Design of a subscale propellant slag evaluation motor using two-phase fluid dynamic analysis

Abstract

Small pressure perturbations in the Space Shuttle Reusable Solid Rocket Motor (RSRM) are caused by the periodic expulsion of molten aluminum oxide slag from a pool that collects in the aft end of the motor around the submerged nozzle nose during the last half of motor operation.It is suspected that some motors produce more slag than others due to differences in aluminum oxide agglomerate particle sizes that may relate to subtle differences in propellant ingredient characteristics such as particle size distributions or processing variations. A subscale motor experiment was designed to determine the effect of propellant ingredient characteristics on the propensity for slag production. An existing 5 inch ballistic test motor was selected as the basic test vehicle. The standard converging/diverging nozzle was replaced with a submerged nose nozzle design to provide a positive trap for the slag that would increase the measured slag weights. Two-phase fluid dynamic analyses were performed to develop a nozzle nose design that maintained similitude in major flow field features with the full scale RSRM. The 5 inch motor was spun about its longitudinal axis to further enhance slag collection and retention. Two-phase flow analysis was used to select an appropdate spin rate along with other considerations, such as avoiding bum rate increases due to radial acceleration effects.Aluminum oxide particle distributions used in the flow analyses were measured in a quench bomb for RSRM type propellants with minor variations in ingredient characteristics.

Assessment of propellant relative slag potential by direct measurement of slag in subscale motors

“…For details of contour design/selection, including two-phase CFD flow simulation results and predictions, the reader is referred to an excellent treatise by Whitesides. 7 The submerged nozzle was developed for three reasons: to increase the amount of slag retained and thereby increase discrimination, to provide an empirical submerged nozzle motor test bed for analytical model calibration, and to provide a slag capture feature outwardly similar to that found in submerged nozzle motors like the RSRM. Initial testing indicated that use of the new nozzle increased slag by only 15 percent.…”