A nozzle is a pipe or tube of varying cross-sectional area at the outlet of a rocket engine which is used to transform the energy from combustion gases into kinetic energy. It is the outlet of a combustion chamber.
A solid rocket nozzle has two purposes: using the conical section of its neck it controls the combustion of the block of powder, and using the shape and size of the divergent part it contributes towards creating the propulsive force due to the more or less extensive expansion of the gases.
It can also have a role in in steering the launcher.
There are three parts to a nozzle:
• the convergent zone;
• the neck zone;
• the divergent zone or skirt of the nozzle, eventually fitted with so-called nozzle flaps.
Because of the high temperatures (around 3500°C) and the need to keep a constant section, the neck of the nozzle is made from refractory material (graphite, carbon, tungsten).
For the divergent zones, metal or ablative laminates can be used.