Aspects of Combustion Stability in Liquid Propellant Rocket Motors Part I: Fundamentals. Low Frequency Instability With Monopropellants

Existing theories are able to explain part of the observed ffects of low-frequency unstable combust ion with the issumption of a constant t ime lag between the propellants ujection and their transformation into hot gases. A discussion shows that the t i m e lag cannot be constant if the pressure oscillates, but m u s t be correlated to the pressure listory during the t ime lag itself. A reasonable formula is uggested to represent this correlation, and this formula s applied to the determination of the critical condit ions, t is first shown that instabil ity can arise even for contant rate of inject ion. Secondly, it is shown how the ariation of the inject ion rate affects the critical condit ions or monopropel lant feeding systems of constant rate or jonstant pressure type, having given characteristics of nertia and elasticity. Finally, the influence of n o n miform values of the t ime lag for different portions of the njected propellant is discussed. The bipropellant case md the case of h igh frequency instabi l i ty will be discussed n Part II, to be published in next issue of the Journal of ;he American Rocket Society.