EVALUATION METHOD OF THRUST OSCILLATIONS IN LARGE SRM - APPLICATION TO SEGMENTED SRM's

Abstract

Large segmented Solid Rocket Motors (SRM) experience pressure oscillations. A number of them, includin g Ariane 5, Space Shuttle or the Titan family, are reported to exhibit this kind of instability during operatio n. Pressure oscillations (ODP) in such SRM could be due to many physical phenomena with different origins : the formation of vortex shedding in relation with geome tric and ballistic factors, combustion instabilitie s linked to the combustion of aluminium particles, etc. Since such oscillations could be penalizing for the launcher, SAFRAN-SME (SME) has been conducting extensive studies in the context of R&D programs, under the a uthority of CNES and together with ONERA to improve pressure oscillation modelling. The aims of the wor k performed have been: to identify the various orig ins of these instabilities, to better understand the conditions in which they develop and the mechanism involved, to consider possible coupling phenomena and to assess their ult imate impact on motor operation. Numerous 2D and 3D geometries have been studied, by considering variou s thermodynamics conditions and numerous models such as turbulence, multi-phase flow, etc. This knowledge is a highly valuable asset when the time comes to design a new motor or even when a modification is planned on existing geometries. At the present t ime, the knowledge acquired provides a means of defining a research methodology for thrust oscillations in the preliminary design phase for future SRM. This methodology is useful in identifying and predicting, as a first ap proximation and at a very early stage, critical geo metries and time points during firing at which thrust oscillations c ould occur, which might compromise booster performance. During a second phase, numerical aerodynamic simulations and in some cases experimental tests on subscale mot ors can be performed to confirm the expected preliminary resul ts and to evaluate the ODP amplitudes. The first part of this paper gives an overview of t he methodology employed by SME to study ODP issue in SRMs. Key themes and main findings are presented. The second part is devoted to some implementation examples on well known segmented SRMs.