Dynamic modeling of hybrid rocket combustion

The combustion dynamics in hybrid rockets is studied to provide a basic inp ut Into transient motor processes. The model treats the time-dependent heat flow into the ablating fuel surface. A variable surface temperature is con sidered with an effective activation energy to describe the surface-tempera ture variation during the transient. Two time scales are observed for throt tling: a short lag near the surface related to the activation energy, and t he 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 ampl ification of the regression-rate oscillations for low frequencies. Although this effect is not the cause of instability, it can aggravate existing osc illations at these low frequencies. We next formulated a quasisteady combus tion 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 linear ized treatment of this coupled system evidences some low frequency instabil ities. The scaling of the oscillation frequencies and the erratic character of the experimentally observed instabilities are successfully explained.