Nozzles and exit cones The basic elements of a solid rocket motor are a combustion chamber (which also acts as a propellant storage tank), an igniter and a nozzle. The main function of the nozzle is to convert the heat energy produced in the combustion chamber into kinetic energy. It does this by transforming the low-speed, high-pressure and high-temperature gases produced in the combustion chamber into a gas that is ejected at high speed, low pressure and lower temperature. The thrust is optimal when the pressure at the outlet from the exit cone is equal to the ambient atmospheric pressure: this condition imposes a particular shape and size on the nozzle, but varies depending on the altitude.
Other criteria have an impact on nozzle design:
- type of propellant, since some erode the nozzle surface more than others,
- burn time, which may require some provision for cooling,
- the vector thrust control system employed,
- size limitations, particularly for upper stages or for embedded motors,
- the operating environment: the presence of other motors operating at the same time, or passing through a depth of water (for submarine-launched missiles),
- specified performance levels,
- reliability,
- cost.
The inner surface of the nozzle, known as the gas stream profile, comprises three main sections:
- The convergent section: located between the combustion chamber and the throat, the gradual reduction in cross-sectional area accelerates the gases.
- The throat: where the diameter of the nozzle reaches its smallest value, and where the thermo-mechanical stresses reach a maximum.
- The exit cone: increases the velocity of the gases as they expand. Metal exit cones must be cooled during operation, which adds to the total mass of the motor. One solution to size constraints, but also to adapt the length of the nozzle to the outside pressure, is to use an extendible exit cone, formed from concentric rings that can be deployed like an optical telescope.
The higher the ratio of the cross-sectional area of the exit cone outlet to the throat, the greater the performance of the motor; hence the benefit of using an erosion-resistant material. Initially, thermostructural carbon/carbon composite materials were used for the nozzle throat section of solid propellant motors only. They simplify the structure of the nozzle, reduce the weight, and improve reliability. This material has now been proven to be suited to the construction of the entire nozzle, including the exit cone.
The flexible bearing nozzle
The flexible bearing is an essential component of the nozzle. It is made of alternate elastomers and metallic or composite shims. When a force is exerted by two actuators arranged perpendicular to each other, it can pivot around a point defined by its spherical geometry and can alter the thrust axis of the rocket motor, offering a thrust vector control angle of up to 7 degrees.
For the first two minutes of the mission the Ariane 5 nozzles can steer the launcher by making a series of adjustments to ensure that the launcher follows the correct trajectory.
