Investigation of Q1D Model for Pressure Oscillations in Solid Rocket Motors
Creators: name:  M. GROSSI 
name:   D. BIANCHI 
name:   B. FAVINI 
Publisher: name: Proceedings of  the 9th European Conference for Aerospace Sciences. Lille, France, 27 June - 1 July, 2022
Year: 2022
Subjects: PROPULSION PHYSICS
SOLID ROCKET MOTOR

description: Solid Rocket Motors (SRMs) are known to be prone to the development of flow instabilities during their quasi-steady state phase. Nowadays such issue still represents a major and open problem for many motors currently flying as Ariane 5 P230 and VEGA first stage P80, but also for future ones as P120C, the largest solid rocket motor employing a monolithic grain which will be used as Ariane 6 strap-on booster and next generations VEGA launchers first stage. In order to guarantee the launcher safety and payload comfort, it is actually of paramount importance to carry out a pressure oscillations risk assessment which address all instability sources. In the current state of the art, two physical phenomena are considered to be the major driving mechanism for pressure oscillations onset in launching vehicle SRMs: hydrodynamic instabilities related to vortex shedding and thermoacoustic instabilities driven by aluminum distributed combustion. Hydrodynamic instabilities involve vortical structures generation and dynamics which lock in with chamber acoustics, giving rise to an aeroacoustic feedback loop. Alongside this mechanism, also a thermoacoustic instability is currently considered a pressure oscillations source in SRMs. Modern solid propellants contain metallic fuel particles, mainly aluminum, whose injection in the combustion chamber gives rise to a multiphase reacting flow. Experimental analysis carried out on aluminum combustion have shown that, once the particles are freed from the grain, a certain amount tend to cluster providing agglomerates whose diffusive chemical reaction is strongly influenced by the environment flow condition. As a consequence, a coupling may actually arise if mass and heat release fluctuations produced by the burning droplets are synchronized with the acoustic field. In the past the authors have been working on pressure oscillations in SRMs characterized by aft-finocyl geometries, employing a Q1D, mono-phase formulation named AGAR in which instability source is strictly related to vortex-shedding phenomena. In AGAR, thermoacoustic coupling driven by aluminum combustion is not evaluated, but roughly taken into account thanks to calibration parameters. In order to overcome such limitations a new Q1D model able to deal with pure thermoacoustic feedback loop in solid motors has been developed and validated on the basis of CFD simulations results. The unification of these two Q1D models has lead to a new formulation, named TAHR, which can therefore deal with both hydrodynamic and thermoacoustic instabilities in solid motors. TAHR has been recently validated against Vega P80 SRM, witnessing the model capability to cope with modern and complex solid motors used in the current space transportation system. A full investigation of TAHR model is the ultimate goal of this work. In the final manuscript an aft-finocyl SRM geometry will be taken into account in order to explore the model response, in terms of instability characterization, to different conditions like aluminum amount and size. Particular care will be given to gas and particles unsteady fields in order to highlight the major characteristics able to drive the motor instability. Finally the interaction between the two kinds of pressure oscillations sources will be taken into account. Such work can be considered as a mandatory step within a project that has as its final objective the investigation of pressure oscillations in solid rocket motors by means of a light and reliable tool.