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 lag as a result of conductivity. The model is also applied to an oscillating surface heat-flux input. me 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 next formulated a quasisteady combustion model, which is then 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 and blowing from the surface. A linearized treatment of this coupled system evidences some low frequency instabilities. The scaling of the oscillation frequencies and the erratic character of the experimentally observed instabilities are successfully explained.