: The objective of this program was to investigate and define the effects of inert binder properties on composite solid propellant burning rate. Experimental pyrolysis data were obtained for many binders of practical interest over a wide range of heating rates and pressures, in several environmental gases, with and without 10-percent ammonium perchlorate (AP) contained in the sample, and in some cases with catalysts. These data were used to extract kinetics constants from Arrhenius plots, and heat of decomposition. In addition, motion pictures were taken of the pyrolyzing surface and gas samples were extracted for analysis. Pyrolysis kinetics varied between binders, but were found to be independent of pressure, the presence of AP, and the presence of burn rate catalysts; however, a chlorine gas environment had a material effect upon the results. All of the binders exhibited molten, boiling surfaces mingled with char, to varying degrees; the amount of char increased with AP present, and in chlorine. Relevant data were input to the Derr-Beckstead-Price combustion model in order to associate binder properties with known binder effects on burning rate. Although the effects were predictable, they stemmed from properties other than pyrolysis kinetics; however, the binder data as applied to the model revealed possible deficiencies in the model, which are discussed. It appears that the approach of combustion tailoring by binder modification would have to involve the gas phase combustion processes rather than surface pyrolysis. Therefore, future work concerning the role of binder should be directed toward the gas phase.

/pdf/role-of-binders-in-solid-propellant-combustion-165jbx73v6.pdf

Role of Binders in Solid Propellant Combustion

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

H + 62 + M -> HO2 + M Reaction in Lean Hydrogen-Oxygen Mixtures," Journal of Chemical Physics, Vol 43, No 9, Nov 1965, pp 3237-3247 7 Getzinger, R W, "A Shock Wave Study of Recombination in Near Stoichiometric Hydrogen-Oxygen Mixtures," Eleventh Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, Pa, 1967, pp 117-124 s Stull, D R, JANAF Thermochemical Tables, Dow Chemical Co, Midland, Mich; may be purchased as Rept PB-168370 ($1000) and addenda PB-168370-1 and PB-168370-2 ($300 each), National Bureau of Standards, CFSTI, US Dept of Commerce, Springfield, Va 9 Bird, P F, Duff, R E, and Schott, G L, "HUG, a Fortran-Fap Code for Computing Normal Shock and Detonation Parameters in Gases," Rept LA-2980, Feb 1964, Los Alamos Scientific Lab, The Univ of California, Los Alamos, N Mex 10 White, D R and Millikan, R C, "Oxygen Vibrational Relaxation in O2-H2 Mixtures," Journal of Chemical Physics, Vol 39, No 8, Oct 1963, pp 2107-2108 11 White, D R and Millikan, R C, "Vibrational Relaxation of Oxygen," Journal of Chemical Physics, Vol 39, No 7 Oct 1963, pp1803-1806 12 White, D R and Millikan, R C, "Oxygen Vibrational Relaxation in O2-He and O2-Ar Mixtures," Journal of Chemical Physics, Vol 39, No 7, Oct 1963, pp 1807-1808 13 Gilbert, R B and Strehlow, R A, "Theory of Detonation Initiation Behind Reflected Shock Waves," AIAA Journal, Vol 4, No 10, Oct 1966, pp 1777-1783 14 Getzinger, R W and Blair, L S, "A Shock Tube Study of the Effects of Nitrogen and Water Vapor on Recombination in the Hydrogen-Oxygen System," 6th International Shock Tube Symposium, Freiberg, Germany, April 12-14, 1967; also The Physics of Fluids, to be published 15 Shchelkin, K I and Troshin, Ya K, Gas Dynamics of Combustion, Mono Book Corp, Baltimore, Md, 1965, pp 1-53 16 Voitsekhovskii, B V, Mitrofanov, V V, and Topchian, M E, Struktura Fronta Detonatsii v Gaza, Akademiya Nauk SSSR Novosibirsk, 1963, pp 81-96; transl as The Structure of a Detonation Front in Gases, Rept FTD-MT-64-527 (AD 633821), Feb 1966, Foreign Technology Div, Wright-Patterson Air Force Base, Ohio, pp 81-97 17 Strehlow, R A and Engels, C D, "Transverse Waves in Detonations II Structure and Spacing in H2—O-2, C2H2—O2, C2H4—O2 and CEU—O2 Systems," AIAA Journal, to be published

Location of action of burning-rate catalysts in composite propellantcombustion.

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 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

: Surface structures of thermoplastic and elastomeric fuels were found to be markedly different upon exposure to conductive heating from a doubly compressed stagnant gas at oxygen concentrations below those required to cause ignition. The thermoplastic polymers exhibited a molten surface, while the elastomeric fuels showed no visible changes. Vaporization followed by a gas- phase reaction is the probable ignition mechanism for the thermoplastic fuels in the shock tube environment. A gas-solid reaction may occur in the ignition of elastomeric polymers. No general conclusion can be drawn regarding the precise nature of the oxygen-polymer ignition process in the shock tube environment as consideration must be given to the physical structure of the polymer surface. Ignition characteristics of several representative ammonium perchlorate composite propellants were studied using the arc-imaging furnace. The nature of the fuel component was found to have the major influence on the ignition time, and the effect is related to the initial pressure. A model describing composite propellant ignition in a neutral environment was formulated and programmed for computer studies.

Larry J Shannon

Papers

Composite solid propellant ignition mechanisms