Rocket is not propelled forwards by the explosive gases rushing from its engine pushing against the surrounding air.For a start there is no air in space. Three centuries ago the great English scientist Isaac Newton explained it this way; for every action there is an equal and opposite reaction. If shot putter wearing ice skates throws the shot forwards, he moves backwards
because of the momentum he has created, not because of the shot pushing against the air. Action and reaction is the principle of rocket engine.
A working rocket engine is a controlled explosion. It burns fuel in an oxidizer usually oxygen, in a combustion chamber. This creates hot gases under enormous pressure.
The gases accelerate out of the back of the chamber.Engineers found that by making exit or throat from the chamber , the gases accelerate even more, giving extra thrust. They then added a conical nozzle to throad.This restricts the gases accelerates them still more and also helps to guide rocket.
The principle of rocket engine is simple, but there are many pratical problems. Engineers have designed various systems to over come these. In te space shuttle main engine , oxygen oxidizer and hydrogen fuel are first pressurized , mixed and preburned , to form hot gases.
These gases then introduced together in an exact mixture in the combustion chamber.The ultra cold fuel circulates n heat exchanger to warm itself before preburning and to cool the chamber nozzle.
Solid Fuel Rocket
The solid fuel rocket engine does not burn gunpowder, lie a toy firework, but a specially mixed propellant. But once ignited it cant be turned off.It is used to mainly as booster , strapped to the main engine.
Appolo, United states moon program was launched by the Saturn V Rocket.This hade a third stage fuelled by liquid hydrogen and Oxygen. These liquid were fed at high pressure and carefully controlled rate into the combustion chamber.