Leonard H. Caveny

Nitramine flame chemistry and deflagration interpreted in terms of a flame model

The diversity of chemical kinetic time scales associated with nitramine decomposition has led to incorporation of two simultaneous overall reactions in the vapor phase mode of deflagration. This allowed derivation of an asymptotic burning rate formula, showing variable pressure dependence. The comprehensive model considers a reacting melt layer, coupled to the gas field through conservation conditions satisfied by all chemical species and enthalpy, and is solved numerically. The structure of the deflagration wave near the propellant surface is obtained, along with the overall pressure dependence of the surface temperature and the flame speed eigenvalue, comparing RDX and HMX. A mechanism of coupling between secondary reactions and heat feedback to the surface is proposed, and a quantitative measure of the effect of condensed phase exothermicity on burning rate is demonstrated.

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

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).

Site and Mode of Action of Platonizers in Double Base Propellants

Certain metal organic salts (e.g., lead or copper salicylate) when used in double-base propellants induce desirable insensitivities of burning rate to pressure and initial temperature. To understand this, the combustion wave zones (luminous flame, dark, fizz, and surface reaction zones) were examined by means of photography and fine thermocouples (4fi bead). The metal salts significantly alter the surface and fizz zones. The surface zone accumulates carbonaceous material coincident with the appearance of an accelerated burning rate in the catalyzed case. No attendant change in surface heat release is detected. Coinciding with this carbonaceous layer occurrence are substantial (50-100%) increases in conductive feedback from the fizz zone. This latter effect is believed directly responsible for the altered burning behavior though its origin may lie in the altered surface chemistry.

Agglomeration and ignition mechanism of aluminum particles in solid propellants

Aluminum powders added to conventional rocket propellants burn either as single particles or agglomerates which contain hundreds or even thousands of the original particles. Combustion efficiency and acoustic stability characteristics are very dependent on the final Al/Al 2 O 3 particle size injected into the chamber flowfield. High-speed photographs of burning homogeneous propellants provided data on agglomeration size and visualization of the flow processes as a function of pressure (1 to 10 MPa), initial particle size (5 to 100 μm), and aluminum mass fraction (0.1 to 13%). Photomicrographs of extinguished surfaces revealed the importance of particle accumulation in a thin mobile reaction layer adjacent to the burning surface. A model was developed that interpreted data and observations from several sources. The model accounts for accumulation of aluminum particles in the mobile reaction layer, retention of particles by surface tension forces, melting, and ignition at the surface. The following agglomeration and particle behavior items are categorized: decreasing agglomerate size with increasing pressure, minimum mass loading required for agglomeration, prominent agglomeration for particles with diameters less than the reaction layer thickness, and sharply reduced agglomeration for larger particles. The model provides an approach for controlling and interpreting agglomerate size behavior.

Aluminized Solid Propellants Burning in a Rocket Motor Flowfield

Combustion and agglomeration processes of aluminum particles emitted from the surface of an aluminized double-base propellant (NC/TMETN) were studied under rocket motor, crossflow conditions. High-speed color photographs (-2000 frames/s) were taken of burning AI/AI2O3 agglomerates forming on the surface, moving along the surface, and entering the flowfield. As an example, a propellant containing 6-^m Al burning at 7 MPa and 6 m/s crossflow produced a mean agglomerate size of about 250 ^m. Analysis of size distributions of the agglomerates leaving the surface revealed that the following parameters decrease with increasing pressure: collision frequency on the surface, the agglomerate stay time on the surface, and mean agglomerate size. Increasing the crossflow velocity decreased the mean agglomerate size. The propellants which contained the large aluminum particles (50 /*m vs 6 j*m) burned without the aluminum igniting or agglomerating on the surface.

The Mechanism of Super-Rate Burning of Catalyzed Double Base Propellants

Previous investigators have offered qualitative explanations for the large, pressure-dependent, burning behavior changes seen in nitrate ester propellants when lead or copper salts are added. The most developed qualitative models are those of Camp and of Powling and co-workers but both exhibit certain discrepancies with experiment. New evidence reported here derives from radiation-assisted burning tests in which the spectral content (particularly ultraviolet) of the impinging radiation was varied; contrary to the original hypothesis of Camp, the UV component of the radiation yielded no special burning rate enhancing effect. Experimental evidence recently presented by the authors shows that the burning rate enhancement by lead or copper compounds is a result of acceleration of the fizz zone reactions; this is accompanied by an increased production of carbonaceous material at the burning surface. Therefore it is hypothesized that the acceleration is due to a shift in equivalence ratio toward the stoichiometric when potentially burnable fuel molecules are instead carried through the fizz zone as solid carbon. A simplified mathematical model of this hypothesis is developed based on representing the equivalence ratio change as a shift from a single normal reaction pathway toward a second, more reactive pathway. This hypothesis successfully explains the appearance of a region of enhanced burning rate and its disappearance at higher pressures but it is shown that sudden disappearance (mesa burning), sometimes seen experimentally, requires a further mechanism whose nature is not yet clear.

