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 e ow 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 heate ux input. Weobserved an amplie cation oftheregression-rateoscillations for low frequencies.Although thiseffect 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 thethermal lag system with boundary-layer delaysthat 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.