BAM!!!
The combustion chamber of a rocket is open to the near-infinite vacuum of space.
Therefore, no gas can even be meaningfully said to exist within it, let alone combust.
Any gas introduced therein when the pressurised fuel tank leading to the combustion chamber is opened will simply expand freely into the enormous, zero-pressure vacuum, following the path of least resistance & doing no work whatsoever.
This will continue for as long as the fuel tank & chamber are open to the vacuum, until both exterior & interior pressures are equalised at zero.
It is a beautifully simple concept, fully supported by All the laws of physics, yet you 'round earthers' (lol!) just can't seem to grasp it...
Plus this:
You all claim that the recoil of a gun is a valid analogy for how a rocket works in a vacuum.
Here is why it is not:
With a gun you have object A, the mass of the gun; the expanding propellant, P, the gunpowder, sited between them; and object B, the mass of the bullet.
But with a rocket you ONLY have object A, the mass of the rocket, & the expanding propellant, P, the fuel.
No object B, see?
Thus, you have removed the necessary recoil mass required to produce motion.
But we know a rocket DOES produce motion, don't we?
Ergo, some other mass MUST be taking the place of object B.
& the ONLY possibility for that other mass is the Atmosphere.
Ergo, NO atmosphere, NO motion; rockets CANNOT function in a vacuum.
Q.E.D.
No matter how hard you try to spin it, cultists, every child knows that You cannot Push on Nothing.
No maths required; only common sense.
Then there's the fact that you are all trying desperately to confine this 'debate' to the wrong branch of physics, i.e. Solid Mechanics rather than Fluid Mechanics...
Kinda dishonest of you, dontcha think?
Pressure-Gradient Forces, Gas Laws, Fluid Mechanics, Continuum Assumption & Joules Expansion are the areas I suggest neutral readers research.
Oh; & Thermodynamics too - thanks for that, markjo!