Why Do Rocket Engines Have Nozzles?

Nozzle design is the most important factor in the generation of rocket thrust and overall rocket engine performance. As opposed to jet engines that use the oxygen in air to oxidize fuel, rocket engines carry their own oxidizer aboard the vehicle, usually in the form of liquid oxygen in liquid fuel engines, and solid oxidizer such as ammonium perchlorate in solid fuel rocket engines. This added weight makes it imperative that every last bit of thrust is extracted from the rocket engine during its relatively short run.

Rocket Nozzle Operation

Rocket engines generate thrust according to the physics formula force equals mass times acceleration, where mass defines the weight of the hot gases leaving the engine and acceleration defines the net change in hot gas velocity as they proceed from the inside of the engine to the environment outside. This is similar to the velocity of water through a constricted garden hose being much greater than that from a hose with no nozzle.

Converging and Diverging Nozzles

Rocket engines use both converging and diverging nozzle sections to derive the greatest thrust from the combustion of fuel and oxidizer within the rocket motor combustion chamber. The combustion results in extremely high temperatures and pressure within the chamber and the gases rapidly seek the path of lowest resistance to exit, which is through the exit nozzle.

Convergent Section

The first section of the rocket engine nozzle is the convergent section, where the nozzle smoothly constricts to a narrow throat where the hot gases achieve a maximum velocity of Mach 1, or the speed of sound through the hot gas, which is much higher than the velocity of sound through ambient air.

Divergent Section

After the throat, the engine nozzle widens again into a concave bell or divergent section, where the rapidly moving gases expand. Total energy in the form of temperature and pressure are converted into kinematic energy and the gases leave the divergent bell section at a velocity many times greater than the speed of sound.

Dispelling Misconceptions

The thrust of a rocket motor is derived purely from the inertial gain of phenomenally accelerating the mass of the extremely hot and high pressure combustion gases through the convergent and divergent sections of the rocket engine nozzle at extremely high velocities. Contrary to common belief, thrust does not come from the flames pushing against the ground or the air behind the rocket.

Summary

Rocket engine nozzles convert the energy from high temperatures and pressures to inertial energy in the form of extremely high velocities to propel rockets into space and beyond.

References

• NASA Rocket Advances