Again, the rocket ignition never occurs in a vacuum; but the exhaust gas from the thrust chamber can be expelled into a vacuum.
Going back to my original post, the second link explains the de Laval nozzle. I will summarize:
In the thrust chamber after ignition, the pressure and temperature reach equilibrium. As the flow of gases moves toward the throat, it accelerates to the local speed of sound, which is roughly proportional to the square root of temperature. Then, after the throat, there is the nozzle, which keeps the flow captured from boundless space. As the flow moves down the nozzle, the pressure and temperature get lower, and the exhaust accerates until it reaches it maximum velocity, as described at the link.
The mathematics of this nozzle date back to 1880’s, and allows for zero exit pressure.
Yes, that is exciting but how can it be tested in a true boundless and absolute vacuum? How could a nozzle have more output than a boundless absolute vacuum?
I suppose the question is, what is the construction of this sort of nozzle that would withstand the pressure of a boundless infinite vacuum?
It’s all interesting stuff, thanks for your posts.