Pyrolysis and combustion of HTPB, GAP, and OXSOL for hybrid rocket motor applications

HTPB was studied under conditions that closely simulate a hybrid rocket motor. The results of the HTPB characterization indicated that the activation energy for pyrolysis was relatively low for surface temperatures between 700 to 1000 K. For the conductive heat technique, the pyrolysis activation energy was 4.9 kcal/mole. The pyrolysis activation energy in an oxidizing environment was 2.6–2.9 kcal/mole and was found to depend on pressure. The characterization of GAP energetic fuel was studied using several techniques: a strand burner, small motor firings, and a hybrid slab motor. The burning rate of GAP was found to have a moderate pressure exponent of 0.56 for pressures up to 13.5 MPa. A slope break in the burn rate curve was found to occur at pressures above 13.5 MPa with a pressure exponent of 0.12. Additionally, the temperature sensitivity of GAP had a pressure dependence below 10 MPa and ranged from 0.015 to 0.008 K−1. Above 10 MPa, the temperature sensitivity was constant at 0.008 K−1. Analysis of the decomposition mechanism of GAP indicated an activation energy for regression rate of 41 kcal/mol at surface temperatures up to 712 K and an activation energy of 6.6 kcal/mol at higher surface temperatures. The gas-phase heat flux was determined to be approximately 20 W/cm2, which is two orders of magnitude lower than the surface heat release rate of 1600 W/cm 2. The pressure dependence of GAP burning rate can be explained as a surface heat release process which is controlled by desorption of energetic GAP fragments. Based upon the small motor firings of GAP and the low gas-phase temperature and heat flux, the gas-phase composition appears to not change much from the initial decomposition products at the surface. Liquid strand burning measurements indicated a dependence of OXSOL regression rate on tube diameter. A diameter sensitivity term, analogous to the temperature sensitivity of solid propellants, was introduced and was found to decrease with pressure from 0.07 to 0.04 mm−1. Analysis of the gas phase temperatures indicated that water vaporization played a key role in the regression rate of OXSOL at pressures below 9 MPa.