Dynamic effects on ignitability limits of solidpropellants subjected to radiative heating

The dynamic response of a solid propellant to a rapidly varying radiation flux comprises a problem representative of the general class of transient responses of heterogeneous flames to rapid disturbances. In the present study, the response of double-base propellants to roughly square-wave radiation pulses is examined. It is found that, when such pulses are used to ignite propellants, they may in some cases produce a flame which persists for whatever duration the pulse persists, but which extinguishes as soon as the pulse stops. This tendency to extinction upon deradiation is found to be lessened by increased pressure or increased time interval for reducing the flux to zero. This extinction response upon deradiation is not limited to the ignition situation; it is shown experimentally that a steadily burning propellant can be extinguished by a radiation pulse of appropriate magnitude, duration, and speed of cut-off. It is proposed that this dynamic extinction behavior results from an imbalance in the heat fluxes to and from the burning surface during deradiation. A mathematical model of this phenomenon, deriving from the nonsteady burning approach of Zeldovich, is solved and shown to predict quite well the same type of behavior as that found experimentally.

Flame zone and sub-surface reaction model for deflagrating RDX

A study of 1,3,5 Trinitro Hexahydro 1,3,5 Triazine, RDX, burning as a monopropellant was undertaken to obtain a better understanding of the important chemical steps that control heat feedback to the condensed phase, to determine the contributions of the liquid layer, and to provide a means of evaluating theories for modifying the burning rate of nitramines. The following chemical mechanism is proposed: first, partial decomposition of the RDX molecule in the liquid phase; second, following vaporization, gas phase decomposition of RDX; third, oxidation of formaldehyde by NO2. The flame structure and liquid layer reactions of deflagrating RDX were expressed in terms of the energy, continuity, and species equations corresponding to RDX decomposing in liquid and gaseous phases and the NO2/CH2O reactions adjacent to the surface. In addition to the temperature profile and burning rate, the numerical solution provides the details of the interactions at the liquid/gas interface and the concentration profiles for the nine most prominent species. Using published kinetic data, the calculated results reveal that even though the liquid layer becomes thinner with increasing pressure, the increase in surface temperature causes its heat feedback contribution to increase. The pressure sensitivity of burning rate between 0.7 and 0.8 is interpreted in terms of the relative contributions of gas phase and liquid layer RDX decomposition and the oxidation of CH2O. In particular, as pressure increases, the contribution from liquid layer reactions and the second order, NO2/CH2O reaction become more prominent.

Influence of Thermal Radiation on Solid Propellant Burning Rate

Radiation assisted burning rate data r(qr] and temperature sensitivity of burning rate data [r(p, T0) and op = dr/dT0)p] were obtained for a well characterized double base propellant (nitrocellulose and metriol trinitrate). For example, at 14.6 atm, 70 cal/sec-cm of xenon arc radiation increased the burning from 0.2 to 0.6 cm/sec. Analysis of the heat feedback from the flame revealed that once r(T0) data are available, r(qr) data does not permit additional properties of the combustion zone to be deduced. However, a simple analytical relationship between r(qr) and op was developed that approximates the measured results. Propellant burning rate responsiveness to external thermal radiation increases with higher ap and lower burning rate.

Low Burning Rate Aluminized Propellants in Acceleration Fields

To reduce slag formation and burning rate increases, the effects of radial acceleration (up to 20 g) and low pressure (2-7 MPa) on the combustion of low-burning rate aluminized propellants (87 formulations) were investigated. Experiments were performed to obtain agglomerate size distribution, slag formation, and burning rate data. Data sources included photographs under cross flow and acceleration conditions, scanning electron probe (Cl and Al) images, particle collection combustors, and rocket motors. The tendency to form slag increases as the agglomeration size at the burning surface increases. A lower burning rate increases acceleration effects. However, residue formation is more sensitive to formulation than is burning rate; for example, bimodal vs trimodal AP can cause significant changes in residue in the absence of burn rate changes; RDX produces larger agglomerates than HMX. Through understanding obtained from this research, reduction in agglomeration, slag formation, and burning rate augmentation are directly attributable to systematic changes in formulation ingredient levels and sizes.