Aluminized Solid Propellants Combustion Under Vibratory Conditions

Paper number

IAC-06-C4.3.01

Author

Mr. Alessio Bandera, Politecnico di Milano, Italy

Coauthor

Dr. Luigi T. DeLuca, Politecnico di Milano, Italy

Coauthor

Dr. Robert L. Glick, United States

Year

2006

Abstract

In the recent past, the influence of steady acceleration fields on the combustion process of solid rocket propellants was analyzed in great details and nowadays a satisfactory knowledge of the related burning rate augmentation mechanisms has been reached.  Theoretical models exist capable of adequately fitting the experimental data for aluminized propellants.  Aluminum and aluminum oxide aggregates on the burning surface, playing the role of thermal short-circuit from the flame to the propellant grain, have been recognized responsible of burning rate augmentation.
On the contrary, the influence of unsteady accelerations, for example due to vibrations experienced by motors during the launch phase,  was never tested.  The objective of this work is to discuss the results of the tests, conducted at the SPLab of Politecnico di Milano, on vibratory combustion for a variety of solid propellant formulations. The commercial Ariane 5 solid booster propellant (0.68 AP + 0.18 Al + 0.14 HTPB) was tested as a reference formulation.
Two different set-ups were especially developed for this investigation: the first one limited to  ambient pressure testing (Basic Set-Up), the latter for pressure testing (Pressure Set-Up). The vibratory motion of the propellant strand is supplied by an electro-magnetic mini-shaker, which forces a sinusoidal accelerated motion. 
The results obtained at ambient pressure tests show, for the reference formulation, a first peak of burning rate augmentation at 25 Hz together with resonance peaks at frequencies multiple of 25 Hz.  By increasing the operating pressure, the first peak and its resonance peaks shifted to higher frequency values.
A possible explanation for this experimental trend is associated with the natural frequency of the propellant condensed-phase thermal wave.  At 1 bar this value is around 23 Hz, thus quite close to the 25 Hz experimentally found for maximum burning rate augmentation. The heat feedback from the gas-phase to the condensed-phase is interrupted, once at every cycle of oscillation, when clusters of solid particles are expelled from the burning surface by inertial forces.  A theoretical model taking these effects into account is presently under development.   

Abstract document

IAC-06-C4.3.01.pdf

Manuscript document

IAC-06-C4.3.01.pdf (🔒 authorized access only).

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