Effect of particle size distribution on the radiosity of solid propellant rocket motor plumes

The feasibility of conducting environmental chamber tests using a small rocket motor to study the physical processes which occur when the exhaust products from solid motors mix with the ambient atmosphere was investigated. Of particular interest was the interaction between hydrogen chloride, aluminum oxide, and water vapor. Several types of instruments for measuring HCl concentrations were evaluated. Under some conditions it was noted that acid aerosols were formed in the ground cloud. These droplets condensed on Al2O3 nuclei and were associated with the rocket exhaust cooling during the period of plume rise to stabilization. Outdoor firings of the solid rocket motors of a 6.4 percent scaled model of the space shuttle were monitored to study the interaction of the exhaust effluents with vegetation downwind of the test site. Data concerning aluminum oxide particles produced by solid rocket motors were evaluated.

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Analysis of the measured effects of the principal exhaust effluents from solid rocket motors

An approximate method of solution for the scattering of radiation by optically thick media is described. Multiple scattering functions, analogous to single scattering functions, are derived by a Monte Carlo technique. For a given particle size distribution, these multiple scattering functions are related only to the single scattering function and the optical depth. For cases such as combustion smoke in which the particle number density varies but the particle size distribution is fairly constant, a straightforwar d method is given for interpreting light scattering data and for predicting light scattering or visibility. For the cases described in this paper, scattering by alumina particle clouds with optical depths of 0.2-10, the multiple scattering functions differ greatly from the single scattering function only in the forward direction. In other directions, the two functions agree within a factor of 4. The differences would be much greater for nonabsorbing particles. The results are compared to earlier rocket exhaust smoke visibility calculations that presumed single scattering. For an optical depth of 10.

Effects of Multiple Scattering on Rocket Exhaust Plume Smoke Visibility

Publisher Summary This chapter presents plume signature and signal interference. The plume or smoke reveals the launch location of a missile and allow the missile to be located in flight. Intense flames at the rear of the missile, caused by the combustion of the motor exhaust products with air, also reveal the launcher location and trajectory of the missile. The flames also cause saturation of the instruments used for optical tracking of the missile or of the target. The flow at the exit plane of the nozzle of a solid propellant rocket motor in operation is supersonic, with an average temperature of the mixture of gases and solid or liquid particles over 1,000K. Condensed exhaust products, solid or liquid, are present downstream of the nozzle exit plane, leading to the presence of a type of plume smoke called primary smoke. Secondary smoke develops downstream and in external regions of the plume, depending on the atmospheric conditions, caused by the condensation of water vapor from both the plume and the atmosphere. Most of the chemical products in the plume come from the combustion of the propellant. Additional contributions to the exhaust gases and particles come from thermal and mechanical erosion, pyrolysis and combustion of the inhibitors, liners and insulators, and from the ablation of the nozzle and blast-tube materials.

Plume, Signal Interference and Plume Signature

Analytic expressions are presented defining the visible contrast of a rocket plume against the sky produced by scattering of sunlight and sky light by primary and secondary smoke in the plume. Allowances are made for cloud cover and atmospheric attenuation. To use these equations for prediction of the probability of detection by an observer, the rocket plume and the formation of smoke must first be described by existing computer models. A correlation available in the literature is used for prediction of the probability of detection as a function of contrast and visual angle. Required propellant data include the number of particles produced per unit mass of propellant by combustion, their size distribution, and their composition. The analytical procedure is encoded in a computer program, VISIG, suitable for use on a microcomputer. An example is given for the prediction of the probability of detection by an observer of a rocket flying at 5000 ft when viewed against the sky for a variety of sun, sky, and atmospheric conditions.

Prediction of the visible signature of solid rocket plumes

Ambient light scattered from metal oxide particles in composite-propellant rocket exhausts makes the missile trail visible at long ranges. In the selection of propellant aluminum concentration, it is important to be able to predict the distance from which the exhaust no longer is visible. In this paper, a simple methodology, amenable to hand calculation, is presented for this purpose. The user need supply only the propellant aluminum concentration, missile velocity and altitude, and the rocket motor thrust level. The methodology predicts smoke trail dimensions, particle concentration as a function of location in the trail, sunlight and skylight scattering, background obscuration, and the range at which the trail may be detected visually for any atmospheric attenuation level. The entire calculation can be carried out with a scientific pocket calculator.

Stuart H. Breil

Papers

A simple method for predicting rocket exhaust smoke visibility