The rocket propellant is the chemical mixture burned to produce thrust in rockets and consists of a fuel and an oxidizer. Fuel is a substance that can be in a liquid or solid state, burns when combined with oxygen producing gases for propulsion. An oxidizer is an agent that releases oxygen in combination with a fuel.
Rockets propellants can be classified according to their state as
- Solid propellant
- Liquid propellant and
- Hybrid (a mixture of solid and liquid propellant).
Let us see in brief about solid and liquid propellants of the rocket.
Solid Propellant Rockets:
The solid propellant rocket motor consists of a seamless tube, usually made of steel, closed solidify at one end. The open end holds the nozzle which may be a single or multi-orifice type.
The solid propellant rockets are divided into two main types according to the amount of surface area exposed to burning. These two types are:
- The restricted burning rocket is one in which the propellant is constrained to burn on only one surface. The manner in which cigarette burns is similar to the type of burning of a restricted burning rocket.
- The unrestricted burning rocket is mostly free to burn on all surfaces at the same time.
The restricted burning rocket delivers a small thrust for a relatively long period for a while the unrestricted flaming rocket provides a relatively large thrust for a short period.
Liquid Propellant Rocket:
Liquid propellant rockets were developed to overcome some of the undesirable features of the solid propellant rockets such as shorter duration of thrust, and no provisions for adequate cooling or control of the burning after combustion starts.
In the liquid propellant rocket, the propellants in a liquid state are injected into a combustion chamber, burned and exhausted at a high velocity through the exit nozzle. It is used to cool the rocket engine by circulation of fuel around the walls of the combustion chamber and the nozzles.
The maximum duration of an uncooled rocket motor is about 25 seconds when the same type of fuel is used as in cooled rocket motor. The use of liquid cooled rocket motors allows them to the operated as long as fuel lasts.
Another primary advantage of liquid propellants for rocket use is that the size of the combustion chamber may be reduced for a given thrust, as compared to the solid propellant rocket. An additional advantage of a liquid propellant rocket over that of a solid propellant rocket engine is the ability to discontinue combustion in the combustion chamber at any time.
The use of solid propellants usually requires that all of the propellants be in the combustion chamber at the start of burning.