Combustion chamber

The combustion chamber is the place where chemical energy is transformed in kinetic energy.
There is generally two components as propellants : Fuel and oxidizer.
They are injected in the combustion chamber under very high pressure (because there is very high pressure inside the combustion chamber) and mixed then lit.
The chemical energy is then transformed in caloric energy, and temperature and pressure are very high. At the opposite of the injectors there is a hole in the combustion chamber.
The burned fuel can escape through it because it's a very small pressure zone.

This is a very dangerous process, with temperatures and gas speed far above industrial standard.
So the failure rate is high!

There is a very simple and awfully imprecise way to calculate the force given by a combustion chamber.
Pressure is force divided by surface.
Here surface is the surface of the hole in the combustion chamber.
Pressure equal pressure inside the combustion chamber minus pressure at the end of the nozzle.
Example :
1MPa - 3kPa =~1MPa
if surface is 1m2 (for the sake of clarity)
force is ~1MNewtons, approximatively the weight of 100 metric tons.

This kind of reasoning is inadequate because it's assuming too much simplifications but it help understanding how it works.
One big mistake is that we assume that the gases in the combustion chambers were at the equilibrium, nothing can be so false, instead they are rushing from the injectors to the nozzle at Mach 25!

More real description of a conventional rocket motor (courtesy Andrew Novicky)

Profile of liquid propellant rocket engine