Rocket Engine Combustion

The rocket engine combustion is the violent chemical reaction that occurs in the machine combustion chamber. It produces hot gases which are necessary for the propulsion of a rocket. In other words, the energy built up in the rocket engine for propulsion purposes arises from the thermochemical energy of the propellants. This means that their chemical reactions (combustion) is exothermic and it is accompanied by the production of large quantities of gases at high temperature. Since the fuels utilized are usually hydrocarbons, the byproducts of the combustion generally contains carbon dioxide (CO2), carbon monoxide (CO), and water (H2O) as its main constituents.

Both the temperature reached by the reaction and the composition of the reaction byproducts depends to some extents on the mixture ratio (oxidizer weight/fuel weight) as well as on the phenomenon of dissociation. The condition for chemical equilibrium between the reaction byproducts determines the final gas composition.

The chemical reaction of the propellants does not go to completion due to the high temperature, which is produced, is not high enough. Therefore, some of the carbon dioxide present dissociates into: CO2 = CO+½O2, while the water vapor dissociates into: H2O = H2 + ½O2. If the temperature is allowed to become sufficiently high, the dissociation results in the further breakdown of these products. Thus, at 4,500° F, approximately, the water vapor dissociates into H + OH. And at 7,200° F, it dissociates into H + H + O. At temperatures above 9,000° F, no molecular components are present in the reaction products.

As a result of the dissociation phenomena, the maximum temperature which can be reached by the combustion gases is limited. The amount of dissociation occurring will depend mainly on the predominantly temperature and pressure. Thus, in general, it can be said that the higher the temperature, the greater the dissociation; and the higher the pressure, the less the dissociation is going to be.

Below, a drawing of a rocket engine combustion chamber, with its parts.