eller blade which pulls the aeroplane along.
Motion of the Propeller
When the aeroplane is moving forward, the propeller will have two different motions.
1. Forward motion
2. Rotational motion
Thus each propeller blade section follows a corkscrew path through the air, called a helix. Hence the propeller can also be called an air screw.
Any means of propulsion which depends on throwing the air backwards is really a form of jet propulsion, and in that sense all power driven aeroplanes are jet propelled. But by common usage the term jet propulsion has come to be applied only to those forms in which there is external propeller.
Theory of jet propulsion
Before studying the various types of jet propulsion, it is well to become familiar with the reasons why such devices operate to produce the thrust.
Application of Newton’s laws of motion
The theory of jet propulsion may be considered the reaction of mass when its velocity is changed.
The term “mass” is similar to “weight” but it is actually a different quantity. Mass refers to the quantity of matter, while weight refers to the pull of gravity on that matter.
Newton’s laws of motion relating to jet thrust may be stated as follows.
It states that the rate of change of momentum in any direction is proportional to the force acting in that direction.
It states that for every action there is an equal and opposite reaction.
Any analogy that may be applied to explain the above principles is the recoil of a pistol.Pistol recoils in accordance with Newton’s Laws. It is interesting to note that whether a piston is fired through water, through ordinary atmosphere, or through a vacuum the recoil is the same in all cases. Still another fact that should be noted is that the thrust is equal against the bullet and the pistol. Figs. given below shows how the recoil of a pistol may be compared to the propulsion of a jet engine.
Rocket propulsion differs from both jet and propeller propulsion in the one important respect that “the things that are thrown backwards” are carried in the rocket itself; in other words, we do not relay on the surrounding air to provide the jet. This fact points immediately to the inherent disadvantage of the rocket system, that “it would need an awful big store”. In practice it is just impossible to provide the storage or to carry the weight that would be required for a flight of any reasonable duration, and so rockets may be considered as boosts for other forms of propulsion, to be used only in special circumstances, or for flights the duration of which is measured in minutes and seconds rather than in hours.
Rocket propulsion is simply a development of the work type of rocket with which every one is familiar. The fuel or fuels are carried in the “engine” which is nothing more nor less than a suitable chamber, when they are ignited, or may be simply omitted, they burn furiously and the products of combustion, gases and so on, are ejected by the pressure produced at high speed through a suitable nozzle. Many different kinds of fuel can be used, solids, liquids, and gases and the system may even lend itself eventually to the use of atomic power.
It is usually used in the case of missiles, satellites and space ships.
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