Fuel reacts with an oxidizer to release chemical energy. Common fuels include hydrogen, methane, and kerosene; liquid oxygen is a common oxidizer. Together, they are called propellants. The rocket carries both, so combustion does not depend on oxygen from the air.
Tank pressure or pumps push the propellants toward the engine at a pressure higher than the combustion chamber. Valves control the flow. An injector distributes and mixes the incoming streams so they can burn steadily inside the chamber.
Once ignited, the reacting propellants form extremely hot gas. Continuous feeding and burning maintain high pressure in the chamber while gas escapes through the nozzle. In normal operation, this is a sustained, controlled burn.
The nozzle narrows to a throat, where the gas reaches the local speed of sound during normal operation. Beyond it, the widening bell lets the gas expand and accelerate to supersonic speed. Its pressure and temperature fall as thermal energy becomes directed motion.
The engine pushes gas backward, and the gas pushes the engine forward with an equal and opposite force: thrust. Gas pressure acts on the chamber and nozzle walls, transmitting that force into the rocket. No ground or surrounding air is needed to push against.
The chamber and bell guide gas rearward; the reaction pushes the rocket forward. Carrying its own oxidizer lets it keep burning even in a vacuum.
Thrust depends on how much mass leaves each second, how fast it leaves, and the pressure difference between the nozzle exit and its surroundings.
Liquid bipropellant engines keep fuel and oxidizer separate until they enter the engine. Solid motors combine both in a solid propellant. Hybrids typically pair solid fuel with a liquid or gaseous oxidizer. All produce hot exhaust to generate thrust.
The gas can be hotter than the engine materials can withstand. Many liquid engines circulate fuel through channels in the chamber and nozzle walls before burning it, carrying heat away. Other designs use protective layers that gradually erode, or radiate heat into space.
A wider exit relative to the throat allows more expansion. Vacuum engines often have large bells to use the low surrounding pressure. At sea level, too much expansion can make the flow separate from the walls. Nozzle shape is matched to the operating conditions.
At liftoff, upward thrust must exceed the rocket’s weight to accelerate it upward. As propellant is used, the vehicle gets lighter. Many rockets steer by pivoting their engines or nozzles, changing the direction of thrust.