Radiative ignition of double base propellants. II - Pre-ignition events and source effects

Abstract

In this second paper of a two-part study, emphasis is on the pressure dependent pre-ignition events in double base propellants and the influence of radiation source (arc image vs laser) on observed ignition behavior. The pressure-dependent (< 21 atm) ignition domain of a PNC/MTN double base propellant is examined using controlled exposure lengths together with high speed movies and an infrared detector to monitor flame development. There is a brief flux-dependent period of transient flame development after gasification begins; this is followed by a relatively long, flux-dependent period of steady-state, radiation assisted burning before a selfsustaining condition is reached. The nature of this condition for self-sustainment is not yet well-defined. The ignition behavior seen with an arc furnace or a laser is generally quite close for both double base and composite propellants, if optical effects (reflection, penetration) are factored out. An exception is fast deradiation extinction, seen only with the laser; greater radiation penetration in the arc image wavelength region precludes this phenomenon. I. Introduction A S pointed out in the companion paper,l thermal zmradiation is a convenient energy input for studying solid propellant ignition behavior. By varying such factors as radiant flux, ambient pressure, and propellant compositions, much can be learned about the processes underlying the macroscopic ignition response, e.g., which of these factors controls that behavior in differing ambient conditions and what is needed to predict ignitability quantitatively in new circumstances. At the same time, one must be aware that the unique nature of radiation and the characteristics of the devices used to produce it can affect the test results. The present study is a systematic survey of radiation ignition behavior of a variety of propellants, intended to illustrate both these positive and negative aspects. In the companion paper,1 a generalized ignition behavior 4'map'* (log °f irradiation time vs log of radiant flux) was presented. The boundaries on this map were discerned by go/no-go testing and by monitoring of IR radiation from the developing flame. For clarity, we summarize them again here in the order they are encountered as irradiation time increases: Lla = first gas evolution; Llb = first IR signal from the surface region indicating the beginning of exothermicity; L]c = incipient flame indicated by a roughly 50-fold increase in IR signal; Lld = self -sustained flame indicated by steady burning if flux is removed; L2 = deradiation extinction boundary indicated by disappearance of a well-developed flame when radiation is removed too quickly. The companion paperl concentrated on certain propellant formulation effects; it illustrated how these boundaries are shifted strongly by the optical properties of the propellants. This was shown to obscure the comparison of different types of propellants, but unmodified double base propellants