fig 1 Expansion into box
from reference below |
From: Joule Thomson
Now, how does this relate to a rocket in space?
For an ideal gas, free expansion does no work, but what does this mean? It is simply that the temperature of the gas is unchanged during the expansion. But, the upper diagram does not represent our rocket in free space. The right half of this should be replaced by "the infinite vacuum of space", more as in fig 2.
Joule-Thomson expansion simply says that the temperature of an (ideal) gas does not change, but this in no way affects Newton's Laws of motion. The whole system is the rocket plus the "near-infinite vacuum of space".
There is nothing in the Joule-Thomson free expansion to "countermand" the momentum of the gas heading right (in the lower diagram) imparting like momentum to the rocket heading left.
As obviously expected there is no conflict between Newton's Laws and the Joule-Thomson free expansion. | | 
Fig 2 Expansion into space
modified from reference |
The gas in these illustrations would have very little mass, but the Saturn V exhaust flow rate averaged over
12,800 kg/s at a velocity of about 2400 m/s.
Now this might be impossible for one like
Poor Pathetic Puppet Papa to comprehend, but real people might cotton onto the
fact that rockets really do work in the infinite vacuum of space.