In an open system propulsion system, such as a rocket engine, the combustion chamber is not pressurized using the surrounding environment. Instead, the combustion chamber is pressurized by using a propellant combination, typically a fuel and an oxidizer, that undergoes a chemical reaction to produce high-pressure gases. These gases are then expelled through a nozzle to create thrust and propel the rocket.
The process begins with the introduction of propellant components into the combustion chamber, which are ignited to initiate combustion. The chemical reaction generates high-pressure and high-temperature gases. The expansion of these gases creates a force, known as thrust, which is the principle of rocket propulsion.
The thrust generated by the expulsion of gases in one direction (action) creates an equal and opposite reaction force that propels the rocket in the opposite direction. This is governed by Newton's third law of motion.
Regarding the mass of the flame, it is important to note that the mass of the combustion products and exhaust gases is not zero. The mass is determined by the mass of the propellant consumed during the combustion process. According to the law of conservation of mass, the total mass of the system remains constant, so the expelled gases have a nonzero mass.
The equation you mentioned, F = m × a (force equals mass times acceleration), is valid in the context of Newtonian mechanics. In the case of a rocket engine, the acceleration (a) is not zero, as the high-velocity expulsion of gases generates a significant acceleration. This acceleration, along with the mass flow rate of the expelled gases, determines the thrust produced by the rocket.
In summary, the pressurization of the combustion chamber in a rocket engine is achieved through the chemical reaction of propellant components, not by relying on the external pressure of space. The generation of thrust and the expulsion of gases follow the principles of action-reaction and conservation of mass, allowing the rocket to accelerate through the expulsion of nonzero-mass gases.