A Study of the Amplitude of Pressure and Thrust Oscillations in a Lab-Scale Hybrid Rocket

Hybrid rockets are being studied as a potential replacement for the solid rocket boosters on the NASAspace shuttle.One physical characteristic of hybrid rockets that must be understood and overcome is potentiallysevere pressure oscillations during combustion.Pressure oscillationsinside the rocket combustion chamber lead to oscillationsin the thrust of the rocket. These oscillationsare damaging to potentialhuman passengers and cargo and must be minimized. Current theories surmise that the oscillationsare causedby combustion chamber geometry, oxygen feedline parameters, and/or fuel combustion char- acteristics. This study focuseson the role of the fuelcharacteristicsin pressure and thrust oscillations.The standardhybridrock- et fuel is hydroxyl-terminated polybutadiene (HTPB).A fuel additive,guanidinium azo-tetrazolate (GAT),has been shown to increase thrust and impulseof the rocket when added as 15%by mass to the fuel. This study compares the amplitude of the pressure and thrust oscillationsof the rocket whenburning HTPB fuelsand whenburning GAT-addedfuels.Data from sever- al firings at oxygen flowrates from 0.018kg/sec to 0.054 kg/sec are analyzed.Results show the GAT-addedfuel combustion showsno significant increase or decreasein the amplitude of the pressure and thrust oscillations.