The combustion chamber of a rocket engine is not an environment where gas is freely expanding.
The only place you could get free expansion, hypothetically, is in a true vacuum.
Anything close to that does not allow FREE expansion, although it does allow a much faster expansion of gases against much less resistance of reactive pressure.
Fuel and oxidizer are burned at such a high rate that the expanding gasses are forced out of the chamber faster than they would normally escape if the combustion was not occurring.
Yes they are forced out faster. I agree, but the issue is about what's happening inside the chamber.
You see, the gases in a sealed chamber would expand and explode if there was no outlet. Like a handgrenade with the pin pulled out. It blows apart and throws shrapnel in ALL directions because the expansion of gases had no seperate outlet.
Now imagine that hand grenade having the pin pulled out but also having a large hole in the top. There's your rocket.
It ignites and expands, except that the expanded gases have an outlet and you would see a spray of burning gases and probably a handgrenade flapping about around the place as it hits ATMOSPHERE and exapnds it due to it's ejected HOT gases expanding the compressed (colder) air around it which reacts in qual measures to compress back, causing the hand grenade in the opposite direction to its ejected flame.
This is how a rocket works, except much more thrust. When I say rocket, I mean much smaller rockets than the ones we are bullshitted with, as in fantasy space rockets.
Think about a water hose. If the water pressure is low, then the water just dribbles out on its own and its easy to handle. Now if you add a high pressure pump, then the water is forced out quite vigorously and the hose becomes much harder to handle.
It becomes harder to handle because you are throwing much denser water at the atmosphere and the atmosphere is being compressed out of the way of the thrustng water. The atmosphere reacts in equal measures by squeezing back onto the water being ejected out of the hose and this is what pushes you back.
Think about it rationally.
If your hose was shooting out barbed wire water thn it would grip the inside of your hose and carry it forward not push it back against you.
Remember that water is moving forward pushed by the pump. The water in the hose cannot push back against you if it's being pushed forward.
The only way you can be pushed back is by the atmosphere friction gripping the more dense water to try and equalise the presasure put into it. Nature has to always equally react to an action in equal measures and it includes equalising pressures or attempting to.
As long as energy is applied against atmospheric pressure, then atmospheric pressure will always try to equalise against that energy by fricton squeezing it.
No different to a rocket burn, except for the actual means of expansion of atmosphere and energy applied.
Same thing with gasses in a rocket engine:
As above.
low pressure = free expansion; very high pressure = thrust.
Low pressure = low expansion; very high pressure = very high expansion as long as it's done in atmospheric conditons.
In your space, low pressure = free expansion; very high pressure = free expansion for anything sealed in a container against a vacuum of any pressure and allowed to be released. It would all be FREE EXPANSION.