Flame plasma and the microwave determination of solid propellant regression rates

Abstract

The microwave determination of detonation wave velocities in explosives and regression rates of solid rocket propellants was initially based on the firm belief of the original workers that the incident microwave in an explosive or propellant strand is totally reflected by the highly conductive flame plasma. Subsequent investigations have shown that the microwave conductivity of flame plasmas is of no consequence in microwave reflection from the strand-flame interface, the flame behaving actually as a medium of refractive index close to unity. This paper reviews the development of ideas in this field and adds new experimental evidence that rocket solid propellant flames are nonreflecting plasmas for frequencies in the microwave region. Microwave measurements are a powerful tool in solid rocket propellant studies and yield a complete regression rate versus pressure curve in a single experiment, thus substituting for many Crawford bomb tests, in fact in improved conditions of combustion, and it appears to be important to absolve the method of criticism relative to flame plasma reflection, of which no experimental evidence was ever produced, and which actually only was a working hypothesis of the inventors of the method.