What is Rocket Engine?

A ROCKET ENGINE is a device to convert chemical energy into kinetic energy and so produce a propulsive effect or a thrust. Both the internal  combustion engine in a motor car and the gas turbine engine in a jet aircraft actually meet these definitions.

In the motor car engine air is taken into the cylinders and used to burn the fuel, the released energy being transmitted through the transmission to the wheels as shaft power. In the gas turbine engine, air is taken in to burn the fuel, but the resultant gases are then expelled in the form of a high velocity jet, and the propulsive effect is obtained by the reaction of this jet.

The rocket engine is similar to the gas turbine in so far as it is a reaction device, but it differs from both the gas turbine and the reciprocating engine in that it does not take in any atmospheric air, but instead carries all its means of propulsion within itself.

This then is the most important thing about a rocket engine. It not only carries its fuel, but it also carries the oxygen, in some form of chemical, with which to burn that fuel. For this reason a rocket engine is to all intents and purposes unaffected by its environment and can therefore operate successfully at high altitudes where the air is very thin, or out in space where there is not any air at all.

As a analogy, let us consider a man in a boat on a canal, and assume that he is propelling the boat by throwing bricks backwards over the stern. The bricks then correspond to the stream of gases being discharged from a gas turbine or rocket engine. Now the bricks could be distributed in a row along the edge of the canal, with the man picking them up, and throwing them backwards as he goes along. This is like the gas turbine in which air, corresponding to the bricks, is taken in all the time.  When the supply of bricks end the man cannot propel the boat any further by this means.

If the other case of the man having a pile of bricks in the boat with him instead of along the bank of the canal is now considered it can be seen now propel the boat quite independently of his surroundings. This corresponds to the use of rocket propulsion, but it should be noted that, in this case, the boat is initially far more chemicals than the gas turbine has to be fed with fuel. This fact alone generally confines the use od rocket engines to rather short period of operation.