: Surface ignition of solid propellants has been represented by several analytical models, each involving obvious compromises with regard to scope of applicability. These models are distinguishable primarily in terms of site of the exothermic reaction governing ignition. Early research with nitrocellulose led to development of a theory involving chemical heat generation in the condensed phase. Two subsequent theoretical models were developed to explain ignition of the solid fuel ingredient of a composite propellant in an oxidizing atmosphere, and these two models were then extended on a heuristic basis to encompass a composite propellant in an inert atmosphere in which the oxidizing gas was produced by decomposition of the solid oxidizer. These two models are distinguished by whether the oxidation occurs at the surface or in the gas film above the surface. This report reviews the solid, heterogeneous, and gas-phase ignition theories and reviews the nature and implications of the assumptions involved. It is concluded that, while possessing certain drastic simplifications in common, the various quantitative models differ so conspicuously in their assumptions regarding external initiating stimulus as to make quantitative comparisons or tests of validity impossible.

Theory of ignition of solid propellants.

In this first paper of a two part study, the ignition response to arc image radiative heating (5 to 100 cal/cm sec) of several double-base propellants is examined; comparisons with certain AP and HMX propellants are made also. Ignition delay is affected by chemical factors in propellant formulation (stability of the condensed phase, reaction rate in the gas phase) and by optical factors in propellant formulation (opacifiers affecting reflectivity and in-depth absorption). The results show that comparisons of the chemical factors in the formulation can only be made properly when the optical factors are minimized (as by carbon addition). When optical factors are minimized by opacifying the propellant, one finds, in order of increasing ease of ignitability, the formulations tested fall as follows: HMX composite, AP composite, double base (noncatalyzed), double base (catalyzed).

Radiative Ignition of Double Base Propellants: I. Some Formulation Effects

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

Radiative Ignition of Double Base Propellants: II. Pre-ignition Events and Source Effects

The arc image furnace is used widely for evaluating solid-propellant ignition characteristics, but the applicability of such data to rocket motors is not well-established. In arc image testing, radiation replaces the normal conductive and convective heating modes that exist in rocket motor ignition; radiation interacts with the propellant in a basically different way. Thus, for example, because radiation is absorbed in depth, the ignition delay is sensitive to propellant opacity. In addition, radiation can cause subsurface disruption of the propellant. A physical model is proposed in which incident radiation penetrates the propellant and is absorbed, causing thermal and photochemical decomposition and a temperature increase. Oxidative reactions start either in the gaseous boundary layer or on the surface and, by thermochemical action, lead to a flame. The system of differential equations for this complete model is complicated and has not been solved. However, an equivalent model of simpler character is solved, and an equation is produced for the ignition time delay as a function of radiation intensity, pressure, and physico-chemical properties. This equation can be used for correcting arc image data to equivalent convective ignition data. The qualitative behavior of experimental radiative ignition data reported by Beyer and Fishman and by Bastress can be rationalized with the aid of this theory.

A Critical Analysis of Arc Image Ignition of Solid Propellants

The radiative ignition behavior of two polymeric fuels—polystyrene and an epoxy—in oxygen/nitrogen mixtures has been determined experimentally. A CO2 laser was adapted for use as the radiation source to simplify interpretation of the results. Ignition delay (as determined by first light emission) was measured as a function of oxygen percentage, pressure, radiant flux, and fuel absorptivity. The observed ignition delays, and the variations with pressure, oxygen fraction, and radiation intensity, conform to the predictions of a theoretical model based on the need to heat the surface to a critical ignition temperature. The most important finding is that ignition of a solid fuel by radiation is much slower than ignition by an identical heat flux applied by a hot gas source. Two main reasons for the retardation can be deduced from the experimental results: (1) the radiative transparency of the fuel, even for seemingly opaque substances, results in slower surface heating, and (2) the cool gas environment in the usual radiation ignition test suppresses the incipient ignition.

Radiative ignition of polymeric materials in oxygen/nitrogen mixtures

: The ignition characteristics of five operational propellants and seven model systems of varying composition were investigated by the use of high flux radiant energy as the ignition stimulus. The study provided considerable information which relates ignition characteristics to compositional factors. Findings of particular importance are: (1) for the types of propellants studied, ignitability is directly related to burning rate; within each system, those factors which increase burning rate also ease ignition; (2) minimum pressure for ignition of ammonium perchlorate propellants appears to be primarily related to the nature of the binder.

http://www.dtic.mil/cgi-bin/GetTRDoc?AD=AD0474229&Location=U2&doc=GetTRDoc.pdf

Norman Fishman

Papers

Solid propellant ignition studies