The vehicle which expels a fluid to gain a forward motion is called a rocket. Rockets may not always be used to launch a satellite, but we will concentrate on constructive uses. Most of the rockets in use today are chemical rockets i.e., rockets which produce thrust by chemical reactions. Now a chemical rocket may use either solid or liquid propellants in which case it is called a Solid Propellant Rocket or a Liquid Propellant Rocket.

SPR has different characteristics from the LPR and so they are used for different roles. Let us discuss their characteristics.

Solid Propellant Rockets

SPR's are simple in structure and the complexity of the rocket is low. The rocket consists of a thrust chamber to which a nozzle has been attached. The thrust chamber itself contains the propellant. The propellant consists of fuel and oxidiser. The fuel burns and the oxidiser releases oxygen to burn the fuel. The chemical reaction causes the formation of hot gases which are expanded in the nozzle. These gases stream out and this pushes the rocket forward.

SPR's have a tendency towards progressive burning. This simply means that as propellant burns more and more, the area of burning surface increases, which means that still more of the propellant is burnt. Some materials like Aluminium powder are also added to the fuel-oxidiser mixture to increase the chamber temperature. Oh, and stabilizers are also added so that a rocket may not go off by itself! This is necessary because usually fuel used is an explosive! (for example, Nitroglycerine.)

The rate at which the fuel of a SPR burns depends on the temperature of the surroundings, and once fired, the rate of burning of fuel cannot be controlled. This means that the rate of burning has to be taken into consideration at the design stage itself, and indeed, it is one of the major parameters upon which the design of a rocket (SPR) is based.

SPR's are generally used as missiles and boosters stages for a Launch vehicle. The middle and upper stage of a launch vehicle are usually...

Liquid Propellant Rockets

The rocket is usually quite complex in this case. It consists of a number of subsystems which combined together, produce the functionality of a rocket. Some of these subsystems are pumps, turbines, coolers, fuel injectors etc. The rocket will have fuel and oxidiser (usually liquefied) stored in tanks. These tanks are connected to the the thrust chambers through valves and some more pumps. The process of operation is like this:

The fuel and oxidiser are pumped into the combustion chamber and are injected into the chamber as a fine spray (through the injectors) there they burn and produces gases which are expanded in the nozzle to produce thrust. Now the system becomes complex because you have to have a turbine to run the pump and to run the turbine you need to somehow produce gas. There are various methods of doing so. We may burn a part of the fuel to do that or provide a separate system for doing that and so on. Since the fuel and oxidiser are stored at a very low temperature, handling of fuel and designing pumps that operate at such a low temperature becomes a problem.

I forgot to mention that LPR's may have only one propellant. In which case they are called mono propellant rockets. The rockets which I described above are called bi propellant rockets, since they have two propellants - fuel and oxidiser. LPR's have the advantage that the fuel burn rate can be controlled.

There are other kind of rockets also, about which I'll add something in future (maybe).