Role of Aluminum in Suppressing Instability in Solid Propellant Rocket Motors.

摘要

Combustion instability in large high-performance solid propellant rocket motors was a major difficulty standing in the way of practical development in the United States, until the discovery in 1956, after much testing, that powdered aluminum added to the propellant suppresses instability. This discovery made it possible for the national solid rocket program to move forward rapidly. The exact mechanism of the action of aluminum has long been a mystery. Aluminum is almost unique in its effectiveness; magnesium is the only other additive known to be equally effective. A theory is advanced in this paper, based on the previously published KTSS theory of nonsteady propellant burning, to the effect that it is the melting of the aluminum on the burning propellant surface, not its particular burning characteristics, that reduces the amplitude of the propellant burning rate fluctuations. The melting leads to the formation of a thin layer of molten aluminum that, through its thermal inertia, reduces the temperature response of the surface and therefore the burning rate response. The stabilizing mechanism is powerful: a very thin layer with only partial coverage of the surface produces a very large reduction in the acoustic admittance. Unfortunately, aluminum is objectionable (magnesium, too) because it produces smoke, flash, and ions in the exhaust jet. The paper concludes with an assessment, on the basis of the KTSS theory, of the possibility of ever finding an equally effective substitue and with some directions for investigation. (Author)