Philosophical Approach to the Basic Understanding of the Mechanics of Jet Propulsion

This paper attempts to bring home the philosophy which lies behind the practical application of air-breathing and rocket engines. The various types of practical engines namely rocket engine, ram jet engine, turbojet engine, turbofan engine (mixed and unmixed exhausts), turboprop engine appear to be principally different. However philosophically, they are all one and the same. To appreciate this philosophy, let us change our frame of interest from the fluid to the jet engine. This leads us to the very first principles of jet propulsion i.e. the generation of thrust from the pressure forces exerted by the fluid on the boundaries and components of the jet engine. Well established mathematical concepts and relationships are available for practically estimating the performance of these engines based on fluid parameters. However philosophically, thrust and propulsive efficiency are sufficient for the philosophical understanding of the generation of variety of aircraft engines.

A fluid exerts pressure normal to the surface over which it flows. In a jet engine, the resultant of the fluid pressure forces in the direction of motion is the thrust. Hence for a surface whose outward normal subtends an angle in the first or fourth quadrant with the direction of motion will give positive thrust and that which subtends an angle in the second or third quadrant will give negative thrust. Hence a convergent nozzle will give negative thrust. Negative thrust may be viewed as a necessary evil to maintain the positive thrust which cannot be maintained by only combustion temperature due to limitation on its maximum value. An engine which can make maximum possible use of the fluid pressure profile for thrust would have the highest possible propulsive efficiency i.e. unity. The bypass concept is an attempt in this direction. The outward normal of the bypass disk subtends an angle of nearly 0° with the direction of motion thereby converting almost 100% of the pressure force acting on it for thrust. Mixing at low bypass ratios reduces the negative thrust by increasing the taper ratio of the combined nozzle when compared with individual cold and hot nozzles for separate jets.

The core and bypass engines are together clubbed into a single theoretical model by means of an appropriate weighting parameter which takes limiting values for bypass ratio of zero and infinity.