Dynamic Modeling of Hybrid Rocket Combustion

The combustion dynamics in hybrid rockets is studied to provide a basic input into transient motor processes. The model treats the time-dependent heat flow into the ablating fuel surface. A variable surface temperature is considered with an effective activation energy to describe the surface-temperature variation during the transient. Two time scales are observed for throttling: a short lag near the surface related to the activation energy, and the larger well-known thermal resulting from conductivity. The model is also applied to an oscillating surface heat-flux input. We observed an amplification of the regression-rate oscillations for low frequencies. Although this effect is not the cause of instability, it can aggravate existing oscillations at these low frequencies. We formulate a quasi-steady combustion model, which is coupled with the thermal lag system with boundary-layer delays that account for the adjustment of the boundary layer to the changes in the freestream conditions. A linearized treatment of this coupled system provides evidence for some low-frequency instabilities. The scaling of the oscillation frequencies and the erratic character of the experimentally observed instabilities are explained. (Author)