Spacecraft contamination from scarfed nozzle exhausts

Abstract

A rocket operating in a vacuum environment will produce a molecular flow that is directed into the nozzle backflow region. This paper discusses the development and application of an analytical model which describes this flow and provides a means of predicting the exhaust plume contamination from a scarfed nozzle. The model is an adaptation of one previously developed for a conventional unscarfed nozzle. It is based upon the existence of a small, conical-shaped continuum region downstream of the nozzle exit from which the directed flow originates as a result of molecular effusion. The model was programmed on the computer and provides a quick and convenient means of estimating the maximum contamination. Results using the earlier model for conventional nozzles compared favorably with limited test data; however, no known contamination data for scarfed nozzles are available for a comparison with this model's predictions. When used to predict contamination from the Peacekeeper missile attitude control engine (ACE) and the Star 30 space motor, the model indicated that scarfing will dramatically increase the contamination of surfaces in the backflow region.y d Nomenclature = coefficient in Maxwell's equation = axial coordinate of point P in backflow region (Fig. 3) = radial coordinate of point P in backflow region (Fig. 3) = distance between point P and cone surface (Fig. 3) = energy = energy flow rate or energy flux at point P = view factor = fraction of all molecules that will impinge at point P = mass function = mass exponent = length = Mach number = molecular mass = mass flow rate or mass flux at point P = molecular density = normal vectors (Fig. 3) = total pressure, static pressure = nozzle or cone radius = distance between point P and origin (Fig. 3) = specific gas constant, ratio = contributing cone surface area = temperature = mass flow velocity = atomic mass unit (1.66X 10 ~2 7 kg) = thermal velocity = molecular weight = Cartesian coordinates = angle (Fig. 3) = plume centerline displacement angle (Fig. 7) = exponent in Maxwell's equation, angle (Fig. 3) = scarf angle = specific heat ratio = boundary layer thickness, cone semivertex angle = nozzle area ratio, angle (Fig. 4) KI = angle (Fig. 4) 0 = wall angle, angular distance of point P (Figs. 1, 3) X = Hill-Draper plume shape parameter p = density X = meridional angle of point P (Fig. 2) da = incremental surface at point P dco = incremental solid angle Subscripts av = average b = boundary layer, cone base c = scarf plane/nozzle centerline intersection station (Fig. 2) d = data match e -nozzle exit / = impinging, incident t = lower min = minimum max = maximum n = nozzle, nozzle wall P = point P r = reflected s = scarf station (Fig. 2) t = throat 0 = stagnation

Molecular Beams in Space: Sources of OH(A→X) Emission in the Space Shuttle Environment

“…Knowledge of the precise orbital conditions and optical interference sources then permits the extraction of rate coefficients for luminescence processes. It is worth mentioning here that contamination, especially H 2 O from outgassing and from thruster exhaust, affects all space or space-bound vehicles, as has been shown for rockets, , for the Space Shuttle ,, and has been demonstrated in laboratory simulations…”

Modeling of atmospherically induced gas phase optical contamination from orbiting spacecraft

“…A number of attempts have been made to model the effect of contamination on sensors flown on the space shuttle. Many of the earlier attempts have treated contamination as being due solely, or primarily, to the exhaust products of thruster motors [e.g., Trinks and Hoffman, 1984;Hoffman et al, 1985;Boraas, 1987]. Of course, spacecraft contamination is indeed related to the flow fields of the exhaust products of spacecraft engines, as has been demonstrated in a Monte Carlo code developed for the specific purpose of understanding the backflow of exhaust products into the shuttle bay [Hueser et al, 1986].…”