THEORETICAL STUDY OF RAM-JET PROPULSION

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Abstract
In ram-jet propulsion, artificial compression before combustion does not exist. Therefore, the only factors which may influence the increase in momentum, i.e., the thrust, are:(1) The degree of initial dynamic compression by a diffuser;(2) The shape of the combustion chamber;(3) The variations of the rate of heat release along the tube;(4) Air excess coefficient;(5) Kind of fuel;(6) Velocity of flight;(7) Aerodynamic and heat losses.This paper deals almost exclusively with the first four factors, i.e., with the theoretical case of an ideal gas and a perfect flow, with no heat or aerodynamic losses involved. The possible appearance of shock waves is disregarded. Neither are boundary layer phenomena discussed in this paper. The external fairing and the relation between the internal and external shape, as well as the influence of this relation on the actually obtainable thrust and efficiency, are also considered to be beyond the scope of the paper. General conclusions are drawn as to the optimum shape of the ideal engine from the standpoint of efficiency. It is found that air should be submitted initially to as high a dynamic compression as possible, i.e., using all kinetic energy available, after which a combustion chamber of constant cross section is used—and finally a nozzle. Another conclusion is that it is impossible to obtain a positive thrust with a combustion chamber of constant cross section, if no dynamic compression, by a diffuser, is initially applied. Special attention is drawn in the paper to the representation and interpretation of the ideal ram-jet cycle on the entropy diagram, this giving a clear picture of the relations between the energy released and the efficiency and thrust obtainable. The fact that in the flow through a tube of constant cross section with heat release (e.g., by combustion), the pressure may decrease even if ideal gas and perfect flow (with no losses involved) are concerned, is emphasized throughout the paper, as it seems that generally this fact is rather overlooked.

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