Infrared signature studies of aerospace vehicles

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

Predicted extents of afterburning in fuel-rich exhausts of metal-modified, double base propellant rocket motors have been investigated by means of a plume prediction code, which incorporates fully coupled turbulent kinetic energy boundary-layer and nonequilibrium chemical reaction schemes. The influences of a number of metals, including potassium, iron, molybdenum, tungsten, barium, tin, and chromium, on afterburning have been examined for a 300 N thrust motor. The effects of changes in total effluent mass flow rate and exit temperature were determined for both modified and unmodified propellants under static and flight conditions at sea level.

Af terburning Predictions for Metal-Modified Propellant Motor Exhausts

The chemically reacting flow in axisymmetric, turbulent free jets is solved numerically by a second-order accurate finite-difference scheme that is unconditionally stable with respect to the chemical rates. The transport properties are obtained from the turbulent kinetic energy model of turbulence. The momentum, energy, and turbulence model equations are solved by an explicit differencing scheme, whereas a hybrid expolcit/implicit scheme is used to solve the species conservation equations. High computational efficiency is attained by solving the equations in a natural coordinate system, and by employing the chemical element conservation principles to minimize the order of the matrix inverted to obtain the species concentrations. The scheme computes reacting jet flows in one-fifth the computer time required by any previously reported scheme.

Efficient Computation of "Stiff" Chemically Reacting Flow in Turbulent Free Jets

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.

/pdf/analysis-of-the-measured-effects-of-the-principal-exhaust-4rwws5iy9g.pdf

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

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.

A simple method for predicting rocket exhaust smoke visibility

Afterburning of combustible plume species with air is the most striking effect noted in observations of lowflying missiles (less than 30-km alt). Because afterburning raises the temperature of the plume, it intensifies the physical and chemical processes that cause plume signature effects. In systems with relatively cool exhausts, the onset of afterburning is delayed until the missile speed exceeds some minimum value (about Mach 2.2). The onset of afterburning in these cases has been attributed to the effect of combustion in a recirculation zone at the base of the missile. A model of equilibrium combustion in the base region was coupled to a chemical kinetic model of downstream plume mixing and afterburning in order to perform the calculations for this study. The calculations made with the combined model confirm observations of plumes which afterburn only after the base region has ignited. The calculations also indicate that the rocket motor mass flow rate has an important influence on plume afterburning. It also is calculated that, for systems with hot exhausts, the base has no important effect on afterburning in the rest of the plume.

Andrew C. Victor

Papers

A simple method for predicting rocket exhaust smoke visibility

Calculations of rocket plume afterburning coupled to reacting base recirculation regions