Hello Reader!! Welcome to another article on Rocket Science. In this article, I will discuss the Solid Propulsion System. In the previous article, I discussed some basics of Rocket Propulsion. If you are new to Rocket Propulsion, please go through the previous article for better understanding.

What is Solid Propulsion System?

A solid propulsion system is a rocket propulsion system that uses propellants in solid form.

In the first article, we discussed the similarity between a firework rocket and a rocket that is used for space exploration. A firework rocket can be considered as the simplest form of solid propulsion rocket. Here, Black Powder is used as a propellent. A paste of black powder and binder is inserted into the frame and it becomes solid after drying. This is ignited with the help of Magnesium Wire. Burning of black powder generates hot gas which is exhausted through a small vent in the downward direction and propels the rocket in the upward direction.

In the case of a heavy rocket, different propellants are used and they are ignited with the help of an Ignitor. The generated hot gases are exhausted through the rocket nozzle with a high velocity generating upward thurst.

Parts of Solid Rocket Motors:

A Solid rocket motor consists of 3 basic parts:

1.Casing

2.Propellant Grain

3. Igniter

4.Nozzle

Casing

The casing is the cylindrical outer shell of a Solid Rocket Motor. It contains the propellant grain. The burning of propellant grain takes place inside the Casing. So, the casing can be considered as a Combustion Chamber.

Propellant Grain

Fuels and Oxidizers are mixed with some additional components (catalysts and binders) to make a paste and then the paste is given a particular shape and heated. Heat sets the paste and makes solid propellant grain.

If the paste is set in the casing of the rocket, the grain is termed as Case Bounded Propellant Grain.

If the grain is prepared in a different case and then inserted to the Casing of Rocket, the grain is termed Free Standing Propellant Grain.

The geometric shape of the propellant grain is very vital for the performance of the rocket motor. We will discuss the Geometry of propellant grain after some time.

Ignitor:

Ignitor is used to ignite the propellant grain. There are different types of ignitors used in Solid Rocket Motors. The most popular type of ignitor is Pyrotechnic Ignitor. In this type of ignitor, pyrotechnic powder (materials that are easy to ignite) is used to generate a flame. Electricity is passed through a Nichrome wire to heat it and ignite the pyrotechnic powder. The pyrotechnic powder creates flame and ignites the propellant grain.

Nozzle:

The generated hot gasses are exhausted through a Convergent-Divergent Nozzle with a high velocity.

Geometry of Propellant Grain:

The generated upward thrust produced by a Solid Rocket Motor depends on the exhaust velocity of the exhaust gases. The exhaust velocity depends upon the pressure inside the chamber. Chamber pressure depends upon the area of the burning surface.

Usually, the outer surface of the propellant grain is cylindrical. The inner surface varies according to the requirement. For different types of inner surfaces, the areas of the burning surfaces are different. So the generated thrusts are also different for different grains.

So, the shape and geometry of propellant grain are designed according to the thrust requirements. But after the ignition, the thurst can not be regulated.

Solid rocket motors are usually used in the Booster Stages of different Launch Vehicles. In the 1st stages (Booster Stages) of PSLV and GSLV, solid rocket motors are used.

In some versions of PSLV, 6 Solid Rocket Strap-On Motors are augmented with the 1st stage.

In GSLV, 4 Liquid Propulsion Strap-Ons are augmented with the 1st stage.